Silicon-Based On-Chip Light Sources: A Review

Abstract

Silicon-based on-chip light sources are important since they can provide a compact solution for various applications in the field of high-speed optical communications, high-precision sensing, quantum information processing, and so on. We review the progress of silicon-based on-chip light sources in various materials. We provide some key parameters like pump thresholds, output powers, and pump schemes of on-chip lasers based on various materials. Finally, we point out the existing issues in the current investigations and possible solutions in the future.

1. Introduction

Emerging applications, such as big data, 5G networks, artificial intelligence, and cloud computing, have posed unprecedented challenges to the processing speed of signals, power consumption of systems, and transmission bandwidth of signals in current integrated optoelectronic devices. Compared with traditional telecom systems, all-optical signal processing, with the advantages of high transmission speed, broad bandwidth, electromagnetic interference immunity, low power consumption, and superior security, has gained significant attention in this high-speed optical communication era. Therefore, there is an urgent need to develop new-generation photonic integrated chips with high integration density, low power consumption, and efficient all-optical operation [1,2,3,4].

Silicon photonics is considered a leading platform for integrated optics due to its compatibility with existing CMOS manufacturing processes, which allows for cost-effective, large-scale production of photonic integrated circuits (PICs). Various materials have been deposited on silicon wafers for the development of various functional devices [1,2,3,4]. Current silicon-based optical components, including passive devices like optical waveguides, filters, and multiplexers, as well as active devices such as optical amplifiers [5], high-speed optical modulators [6], optical switches [7], and photodetectors [8], have attracted extensive attention in new-generation optical communication technologies. However, the slow development of on-chip silicon light sources has hindered the realization of large-scale integration for silicon photonic devices. This remains a major challenge in telecom systems. As an indirect bandgap semiconductor material [9,10,11], silicon exhibits low luminous efficiency. Moreover, silicon’s 1.1 eV bandgap limits emitted light wavelengths to <1.1 μm. Clearly, developing on-chip light sources directly from silicon faces significant limitations.

To address these challenges, researchers have developed various on-chip light generation methods on silicon platforms, including stimulated Raman scattering [12], hybrid III-V/Si bonding [13], and rare-earth-doped gain media [14]. Raman-based approaches are constrained by silicon’s narrow Raman gain bandwidth and strong two-photon absorption. III-V/Si heterogeneous integration represents a primary direction for silicon light sources, but suffers from limitations including short carrier lifetime, refractive index variations induced by electron–hole pair excitation, strong nonlinear optical responses, and spatiotemporal gain pattern effects caused by low emission efficiency in extended near/mid-infrared wavelengths. Moreover, the expensive and complex integration processes required for III-V semiconductors with silicon devices hinder their compatibility with standard CMOS processes.

In contrast, rare-earth (RE)-doped optical gain materials have recently shown significant promise for silicon-based light sources. These materials can be either doped into silicon or monolithically deposited on silicon, enabling low-cost wafer-scale manufacturing and universal device design. They exhibit advantages including high thermal stability, narrow intrinsic linewidth, and broadband optical gain/lasing through optical pumping [15,16]. However, silicon’s low rare-earth doping concentration and high optical loss limit its luminous efficiency. An alternative effective approach involves integrating rare-earth-doped thin-film materials or gain devices with silicon chips. Erbium-doped systems, for instance, achieve population inversion through optical pumping and emit at 1.53 μm wavelength—corresponding to the minimum absorption window of silica fibers in optical communications. This wavelength remains unaffected by pump power or environmental temperature, serving as the standard wavelength in silicon photonics that meets communication-band signal processing requirements. Erbium-doped thin-film fabrication processes are compatible with silicon photonic manufacturing, offering attractive cost-effectiveness and design flexibility. Furthermore, combining population inversion in gain films with resonant enhancement in on-chip microcavities could ultimately enable on-chip laser emission. Therefore, developing novel erbium-integrated on-chip light sources compatible with silicon photonic systems represents an excellent solution to enhance silicon photonics’ applicability in optical communications, holding significant importance for the advancement of silicon photonic technology.

In the present review, we concentrated on various on-chip lasers that can be integrated with silicon wafers. Especially, we focus on those electrically- and optically-pumped light sources with emission wavelengths around the telecom band. While the optically pumped light sources are dominant in most of the literature, electrically pumped sources may be more practical in the applications of integrated photonic devices. For other kinds of on-chip light sources, the readers can refer to other excellent review papers in optical frequency combs [17], supercontinuum sources [18], Raman lasers [19], and so on.

2. Hybrid III-V/Si Bonding Laser

It is challenging to realize high-efficiency light emission in silicon due to its indirect bandgap, leading to difficulty in achieving direct interband transitions and efficient radiative recombination of carriers. To overcome this physical constraint, researchers focus on direct bandgap semiconductor materials, like III-V compound semiconductors combining group III elements (Al, Ga, In) with group V elements (P, As, Sb). These materials exhibit multiple critical advantages in laser applications. Firstly, their intrinsic direct bandgap enables over 90% electron–hole radiative recombination efficiency, achieving orders-of-magnitude improvement in quantum efficiency compared to indirect bandgap materials like silicon. Secondly, precise control of ternary (e.g., InGaAsP) or quaternary (e.g., AlGaInAs) alloy compositions allow flexible bandgap engineering across 0.4–4.5 μm, meeting broad-spectrum applications from visible to mid-infrared. Moreover, exceptional high-frequency characteristics (carrier mobility > 3000 cm²/(V·s), saturation velocity > 1 × 10⁷ cm/s) support high-speed modulation beyond 10 Gb/s, fully compatible with modern optical communication standards. Furthermore, heterostructure designs such as quantum wells and quantum dots enable three-dimensional localization of carriers and optical modes, reducing threshold current density to the μA level while significantly enhancing thermal stability (characteristic temperature T₀ > 120 K). These properties establish III-V compounds as the cornerstone material system for high-performance lasers. Current research prioritizes developing silicon-compatible integration technologies, including low-temperature bonding and selective epitaxy, to achieve scalable photonic integration [20,21,22].

Various materials, including semiconductor quantum wells, nanowires, and quantum dots, have been deposited, and the corresponding devices have been fabricated for the development of on-chip lasers; below are the typical results.

2.1. Quantum Well and Nanowire

L. Cerutti et al. explored the viability of GaSb-based lasers for monolithic silicon integration toward 1.55 μm emission. By designing an active region based on strained Ga_0.8In_0.2Sb quantum wells, effective carrier confinement was achieved. Edge-emitting lasers fabricated on GaSb substrates demonstrated continuous-wave operation at 1.56 μm up to 318 K. The same structure grown on silicon substrates exhibited room-temperature pulsed operation at 1.55 μm, achieving a threshold current density of 5 kA/cm² and an output power of 12 mW at 288 K under 200 mA driving current [23].

Dhruv Saxena et al. first demonstrated room-temperature lasing in GaAs/AlGaAs/GaAs core-shell-cap nanowires through precise Fabry-Pérot cavity design. Under 522 nm optical pumping, the device exhibited threshold pump fluence of 96 μJ/cm² at 6 K and 207 μJ/cm² at 300 K, with a threshold gain of 1820 cm⁻¹. This breakthrough marks a critical step toward integrating GaAs nanowire lasers into near-infrared nanophotonic devices [24].

Jun Wang et al. reported an InGaAs/AlGaAs quantum well laser structure prepared by metal–organic chemical vapor deposition (MOCVD). Material characterization revealed that this method effectively reduces threading dislocation density and interfacial roughness, allowing the optical performance of the silicon-based active region to approach levels achieved in structures grown on native GaAs substrates. Fabricated broad-area lasers (1 mm cavity length, 15 μm stripe width) demonstrated an exceptionally low threshold current density (313 A/cm²) under room-temperature pulsed operation. The laser achieved continuous-wave operation at 240 K [25].

Alexander Spott et al. reported the first heterogeneously integrated 4.8 μm quantum cascade laser (QCL) on a silicon platform, utilizing an InP-based InGaAs/InAlAs quantum cascade active region integrated with a silicon-on-nitride-on-insulator (SONOI) waveguide structure (as shown in Figure 1a). The device showed room-temperature pulsed operation with a threshold current of 387–388 mA, maximum output power of 31 mW, and slope efficiency of 170 mW/A at 293 K (as shown in Figure 1b). Future optimizations in thermal management and waveguide design may enable continuous-wave operation [26].

Meixin Feng et al. proposed a preliminary on-chip integration scheme featuring GaN-based lasers, modulators, and photodetectors monolithically grown on silicon substrates. The modulator, integrated and sharing the same InGaN quantum well active region with the laser, enabled light intensity control by modulating applied biases to induce energy band bending in the active region, thereby adjusting absorption characteristics. This approach achieved threshold current and optical power modulation, with a spectral linewidth of 1.0 nm and a tunable threshold current (minimum at +2 V and increased to 770 mA at −4 V) accompanied by a slope efficiency of 98.1 mW/A. The photodetector effectively monitored the modulated laser output, demonstrating the feasibility of voltage-tunable intra-system light detection [27].

The team subsequently reported a room-temperature electrically pumped InGaN-based microdisk laser grown on silicon substrate. The device, fabricated via AlGaN cladding layers (vertical optical confinement factor 1.84%) and microring resonator configuration (R = 20 μm/r = 10 μm), achieved lasing at 412.4 nm under 250 mA threshold current with spectral linewidth narrowed to 0.4 nm, as shown in Figure 2a,b. Electroluminescence spectra exhibited distinct slope discontinuity in light–current curves in Figure 2c, providing conclusive evidence of lasing. This work marks the first observation of room-temperature electrical pumping in InGaN-based microdisk lasers directly grown on silicon substrates [28].

Hoang Nguyen-Van et al. reported QCLs directly grown on silicon substrates with an emission at 11 μm. The devices exhibited a room-temperature (300 K) threshold current density as low as 1.3 kA/cm² (3 mm cavity length) with differential gain of 16.9 cm/kA and maximum operating temperature exceeding 380 K, matching the performance of InAs-substrate counterparts (threshold current density 1.03 kA/cm²) [29].

Z. Loghmar et al. demonstrated monolithically integrated indium arsenide/aluminum antimonide (InAs/AlSb) QCLs on (001) silicon substrates. By utilizing a 1.5-μm-thick GaSb buffer layer and high-temperature substrate pretreatment (1000 °C annealing), antiphase-domain-free epitaxial growth was realized on silicon substrates. The lasers emitted at 8 μm wavelength under room-temperature (300 K) operation, with threshold current densities of 0.92–0.95 kA/cm² for 3.6 mm-long device in pulsed mode. Comparative studies showed that QCLs with identical designs grown on native InAs substrates exhibited lower threshold current densities (0.75 kA/cm²) while maintaining equivalent maximum operating temperatures as shown in Figure 3. The low threshold current density characteristics of these silicon-based QCLs make them particularly suitable for photonic integrated sensor applications [30]. The relatively high threshold current density in the silicon-based InAs/AlSb QCL is primarily attributed to crystal defects introduced during heteroepitaxial growth. High dislocation density resulting from the lattice mismatch between the silicon substrate and III-V materials significantly increases non-radiative recombination and reduces carrier efficiency. Additionally, the rougher heterointerface of the silicon-based material (RMS reaching 9.9 nm) increases optical scattering losses, while material inhomogeneity slightly broadens the gain spectrum, collectively pushing up the threshold current.

2.2. Quantum Dots

Recent research on quantum dot lasers based on the InAs/GaAs material system has garnered significant attention, with scientific efforts focusing on structural design, fabrication processes, and optoelectronic performance optimization [31,32,33,34,35]. Notably, Chen et al. achieved the first electrically driven continuous-wave InAs/GaAs quantum dot laser. Using metal–organic chemical vapor deposition (MOCVD), a 400-nm-thick anti-phase boundary-free GaAs film with a root-mean-square (RMS) surface roughness as low as 0.86 nm was epitaxially grown on silicon substrates, forming a high-quality virtual substrate. Subsequent molecular beam epitaxy (MBE) enabled the fabrication of quantum dot laser structures on this GaAs/Si (001) platform, delivering breakthrough performance: under room-temperature continuous-wave operation, the laser demonstrated a threshold current density as low as 425 A/cm² with single-facet output power reaching 43 mW at a stable emission wavelength of 1.3 μm. In pulsed mode, it achieved a threshold current density to 250 A/cm² and single-facet output power surpassing 130 mW at room temperature, and the device worked even at a temperature of 375 K [33].

Liu et al. reported the electrically pumped continuous-wave (CW) III-V semiconductor laser monolithically grown on (001) silicon substrate. This achievement was realized using InAs/GaAs quantum dots as the active region and incorporating a GaP buffer layer between the silicon substrate and device layer. The laser had a threshold current density of ~860 A/cm² and single-facet output power of 110 mW. This work validates the compatibility of high-performance III-V light sources with standard silicon substrates, providing a critical technical pathway for silicon photonic integration [34].

Chen Shang et al. demonstrated the electrically pumped quantum dot laser grown via molecular beam epitaxy on 300-mm patterned (001) silicon wafers, employing InAs/GaAs quantum dots as the gain medium combined with a buffer layer architecture based on AlGaAs/GaAs material systems. At 20 °C, the maximum double-side output power reaches 126.6 mW, and a threshold current is 47.5 mA in Figure 4a. The device operates in continuous-wave (CW) mode up to 60 °C, as shown in Figure 4b, exhibiting a distinct lasing peak at 1300 nm above the threshold in Figure 4c. Figure 4d shows minimal variation in the bias voltage required to maintain 10 mW output power over 350 h, indicating device reliability approaching that of similar lasers grown on bare silicon substrates. This heterogeneous epitaxial integration platform on silicon holds the potential for robust on-chip laser operation and efficient low-loss coupling to silicon photonic circuits, offering prospects for scalable and low-cost mass production [35].

2.3. InP

InP-based semiconductor lasers demonstrate superior device performance in the near-infrared (1.3–1.6 μm) band, characterized by low threshold current density and high energy conversion efficiency, owing to their direct bandgap characteristics, high carrier mobility, and exceptional optical field confinement.

Recent studies [36,37,38,39,40,41,42,43,44,45,46,47] have achieved breakthrough improvements in device performance through heterostructure design and interface engineering optimization. For example, Wang et al. developed a III-V/Si hybrid integrated single-microcavity narrow-linewidth laser combining an InP multiple quantum well gain section with silicon-based microring resonators (Si/SiN waveguides). The device achieved single-mode lasing with a side-mode suppression ratio (SMSR) exceeding 45 dB; the peak value reaches 48 dB, as shown in Figure 5, and a maximum output power of 16.4 mW. Compared to current hybrid/heterogeneous integrated laser architectures requiring multiple complex controls, this design simplifies laser characterization complexity for mass production through only two control parameters. This streamlined structure and control framework simultaneously realized an ultranarrow linewidth of 2.79 kHz and a low relative intensity noise (RIN) of −135 dB/Hz. The system demonstrated a 10 dB signal-to-noise ratio (SNR) in 12.5 Gb/s optical data transmission experiments, showing potential for phase noise-sensitive applications such as coherent optical communications and LiDAR systems [38].

Chao Xiang et al. demonstrated optical amplification via current injection in a hybrid InP/Si gain waveguide, combined with an extended distributed Bragg reflector (E-DBR) design: one side features a 20 mm long SiN grating (providing narrowband feedback with κ values of 0.25–0.875 cm⁻¹) in Figure 6a,b, while the opposite side integrates a silicon-based Mach–Zehnder interferometer (MZI) broadband tunable reflector, forming a hybrid integrated external-cavity laser. For a laser operating at telecommunication wavelengths (1548 nm in Figure 6c), Figure 6d reveals mode hopping induced by intracavity phase variations in the laser; this laser delivers >10 mW output power through SiN waveguides, as shown in Figure 6a, achieving threshold current densities as low as 580 A/cm², baseband linewidths <1 kHz in Figure 6e, and relative intensity noise (RIN) < −150 dBc/Hz (up to 20 GHz) in Figure 6f. Through self-injection locking coupled with a SiN microring resonator (Q-factor of 42 million), the frequency noise is reduced by 20–30 dB, realizing a Lorentzian linewidth of 3 Hz [46].

Table 1. Summary of the typical results in various III-V-based semiconductor lasers.

Material	Threshold (Threshold Intensity)	Working Wavelength	Maximum Output Power	Pumping Scheme	Ref.
GaSb	5000 A/cm2	1550 nm	12 mW	Electrical	[23]
GaAs	207 μJ /cm2 per pulse	830–870 nm	/	Optical	[24]
GaAs	313 A/cm2	976.8 nm	>300 mW	Electrical	[25]
InP	0.9 mA	1526–1532 nm	0.93 mW	Electrical	[36]
InP	22 ± 2.1 mW	930.5 nm	6.4 mW	Optical	[37]
InP	52 mA	1563–1566 nm	16.4 mW	Electrical	[38]
InAs/GaAs	650 A/cm2	1290 nm	/	Electrical	[31]
InAs/GaAs	62.5 A/ cm2	1310 nm	>105 mW	Electrical	[32]
InAs/GaAs	425 A/cm2	1285–1292 nm	43 mW	Electrical	[33]
InAs/GaAs	860 A/cm2	1280 nm	110 mW	Electrical	[34]
InAs/GaAs	47.5 mA	1300 nm	126.6 mW	Electrical	[35]
InGaAs/InAlAs	387–388 mA	4800 nm	31 mW	Electrical	[26]
GaN	700 mA	405 nm	/	Electrical	[27]
InGaN	250–1600 mA	412.4 nm	/	Electrical	[28]
InAs/AlSb	920–950 A/cm2	8000 nm	/	Electrical	[30]
InAs/AlSb	1300 A/cm2	1050–1110 nm	>100 mW/facet	Electrical	[29]
InP	36 mA	1310 nm	22 mW	Electrical	[39]
InP	17 mA	1566 nm	6 mW	Electrical	[40]
InP	17 mA	1566 nm	6 mW	Electrical	[41]
InP	100 μA	1560 nm	95 ± 2 μW	Electrical	[42]
InP	51 mA	1544 nm	/	Electrical	[43]
InP	100 mA	1480–1590 nm	3 mW	Electrical	[44]
InP	50 μJ/cm2 per pulse	870–910 nm	/	Optical	[45]
InP	42 mA	1548 nm	>10 mW	Electrical	[46]
InP	30 mA	1541–1589 nm	/	Electrical	[47]

lists the typical results of the III-V-based semiconductor laser integrated with silicon, including emission wavelength, threshold pump power, and maximal output power. In most cases, the output powers are limited at around few or tens of mW; for practical applications, it is highly desired to have a high-power laser output with relatively lower pump power. Especially for the on-chip applications, it might be more important to have a laser with low pump power and high conversion ratio of pump to output power.

3. Al₂O₃

Al₂O₃ is probably most investigated host materials for the development of rare-earth ions doped on-chip lasers. Several important material properties, like low propagation loss, good rare-earth ion solubility, and high gain characteristics, make Al₂O₃ an ideal platform for the development of RE-doped on-chip lasers [48]. The Al₂O₃-based on-chip lasers can be categorized into different types: microring resonator-based lasers and distributed Bragg reflector-based lasers, including distributed feedback lasers (DFB); and distributed Bragg reflector lasers (DBR).

3.1. Microring Resonator Lasers

Microring resonator lasers offer advantages such as compact size (typically on the order of tens of micrometers), high integration density, high quality factor (Q values up to 10⁵–10⁶), low threshold, and CMOS compatibility. Starting from the initial observation of net optical gain in Al₂O₃:Er³⁺ thin films by Polman’s team in 1996 [49], and after over a decade of exploration, Bradley’s team achieved on-chip lasing in 2009 using a microring resonator structure, demonstrating the first low-threshold laser emission at communication wavelengths on a silicon platform [50]. This breakthrough provided a CMOS-compatible light source solution for photonic integrated circuits. The device balanced high pump coupling efficiency and low signal output coupling through optimized microring resonator design. A 500-nm-thick Al₂O₃:Er³⁺ layer was deposited on a silicon substrate via reactive co-sputtering, followed by photolithography and reactive ion etching to form 1.5-μm-wide single-mode waveguides, capped with a 5-μm-thick SiO₂ upper cladding layer. By adjusting the directional coupler length (350–600 μm) and microring cavity length (2.0–5.5 cm), broad wavelength selectivity could be achieved, as shown in Figure 7. Within the 1530–1557 nm range, the lasing wavelength was tuned by modifying the output coupling ratio. The device exhibited a maximum output power of 9.5 μW, a slope efficiency of 0.11%, and a threshold pump power as low as 6.4 mW, outperforming previously reported Er³⁺-doped microring lasers. Bradley’s team further explored optimization strategies such as ytterbium (Yb³⁺) co-doping, unidirectional output structures, and distributed feedback to enhance device efficiency, reduce size, and achieve single-frequency operation.

In 2014, Bradley et al. demonstrated a monolithically integrated erbium (Er³⁺)/ytterbium (Yb³⁺)-doped microring laser based on a silicon-compatible process. The laser employs a dual-SiN-layer structure, with a microring outer diameter of 160 μm and an Al₂O₃:Er³⁺ gain layer thickness of 2 μm. For the undoped microring, the measured maximum intrinsic quality factors (Qᵢ) were 3.8 × 10⁵ at 980 nm and 5.7 × 10⁵ at 1550 nm. The Er³⁺-doped device achieved single-mode lasing at 1559.82 nm with a threshold of 0.5 mW, a double-ended slope efficiency of 0.3%, and a maximum output power of 2.4 μW. The Yb³⁺-doped device demonstrated single-mode operation at 1042.74 nm, exhibiting a threshold of 0.7 mW, a slope efficiency of 8.4%, and an output power exceeding 100 μW. The devices are coupled via 4 μm deep trenches to SiN waveguides with widths of 0.4/0.9 μm and optimized gap dimensions of 0.3–0.5 μm. This design enables high-density silicon-based on-chip integration through a SiN-Al₂O₃ composite cavity structure, reducing the device footprint by 500 times and lowering the threshold by over an order of magnitude compared to traditional waveguide lasers. It provides a scalable micro/nano light source solution for optical communications (1.5 μm) and biophotonics (1.0 μm) applications [51].

In 2023, Wang et al. reported a photonic chip-integrated titanium–sapphire (Ti:Sa) laser by combining Ti:Sa gain media with a silicon nitride-sapphire integrated photonic platform. The research team designed a microring resonator with a 200 μm radius, achieving laser emission across the 730–830 nm wavelength range. By confining both pump and lasing modes within a single microring, the device reduced the threshold power to 6.5 mW—two orders of magnitude lower than conventional Ti:Sa lasers. Single-mode operation was realized through a distributed Bragg reflector (DBR) and feedback loop structure, yielding a side-mode suppression ratio (SMSR) of 16 dB, along with a 0.5 mW fiber-coupled output power and a narrow linewidth of 120 kHz [52].

In 2024, Yang et al. reported a photonic integrated platform based on monocrystalline titanium–sapphire-on-insulator (Ti:SaOI). Utilizing a 50 μm-diameter microring resonator, they achieved a Q factor of 1.3 × 10⁶ and demonstrated continuous-wave laser emission with an ultralow threshold of 290 μW under 532 nm pumping. High-confinement waveguides enabled amplified spontaneous emission (ASE) with a peak gain of 64 dB/cm and a bandwidth of 100 THz. An 8 mm spiral waveguide achieved 20 dB total gain under 175 mW pumping, successfully amplifying 2.2 ps pulses without distortion to 1.0 kW peak power and 2.3 nJ energy. Integration of a platinum micro-heater enabled 50 nm wavelength tuning in an 80 μm microring, yielding 140 kHz linewidth and 1.8 mW output power when pumped by commercial laser diodes. The platform manipulated silicon carbide quantum emitters via laser arrays. This system reduces the size and cost of traditional Ti:sapphire setups by three orders of magnitude, achieves the first solid-state amplification in sub-micrometer bands, and provides miniaturized solutions for quantum optics and high-power frequency comb applications [53].

3.2. DBR and DFB

Generally, microring lasers are prone to multimode oscillation, requiring coupling gap optimization or auxiliary structures to maintain single-mode lasing. Additionally, limited by their small mode field volume, microring lasers typically exhibit low output power (e.g., microwatt-level), making them unsuitable for direct high-power applications. In contrast, DBR lasers demonstrate superior single-mode stability and higher output power capabilities.

In 2013, Purnawirman et al. utilized a wafer-scale immersion lithography process to fabricate grating structures combined with silicon nitride waveguides, forming a distributed Bragg reflector (DBR). A layer of erbium-doped aluminum oxide (Al₂O₃:Er³⁺) was then deposited atop as the gain medium, creating an inverted ridge waveguide. The DBR cavity achieved an optical power output exceeding 5 mW, with laser emission observed across a broad wavelength range, including peaks at 1536 nm, 1561 nm, and 1596 nm [54].

Compared to DBR lasers, DFB lasers exhibit narrower linewidths and higher side mode suppression ratios (SMSR). These characteristics are critical for devices requiring long coherence lengths and extremely high phase sensitivity, such as those used in coherent optical communication systems. Michael Belt et al. reported an arrayed erbium-doped aluminum oxide (Al₂O₃:Er³⁺) waveguide distributed feedback (DFB) laser based on an ultra-low-loss silicon nitride (Si₃N₄) platform. This laser employed 248 nm stepper lithography to define sidewall grating structures within the Si₃N₄ layer, combined with a reactive co-sputtering process to fabricate the erbium-doped gain layer, achieving a monolithically integrated design. A five-channel laser array with wavelengths distributed across 1531–1543 nm (spanning 12 nm) demonstrated a single-mode output power of 8 μW, a side mode suppression ratio (SMSR) exceeding 35 dB, and a linewidth of 501 kHz [55].

Subsequently, Michael Belt et al. further refined the integration of erbium-doped waveguide distributed Bragg reflector (DBR) and distributed feedback (DFB) laser arrays on an ultra-low-loss silicon nitride (Si₃N₄) waveguide platform. By optimizing cavity design and leveraging sidewall grating technology enabled by the ultra-low-loss platform, they achieved low threshold power and high slope efficiency. The DBR lasers exhibited a threshold as low as 11 mW under 974 nm pumping, with a pump-to-signal conversion efficiency of 5.2%. For the DFB lasers, extending the cavity length to 21.5 mm (nearly three times longer than previous designs) enhanced pump efficiency by over two orders of magnitude, reaching up to 0.77%. Figure 8 demonstrates the spectral performance of the DBR and DFB laser arrays: Figure 8a displays the output spectra of five DBR lasers with distinct grating periods (ranging from 478 nm to 486 nm), emitting at wavelengths of 1535 nm, 1541 nm, 1547 nm, 1554 nm, and 1560 nm, respectively; Figure 8b shows the output spectra of four DFB lasers with grating periods (from 478 nm to 490 nm), operating at wavelengths of 1534 nm, 1546 nm, 1558 nm, and 1570 nm; all devices exhibit side mode suppression ratios exceeding 50 dB, and the output power variations arise from the wavelength-dependent gain threshold and maximum small-signal gain spectrum of the erbium-doped gain layer. The devices demonstrated stable performance under uncooled conditions, and future improvements could further boost efficiency by adopting 1480 nm pumping to mitigate energy-level bottlenecks, introducing Yb sensitizers, or integrating pump filters. This technology offers a scalable solution for multi-wavelength array integration with high thermal stability and narrow linewidth [56].

Purnawirman et al. subsequently reported an erbium-doped aluminum oxide (Al₂O₃:Er³⁺)-based distributed Bragg reflector (DBR) and distributed feedback (DFB) laser utilizing a multi-segment silicon nitride waveguide architecture. Employing a five-segment silicon nitride waveguide design, they embedded the waveguides in silicon dioxide (SiO₂) and integrated them with a top erbium-doped aluminum oxide layer to achieve lasing in two configurations: a DBR cavity with a 13.8 mm gain length (featuring 5 mm periodic sidewall gratings) and a DFB cavity with a 20 mm full-gain region (incorporating 550 nm spaced gratings). The DBR laser delivered a maximum output power of −3.6 dBm (0.44 mW) at 1565 nm, with a slope efficiency of 1.4% and a threshold pump power of 64 mW. The DFB laser, benefiting from grating optimization, achieved a lower threshold (14 mW), an output power of −7.3 dBm (0.18 mW), and an improved slope efficiency of 2.7%. By thickening the silicon nitride layer to 200 nm, this design overcame the integration limitations of conventional thin-layer waveguides while maintaining efficient mode confinement in the gain medium, paving the way for monolithic integration in communication, sensing, and multi-wavelength photonic systems [57].

In 2017, Purnawirman et al. reported a wavelength-insensitive waveguide design based on a silicon nitride platform, achieving ultra-narrow-linewidth erbium-doped aluminum oxide (Al₂O₃:Er³⁺) distributed feedback (DFB) lasers. The waveguide consists of five segmented silicon nitride (SiNx) sections (200 nm thick, 450 nm wide, with 400 nm gaps) embedded in silicon dioxide (SiO₂), topped with a 1100 nm thick Al₂O₃:Er³⁺ gain layer. The schematic diagram of the device structure is shown in Figure 9a. By comparing discrete quarter-phase-shifted (QPS-DFB) and distributed-phase-shifted (DPS-DFB) cavity configurations, the QPS-DFB lasers achieved on-chip output powers of 0.41 mW, 0.76 mW, and 0.47 mW at C/L-band wavelengths (1536 nm, 1566 nm, and 1596 nm), with slope efficiencies of 0.3–0.6% and threshold powers ranging from 55 mW to 105 mW. Figure 9b and Figure 9c respectively illustrate the optical spectra at different grating periods and the on-chip output power versus pump characteristics of the distributed-phase-shifted distributed feedback (DPS-DFB) lasers. In contrast, the DPS-DFB laser demonstrated significantly enhanced performance at 1565 nm, delivering an output power of 5.43 mW (slope efficiency: 2.9%, threshold: 14 mW) and a side mode suppression ratio (SMSR) exceeding 59.4 dB. This work provides a critical reference for integrating narrow-linewidth lasers on silicon photonics platforms [58].

Table 2. Overview of on-chip lasers on rare-earth ion-doped Al₂O_3.

Cavity Type	Rare-Earth Dopant	Laser Wavelength	Maximal Slope Efficiency	Maximal Laser Output Power	Linewidth	Reference
Ring	Er/Yb	1560 nm/1042 nm	0.3%/8.4%	2.4 µW/100 µW	/	[51]
Ring	Tm	1760–1920 nm	24%	230 µW	/	[59]
Ring	Ti	790–930 nm	2.9%	1.8 mW	140 kHz	[53]
DFB	Er	1543 nm	0.006%	9 µW	501 kHz	[55]
DFB	Er	1560 nm	0.77%	270 µW	/	[56]
DFB	Er	1563 nm	2.2%	1.0 mW	/	[60]
DFB	Er	1563–1580 nm	/	20 µW	/	[61]
DFB	Er	1553 nm	1.3%	2.6 mW	/	[62]
DFB	Er	1592 nm	0.7%	1.2 mW	/	[63]
DFB	Er	1599 nm	0.01%	/	/	[64]
dps-DFB	Er	1565 nm	2.9%	5.43 mW	5.3 kHz	[58]
qps-DFB	Er	1536, 1566, 1596 nm	0.6%	0.76 mW	30.4 kHz	[58]
DFB	Tm	1861 nm	14%	267 mW	/	[65]
DFB	Ho	2022–2101 nm	2%	15 mW	/	[66]
DBR	Er	1536, 1561, 1596 nm	2.6%	5.1 mW	/	[54]
DBR	Er	1546 nm	5.2%	2.1 mW	/	[56]
DBR	Er	1564 nm	0.02%	/	/	[14]
DBR	Tm	1881 nm	23%	387 mW	/	[65]

summarizes the current development of on-chip laser technologies utilizing RE-doped alumina materials, with key performance parameters covering laser wavelength, maximum slope efficiency, maximum output power, and linewidth. Microring resonator lasers feature miniaturized size, high integration density, and low threshold, yet their output power is typically below 10 μW due to the limitation of small mode volume; in contrast, DBR lasers combine single-mode stability with high-power output, while DFB lasers demonstrate unique advantages in phase-sensitive scenarios like coherent optical communications by offering ultra-narrow linewidth and high side-mode suppression ratio.

4. LiNbO₃

As a representative electro-optic material, lithium niobate exhibits a strong electro-optic effect, enabling rapid dynamic modulation of laser wavelength, phase, and output intensity through external electric fields—a critical capability for designing tunable narrow-linewidth lasers. Additionally, its remarkable nonlinear optical properties support efficient frequency conversion processes (e.g., frequency doubling and parametric oscillation), facilitating multi-wavelength laser output on a single chip and expanding spectral coverage.

In 1992, P. Becker et al. pioneered the first Er-diffused Ti:LiNbO₃ single-mode waveguide laser, achieving continuous-wave and pulsed operations at 1576 nm (threshold: 13 mW under 1479 nm π-polarized pumping) and 1563 nm (activated at 25 mW). Subsequent advancements by J. Amin et al. demonstrated Nd-diffused and rare-earth-doped LiNbO₃ lasers, where optimizing cavity mirrors with 95% reflectivity reduced thresholds from 39 mW to 4.2 mW and achieved 8.5% slope efficiency at 1531.4 nm. J.K. Jone et al. reported Yb-doped waveguide lasers, addressing photorefractive instability via wet-oxygen annealing (500 °C, 14 h), enhancing a 14 mm device to 16% slope efficiency. Ingo Baumann et al. optimized Ti:Er:LiNbO₃ lasers theoretically, attaining 37% slope efficiency (1561 nm, 63 mW output at 210 mW pump), highlighting communication compatibility. S. BALSAMO et al. developed a Q-switched Ti:Er:LiNbO₃ laser, delivering 2.4 W peak power and 0.18 μJ pulses at 2 kHz under 100 mW pumping. These milestones underscore LiNbO₃-based lasers’ potential for high-efficiency, stable, and pulsed applications in photonic technologies [67,68,69,70,71,72].

Das, BK and collaborators first demonstrated a single-frequency Ti:Er:LiNbO₃ distributed Bragg reflector (DBR) waveguide laser containing two thermally fixed photorefractive Bragg gratings. The laser initiated lasing at 70 mW pump power, achieving 1.1 mW output power at 120 mW pump power with a slope efficiency of approximately 2%. Operating in TE-polarized single longitudinal mode, it exhibited a linewidth of ~0.75 GHz. Temperature adjustment of the output coupling grating enabled a 80 pm tuning range with a tuning rate of 8 pm/°C. Experiments confirmed that resonator optimization could enhance output power and slope efficiency, suggesting thermal tuning and electro-optic tuning as promising methods for developing tunable single-frequency light sources [73].

R. E. Di Paolo and colleagues reported continuous-wave laser operation in Zn-diffused LiNbO₃:Nd³⁺ channel waveguides at room temperature. Measured in a 10 μm wide, 0.9 cm long waveguide, the laser exhibited a threshold pump power of 1.25 mW and a slope efficiency of 20%. Sustained stable emission without photorefractive damage was achieved for over 20 min under maximum pump power, reaching a maximum output power of 0.14 mW. The laser operated in TM polarization, providing a novel approach for developing LiNbO₃-based waveguide lasers [74].

Masatoshi Fujimura et al. first demonstrated a Yb-diffused LiNbO₃ annealed/proton-exchanged (APE) waveguide laser with a wavelength bandwidth of 0.36 nm. The device achieved stable continuous-wave lasing at 1061 nm under 918 nm pumping. For a 5 μm channel-width device, the threshold pump power was 40 mW, delivering a maximum output power of 1.2 W with a slope efficiency of 3 × 10⁻⁵ [75].

Fujimura et al. proposed and demonstrated a Yb:LiNbO₃ APE waveguide laser pumped by a 980 nm InGaAs laser diode, achieving lasing at 1061 nm. The device exhibited a spectral bandwidth of 0.3 nm, a threshold pump power of 30 mW, and an output power of 4.3 mW at 90 mW pump power—three orders of magnitude higher than previous reports. This marked a significant advancement in output power, offering critical insights for related fields [76]. In the waveguide with an optimized cavity length of 1.7 cm, they achieved laser emission with a threshold pump power of 57 mW and a maximum output power of 0.32 mW at 75 mW pump power [77].

H. Matsuura et al. developed a low-noise yellow-green laser for fluorescence microscopy using a Yb-doped double-clad fiber laser integrated with a periodically poled lithium niobate (PPLN) waveguide crystal. The laser delivered a maximum output of 30 mW with <2% RMS noise across its operating temperature range and an extinction ratio > 22 dB, providing an enhanced light source for fluorescence imaging [78].

Sohel Mahmud Sher et al. proposed the fabrication of embedded channel waveguides in z-cut Er:LiNbO₃ substrates via high-energy ion implantation and detailed the theoretical design of continuous-wave (CW) waveguide lasers. Without considering excited-state absorption (ESA), their study revealed an optimal waveguide length of 15–20 mm with a threshold coupled pump power of 25 mW. Introducing ESA significantly reduced output power as the ESA cross-section increased, although the threshold remained unaffected. This work provides critical guidelines for optimizing ion-implanted waveguide lasers [79].

Mathew George et al. reported the first Ti:Tm:LiNbO₃ waveguide amplifier operating under 1650 nm in-band pumping. The device exhibited broadband gain spanning 1750–1900 nm, with TE-polarized lasing at 1890 nm achieving a threshold coupled pump power of 4 mW and a slope efficiency of 13.3%. Performance optimization through output mirror reflectivity adjustments highlights its potential for photonic device development [80].

Shilei Ji et al. introduced a novel erbium-doped Ti:LiNbO₃ spatial-modulation-gain (SMG) waveguide laser. The device exhibited a lasing threshold at 55 mW pump power, with spontaneous emission noise below threshold and single-frequency operation at 1.5 μm above threshold. At a maximum pump power of 100 mW, this laser demonstrates promising applications in optical communications and LiDAR systems [81].

Yang Tan et al. fabricated a Type II waveguide in Nd:MgO:LiNbO₃ crystals using femtosecond laser writing and achieved 1085 nm waveguide lasing. Under 808 nm pumping, the laser demonstrated a slope efficiency of 27%, a threshold of 71 mW, and a maximum output power of 8 mW. These results advance the development of LiNbO₃-based integrated photonic devices [82].

B.K. Das et al. reported a novel single-frequency Ti:Er:LiNbO₃ distributed Bragg reflector (DBR) waveguide laser, initiating lasing at ~70 mW pump power with an emission wavelength of 1561.84 nm. The device operated in single-longitudinal mode for both TE and TM polarizations, exhibiting a TE-polarized linewidth of 1.11 GHz and delivering 0.72 mW output power at 140 mW pump power. Integrated with two photorefractive gratings as cavity mirrors, this laser shows significant potential for fiber-optic communications and interferometric sensing applications [83].

Jon Martínez de Mendívil et al. fabricated a ridge waveguide laser in Nd-doped LiNbO₃ by combining femtosecond laser writing with Zn-diffusion. The laser cavity, formed by bonding multilayer dielectric mirrors to both waveguide ends, achieved TM-polarized lasing at 1085 nm under 815 nm continuous-wave Ti:sapphire pumping. The device demonstrated a threshold of 31 mW and a slope efficiency of 7% [84].

D.B. Rüske et al. demonstrated a high-efficiency Nd-doped Ti-diffused ridge waveguide laser. A 9.5 μm wide, 12 mm long waveguide delivered optimal performance, exhibiting multi-longitudinal-mode operation centered at 1084.7 nm with a full width at a half-maximum (FWHM) of 0.4 nm in Figure 10a. This output validated the laser’s wavelength stability and modal characteristics. With only Fresnel end-face reflections (~14%), it achieved a threshold of 83 mW, a maximum output of 108 mW, and a record slope efficiency of 34% for ridge waveguide lasers. Adding high-reflectivity (HR) mirrors reduced the threshold to 59 mW but lowered the slope efficiency to 18%, as shown in Figure 10b, and maximum power to 46 mW. Future work will focus on mitigating photorefractive damage, optimizing output coupling, and refining fabrication parameters to enable single-transverse-mode operation and reduced propagation losses [85].

Viacheslav Snigirev et al. reported a rapidly tunable laser based on a hybrid Si₃N₄/LiNbO₃ photonic platform for optical ranging. Figure 11a presents the schematic diagram of the experimental setup, while Figure 11b displays the beat frequency spectra of the targets under different integration times (as indicated by the time slices of 92 ms, 5 ms, and 10 ms in the context of the experiment). The system achieved ~15 cm spatial resolution, detecting target reflections at 2.1 m and wall reflections at 2.8 m in Figure 11c. Processed 3D point-cloud data confirmed its feasibility in LiDAR applications [86].

Cornelis A. A. Franken et al. reported a high-power, narrow-linewidth thin-film lithium niobate (TFLN) laser fabricated using photonic wire bonding technology. The device exhibited a threshold current of 100 mA, with output power increasing approximately linearly with drive current, achieving on-chip output power up to 76.2 mW, a high side-mode suppression ratio (SMSR) across varying currents, and single frequency operation with 61 dB side mode suppression. Wavelength tuning over 43.7 nm near 1530 nm was achieved by adjusting the effective length of microring resonators in a Vernier filter, as shown in Figure 12. Measured via a delayed self-heterodyne setup, the laser demonstrated an intrinsic linewidth as low as 550 Hz. Remarkably, without active stabilization, it maintained continuous operation for 58 h without mode hopping, with a frequency drift trend of only 4.4 MHz/h [87].

5. TeO₂

Tellurium dioxide (TeO₂) exhibits significant application potential in laser technology, with its advantageous characteristics primarily stemming from the synergistic effects of multidimensional physical properties. From a material perspective, TeO₂’s low phonon energy (700–800 cm⁻¹) effectively suppresses non-radiative transitions, substantially enhancing the radiative transition efficiency of rare-earth ions (e.g., Er³⁺, Tm³⁺), making it particularly advantageous for mid-infrared (2–5 μm) laser emission. Studies demonstrate that TeO₂ glass’s high refractive index (1.9–2.3), calculated through Judd–Ofelt theory, strengthens electric dipole transition intensity, thereby improving laser gain coefficients and absorption/emission cross-section values. The material system also exhibits exceptional rare-earth ion solubility (e.g., up to 25 mol% Er₂O₃ dissolution), combined with structural diversity (coexistence of TeO₄ bipyramids and TeO₃ trigonal pyramids), which significantly broadens the fluorescence spectral line full width at half maximum (FWHM) (e.g., 60 nm bandwidth for Er³⁺ at 1530 nm), providing a material foundation for broadband tunable lasers and fiber amplifiers. Notably, TeO₂’s remarkable nonlinear optical properties (second-order nonlinear refractive index n₂ ≈ 2.5 × 10⁻¹⁹ m²/W) and high thermal stability (glass transition temperature Tg ≈ 280–430 °C) synergistically support high-power laser operation and ultrafast laser applications. These comprehensive characteristics establish TeO₂-based material systems as ideal candidates for near- to mid-infrared laser devices [88].

Khu Vu et al. achieved the first demonstration of an erbium-doped tellurium oxide (TeO₂) planar rib waveguide laser under 980 nm pumping. Random mode-hopping lasing emission, triggered by end-face reflections (~8.5%), was observed at 1532 nm with an output power of 0.1 mW (pump power: 300 mW). By introducing gold-mirror reflections (reflectivity enhanced to >80%) and fiber Bragg grating (FBG) feedback (95% reflectivity), the lasing threshold was reduced to 100 mW, and a narrow-linewidth (<1 GHz) output at 1550.27 nm (as shown in Figure 13a,b) and an output power of 1 mW were achieved. The study revealed significant green upconversion (546 nm emission) under strong pumping but no observable photodarkening effects. Simulations indicate that reducing waveguide losses (<0.2 dB/cm) and optimizing cavity mirror reflectivity could enable lower thresholds (<50 mW) and high-power laser operation, in Er-doped TeO₂ waveguide lasers [89].

Khadijeh et al. successfully demonstrated a compact, monolithically integrated thulium-doped tellurium oxide (TeO₂:Tm³⁺) microring laser consisting of a 300 μm radius Si₃N₄ microring structure coated with a 0.39-μm-thick TeO₂ gain layer (Tm³⁺ concentration: 4.1 × 10²⁰ cm⁻³), where 66.7% of the pump energy was confined within the TeO₂ layer. Multimode lasing across 1815–1895 nm with a maximum single-ended output power of 4.5 mW was achieved (as shown in Figure 14), with key performance metrics including a threshold pump power of 18 mW (waveguide-coupled) and 11 mW (microring absorbed), single-ended slope efficiencies of 11% (waveguide-coupled) and 17% (absorbed power), and a bidirectional total efficiency of 34% [90].

Bruno et al. designed an asymmetric grating cavity (input/output grating lengths of 6/3 mm), and demonstrated an integrated on-chip erbium-doped tellurite (TeO₂:Er³⁺) distributed Bragg reflector (DBR) laser by depositing a 0.352-μm-thick TeO₂:Er³⁺ active layer onto 1.2/1.6 μm wide, 22-mm-long SiN waveguide, where C-band (1530–1565 nm) lasing emission was achieved. The device exhibited a threshold power of 13 mW, a maximum slope efficiency of 0.36% (forward: 0.33%), and a peak output power of 0.28 mW (dual-end pumping). The further improvement of the performance was limited by the background propagation loss (1.0–1.1 dB/cm) and low Er³⁺ ion activation ratios [91].

6. Ta₂O₅

Tantalum pentoxide has a high refractive index (approximately 2.1) that can effectively improve the optical mode constraint capabilities of optical waveguides. Since 2005, a number of studies have successively explored the design and performance optimization of rare earth (Nd³⁺, Er³⁺, Yb³⁺, and Tm³⁺)-doped Ta₂O₅ waveguide lasers. In recent years, advancements in nanofabrication technologies have further extended the Ta₂O₅ platform to applications such as microcavity lasers, waveguide amplifiers, and Kerr nonlinear photonic devices, while heterogeneous integration has enhanced its compatibility with other photonic materials and expanded its functional capabilities.

Unal et al. reported a neodymium-doped tantalum pentoxide (Nd:Ta₂O₅) channel waveguide laser with emissions at 1.066 μm and 1.375 μm. The waveguide loss was 0.2dB/cm. Lasing was achieved at both 1.066 μm and 1.375 μm with a threshold pump power as low as 2.7 mW, and a slope efficiency of 21% was measured [92].

In 2010, Subramanian et al. reported an erbium-doped tantalum pentoxide (Er: Ta₂O₅) bar waveguide laser prepared on silicon oxide using magnetron sputtering. The waveguide is 2.3 cm long, and laser output is realized in the 1558–1562 nm wavelength range under 977 nm laser diode pumping. The launched pump power threshold and slope efficiency were measured to be 14 mW and 0.3%, respectively. The threshold and slope efficiency with respect to absorbed pump power are estimated to be 4.3 mW and 1%, respectively [93].

In 2015, Aghajani et al. reported a Yb:Ta₂O₅ waveguide laser. When pumped with a 977 nm laser diode, laser emission was achieved in the range of 1015–1030 nm. For a cavity formed by a high reflector and about 12% Fresnel reflection at the output end, a maximum output power of 25 mW was obtained at a 1025 nm wavelength, with an absorbed pump power threshold of about 29 mW and a slope efficiency of about 27% [94].

In 2020, Tong et al. reported the spectroscopic characteristics and lasing performance of thulium-doped tantalum pentoxide (Tm:Ta₂O₅) waveguides. They used a 3 µm wide, 1 cm long Tm:Ta₂O₅ rib waveguide, which was end-fire-pumped at 795 nm. At an incident pump power of 170 mW—equivalent to a launched pump power of approximately 44 mW, calculated based on the overlap between the modal field and the incident beam-distinct emission—peaks appeared at 1238 nm and 1855/1858 nm, corresponding to the ³H₅→³H₆ and ³F₄→³H₆ transitions, respectively. The threshold launched pump power for lasing at 1858 nm was estimated to be around 15 mW, and a gain of at least 9 dB/cm was achieved [95].

In 2021, Jung et al. reported a new platform for integrated nonlinear photonics based on tantalum pentoxide (Ta₂O₅; hereafter, tantala). Tantala films with thicknesses exceeding 800 nm were deposited on oxidized silicon wafers using ion beam sputtering. These films exhibit a low residual tensile stress of 38 MPa and possess a Kerr nonlinear index of n₂ = 6.2 ± 2.3 × 10⁻¹⁹ m²/W, which is approximately three times greater than that of silicon nitride. Resonators fabricated from these films achieve optical quality factors as high as 3.8 × 10⁶, enabling the generation of ultrabroadband Kerr soliton frequency combs with low threshold power. Figure 15 illustrates soliton spectra and temporal properties in two RW geometries of tantala resonators, demonstrating efficient outcoupling, reduced thermo-optic effects, and sub-35 fs pulse durations. Leveraging this platform, low-threshold Kerr soliton combs and supercontinuum generation were demonstrated with coupled pulse energies as low as 60 pJ. The nonlinear performance of tantala waveguides was further evaluated through supercontinuum generation using low-energy ultrafast seed pulses, underscoring the potential of tantala for future applications in integrated nonlinear photonics [96].

In 2025, Jian Liu et al. reported the realization of on-chip microdisk lasers and waveguide amplifiers based on erbium-doped tantalum pentoxide (Er:Ta₂O₅). Er:Ta₂O₅ thin films with a thickness of approximately 700 nm were deposited via radio frequency (RF) magnetron sputtering. Subsequent fabrication of photonic structures was carried out using femtosecond direct laser writing in conjunction with chemical mechanical polishing. A suspended Er:Ta₂O₅ microdisk laser with a diameter of 36 µm demonstrated multimode lasing under 980 nm optical excitation, achieving a low threshold of 225 µW and exhibiting an intrinsic quality factor of 1.89 × 10⁶ at 951 nm. These results underscore the viability of Er:Ta₂O₅ thin films for the development of high-performance active photonic devices, offering a cost-effective and CMOS-compatible platform for integrated photonic applications [97].

In 2025, a research team reported a heterogeneous integration platform combining InGaAs/GaAs quantum well gain materials with tantala (Ta₂O₅) waveguides, enabling high-yield fabrication of both active and passive photonic integrated circuits (PICs). Distributed feedback (DFB) lasers operating at 976 nm exhibited robust single-mode emission, characterized by a side-mode suppression ratio (SMSR) of 43 dB and a white noise floor in the frequency noise power spectral density (PSD) of approximately 4 × 10⁵ Hz²/Hz, corresponding to a fundamental linewidth of 1.2 MHz. Integrated semiconductor optical amplifiers (SOAs) achieved a maximum on-chip gain of 24.5 dB at 987.4 nm, with a 3 dB bandwidth of 8.5 nm. Utilizing the DFB lasers to pump high-Q microring resonators (Q ≈ 2.5 × 10⁶) fabricated in Ta₂O₅, the team demonstrated wide-span degenerate optical parametric oscillation (OPO) over 183 THz, generating short-wavelength signal outputs at 778 nm and 752 nm. Figure 16 showcases a heterogeneous III-V/tantala PIC integrating diverse passive and active components for advanced photonic functionality. This study underscores the promise of Ta₂O₅-based heterogeneous PICs as a scalable and versatile platform for realizing integrated lasers, amplifiers, and nonlinear photonic devices, thereby advancing the development of quantum photonics and compact photonic systems [98].

7. Outlook and Summary

On-chip integrated lasers are the core of photonic integrated circuits (PICs). They are widely used in high-speed optical communications, high-precision sensing, quantum information processing, and biomedical detection. Their miniaturization, low power consumption, and high stability provide key support for the next generation of optoelectronic systems.

Here we summarize and compare the different on-chip lasers from two aspects:

• Materials

The gain media cited in the paper can be catalogued into semiconductors and RE-doped gain materials. For the former, since III-V semiconductors are epitaxially grown on silicon wafer, the lattice mismatch between conventional materials (e.g., InP, GaAs) and silicon-based waveguides necessitates complex heterogeneous integration processes with low yields. As a matter of fact, many efforts have been devoted to decreasing the density of various defects in the epitaxial films and increasing the electro-optical conversion efficiency—for example, introducing various buffer layers to reduce the defects or designing the different structures to increase the electro-optical conversion efficiency—but basically, the methods and technologies used in the past or future research for the growth of epitaxial films could be similar. Further improvement of the performance in the semiconductor-based on-chip lasers mostly relies on how to get high-quality semiconductor thin films.

On the other hand, RE-doped materials represent another active research direction in the development of on-chip lasers. One of the key factors for various host materials of RE doping is the low optical loss. Generally, loss less than one-tenth of dB/cm is necessary. In terms of this, SiN and LiNbO₃ with low optical losses could be good options, but RE doping into SiN requires ion-plantation, while single crystalline Er:LiNbO₃ needs to be bonded into silicon with extra costs. In contrast, various amorphous oxides and nitrides provide opportunities for the development of on-chip lasers, although there are more spaces to reduce the optical loss in these materials. For example, optical loss less than 0.1 dB/cm has been reported in Er:Ta₂O₅ waveguides [98], and thus it is possible to develop high-performance on-chip lasers based on Er:Ta₂O₅. Moreover, we also want to point out that, while RE ions are doped into host materials, host materials with large metallic atomic radius or large bonding length of metal–oxygen are preferred, since this can effectively separate the doped RE ions. In this case, the formation of RE-ion clusters can be avoided, and the emission from the transition between the different energy level of RE ions can be maximized. In terms of this, Ta has a larger atomic radius than Te and Al, and thus, Ta₂O₅ is expected to have better performance in the emission of light. Of course, screening new materials with better RE solubilities, low optical losses, and high gain are always expected for the exciting area.

2.

Pump schemes

Comparing different pump schemes, electrically pumped on-chip lasers demonstrate unique advantages. First, the large volume of an internal pump source is replaced by on-chip micro-electrodes, enabling miniaturization of the entire system to the centimeter scale. Second, while optically pumped lasers suffer significant pump energy loss due to coupling inefficiencies between external pump sources and chips, electrically pumped lasers substantially reduce this loss, thereby lowering overall power consumption. Finally, electrically pumped lasers integrate electrodes directly onto the waveguide. In contrast, optically pumped lasers typically require fiber-optic or free-space coupling between the pump laser and chip, making the electrically pumped approach inherently more robust and stable. For practical applications, we believe electrically pumped on-chip lasers are better suited for large-scale production, but this usually causes serious issues of heating management, especially in the case of on-chip lasers with high output power.

Currently, on-chip integrated lasers are facing multiple technical bottlenecks. Regarding fabrication, the lattice mismatch between conventional materials (e.g., InP, GaAs) and silicon-based waveguides necessitates complex heterogeneous integration processes with low yields. In terms of performance, on-chip light sources generally exhibit insufficient output power (typically tens of mW), fundamentally limiting their applicability in long-distance communications or high-sensitivity sensing systems. Concurrently, these devices exhibit low electro-optical conversion efficiency (<30%), inadequate thermal stability, and high power consumption. Additionally, mode–field mismatch results in substantial optical coupling loss (typically 3–5 dB), which severely constrains co-integration efficiency between lasers and passive components.

In response to the above challenges, future research can focus on material innovation and process optimization, including the following:

Heterogeneous integration technology: through wafer bonding and selective epitaxial growth, the efficient integration of Group III-V materials and silicon waveguides can be achieved to reduce the interface defect density.

Mode field matching design: using conical waveguides, metasurface couplers or reverse design algorithms to reduce coupling loss to less than 1 dB.

Thermal management optimization: implementation of an aluminum nitride (AlN) heat dissipation layer or microfluidic cooling structure to improve power density and long-term stability. This is particularly critical for electrically pumped lasers, where even low currents applied to the chip cause significant heat accumulation.

New resonant cavity structure: utilizing distributed feedback (DFB) lasers, a microring resonant cavity, or topological photonic design to enhance light confinement capabilities, reduce threshold current (<1 mA), and improve efficiency. In addition, silicon-based quantum dot lasers and lithium niobate thin-film lasers show higher performance potential through energy band engineering and localization of bright light fields.

A laser light source that is easier to integrate on the chip may be a direct electrically pumped rare-earth-doped material. For example, erbium-doped aluminum nitride (Er:AlN) can achieve 1.55 μm band luminescence through electric field regulation, without complex epitaxial processes, and is compatible with CMOS production lines. By improving the efficiency of rare-earth ions, it is possible to solve the problem of low carrier mobility of oxide materials. It is expected to generate a narrow line–wide laser source from the electric pumped LED source through later mode selection and other devices; another class of gain media receiving significant attention is organic gain media. Organic gain media (such as PMN:PMMA organic dyes and conjugated polymers) can cover spectral ranges from visible to near-infrared light and possess high gain characteristics. Furthermore, solution-based processing methods (like spin coating and inkjet printing) enable localized deposition, offering higher material utilization efficiency. Organic gain media offer a viable pathway for low-cost, integrable visible-light lasers, showing significant potential, especially in silicon photonics. Current challenges require breakthroughs in electrical pumping and stability to achieve commercialization [99]. Additionally, biological protein-based light sources (e.g., fluorescent proteins), through genetic engineering and molecular structure optimization, can enhance output power. These bio-light sources combine biocompatibility with ultra-low power consumption characteristics, making them suitable for applications in implantable medical devices or environmental monitoring [100,101].

On-chip integrated lasers are developing towards high performance, multi-material fusion and interdisciplinary applications. Through breakthroughs in process innovation and new material systems, it is expected to open up new application scenarios in the fields of communications, biology, and quantum information processing.