Photonics, Volume 9, Issue 7 (July 2022) – 72 articles

Structured light fields have attracted much attention due to rich spatial degrees of freedom. The tailoring of an arbitrary structured light field on demand is the precondition for the application of structured light. Therefore, the computer holography method used to reconstruct a coherent light field wavefront has been naturally applied for generating structured light. In this work, we comprehensively demonstrate the principles and procedures of pure-phase computer-generated holography (PP-CGH) and binary-amplitude computer-generated holography (BA-CGH) methods for tailoring structured light, realized by two digitally programmable devices: liquid-crystal spatial light modulators ($\mathrm{Lc}\text{-}\mathrm{SLM}$) and digital micromirror devices (DMD), respectively. Moreover, we first compare the two approaches in detail and clarify the recipe to obtain a high tailoring accuracy and efficiency, which will help researchers to better understand and utilize the holographic tailoring of structured optical fields. Full article

Pulsed laser range-gated imaging (PLRGI) is one of the most effective methods to achieve underwater high-resolution imaging. When searching target, there are two methods can be used: fixed gate and sliding gate. In practice, fixed gate has a small depth of field of view and sliding gate cannot meet the need of real time. In order to overcome these problems, multi-slice integration (MSI) method is proposed in this paper. First, the laser energy received by the PLRGI system is derived from radiative transfer theory. In addition, range intensity profiles (RIP) of MSI method is established. Experiments are carried out in lab to validate the RIP model and results show that theoretical data and experimental data are in good coincidence. Then the gate width and the number of pulse assigned to each slice are discussed. Finally, to prove the effectiveness of the MSI method, experiments are carried out in a boat tank. Results show that the MSI method is better than the fixed gate method for surveillance, and can acquire a clear image of the target at 18 m (4.5 times of the attenuation length) in the water with attenuation coefficient of 0.25 m⁻¹. Full article

We propose and theoretically investigate two-photon orbital angular momentum (OAM) correlation through spontaneous parameter down-conversion (SPDC) processes in three-dimensional (3D) spiral nonlinear photonic crystals (NPCs). By properly designing the NPC structure, one can feasibly modulate the OAM-correlated photon pair, which provides a potential platform to realize high-dimensional entanglement for quantum information processing and quantum communications. Full article

Despite the literature providing compelling evidence for the medical efficacy of photobiomodulation (PBM) therapy, its consistency in terms of accuracy and standardization needs improving. Identification of new technology and reliable and ethical biological models is, therefore, a challenge for researchers working on PBM. We tested the reliability of PBM irradiation through a novel delivery probe with a flat-top beam profile on the regenerating amphioxus Branchiostoma lanceolatum. The caudalmost 9 ± 1 myotomes, posterior to the anus, were excised using a sterile lancet. Animals were randomly split into three experimental groups. In the control group, the beam area was bounded with the 635-nm red-light pointer (negligible power, <0.5 mW) and the laser device was coded to irradiate 810 nm and 0 W. In Group laser-1, the beam area was bounded with the same 635-nm red-light pointer and irradiated at 810 nm, 1 W in CW for 60 s, spot-size 1 cm², 1 W/cm², 60 J/cm², and 60 J; irradiation was performed every day for two weeks. In Group laser-2, the beam area was bounded with the same 635-nm red-light pointer and irradiated at 810 nm, 1 W in CW for 60 s, spot-size 1 cm², 1 W/cm², 60 J/cm², and 60 J; irradiation was performed on alternate days for four weeks. We observed that PBM improved the natural wound-healing and regeneration process. The effect was particularly evident for the notochord. Daily irradiation better supported the regenerative process. Full article

Low-level laser therapy (LLLT) has become an important part of the therapeutic process in various diseases. However, despite the broad use of LLLT in everyday clinical practice, the full impact of LLLT on cell life processes has not been fully understood. This paper presents the current state of knowledge concerning the mechanisms of action of LLLT on cells. A better understanding of the molecular processes occurring within the cell after laser irradiation may result in introducing numerous novel clinical applications of LLLT and potentially increases the safety profile of this therapy. Full article

Fluorescence microscopy provides an unparalleled tool for imaging biological samples. However, producing high-quality volumetric images quickly and without excessive complexity remains a challenge. Here, we demonstrate a four-camera structured illumination microscope (SIM) capable of simultaneously imaging multiple focal planes, allowing for the capture of 3D fluorescent images without any axial movement of the sample. This setup allows for the acquisition of many different 3D imaging modes, including 3D time lapses, high-axial-resolution 3D images, and large 3D mosaics. We imaged mitochondrial motions in live cells, neuronal structure in Drosophila larvae, and imaged up to 130 µm deep into mouse brain tissue. After SIM processing, the resolution measured using one of the four cameras improved from 357 nm to 253 nm when using a 30×/1.05 NA objective. Full article

We analyzed a Mach–Zehnder electro-optical modulator based on a silicon nitride strip–loaded waveguide on 0.5 μm thick x-cut lithium niobate thin film. The optical and radio frequency parameters for two different modulator structures (Type I: packaged with 2 μm thick SiO₂ and Type II: unpackaged) were simulated, calculated, and optimized. The Optical parameters included the single-mode conditions, effective indices, the separation distance between the electrode edge and the Si₃N₄-strip-loaded edge, optical power distribution, bending loss, optical field distribution, and half-wave voltage. The radio frequency parameters included the characteristic impedance, attenuation constant, radio frequency effective index, and −3 dB modulation bandwidth. According to the numerical simulation and theoretical analysis, the half-wave voltage product and the −3 dB modulation bandwidth were, respectively, 2.85 V·cm and 0.4 THz for Type I modulator, and 2.33 V·cm and 1.26 THz for Type II modulator, with a device length of 3 mm. Full article

Spectroscopic reflectance is a versatile optical methodology for the characterization of transparent and semi-transparent thin films in terms of thickness and refractive index. The Fresnel equations are used to interpret the measurements, but their accuracy is limited when surface roughness is present. Nanoroughness can be modelled through a discretized multi-slice and effective medium approach, but to date, this offered mostly qualitative and not quantitative accuracy. Here we introduce an adaptive and nonlocal effective medium approach, which considers the relative size and environment of each discretized slice. We develop our model using finite-difference time-domain simulation results and demonstrate its ability to predict nanoroughness size and shape with relative errors < 3% in a variety of test systems. The accuracy of the model is directly compared to the prediction capabilities of the Bruggeman and Maxwell–Garnett models, highlighting its superiority. Our model is fully parametrized and ready to use for exploring the effects of roughness on reflectance without the need for costly 3D simulations and to be integrated into the Fresnel simulator of spectroscopic reflectance tools. Full article

Photopolymer resins used in stereolithographic 3D printing are limited to penetration depths of less than 1 mm. Our approach explores the use of near-infrared (NIR) to visible upconversion (UC) emissions from lanthanide-based phosphors to initiate photopolymer crosslinking at a much higher depth. This concept relies on the use of invisibility windows and non-linear optical effects to achieve selective crosslinking in photopolymers. SLA resin formulation capable of absorbing light in the visible region (420–550 nm) was developed, in order to take advantage of efficient green-UC of Er³⁺/Yb³⁺ doped phosphor. NIR-green light UC shows versatility in enhancing curing depths in laser patterning. For instance, a structure with a curing depth of 11 ± 0.2 mm, cured width of 496 ± 5 µm and aspect ratios of over 22.2:1 in a single pass via NIR-green light UC. The penetration depth of the reported formulation approached 39 mm. Therefore, this technique would allow curing depths of up to 4 cm. Moreover, it was also demonstrated that this technique can initiate cross-linking directly at the focal point. This shows the potential of NIR-assisted UC as a low-cost method for direct laser writing in volume and 3D printing. Full article

We show that a product of two Laguerre–Gaussian (pLG) beams can be expressed as a finite superposition of conventional LG beams with particular coefficients. Based on such an approach, an explicit relationship is derived for the complex amplitude of pLG beams in the Fresnel diffraction zone. Two identical LG beams of the duet produce a particular case of a “squared” Fourier-invariant LG beam, termed as an (LG)² beam. For a particular case of pLG beams described by Laguerre polynomials with azimuthal numbers n − m and n + m, an explicit expression for the complex amplitude in a Fourier plane is derived. Similar to conventional LG beams, the pLG beams can be utilized for information transmission, as they are characterized by orthogonal azimuthal numbers and carry an orbital angular momentum equal to their topological charge. Full article

A flat optical frequency comb (OFC) is generated by using a monolithic integrated electro-optic intensity and phase modulator fabricated on lithium niobite on insulator (LNOI) platform. The LNOI-based modulation chip consists of a push–pull Mach–Zehnder modulator (MZM) and a U-shaped phase modulator (PM) connected by a curved optical waveguide. Microwave and optical packaging are implemented for the modulation chip, where the input and output pigtails of the packaged modulation device are polarization-maintaining fibers, with a core diameter of 6.5 μm. The packaged LNOI-based modulation device is featured by a fiber-to-fiber insertion loss as low as 6.97 dB. The half-wave voltages of the MZM and the PM are measured to be 3.6 V and 3.4 V at 5 GHz, respectively. By using the modulation device, an OFC with a tooth spacing of 5 GHz is generated, and the 13 comb teeth in the generated OFC are with a power flatness of 2.4 dB. The measured results of this device indicate that the tooth spacing of the generated OFC can be extended to tens of gigahertz by using a microwave source with a higher output frequency. In addition, the number of the comb teeth can be enhanced beyond 20 by increasing the power of the radio-frequency signal applied to the PM or by further reducing the half-wave voltage of the PM. Full article

Sparse wavelength division multiplex (WDM) enabled distributed satellite cluster networks (DSCNs) have emerged as a promising architecture to accommodate future extensive applications. Networking of the DSCNs will face the challenges of explosively increasing traffic requests, the limited number of wavelengths, and restricted energy provisioning. To address these issues, a novel approach, the two-phase traffic grooming based on the matching algorithm (TPTG_MA), is proposed in this paper. To analyze resource utilization, energy- and wavelength- minimized models are established. After that, we develop the MA to tackle the traffic grooming problem in two phases, including the first phase for traffic aggregation and sub-wavelength assignment (TAASA) and the second phase for sub-wavelength grooming (SG). To evaluate the performance of the proposed TPTG_MA, the direct lightpath grooming (DLG) heuristic and the genetic algorithm (GA) are simulated for comparison. The results demonstrate that the TPTG_MA and DLG_GA outperform TPTG and DLG in the average wavelength utilization ratio (AWUR), the energy consumption saving (ESC), and the blocking probability. Compared with the DLG_GA, the TPTG_MA achieves at most 18% and 23% higher AWUR in the 12-node and 22-node topologies, respectively. In addition, the TPTG_MA can actualize at most 10% ECS improvement over the DLG_GA. At last, the influence of the network size, the number of wavelengths, and the number of hops are discussed. Full article

Laser wireless power transmission (LWPT) has various applications for mobile devices and specific equipment under extreme conditions. The light spot received by laser photovoltaics is usually non-uniform, resulting in system efficiency reduction. The output characteristics of 1 × 1 cm² GaAs laser photovoltaics were measured under various illuminated areas. The experimental results showed that the efficiency decreased from 40.8% at the full irradiated area to 26.7% at 1/10 irradiated area. Furthermore, the drop in short-circuit current was the main factor for decreasing the efficiency. A three-dimensional (3D) finite element model was used to investigate this factor. The simulation results indicated that non-uniform irradiation could increase the total non-radiative recombination rate. The recombination rate of the absorption region increased from 6.0 × 10²⁰ cm⁻³/s to 2.5 × 10²¹ cm⁻³/s, reducing the short-circuit current. Full article

N-ary gray-level (nGL) coding strategy is an effective method for absolute phase retrieval in the fringe projection technique. However, the conventional nGL method contains many unwrapping errors at the boundaries of codewords. In addition, the number of codewords is limited in only one pattern. Consequently, this paper proposes a new gray-level coding method based on half-period coding, which can improve both these two deficiencies. Specifically, we embed every period with a 2-bit codeword, instead of a 1-bit codeword. Then, special correction and decoding methods are proposed to correct the codewords and calculate the fringe orders, respectively. The proposed method can generate n² codewords with n gray levels in one pattern. Moreover, this method is insensitive to moderate image blurring. Various experiments demonstrate the robustness and effectiveness of the proposed strategy. Full article

We demonstrated that a ring-core erbium-doped fiber amplifier (RC-EDFA) can support orbital angular momentum (OAM) modes with topological charges (|l| = 1~3). The dependence of the characteristics on the length of the RC-EDF was investigated experimentally, including an investigation of gain and 3 dB gain bandwidth over the whole C band (i.e., 1530~1565 nm). The 3 dB gain bandwidth was improved to 21 nm. At a signal wavelength of 1550 nm, the maximum gain of all signal modes was up to 30.1 dB. Differential modal gain was maintained below approximately 1.3 dB. Full article

A prismatic excitation-based affinity biosensor consisting of the prism (BK7), WS₂/graphene hybrid nanosheets, and silver (Ag) as the active metal for the surface plasmon resonance is proposed in this present research. The introduction of the transition metal WS₂/graphene layer protected the silver substrate and enhanced the adsorption of biomolecules, which facilitated the quality and performance of detection. Here, we improved the detection structure by focusing on the metallic materials, graphene and WS₂ film layers, and the thickness of the measured medium on the sensing effect. The results show that the silver film had a more desirable resonance effect, and the design of the symmetric detection structure produced a double resonance peak, and it provides a reference for distributed sensing. Changing the thickness of the detection medium can dynamically adjust the wave vector matching conditions, which gives the sensor a certain tunability. In the bilayer WS₂ and monolayer graphene (W = 2, G = 1) configuration, the sensitivity was up to 224 deg/RIU with a quality factor of 96.97 RIU⁻¹, which has potential for clinical analytic and biochemical detecting applications. Full article

Optical solitons in ultrafast fiber lasers, as a result of dual balances between dispersion and nonlinearity as well as gain and loss, enable various soliton interactions. Soliton collisions are among the most intriguing soliton interactions, which fuel the understanding for particle-like properties of solitons. Here, we experimentally investigate the transient dynamics of collisions between a two-soliton molecule and a soliton singlet in a mode-locked fiber laser. By means of the dispersive Fourier transform technique, the evolving spectral interferograms of different collision scenarios are measured in real time. In particular, the “quasi-elastic” collision is observed, which shows that the soliton-molecule state remains unaltered after the collision and the group-velocity difference between the soliton molecule and the singlet is changed. It is directly demonstrated that a bond exchange occurs between the colliding solitons. By tuning the intra-cavity polarization controller, the dynamic processes of other collision outcomes, including the annihilation of a soliton in the soliton molecule as well as the formation of a stable unequally spaced soliton triplet, are also revealed. Our work facilitates a deeper understanding of soliton collision dynamics in ultrafast fiber lasers. Full article

In this paper, we propose a new algorithm called the partial-failure segregated multicasting routing and spectrum assignment (PFS MRSA) algorithm to improve the service blocking performance of the multicast transmission in flex-grid optical networks (FGONs). By segregating one failure destination leaf-node from a blocked multicast request and accommodating the failure destination leaf-node and the remaining multicast request independently, the success probability of accommodating the originally blocked multicast request can be greatly increased. In this way, the proposed PFS MRSA algorithm can effectively reduce the service blocking probability for the multicast services in FGONs. Simulation results show that the proposed PFS MRSA algorithm achieves significant reduction in service blocking probability when compared with the conventional MRSA algorithms, and such reduction can even reach 100% in some scenarios with low traffic load. Full article

This paper investigates the employment of reconfigurable intelligent surfaces (RISs) to improve the asymptotic capacity of the multiple-input single-output (MISO) visible light communication (VLC) system in the case of high signal-to-noise (SNR). For the RIS-aided MISO-VLC system based on mirror array, we regard the high-SNR asymptotic capacity with the input subject to peak-intensity constraints as a goal and formulate an asymptotic capacity maximization problem to find the optimal orientations of mirrors. As for the non-convex optimization problem, we convert it into a quadratic programming (QP) problem with hemispherical constraints and prove that it can be solved by computing the maximum eigenvalue of an equivalent matrix. Simulation results indicate that the asymptotic capacity is able to be improved significantly by adopting RIS in MISO-VLC systems. Meanwhile, we observe that the proper deployment scheme of RIS is able to enhance the degree of improvement through several simulations. Full article

Mode-locking is an ultra-short pulse laser generation technique. The range of pulse duration may vary from picoseconds to femtoseconds. Yttrium Oxide (Y₂O₃) based saturable absorber (SA) was appropriately revealed in the mode-locked method within the 1.55-micron regime. Y₂O₃ is perfect for strength, melting point, and chemical stability and can be used as a laminated insulator due to its properties. Moreover, Y₂O₃ also owns broadband service, switching speed, and engineering features. The Y₂O₃-PVA film was produced by combining the 50 mg Y₂O₃ powder into a 50 mL polyvinyl alcohol (PVA) solution and stirring it at room temperature for about 24 h. A mode-locked pulse was recorded with the integrated Y₂O₃-PVA SA in the erbium-doped fiber laser (EDFL) cavity, and the output spectrum optical spectrum analyzer displayed was around 1560.66 nm. In addition to the sustained mode-locked pulse, a nearly constant repetition rate of 1.01 MHz at a specific pump power begins from 175.87 mW to 228.04 mW while the pulse duration is 4.15 ps. Lastly, the mode-locked pulse had been evaluated, which showed the peak power started from 4.94 kW to 6.07 kW. Full article

We design and evaluate the performance of a one-dimensional photonic crystal (PhC) optical filter that comprises the integration of alternating layers of a barium titanate ferroelectric $\left({\mathrm{BaTiO}}_3\right)$ and an yttrium oxide dielectric $\left({\mathrm{Y}}_2{\mathrm{O}}_3\right)$, with a critical high-temperature superconductor defect, yttrium–barium–copper oxide $\left({\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\mathrm{X}}\right)$, resulting in the ${(\mathrm{BTO}/{\mathrm{Y}}_2{\mathrm{O}}_3)}_{\mathrm{N}}/\mathrm{YBCO}/$${({\mathrm{Y}}_2{\mathrm{O}}_3/\mathrm{BTO})}_{\mathrm{N}}$ multilayered nanostructure array. Here, we demonstrate that such a nanosystem allows for routing and switching optical signals at well-defined wavelengths, either in the visible or the near-infrared spectral regions—the latter as required in optical telecommunication channels. By tailoring the superconductor layer thickness, the multilayer period number N, the temperature and the direction of incident light, we provide a computational test-bed for the implementation of a PhC-optical filter that works for both wavelength-division multiplexing in the 300–800 nm region and for high-Q filtering in the 1300–1800 nm range. In particular, we show that the filter’s quality factor of resonances Q increases with the number of multilayers—it shows an exponential scaling with N (e.g., in the telecom C-band, $Q\approx 470$ for $N=8$). In the telecom region, the light transmission slightly shifts towards longer wavelengths with increasing temperature; this occurs at an average rate of 0.25 nm/K in the range from 20 to 80 K, for $N=5$ at normal incidence. This rate can be enhanced, and the filter can thus be used for temperature sensing in the NIR range. Moreover, the filter works at cryogenic temperature environments (e.g., in outer space conditions) and can be integrated into either photonic and optoelectronic circuits or in devices for the transmission of information. Full article

In this paper, we demonstrate 3D-printed 1-DOF (one torsional axis; 1 degree of freedom) optical scanners with large mirror areas (up to 20 × 20 mm²). Each device consists of an aluminum-coated square silicon substrate serving as the mirror, two miniature permanent magnets, an electromagnet, and a 3D-printed structure including the mirror frame, torsion springs, and base. One device can reach a static half optical scan angle of 14.8 deg., i.e., a full optical scan angle of 29.6 deg., at 12 VDC; this particular device exhibits a mechanical resonance frequency of 84 Hz. These scanners can be a potential, low-cost alternative to the expensive conventional galvanometer scanners. Full article

In this paper, the transmission mechanism of the spike information embedded in the low frequency fluctuation (LFF) dynamic in a cascaded laser system is numerically demonstrated. In the cascaded laser system, the LFF waveform is first generated by a drive laser with optical feedback and is then injected into a response laser. The range of crucial system parameters that can make the response laser generate the LFF dynamic is studied, and the effect of parameter mismatch on the transmission of LFF dynamics is explored through a method of symbolic time-series analysis and the index, such as the spike rate and the cross-correlation coefficient. The results show that the mismatch of the pump current has a more significant influence on the transmission of LFF waveforms than that of the internal physical parameter of the laser, such as the linewidth enhancement factor. Moreover, increasing the injection strength can enhance the robustness of LFF transmission. As spikes of the LFF dynamic generated by lasers with optical feedback is similar to the spike of neurons, the results of this paper can help understanding the information transporting and processing inside the photonic neurons. Full article

The analysis of age-related changes in skin vessels based on optoacoustic angiographic images during the in vivo skin monitoring of healthy volunteers at different ages is reported. As a result of a quantitative analysis of the three-dimensional OA images, the age-associated differences in the following image parameters were revealed: image intensity, ratio of blood content at different characteristics depths, total vessel length, and number of branches. The reported approach can be effectively employed for automatic assessment and monitoring of age-related vascular changes in the skin and underlying tissues. Full article

This study verifies the acute dose response effect of photobiomodulation (PBM) by light emitting diodes (LEDs) on hemodynamic and metabolic responses in individuals with type 2 diabetes mellitus (T2DM). Thirteen participants with T2DM (age 52 ± 7 years) received PBM by a light-emitting diode array (50 GaAIAs LEDs, 850 ± 20 nm, 75 mW per diode) on the rectus and oblique abdomen, quadriceps femoris, triceps surae, and hamstring muscle areas, bilaterally, using different energy treatments (sham, 75, 150, 300, 450, and 600 Joules) in random order with a washout of at least 15 days apart. The PBM by LEDs statistically decreased plasma glucose levels (primary outcome) in 15 min after application of the 75 and 450 J irradiation protocol, reduced blood lactate levels 15 min after application of the 75, 450, and 600 J irradiation protocol, increased cardiac output ($\dot{Q}$) and cardiac index (CI) in the 1st minute after application of the 75 and 300 J irradiation protocol, and reduced $\dot{Q}$ and heart rate (HR) in the 15 min after application of the 300 J and 600 J irradiation protocol, respectively. For hemodynamic variables, including $\dot{Q}$, total peripheral resistance (TPR), and HR, we observed that the ideal therapeutic window ranged between 75 and 300 J, while for metabolic variables, glucose and lactate, the variation was between 450 and 600 J. Full article

Background: Immediate implant is a subtype of implant that is placed following tooth extraction within the socket without further delay. These implants are known to preserve the alveolar bone and minimize the total number of surgical interventions in a patient. ⁴ Photodynamic therapy (PDT) augments nonsurgical periodontal therapy using antibacterial mechanisms. PTD can be more effective in conjunction with scaling and root planing (SRP). The aim of this study is to assess the effects of PDT on the early outcomes of implants placed on patients with periodontitis with and without SRP at 9 months of follow-up. Materials and methods: A total of 23 implants were placed in 14 patients, with 11 in the test group and 12 in the control group. SRP was carried out prior to immediate implant placement in control sites, and PDT adjunctive to SRP (SRP + PDT) was performed in test sites before immediate implant placement. Plaque index, gingival index, probing pocket depth, clinical attachment level, and radiovisiographs were procured at baseline, 3 months, 6 months, and 9 months. Primary stability was examined at the time of implant placement, and the healing index was recorded a week later. Results: At the end of the 9 months of the study period, (SRP + PDT) group had a mean marginal bone loss of 0.95 mm, and the control group had 1.08 mm. Clinical parameters such as plaque index, gingival index, clinical attachment level, and probing depth showed definitive improvement after 9 months, compared with the baseline, but when the test and control groups were compared, the difference was statistically significant for plaque index and probing depth. The implants in both groups were followed up for a period of 9 months. There was an improvement in marginal bone loss but was not statistically significant. The survival of immediate implants in the PDT group was not different from those in the scaling and root planing group. Conclusion: The effect of PTD can be beneficially used as an adjunct to SRP. However, the effects were not significant. Photodynamic therapy can be effectively used as an adjunct to SRP owing to the better outcomes using PDT. Full article

Fractional vortex beams (FVBs) possess unique topological properties that are manifested in the vortex distribution. However, there are still discrepancies in the value of the vortex strength of FVBs at the far field. In this work we present a complete picture of the behavior of the phase singularities of non-integer (commonly known as fractional) beams in the Fraunhofer diffraction region and demonstrate a very good correspondence between experiments and simulations. As shown in the text, the original beam waist ${\omega }_0$ was found to be a key factor relating to the beam profile topology. This variable was measured in the process of calibrating the experiment. Finally, an experimental method to obtain the non-integer topological charge is proposed. This method only requires an analysis of the intensity, knowledge of the transition behaviors, and the beam waist. Full article

Hermite–Gaussian (HG) beams have significant potential to improve the capacity of free-space optical communication (FSOC). The influence of pointing error on the propagation characteristics of an HG beam cannot be ignored in the FSOC system. Although the average irradiance of the HG beam under a small pointing error from the FSOC tracking mechanism has been investigated through Taylor series approximation, here, we propose that the average irradiance of the HG beam under an arbitrary magnitude pointing error can be deduced through a statistical averaging method. We firstly found that the average irradiance profile of an HG beam finally changes into an approximately Gaussian shape with the increase in pointing error and propagation distance and a larger beam waist at the transmitter could mitigate the profile change. The correlation coefficient between deduced theoretical expression and Monte Carlo simulation reaches 0.999. Additionally, the effective spot size, location of the local extreme value, average received power and signal-to-noise ratio (SNR) loss for an HG beam under pointing error were theoretically deduced and analyzed for the first time. We found that the effective spot size of the higher-order HG beam experiences less broadening under the pointing error than that of the lower-order HG beam. The fundamental theoretical expressions of average irradiance for an HG beam under pointing error have provided effective guidance for analyzing the propagation characteristics and link performance. Full article

Carrier-selective passivating contacts for effective electron and hole extraction are crucial to the attainment of high efficiency in crystalline silicon (Si) solar cells. In this comprehensive review, the principle of carrier extraction and recombination mechanisms in conventional industrial Si solar cells are discussed first. Passivating contacts based on (i) amorphous hydrogenated Si and (ii) polysilicon/silicon oxide are next reviewed, with emphasis on carrier selectivity mechanisms including contact layer band alignment with silicon, and localized carrier transport in ultrathin oxides. More recent developments in dopant-free passivating contacts deposited by lower-cost fabrication processes with lower thermal budget are then described. This third category of non-Si based electron- and hole-selective passivating contacts include transition metal oxides, alkali/alkali earth metal fluorides and organic conjugated polymers. The photovoltaic performance of asymmetric double heterojunction Si solar cells fabricated using these non-Si passivating contacts and their stability in damp heat conditions are discussed and compared with Si based passivating contacts. Full article