Efficient Optical Waveguiding Enabled by Focused Proton Beam Writing in Nd:YCOB Crystal

Abstract

We report on microfabrication and optical characterization of buried channel waveguides defined in Nd:YCOB crystal by focused proton beam writing (PBW). In the fabrication process, the focused proton beam irradiation creates a local material modification region with geometrically symmetric positive index changes at the end of the proton trajectory, where efficient optical waveguiding can be locally supported within a fiber-like channel structure. The impact of the proton fluence (with different values ranging from 10¹⁵ to 10¹⁶ cm⁻²) on the optical waveguiding performance is well studied. The experimental results of the optical waveguide properties are in fairly good agreement with the simulation results.

1. Introduction

Waveguide-based devices combine the optical guiding properties of waveguides and the optical functionalities of substrates well, serving as one of the most critical elements in complex photonic integrated circuits (PICs) because they can incorporate both light processing and transmitting based on a monolithic architecture [1,2]. Multi-functionality is generally the very first factor to be considered when choosing an appropriate material for waveguide device definition. Such a train of thought makes dielectric crystals stand out because of their rich properties for potential applications in photonics [3,4,5,6,7]. Therefore, dielectric waveguides, which combine the excellent optical properties in bulk dielectrics and the compact geometries of waveguide structures, have aroused increasingly more attention due to their functionalities and miniaturization, opening up exciting possibilities and opportunities for integrated photonics research [3,4].

For practical photonic applications, 2D waveguides (typically in channelized configurations, in which optical propagation is restricted in both horizontal and vertical directions) are more attractive than 1D structures (such as slab waveguides) because of the more compact geometry and higher intra-cavity light intensity in the former cases [3,4,8]. Furthermore, 2D waveguide structures are more favorable for integrated photonics applications since they provide noticeable advantages in terms of miniaturization, flexibility, and integration [1].

Several micro/nanomachining techniques have been applied for waveguide formation in different dielectric crystals [3,4,8]. Energetic ion implantation/irradiation is a versatile technique for localized material modification and thus for introducing refractive index contrast at the target material surface [9,10]. It has been proven to be a powerful technique for waveguide fabrication in many dielectric materials [9,10]. The standard structure of ion-implanted/irradiated waveguides is usually in a slab format. By applying patterned stripe microstructures as a mask (e.g., patterned photoresist on the target surface), fabrication of 2D structures by ion implantation/irradiation is also possible. However, such embedded waveguides (with structurally asymmetric air–dielectric interfaces) are incompatible with 3D photonic designs supporting symmetric refractive index profiles. In contrast to conventional ion implantation/irradiation, the proton beam writing (PBW) technique involves scanning of a focused beam of high-energy (MeV) protons for localized material modification at micron or submicron metric scales [11,12,13,14,15,16]. This feature makes PBW very suitable for direct 3D micromachining of dielectric waveguides with flexible geometries without masking the substrate [9]. By adjusting the beam energy, channel waveguides at controllable depths can be well defined within the bulk substrate. In this manner, PBW has been successfully applied for fabrication of buried waveguides in various materials [9,10,11,12,13,14,15,16,17,18,19].

As one of the most important gain media for self-frequency-doubling (SFD) lasers, neodymium-doped yttrium calcium oxoborate (Nd:YCa₄O(BO₃)₃ or Nd:YCOB) shows a promising prospect of generating visible laser emission in a small-footprint cavity format, e.g., in a waveguide structure [3,4,20,21,22]. In particular, Nd:YCOB waveguides fabricated by femtosecond laser direct writing and conventional ion implantation/irradiation have been experimentally demonstrated [23,24,25]. However, experimental demonstration of Nd:YCOB waveguides fabricated by PBW is still missing.

In this work, we report on fabrication of buried waveguides in Nd:YCOB crystal by employing 2 MeV PBW with diverse proton fluences. The formation mechanism of the optical waveguide structures is analyzed. The optical guiding properties of different waveguides are experimentally characterized and compared. The experimental results achieved in this work show a promising potential of using PBW waveguides in Nd:YCOB for integrated SFD lasers.

2. Materials and Methods

The Nd:YCOB crystal (doped by 1 at. % Nd³⁺ ions) used in this work is cut to the size of 8 (x) × 8 (y) × 2 (z) mm³ and optically polished. The PBW process is carried out by using the facilities at the Centre for Ion Beam Applications, National University of Singapore. The accelerated proton beam energy is fixed at 2 MeV with different writing fluences of 1 × 10¹⁵, 2 × 10¹⁵, 5 × 10¹⁵, and 1 × 10¹⁶ cm⁻² (hereafter labeled as WG1–4, respectively), and focused down to a beam diameter of approximately 1 μm. During the fabrication process, the Nd:YCOB sample is placed on a PC-controlled XYZ microposition stage. To ensure an appropriate cross-sectional size of the formed waveguide, the proton beam is magnetically scanned over a distance of 4 μm in a perpendicular direction to the scan pathway. Figure 1a schematically illustrates PBW fabrication of Nd:YCOB channel waveguides, which are defined along the y-axis of the Nd:YCOB. The cross-sectional microscopic image of the fabricated waveguide (with a writing fluence of 1 × 10¹⁵ cm⁻²) is shown in Figure 1b. It is easy to identify the material modification region and thus the induced localized refractive index contrast at the PBW focal point from the microscopic image. No significant material modification along the irradiated proton trajectory can be identified (the crystalline cracks on the sample surface are mainly induced by polishing processes, and there is no influence on the optical performance of the fabricated waveguides). Here, we note that the cross-sectional images of four waveguides fabricated by using different proton fluences are identical.

We use the Stopping and Range of Ions in Matter code (SRIM 2013) [26] to simulate the penetration process of 2 MeV proton beam into the Nd:YCOB crystal. Figure 2a,b depict the distribution of H⁺ concentration and defects per atom (DPAs) along the depth beneath the Nd:YCOB surface with diverse fluences. As we can see, the H⁺ concentration and induced crystalline defects remain at a very low level during the first 30 μm of the proton trajectory, while peaking (with maximum DPA values of only 5.8‰, 2.9‰, 1.16‰, and 0.58‰, respectively) at a depth of approximately 34 μm (which is in good agreement with the waveguide location shown in Figure 1b), where the proton beam loses its energy and subsides. The relatively low DPA values suggest that the nuclear collisions mainly happened at the end of proton trajectory and the original crystalline lattice is well preserved even in the most disordered regions [9,10]. In addition, the induced localized DPA increases with the injected proton fluence. This implies that the waveguides fabricated by using higher proton influence may offer higher refractive index modification but also higher losses due to more localized crystalline defects. To verify this speculation, we carry out optical guiding characterizations of the fabricated waveguides, and the experimental results are discussed in the following.

The modal profiles of the Nd:YCOB channel waveguides are measured by using a conventional end-face coupling optical system. A polarized 632.8 nm light beam from a He-Ne laser is focused by using a 20× microscope objective and coupled into the waveguides. The output light beam is collected by using another 20× microscope objective, and imaged by a CCD. The numerical apertures (N.A.) of both microscope objectives are 0.4.

3. Results and Discussion

Figure 3a–d show the near-field modal profiles of fabricated Nd:YCOB channel waveguides fabricated by 2 MeV PBW with fluences of 1 × 10¹⁵, 2 × 10¹⁵, 5 × 10¹⁵, and 1 × 10¹⁶ cm⁻², respectively. As we can see, the 632.8 nm light field is well confined in the channel waveguides WG2–4, while leaky for the WG1 case. We attribute this result to the relatively low proton fluence of 1 × 10¹⁵ cm⁻², which is not able to introduce sufficiently high refractive index contrast in Nd:YCOB crystal lattice, leading to quite weak optical confinement. In the other three cases, nearly symmetric mode profiles with circular-like shapes are obtained, suggesting their good optical waveguiding properties and potential applications for SFD lasers.

The optical waveguiding results in Figure 3 confirm a local increment of the refractive index at the end of the proton path, and this increment is very likely due to a local enhancement in the electronic polarizability caused by nuclear collisions considering the relatively low DPA values (as indicated in Figure 2b) [9,10]. To obtain the refractive index modification induced by the incident proton beams, we estimate the maximum refractive index change by experimentally determining the N.A. of the waveguides and using the formula of Δn ≈ sin²Θ_m/2n [27]. The Θ_m is the maximum incident angular deflection at which no transmitted power change occurs, and n ≈ 1.72 is the refractive index of the unmodified substrate (measured by using a prism coupler, Metricon Model 2010/M), and the maximum refractive index increase (Δn) was calculated to be ∆n ≈ +0.003 (for WG2). With this value, we reconstruct the refractive index profile (assuming a Gaussian profile) of the fabricated Nd:YCOB channel waveguide (WG2), see Figure 4a. Based on the reconstructed refractive index profile, we can obtain the optical modal profile of WG2 (see Figure 4b) by using the commercial software BeamPROP^©, Rsoft, based on the finite-difference beam propagation method (FD-BPM). During the simulation, a combination of triangular and rectangle meshes are used with an average grid size of 40 × 40 nm². It is clear to see that the experimentally measured beam profile shown in Figure 3b and the simulation result given by Figure 4b are quite similar (in terms of the circular shape and the intensity distribution), suggesting that the reconstructed refractive index profile of the PBW buried waveguide is a reasonable estimation. It is worth noting that the estimated ∆n for WG1 is <0.001, while for WG3 and WG4, ∆n identical to that of WG2 can be determined.

The total losses of different waveguides at 632.8 nm are determined to be 3, 0.8, 1.1, and 1.2 dB for WG1–4, respectively, by measuring the optical powers at the input and output waveguide facets. Here, the total loss value is the sum of coupling and propagation losses (the latter corresponds to the effective waveguide loss). The coupling losses (which is inversely proportional to the overlap integral of the incident Gaussian beam and the fundamental waveguide mode [28]) for different waveguides are calculated to be 0.5 (WG1) and 0.2 (WG2–4), respectively, by using the software Rsoft and considering their refractive index profiles as well as the focal spot size (around 10 μm) of the 632.8 light. Thus, considering the waveguide length of 8 mm, the propagation losses for all the fabricated waveguides are determined to be 3.125 (WG1), 0.75 (WG2), 1.125 (WG3), and 1.25 dB/cm (WG4), respectively. Here, the higher loss in the WG1 case is due to its lower ∆n and thus weaker optical confinement. In contrast to WG2, the slightly higher propagation losses for WG3 and WG4 are mainly attributed to the higher scattering losses caused by slightly higher DPA values. Therefore, we can conclude the following: A. the proton fluence has a significant impact on the waveguiding properties, and B. an appropriate compromise of sufficient refractive index modification and reduced DPAs should be considered when using PBW for waveguide fabrication. We note here that thermal annealing treatment is also conducted for all the waveguides, and no significant improvement in the waveguide quality can be identified.

We further explore the potential lasing performance of the fabricated waveguides. However, the maximum laser output power at 1.06 μm is lower than 3 mW when pumping at 810 nm (with a maximum pump power 1 W) by using a continuous-wave Ti:Sapphire laser. With such a low lasing output, no SFD can be observed. To this end, further optimization of the waveguide quality and coupling efficiency (less than 80% in the lasing experiment according to optical simulation) via systematically researching the fabrication parameters and designing the waveguide cross-section size well will be performed.

In contrast to Nd:YCOB waveguides fabricated by standard ion implantation/irradiation [23,24] and femtosecond laser direct writing [25], PBW waveguides in this work show single-mode guiding properties, leading to relatively lower waveguide losses (multi-mode properties can be identified but no loss information is given in [23,24,25]). However, due to the much smaller waveguide cross-sectional sizes defined by PBW (in contrast to the cladding waveguide demonstrated in Ref. [25]), the optical gain given by the waveguides in this work is not sufficient for efficient SFD. This can be optimized in the future work. Moreover, similar to PBW, microbeam direct writing using higher ion mass and energy, e.g., carbon ion microbeam [9,15,16], is also an alternative method for fabrication of high-quality channel waveguides. In this way, the required fluence is much lower (at least an order of magnitude lower than using light ions, e.g., H⁺ and He⁺ [9]) because the electronic interaction becomes high enough for material modification.

4. Conclusions

We have fabricated buried waveguides in Nd:YCOB crystals by using 2 MeV PBW at different fluences (with values ranging from 10¹⁵ to 10¹⁶ cm⁻²). Based on both simulation and experimental results, the influence of proton fluence on the optical waveguiding properties has been studied and discussed in detail. The minimum waveguide loss of 0.75 dB/cm is determined from the one fabricated by using ion fluence of 1 × 10¹⁶ cm⁻². In addition, thermal annealing treatment has no identified impacts on the waveguide quality. Further investigations will be performed on the improvement of the waveguide quality as well as the enhancement of lasing and nonlinear optical properties in the waveguide structures.