Surface Gratings Research Articles

High-power semiconductor laser with a narrow linewidth based on transverse photonic crystal

Broad-area lasers (BA) are practical for producing high output power. However, under a high current operation, high-order modes are easily excited, resulting in the broadened linewidth. Here, based on mode engineering of double-side transverse photonic crystals (TPCs) combined with a longitudinal high-order surface grating, a narrow-linewidth electrically-pumped broad-area laser with high power emission only using I-line lithography is demonstrated. By matching the high-order modes of the wide main waveguide with TPC bands, the effective volume of the high-order modes is expanded, while the fundamental mode remains unchanged. Then, single-lateral mode operation is achieved by selective pumping only for the main waveguide due to the significant distinction in modal gain between the fundamental mode and the high-order modes. In addition, a 27-order grating is constructed above the main waveguide to keep the laser operating in single-longitudinal mode. In the experiment, the device shows an output power of 115 mW, a lasing wavelength of 1552.94 nm with a side-mode suppression ratio (SMSR) of 59.26 dB, a narrow linewidth of 443 kHz, and a relative intensity noise (RIN) < -135 dB/Hz at 600 mA, thus has the potential to meet the needs in fields such as coherent optical communication and LiDAR.

Enantioselective Molecular Detection by Surface Enhanced Raman Scattering at Chiral Gold Helicoids on Grating Surfaces.

Distinct advantages of surface enhanced Raman scattering (SERS) in molecular detection can benefit the enantioselective discrimination of specific molecular configurations. However, many of the recent methods still lack versatility and require customized anchors to chemically interact with the studied analyte. In this work, we propose the utilization of helicoid-shaped chiral gold nanoparticles arranged in an ordered array on a gold grating surface for enantioselective SERS recognition. This arrangement ensured a homogeneous distribution of chiral plasmonic hot spots and facilitated the enhancement of the SERS response of targeted analytes through plasmon coupling between gold helicoid multimers (formed in the grating valleys) and adjacent regions of the gold grating. Naproxen enantiomers (R(+) and S(-)) were employed as model compounds, revealing a clear dependence of their SERS response on the chirality of the gold helicoids. Additionally, propranolol and penicillamine enantiomers were used to validate the universality of the proposed approach. Finally, numerical simulations were conducted to elucidate the roles of intensified local electric field and optical helicity density on the SERS signal intensity and on the chirality of the nanoparticles and enantiomers. Unlike previously reported methods, our approach relies on the excitation of a chiral plasmonic near-field and its interaction with the chiral environment of analyte molecules, obviating the need for the enantioselective entrapment of targeted molecules. Moreover, our method is not limited to specific analyte classes and can be applied to a broad range of chiral molecules.

Study on the Thermal Radiation Characteristics of Tungsten Surface Grating Structures Prepared by Femtosecond Laser Direct Writing

Study on the Thermal Radiation Characteristics of Tungsten Surface Grating Structures Prepared by Femtosecond Laser Direct Writing

Dynamic surface gratings for optical encryption: A simple approach based on azobenzene photoisomerization and solvent engineering

Dynamic diffraction gratings are optical devices that allow for non-mechanical, non-contact and remote on-demand control, and have garnered significant interest in optical encryption. Azobenzene materials are ideal materials for developing dynamic gratings owing to the advantages of fast photo isomerization and good reversibility. It is still challenging to precisely fabricate ordered periodic gratings on the surfaces of azobenzene materials. Here, controllable dynamic grating patterns were successfully fabricated by mask exposure and solvent engineering on azobenzene polyamide acid (PAA-Azo)-solvent system. The movement and mass migration of polymer chains can be regulated by modulating the intermolecular hydrogen bonding and free volume based on different solvents and solvent contents within the system, resulting in the formation of surface gratings with customizable morphology and light-splitting ability. Finally, by regulating the periodicity and amplitude of gratings, colorful dynamic patterns were obtained to monitor the expiration of light-sensitive and short shelf-life products, based on the azobenzene polyamide acid (PAA-Azo)-solvent system. This work provides a new and simple approach for controllable preparation of high-regularity and high-performance dynamic gratings.

Room-temperature DBR-free VCSEL operation on an InGaN/GaN thin-film platform with a monolithic surface grating.

Vertical-cavity surface-emitting lasers (VCSELs) are pivotal in various applications ranging from data communication to sensing technologies. This study introduces a VCSEL design featuring a monolithic top surface high contrast grating (HCG) reflector on a thin-film substrate, aimed at improving lasing performance while reducing fabrication costs by omitting the use of distributed Bragg reflectors (DBRs). We fabricated the proposed VCSEL with the surface grating and characterized its performance through micro-photoluminescence measurements. The laser demonstrated room-temperature lasing at 436.2 nm with a Q factor of 4600 and a lasing threshold of 5.5 kW/cm2 under optical pumping. The implementation of the surface grating reflector was instrumental in facilitating vertical lasing, significantly improving surface reflectivity compared to conventional flat GaN/air interfaces. This innovative design holds significant promise for the development of cost-effective, DBR-free VCSELs, with potential applications extending to photonic integrated circuits and light detection and ranging (LiDAR) systems.

Blue GaN-based DFB laser diode with sub-MHz linewidth.

Distributed feedback laser diodes (DFBs) serve as simple, compact, narrow-band light sources supporting a wide range of photonic applications. Typical linewidths are on the order of sub-MHz for free-running III-V DFBs at infrared wavelengths, but linewidths of short-wavelength GaN-based DFBs are considerably worse or unreported. Here, we present a free-running InGaN DFB operating at 443 nm with an intrinsic linewidth of 685 kHz at a continuous wave output power of 40 mW. This performance is achieved using a first-order embedded hydrogen silsesquioxane (HSQ) surface grating. The frequency noise is measured using a cross-correlated self-heterodyne frequency discriminator, and two estimations of integrated linewidth are evaluated using 1/π integration and β-separation line integration methods.

Monolithically integrated four-channel laser array based on high-order surface gratings for O-band CWDM systems

An O-band monolithically integrated four-channel laser array based on high-order surface gratings is demonstrated. The fabricated laser array exhibits a wide bandwidth of up to 60 nm, with threshold currents ranging from 17 to 25 mA and slope efficiencies between 0.22 and 0.29 W/A. At an injection current of 200 mA, all side-mode suppression ratios (SMSRs) are above 40 dB, and the output power exceeds 36 mW. The high-order surface gratings are fabricated by standard lithography, which avoids high-precision lithography and a complex regrowth technique. A potentially cost-effective light source for coarse wavelength division multiplexing (CWDM) systems is provided.

Optical Response of PDMS Surface Diffraction Gratings under Exposure to Volatile Organic Compounds.

Monitoring volatile organic compounds (VOCs) in indoor air is significantly gaining importance due to their adverse effects on human health. Among the diverse detection methods is optical sensing, which employs materials sensitive to the presence of gases in the environment. In this work, we investigate polydimethylsiloxane (PDMS), one of the materials utilized for gas sensing, in a novel transducer: a surface relief diffraction grating. Upon adsorption of the volatile analyte, the PDMS grating swells, and its refractive index changes; both effects lead to increased diffraction efficiency in the first diffraction order. Hence, the possibility of VOC detection emerges from the measurement of the optical power transmitted or diffracted by the grating. Here, we investigated responses of PDMS gratings with varying surface profile properties upon exposure to VOCs with different polarities, i.e., ethanol, n-butanol, toluene, chloroform, and m-xylene, and compared their response in the context of the Hansen theory of solubility. We also studied the response of the grating with a 530 nm deep surface profile to different concentrations of m-xylene, showing a sensitivity and limit of detection of 0.017 μW/ppm and 186 ppm, respectively. Structures in the PDMS were obtained as copies of sinusoidal surface gratings fabricated holographically in acrylamide photopolymer and revealed good sensing repeatability, reversibility, and a fast response time. The proposed sensing technique can be directly adopted as a simple method for VOC detection or can be further improved by implementing a functional coating to significantly enhance the sensitivity and selectivity of the device.

High-efficiency self-focusing metamaterial grating coupler in silicon nitride with amorphous silicon overlay

Efficient fiber-chip coupling interfaces are critically important for integrated photonics. Since surface gratings diffract optical signals vertically out of the chip, these couplers can be placed anywhere in the circuit allowing for wafer-scale testing. While state-of-the-art grating couplers have been developed for silicon-on-insulator (SOI) waveguides, the moderate index contrast of silicon nitride (SiN) presents an outstanding challenge for implementing efficient surface grating couplers on this platform. Due to the reduced grating strength, a longer structure is required to radiate the light from the chip which produces a diffracted field that is too wide to couple into the fiber. In this work, we present a novel grating coupler architecture for silicon nitride photonic integrated circuits that utilizes an amorphous silicon (α-Si) overlay. The high refractive index of the α-Si overlay breaks the coupler’s vertical symmetry which increases the directionality. We implement subwavelength metamaterial apodization to optimize the overlap of the diffracted field with the optical fiber Gaussian mode profile. Furthermore, the phase of the diffracted beam is engineered to focalize the field into an SMF-28 optical fiber placed 55 µm above the surface of the chip. The coupler was designed using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations supported by genetic algorithm optimization. Our grating coupler has a footprint of 26.8 × 32.7 µm2 and operates in the O-band centered at 1.31 μm. It achieves a high directionality of 85% and a field overlap of 90% with a target fiber mode size of 9.2 µm at the focal plane. Our simulations predict a peak coupling efficiency of − 1.3 dB with a 1-dB bandwidth of 31 nm. The α-Si/SiN grating architecture presented in this work enables the development of compact and efficient optical interfaces for SiN integrated photonics circuits with applications including optical communications, sensing, and quantum photonics.

Stably polarized 795 nm vertical-cavity surface-emitting lasers with anti-phase SiNx surface gratings

795 nm vertical-cavity surface-emitting lasers (VCSELs) with dielectric surface gratings to control the output polarization are designed and fabricated. The calculated results demonstrate that a well-designed SiNx surface grating positioned on the surface of an anti-phase VCSEL structure enhances the reflectivity difference between the two polarization modes compared to a conventional GaAs surface grating, consequently resulting in a larger gain anisotropy in VCSELs and a high orthogonal polarization suppression ratio (OPSR). Characterization shows that a peak-to-peak OPSR of 30.3 dB is achieved at 85°C for 795 nm VCSELs with a SiNx surface grating of 5 µm in diameter and an oxide aperture of ∼4µm, demonstrating the effectiveness of the SiNx surface grating in polarization control for 795 nm VCSELs.

Single-shot pump-probe technique by the combination of an echelon and a grating with a time window of 109 ps

In this study, using only a single pulse, pump-probe measurement with a large time window of more than 100 ps is implemented. A commercial grating is used to encode a time window of ∼56 ps in a single pulse; therefore, there is no need for machining customization. In addition, in this technique, the grating surface is accurately imaged, eliminating the image blur problem caused by phase differences in previous echelon-based techniques. Moreover, to make full use of the grating surface and obtain a larger time window, a simple reflection echelon is combined that matches the grating in the time window. This combination encoding strategy results in a total time window of ∼109 ps and maintains accurate imaging of the grating surface. This time window is an order of magnitude greater than the maximum reported values of the echelon encoding strategy and the angle beam encoding strategy. To demonstrate this single-shot pump-probe technique, the two-photon absorption process of ZnSe and the excited-state absorption process of a symmetrical phenoxazinium bromine salt were studied. The possibility of further improving the experimental setup is also discussed.

Spatially synchronous fringe detection: a method to facilitate the alignment of an echelle grating mosaic by separating the compensative angular errors

Alignment of mosaic gratings is traditionally supported by two interferometric verifications: on the zero order to verify the grating surfaces and on the blaze to verify the groove direction. In the case of low frequency echelle grating an interferometric measurement on the zero order is hardly feasible due to extremely low contrast of the fringes. The complete alignment has then to be carried out on high order (close to the blaze) where the two misalignment errors (the tip and rotation) show the same effect on the interferogram. The acquisition of a low and a high diffraction order image simultaneously, referred to as the spatially synchronous fringe detection method (SSFD), is used to analyze the misalignment. Iterative adjustment with the autocollimation configuration at the low and high order is used to separate the compensative errors of Δθ x and Δθ z . A prototype mosaic with two 110mm×220mm segments has been aligned with the support of this method. A numerical simulation of the alignment procedure as well as the error orientation analysis of this mosaic grating are presented. The mosaic grating with an accuracy of Δθ x <0.64µrad, Δθ y <1.13µrad, Δθ z <0.65µrad and a wavefront RMS error of 0.149λ has been completed. This method can greatly facilitate the alignment of an echelle mosaic for an astronomical spectrograph.

Hybrid graphene-high-aspect ratio plasmonic nanograting systems

One-dimensional plasmonic nanogratings (1D-PNGs) with high aspect ratios and narrow grooves promise enhanced coupling for hybrid graphene systems with the localized surface plasmon of the metallic grating and graphene surface plasmons. However, both the fabrication of the 1D-PNG and the application of graphene to it are difficult. We developed 1D-PNGs with a high aspect ratio of 15 and narrow grooves of 100 nm in width using the tapered mold method and a dry graphene-transfer procedure. Raman spectroscopy measurements showed that monolayer graphene was successfully transferred onto the 1D-PNGs, and the graphene was strongly doped with Au in the 1D-PNGs. Graphene on narrow grooves (free-standing graphene) demonstrated an almost identical p-doping level to graphene on Au because the narrow groove width allowed sufficient doping by Au for graphene on grooves. Reflectance measurements showed that the 1D-PNGs exhibited polarization- and wavelength-selective absorption at infrared (IR) wavelengths, and the effect of graphene blue-shifted the absorption peak wavelength induced by the surface plasmon resonance of 1D-PNGs. Numerical calculations agree well with these experimental results and indicate that the electric field strongly localizes on graphene in the grooves. Moreover, the doping level tunes the absorption wavelength owing to the coupling with graphene plasmons and the surface plasmon resonance of 1D-PNGs. This could provide electrical tunability to the graphene plasmons. Our fabrication procedure produced hybrid graphene-1D-PNGs with high aspect ratios and narrow groove systems for IR wavelengths. This system can contribute to developing high-performance electrically tunable graphene-based IR photodetectors, tunable IR emitters/absorbers, and biological sensors.

High-Efficiency Metamaterial-Engineered Grating Couplers for Silicon Nitride Photonics.

Silicon nitride (Si3N4) is an ideal candidate for the development of low-loss photonic integrated circuits. However, efficient light coupling between standard optical fibers and Si3N4 chips remains a significant challenge. For vertical grating couplers, the lower index contrast yields a weak grating strength, which translates to long diffractive structures, limiting the coupling performance. In response to the rise of hybrid photonic platforms, the adoption of multi-layer grating arrangements has emerged as a promising strategy to enhance the performance of Si3N4 couplers. In this work, we present the design of high-efficiency surface grating couplers for the Si3N4 platform with an amorphous silicon (α-Si) overlay. The surface grating, fully formed in an α-Si waveguide layer, utilizes subwavelength grating (SWG)-engineered metamaterials, enabling simple realization through single-step patterning. This not only provides an extra degree of freedom for controlling the fiber-chip coupling but also facilitates portability to existing foundry fabrication processes. Using rigorous three-dimensional (3D) finite-difference time-domain (FDTD) simulations, a metamaterial-engineered grating coupler is designed with a coupling efficiency of -1.7 dB at an operating wavelength of 1.31 µm, with a 1 dB bandwidth of 31 nm. Our proposed design presents a novel approach to developing high-efficiency fiber-chip interfaces for the silicon nitride integration platform for a wide range of applications, including datacom and quantum photonics.

Stochastic and multi-objective design of photonic devices with machine learning

Compact and highly performing photonic devices are characterized by non-intuitive geometries, a large number of parameters, and multiple figures of merit. Optimization and machine learning techniques have been explored to handle these complex designs, but the existing approaches often overlook stochastic quantities. As an example, random fabrication uncertainties critically determines experimental device performance. Here, we present a novel approach for the stochastic multi-objective design of photonic devices combining unsupervised dimensionality reduction and Gaussian process regression. The proposed approach allows to efficiently identify promising alternative designs and model the statistic of their response. Incorporating both deterministic and stochastic quantities into the design process enables a comprehensive analysis of the device and of the possible trade-offs between different performance metrics. As a proof-of-concept, we investigate surface gratings for fiber coupling in a silicon-on-insulator platform, considering variability in structure sizes, silicon thickness, and multi-step etch alignment. We analyze 86 alternative designs presenting comparable performance when neglecting variability, discovering on the contrary marked differences in yield and worst-case figures for both fiber coupling efficiency and back-reflections. Pareto frontiers demonstrating optimized device robustness are identified as well, offering a powerful tool for the design and optimization of photonic devices with stochastic figures of merit.

A Study on Length Traceability and Diffraction Efficiency of Chromium Gratings

Measurement traceability is a prerequisite for achieving accurate and reliable results as well as technical standardization. The period of Chromium (Cr) gratings fabricated by atomic lithography can be directly traced back to natural constants. Applying the Cr grating to grating interferometry can achieve nanometer measurement traceability. This research aims to analyze the diffraction efficiency characteristics of self-traceable Cr gratings to provide a theoretical basis for the fabrication and application of Cr gratings. In this regard, we establish the theoretical model of the laser beam incident angle and grating diffraction efficiency using the rigorous coupled-wave method. Then, we analyze the influence of the laser beam incident angle on grating diffraction efficiency by simulation, verify the accuracy of the theoretical model, and finally build a measurement system for grating diffraction efficiency. Through experiments, we find that the diffraction efficiency of the grating shows a rapid increase to reach a stable maximum value followed by a decrease, when a laser beam with a wavelength of 405 nm is incident on the surface of a self-traceable grating in Transverse Magnetic (TM) polarization and the incident angle changes within an effective range. The experimental results are consistent with the trend of theoretical calculation results.

975 nm multimode semiconductor lasers with high-order Bragg diffraction gratings

The 975 nm multimode diode lasers with high-order surface Bragg diffraction gratings have been simulated and calculated using the 2D finite difference time domain (FDTD) algorithm and the scattering matrix method (SMM). The periods and etch depth of the grating parameters have been optimized. A board area laser diode (BA-LD) with high-order diffraction gratings has been designed and fabricated. At output powers up to 10.5 W, the measured spectral width of full width at half maximum (FWHM) is less than 0.5 nm. The results demonstrate that the designed high-order surface gratings can effectively narrow the spectral width of multimode semiconductor lasers at high output power.

Slotted surface gratings fabricated by selective area growth of the p-InP cladding layer for BH lasers.

In this paper, we present a novel, to our knowledge, method for the fabrication of slotted surface gratings for buried heterostructure (BH) lasers. In the device fabrication process, SiO2 strips needed for InP current blocking layer growth are reused for the formation of slot grating pattern masks. In the following growth of the p-InP cladding layer, because the slot areas are covered by SiO2, the InP material is grown selectively in only the areas outside the slot areas, forming slots of the surface gratings in the p-InP layer at the same time as the cladding layer growth. Single longitude mode BH lasers having slotted surface gratings have been fabricated successfully, and the spectra show higher than 40 dB side mode suppression ratio (SMSR). The adoption of the method helps to simply the device fabrication and thus lower the device fabrication cost notably.

Analysis of High-Order Surface Gratings Based on Micron Lasers on Silicon

High-quality silicon-based lasers are necessary to achieve full integration of photonic and electronic circuits. Monolithic integration of III–Vmicron lasers on silicon by means of the aspect ratio trapping (ART) method is a promising solution. To obtain sufficient optical feedback to excite the laser without introducing complex fabricating processes, we have designed a high-order surface grating on micron lasers which was epitaxially grown by the ART method and can be fabricated by common UV lithography. The performance of the grating was analyzed by the finite-difference time-domain (FDTD) method and eigenmode expansion (EME) solver. After simulation optimization, the etching depth was set to 0.6 μm to obtain proper reflection. The width of the slots and the slot spacing were selected to be 1.12 μm and 5.59 μm, respectively. Finally, we obtained results of 4% reflectance and 82% transmittance at a 1.55 μm wavelength at 24 periods.

Bottom angle control mechanism and optimization of slanted gratings for optical applications

Slanted gratings serve as a crucial component in the optical waveguide couplers utilized in augmented virtual reality (AVR) devices. The flat bottom surface of slanted gratings ensures excellent diffraction efficiency of the couplers. However, achieving a flat bottom surface during the fabrication process poses challenges. This study presents a comprehensive investigation on the process control of the bottom angle using both simulation predictions based on available process parameters and experimental methods. The results obtained from the experimental procedures clearly demonstrate that the bottom angle is significantly influenced by the angular dependence of etching yield. Various ion incident energies and Ar/CHF3 gas components were employed to modulate the angular dependence of etching yield, thereby controlling the bottom angle in the reactive ion beam etching system. The effects of the ion incident energies and the flow ratios of the Ar/CHF3 gases on the bottom angle were thoroughly investigated. The findings of this study provide valuable insights for manufacturing slanted gratings and contribute to the advancement of AVR technology.