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Flash light sources with a wide field of view (FOV) are indispensable in various fields such as light detection and ranging (LiDAR), optical wireless communication, and adaptive lighting. However, conventional flash light sources, which combine lasers with external optical elements, tend to suffer from high complexity, large size, and high cost. In this study, we investigate a new wide-FOV flash light source which does not require external optical elements, based on a dually modulated photonic crystal surface-emitting laser (PCSEL). First, we propose and design the concept of a photonic crystal into which information of gradually varying diffraction vectors is introduced in order to artificially broaden the divergence angle. We then experimentally demonstrate photonic crystals based on this concept. Finally, by arraying 100 such lasers with mutually different central emission angles and driving all of these lasers simultaneously, we successfully achieve optics-free, 4-W flash illumination over a FOV of 30° × 30° at a wavelength of 940 nm.

The photonic-crystal surface-emitting laser is a new-generation semiconductor laser capable of emitting a high-power, high-quality (i.e., high-brightness) beam, as well as on-chip two-dimensional beam steering, lasing in various wavelength regimes, and on-chip ultra-short self-pulsation, owing to their freedom of light-matter control. Here we introduce light detection as a new functionality, wherein the photonic-crystal laser is operated under reverse-bias conditions. We find that the photonic-crystal laser operated under reverse-bias conditions has a very high shunt resistance and a low dark current, which are essential qualities to achieve high detectivity. In addition, high responsivity is achieved by utilizing proper band-edge-modes and $Q$ -matching conditions for resonant light absorption. Furthermore, by employing photonic-crystal lasers as the detector as well as the laser source, a direct time-of-flight distance measurement is successfully demonstrated.

Realizing high-peak-power (tens to hundreds of watts or higher) short-pulse (tens of picoseconds or less) operation in semiconductor lasers is crucial for state-of-the-art applications including eye-safe high-resolution remote sensing and non-thermal ultrafine material processing. However, it has been challenging to introduce mechanisms that enable stable high-peak-power short-pulse operation in conventional semiconductor lasers. Here, we propose photonic crystal lasers that have two-dimensionally arranged gain and loss sections to enable high-peak-power short-pulse operation in the fundamental mode while suppressing lasing in higher-order modes to avoid laser instability. On the basis of this concept, we experimentally realize a high peak power of ~20 W and a short pulse width of ~35 ps with an injection current of only 3-4 A using a 400-μm-diameter device and theoretically predict that even higher peak power (>300 W) can be achieved in a 1-mm-diameter device. Our results will contribute to the realization of next-generation laser sources for the aforementioned applications. By using engineered gain and loss sections in a photonic crystal laser, pulses with a peak power of ~20 W and pulse width of ~35 ps have been experimentally demonstrated and even higher peak power operation (>300 W) has been theoretically predicted.

Photonic-crystal surface-emitting lasers (PCSELs) with modulated photonic crystals have attracted much attention for their unrivaled capabilities, such as broad area coherent resonance, and lens-free beam scanning and flash illumination. In this paper, we first explain the principles and the development of PCSELs with modulated photonic crystals toward non-mechanical two-dimensional (2D) beam-scanning applications. Then, we show PCSELs with modulated photonic crystals, whose modulation is designed based on an inverse Fourier transform to enable the emission of various beam patterns, such as flash patterns and multi-dot patterns, from a single photonic crystal without using external optical elements. This demonstration underscores the flexibility of PCSELs with modulated photonic crystals as compact, highly functional light sources for a wide range of applications, including not only beam-scanning-type, flash-type, and multidot-type light detection and ranging but also advanced object recognition and adaptive illumination.

We propose and develop a new type of non-mechanical three-dimensional (3D) light detection and ranging (LiDAR) system, which integrates a beam-scanning-type laser source and a flash-type laser source. Employing on-chip dually modulated photonic crystal lasers for both flash and beam-scanning sources, we develop a LiDAR system that is small enough to fit in the palm of the hand. This system is capable of measuring poorly reflective objects in the field of view (FoV) by selectively illuminating these objects with sufficient power by the beam-scanning laser, thereby overcoming the intrinsic issue affecting conventional flash-type 3D LiDAR systems. In addition, we develop and implement a program to have the LiDAR system automatically recognize poorly reflective objects in the FoV and measure their distances by selective illumination. Furthermore, we use the LiDAR system to perform long-distance ranging while including a poorly reflective object in the FoV.

Dually modulated photonic-crystal surface-emitting lasers (DM-PCSELs) are a new type of semiconductor laser that enable on-chip, mechanical-free, high-power, high-beam-quality 2D beam scanning over a wide field of view. These lasers are attracting attention for application in light detection and ranging, and the improvement of their slope efficiency is desired for this application. Thus far, the highest experimentally demonstrated slope efficiency is approximately 0.4 W/A at wavelengths of around 940 nm, which was limited by the fact that roughly half of the laser light was emitted toward and absorbed by the backside electrode. In this work, in order to improve the slope efficiency of DM-PCSELs, we utilize the light emitted toward the backside of the device by introducing a distributed Bragg reflector (DBR) as a backside reflector. In consideration of this laser’s ability to emit beams over a wide field of view, we design laser structures that facilitate a commensurate enhancement of efficiency at various emission angles. Next, we discuss the effect of the DBR on the confinement of transverse modes in thickness direction as well as the suppression of higher-order transverse modes. Then, we analyze the emission characteristics of a DM-PCSEL with a DBR and calculate that the theoretical slope efficiency is enhanced to 0.7–0.8 W/A, which is twice that of the device without a DBR, over a wide range of emission angles from 0° to 30°. We then fabricate the devices and experimentally demonstrate the emission of a single-lobed beam with a high slope efficiency of 0.7–0.8 W/A.

Photonic-crystal surface-emitting lasers (PCSELs) are attractive for a wide range of applications such as material processing and remote sensing owing to their advantage of broad-area coherent lasing supported by a 2-D photonic crystal. Recently, we proposed double-lattice photonic-crystal resonators and achieved 10-W-class high-power and high-beam-quality (namely, high-brightness) operation. To further increase the brightness, it is important to understand the lasing mode behavior of PCSELs in detail. In this article, we experimentally investigate the change in the lasing mode depending on the current injection levels and compare the results with a theoretical analysis that accounts for the change in refractive index and gain distributions due to the current injection. We observed a transition from single-mode lasing to two-mode lasing at high injection currents. We develop a technique for resolving the near-field profiles of individual lasing modes and show that these profiles coincide with theoretical predictions. A comparison of experimental and theoretical results reveals that high-order modes appear due to a bandgap confinement effect induced by a change in the refractive index in the carrier injection area. To effectively remove this confinement effect and suppress the oscillation of high-order modes, we incorporate an in-plane heterostructure into the PCSEL.

We develop a self-consistent theoretical model for simulating the lasing characteristics of photonic-crystal surface-emitting lasers (PCSELs) under continuous-wave (CW) operation that takes into account thermal effects caused by current injection. Our model enables us to analyze the lasing characteristics of PCSELs under CW operation by solving self-consistently the changes in the in-plane optical gain and refractive index distribution, which is associated with heat generation and temperature rise, and the change in the oscillation modes. We reveal that the lasing band-edge selectivity and beam quality of the PCSELs are affected by the spatial distribution of the band-edge frequency of the photonic crystal formed by the refractive index distribution, which depends on the temperature distribution in the resonator. Furthermore, we show that single-mode lasing with narrow beam divergence can be realized even at high current injection under CW operation by introducing a photonic-crystal structure with an artificially formed lattice constant distribution, which compensates such band-edge frequency distribution.

We demonstrate high-power continuous-wave (CW) lasing oscillation of 1.3-µm wavelength InP-based photonic-crystal surface-emitting lasers (PCSELs). Single-mode operation with an output power of over 100 mW, a side-mode suppression ratio (SMSR) of over 50 dB, and a narrow single-lobe beam with a divergence angle of below 1.2° are successfully achieved by using a double-lattice photonic crystal structure consisting of high-aspect-ratio deep air holes. The double lattice is designed to enhance both the in-plane optical feedback and the surface radiation effects in the photonic crystal. The coupling coefficients for 180 ∘ , +90 ∘ , and -90 ∘ diffractions are estimated from the measurements of the photonic band structure as κ1D = 417 cm−1, κ2D+ = 135 cm−1, and κ2D− = 65 cm−1, respectively. The stable single-mode, high-beam-quality operation is attributed to these large coupling coefficients introduced by the asymmetric double-lattice structure.

Light detection and ranging (LiDAR) is a key technology for smart mobility of robots, agricultural and construction machines, and autonomous vehicles. However, current LiDAR systems often rely on semiconductor lasers with low-quality, large-divergence, and asymmetric beams, requiring high-precision integration of complicated lens systems to reshape the beam. Also, due to the broad linewidth and the large temperature dependence of their lasing spectrum, a bandpass filter with broad bandwidth must be used in front of the detector, so the detected signal is affected by noise from background light such as sunlight. These critical issues limit the performance, compactness, affordability, and reliability of the LiDAR systems. Photonic-crystal surface-emitting lasers (PCSELs) have attracted much attention as novel semiconductor lasers that can solve the issues of conventional semiconductor lasers owing to their capability of high-quality, very-narrow-divergence, and symmetric beam operation supported by broad-area band-edge resonance in their two-dimensional photonic crystal. In this paper, we show the progress and the state of the art of broad-area coherent PCSELs and their application to a time-of-flight (ToF) LiDAR system. We first review the progress of PCSELs made so far. Next, we show recent progress based on PCSELs with a double-lattice structure that enables higher-power and narrower-divergence operation while keeping a symmetric beam shape. By optimizing the double-lattice photonic crystal and the reflective properties of a backside distributed Bragg reflector (DBR), we achieve a high peak power of 10 W while maintaining a nearly diffraction-limited beam divergence of ∼0.1° (FWHM) from a 500 µm diameter resonator. Using this PCSEL, we construct a LiDAR system that uses no external lens system in its light source and demonstrate highly spatially resolved ToF sensing (measurement range of ∼20 m), which is appropriate for autonomous robots and factory automation.

We report on photonic crystal lasers (PCSELs) with high power and high beam quality. The PCSELs have been developed with AI-assisted technologies. The developed devices with a 500µm diameter successfully oscillated with a high, 10W-class peak output power and a very narrow divergence angle of 0.1°. This fact indicates that the devices operate in a complete single lateral and longitudinal mode even over the large area of 500µm diameter. The devices have been installed in time-of-flight LiDAR system. Very high-resolution operation has been successfully realized even though a lens system is not utilized, clearly demonstrating the advantage of high-brightness PCSELs.

We report on electrically driven InP-based photonic-crystal surface-emitting lasers (PCSELs), which possess a deep-air-hole photonic crystal (PC) structure underneath an active region formed by metal-organic vapor-phase-epitaxial (MOVPE) regrowth. Single-mode continuous-wave (CW) lasing operation in 1.3-μm wavelength is successfully achieved at a temperature of 15°C. It is shown that the enhancement of lateral growth during the MOVPE regrowth process of air holes enables the formation of deep air holes with an atomically flat and thin overlayer, whose thickness is less than 100 nm. A threshold current of 120 mA (threshold current density = 0.68 kA/cm2) is obtained in a device with a diameter of 150 μm. A doughnut-like far-field pattern with the narrow beam divergence of less than 1° is observed. Strong optical confinement in the PC structure is revealed from measurements of the photonic band structure, and this strong optical confinement leads to the single-mode CW lasing operation with a low threshold current density.

Mechanical-free, high-power, high-beam-quality two-dimensional (2D) beam scanning lasers are in high demand for various applications including sensing systems for smart mobility, object recognition systems, and adaptive illuminations. Here, we propose and demonstrate the concept of dually modulated photonic crystals to realize such lasers, wherein the positions and sizes of the photonic-crystal lattice points are modulated simultaneously. We show using nano-antenna theory that this photonic nanostructure is essential to realize 2D beam scanning lasers with high output power and high beam quality. We also fabricate an on-chip, circuit-driven array of dually modulated photonic-crystal lasers with a 10 × 10 matrix configuration having 100 resolvable points. Our device enables the scanning of laser beams over a wide range of 2D directions in sequence and in parallel, and can be flexibly designed to meet application-specific demands., 電気的に2次元ビーム走査可能な新たなフォトニック結晶レーザーチップの開発に成功. 京都大学プレスリリース. 2020-07-22., Order from chaos. 京都大学プレスリリース. 2020-11-18.

Photonic-crystal surface-emitting lasers (PCSELs) use the two-dimensional (2D) resonance at the band-edge of a photonic crystal for lasing, and they feature various outstanding functionalities such as high-brightness lasing, arbitrary shaping of beam patterns and on-chip 2D beam steering. In this paper, to investigate the applicability of PCSELs for high-speed operation, we design PCSELs with enhanced in-plane optical feedback, which enable single-mode lasing inside a circular region the diameter of which is less than 10 µm. To realize a strong in-plane confinement of the lasing mode, we increase the one-dimensional coupling coefficients between counter-propagating waves through the careful design of the lattice points. We also introduce an in-plane heterostructure composed of two photonic crystals with different photonic bandgaps and utilize reflection at the boundary of the two photonic crystals in addition to the optical feedback at the band-edge of each photonic crystal. By using three-dimensional finite-difference time-domain method (3D-FDTD), we confirm that the proposed hetero-PCSELs can achieve single-mode lasing operation inside a 9-µm-diameter and possibly realize a 3-dB modulation bandwidth larger than 40 GHz.

We investigate microcrystalline-silicon (μc-Si) solar cells with photonic crystals on the top surface, which exploit the large-area resonant effect in photonic crystals to enhance light absorption, for the first time. Guidelines for designing the surface photonic crystals are discussed, showing the importance of oblique leakage loss suppression. The influence of direct surface etching of the intrinsic μc-Si layer (or photovoltaic layer) is studied, and it is shown that unbalanced etching of the amorphous and crystalline components of the μc-Si layer deteriorates the electronic performance, in particular, the open-circuit voltage and fill factor. We then introduce photonic crystals under an adequate etching condition, and confirm that the light absorption (or the external quantum efficiency) is successfully enhanced by improving the incident light coupling to the resonant modes. Finally, we fabricate a few-micrometer-thick μc-Si solar cell with a photonic crystal on the top surface and demonstrate an active-area efficiency of 11%, resulting from enhancement of the short-circuit current density, while simultaneously suppressing the deterioration of open-circuit voltage and fill factor. The obtained efficiency is the highest reported for a category of μc-Si solar-cell in which all the n/i/p layers are composed of Si, excluding wide-gap materials such as silicon oxide.

We develop a self-pulsating photonic-crystal surface-emitting laser with a ring-shaped saturable absorber inside the current injection region, and realize self-pulsation with a peak power of 8W, a pulse width of less than 100ps, and a beam divergence angle of 0.5°.

We develop a time-dependent three-dimensional coupled-wave theory (3D-CWT) for the transient analysis of photonic-crystal surface-emitting lasers (PCSELs). Our model takes into account the temporal evolution of both the photon and carrier distribution inside PCSELs, which enable the analysis of various above-threshold lasing characteristics including the relaxation oscillation, spatial hole burning, and multimode lasing. With the developed time-dependent 3D-CWT, we perform transient analysis of the high-power, high-beam-quality PCSELs with double-lattice photonic crystals and reproduce the experimental results of near-field patterns and lasing spectra under high current injection. Our theory enables the comprehensive understanding of the device physics of PCSELs toward the realization of higher-power continuous-wave lasing and short-pulse lasing.

Thermal management for the continuous wave (CW) operation of large-area double-lattice photonic-crystal surface-emitting lasers (PCSELs) is discussed. Thermal analysis is conducted to calculate the temperature of PCSELs under CW operation with heat dissipation. We assemble a double-lattice PCSEL in a water-cooling package with a highly thermally conductive sub-mount for heat dissipation. We measure the device temperature by using a thermographic camera and compare the measured values with the calculations. Owing to proper heat dissipation, we successfully realize ∼ 8 W of output power under CW operation of a single-chip double-lattice PCSEL.

We develop double-lattice photonic-crystal surface-emitting lasers, which are designed to achieve high-power and high-beam-quality operation. CW output power of ∼7W is successfully achieved from a single-chip with a diameter of 800μm.

Achieving high brightness (where brightness is defined as optical power per unit area per unit solid angle) in semiconductor lasers is important for various applications, including direct-laser processing and light detection and ranging for next-generation smart production and mobility. Although the brightness of semiconductor lasers has been increased by the use of edge-emitting-type resonators, their brightness is still one order of magnitude smaller than that of gas and solid-state/fibre lasers, and they often suffer from large beam divergence with strong asymmetry and astigmatism. Here, we develop a so-called ‘double-lattice photonic crystal’, where we superimpose two photonic lattice groups separated by one-quarter wavelength in the x and y directions. Using this resonator, an output power of 10 W with a very narrow-divergence-angle (

We demonstrate self-pulsation with a pulse width of 100 ps and a beam divergence angle of 0.35° in a photonic-crystal surface-emitting laser by introducing a saturable absorber section and employing a new double-hole photonic crystal.

We develop time-dependent three-dimensional coupled-wave theory for photonic-crystal surface-emitting lasers (PCSELs), which enables comprehensive analyses of lasing characteristics of high-power PCSELs under high current injection including relaxation oscillation, spatial hole-burning, and lasing spectra.

Enhancing the absorption of thin-film microcrystalline silicon solar cells over a broadband range in order to improve the energy conversion efficiency is a very important challenge in the development of low cost and stable solar energy harvesting. Here, we demonstrate that a broadband enhancement of the absorption can be achieved by creating a large number of resonant modes associated with two-dimensional photonic crystal band edges. We utilize higher-order optical modes perpendicular to the silicon layer, as well as the band-folding effect by employing photonic crystal superlattice structures. We establish a method to incorporate photonic crystal structures into thin-film (~500 nm) microcrystalline silicon photovoltaic layers while suppressing undesired defects formed in the microcrystalline silicon. The fabricated solar cells exhibit 1.3 times increase of a short circuit current density (from 15.0 mA/cm2 to 19.6 mA/cm2) by introducing the photonic crystal structure, and consequently the conversion efficiency increases from 5.6% to 6.8%. Moreover, we theoretically analyze the absorption characteristics in the fabricated cell structure, and reveal that the energy conversion efficiency can be increased beyond 9.5% in a structure less than 1/400 as thick as conventional crystalline silicon solar cells with an efficiency of 24%. © 2015 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.

Recently, W-class photonic-crystal surface-emitting lasers (PCSELs) with both a single spectrum and narrow spot beam pattern are reported. These highly coherent PCSEL properties cause a highly bright laser light that is useful for various applications. To improve the PCSEL output power, it is important to enlarge the emitting area to reduce the heat generation effect. However, multi-mode oscillation occurs in a broad emitting area because the difference in the threshold gain between the fundamental and higher modes becomes narrower as the emitting area is broadened. In this work, we fabricate PCSELs with double-hole lattice points that decrease the optical confinement to prevent multi-mode oscillation. The fabricated device, consisting of an AlGaAs/InGaAs material system designed to be oscillated at a wavelength of 940nm, has an emitting area of 300 × 300 μm 2 . In a square lattice photonic crystal whose lattice period equals the lasing wavelength embedded in PCSELs, the distance between the centers of the double hole is set to one quarter of the lasing wavelength to decrease in-plane coupling caused by interference. We confirm that this device is oscillated at the Γ point of band edge A in the photonic band structure. The peak power is more than 5 W under pulse operation at 10 A. The device has a narrow beam divergence of less than 1° and single lobe spectrum in spite of the broad emitting area, so these double-hole lattice points are an effective structure to improve the PCSEL output power.

A Si nanorod array enables the concentration of thermal emission in the near-infrared range while suppressing other components., Control of the thermal emission spectra of emitters will result in improved energy utilization efficiency in a broad range of fields, including lighting, energy harvesting, and sensing. In particular, it is challenging to realize a highly selective thermal emitter in the near-infrared–to–visible range, in which unwanted thermal emission spectral components at longer wavelengths are significantly suppressed, whereas strong emission in the near-infrared–to–visible range is retained. To achieve this, we propose an emitter based on interband transitions in a nanostructured intrinsic semiconductor. The electron thermal fluctuations are first limited to the higher-frequency side of the spectrum, above the semiconductor bandgap, and are then enhanced by the photonic resonance of the structure. Theoretical calculations indicate that optimized intrinsic Si rod-array emitters with a rod radius of 105 nm can convert 59% of the input power into emission of wavelengths shorter than 1100 nm at 1400 K. It is also theoretically indicated that emitters with a rod radius of 190 nm can convert 84% of the input power into emission of

Photonic-crystal surface-emitting lasers (PCSELs) have attracted considerable attention as a novel semiconductor laser that surpasses traditional semiconductor lasers. In this review article, we review the current progress of PCSELs, including the demonstration of large-area coherent oscillation, the control of beam patterns, the demonstration of beam steering, and the realization of watt-class and high-beam-quality operation. Furthermore, we show very recent progress in the exploration of high brightness of more than 300 MW cm−2 sr−1, obtained with a high output power of about 10 W while maintaining a high beam quality M2 ~ 2. The PCSELs with such high performances are expected to be applied to a variety of fields, such as laser-based material processing, optical sensing (light-detection and ranging (LiDAR)), and lighting, as they retain the benefits of compact and high-efficiency semiconductor lasers.

We investigate fabrication of a photonic-crystal structure by air-hole retained crystal regrowth using TBAs-based MOVPE for GaAs based photonic-crystal lasers. Air holes having a filling factor of 10 % (the depth was 250 nm and the width was 110 nm) are successfully embedded. The embedded air holes show characteristic shapes due to anisotropy of growth rate along different crystal planes, such as gallium face and arsenic face. Furthermore, a low threshold of 0.5 kAcm−2 lasing is achieved with the fabricated structure.

We experimentally demonstrate a photonic crystal (PC) thermal emitter that enables the electrical switching of dual emission peaks from a single light-emitting surface. The emitter consists of an n-GaAs/p-GaAs/n-GaAs stack that incorporates multiple quantum wells (MQWs) inside each pn junction. The MQWs induce narrowband thermal emission at two different wavelengths owing to the intersubband transition (ISB-T), which is enhanced by the second and third-order Γ-point resonant modes of the PC. By changing the electron density in one of the two MQWs with a reverse bias to each pn junction, we realize the separate control of the thermal emission intensity of the two peaks. Such two-wavelength switchable mid-infrared narrowband thermal emitters will benefit a number of practical applications including environmental monitoring and health diagnosis.

We demonstrate electrical high-speed wavelength switching of narrowband thermal emission on a single chip by using intersubband transitions in multiple quantum wells and optical resonances of photonic crystals.

Converting from a broadband to a narrowband thermal emission spectrum with minimal loss of energy is important in the creation of efficient environmental sensors and biosensors1,2 as well as thermo-photovoltaic power generation systems3,4. Here, we demonstrate such thermal emission control by manipulating photonic modes with photonic crystals as well as material absorption with quantum-well intersubband transitions. We show that the emission peak intensity for our device can be more than four times greater than that of a blackbody sample under the same input power and thermal management conditions due to an increase in the temperature compared to the blackbody reference, and the emission bandwidth and angular spread are narrowed by a factor of 30 and 8, respectively. These results indicate that the energy saved by thermal emission control can be recycled and concentrated to enhance the narrow peak emission intensity. By constructing a thermal emission control device based on a multiple-quantum-well layer embedded in a two-dimensional photonic crystal, researchers demonstrate that they can convert a broadband thermal emission spectrum into a narrowband spectrum with minimal loss of energy.

We review the recent progress in thin-film silicon solar cells with photonic crystals, where absorption enhancement is achieved by using large-area resonant effects in photonic crystals. First, a definitive guideline for enhancing light absorption in a wide wavelength range (600–1100 nm) is introduced, showing that the formation of multiple band edges utilizing higher-order modes confined in the thickness direction and the introduction of photonic superlattice structures enable significant absorption enhancement, exceeding that observed for conventional random scatterers. Subsequently, experimental evidence of this enhancement is demonstrated for a variety of thin-film Si solar cells: ~500-nm-thick ultrathin microcrystalline silicon cells, few-µm-thick microcrystalline silicon cells, and ~20-µm-thick thin single-crystalline silicon cells. The high short-circuit current densities and/or efficiencies observed for each cell structure confirm the effectiveness of using multiple band-edge resonant modes of photonic crystals for enhancing broadband absorption in actual solar cells.