High-brightness scalable continuous-wave single-mode photonic-crystal laser.

Realizing large-scale single-mode, high-power, high-beam-quality semiconductor lasers, which rival (or even replace) bulky gas and solid-state lasers, is one of the ultimate goals of photonics and laser physics. Conventional high-power semiconductor lasers, however, inevitably suffer from poor beam quality owing to the onset of many-mode oscillation1,2, and, moreover, the oscillation is destabilized by disruptive thermal effects under continuous-wave (CW) operation3,4. Here, we surmount these challenges by developing large-scale photonic-crystal surface-emitting lasers with controlled Hermitian and non-Hermitian couplings inside the photonic crystal and a pre-installed spatial distribution of the lattice constant, which maintains these couplings even under CW conditions. A CW output power exceeding 50 W with purely single-mode oscillation and an exceptionally narrow beam divergence of 0.05° has been achieved for photonic-crystal surface-emitting lasers with a large resonant diameter of 3 mm, corresponding to over 10,000 wavelengths in the material. The brightness, a figure of merit encapsulating both output power and beam quality, reaches 1 GW cm-2 sr-1, which rivals those of existing bulky lasers. Our work is an important milestone toward the advent of single-mode 1-kW-class semiconductor lasers, which are expected to replace conventional, bulkier lasers in the near future.

High-power and high-efficiency operation of 1.3 µm-wavelength InP-based photonic-crystal surface-emitting lasers with metal reflector.

We demonstrate high-output-power and high-efficiency operation of 1.3-µm-wavelength InP-based photonic-crystal surface-emitting lasers (PCSELs). By introducing a metal reflector and adjusting the phase of the reflected light via optimization of the thickness of the p-InP cladding layer, we successfully achieve an output power of approximately 400 mW with the slope efficiency of 0.4 W/A and the wall-plug efficiency of 20% under CW conditions. In addition, this PCSEL exhibits a narrow circular beam with a divergence angle below 1.6° even at high output powers under CW conditions at temperatures from 15°C to 50°C. We have also demonstrated an output power of over 12 W under pulsed conditions at room temperature.

High-order optical vortex beam generation based on watt-class spatial phase plate-integrated photonic-crystal surface-emitting lasers.

We develop spatial phase plate (SPP)-integrated photonic-crystal surface-emitting lasers featuring double-lattice photonic-crystal structures embedded using a crystal regrowth technique. SPPs possessing eight segmentations per phase rotation number l are fabricated on the top surface to generate optical vortex beams (OVBs) with l = 1-3. The beams exhibit a high output power of ∼5 W and high mode purities of 85%, 78%, and 72% for l = 1-3, respectively. These purity values are comparable with those of a pure Gaussian mode passing through an SPP. The compact, high-power, and high-purity OVB sources can be used in the fields of material processing, optical manipulation, and microscopy.

Raman silicon nanocavity laser with efficient light emission from the edge of an adjacent waveguide.

A Raman nanocavity laser can emit light into free space and into a properly designed waveguide adjacent to the cavity by mode coupling. In common device designs, the emission from the edge of this waveguide is relatively weak. However, a Raman silicon nanocavity laser with strong emission from the waveguide edge would be advantageous for certain applications. Here we investigate the increase in the edge emission that can be achieved by adding photonic mirrors to the waveguides adjacent to the nanocavity. We experimentally compare devices with and without photonic mirrors: the edge emission for devices with mirrors is 4.3 times stronger on average. This increase is analyzed using coupled-mode theory. The results indicate that the control of the round-trip phase shift (between the nanocavity and the mirror) and an increase of the quality factors of the nanocavity are important for further enhancement.

Suppressing the sample-to-sample variation of photonic crystal nanocavity Q-factors by air-hole patterns with broken mirror symmetry.

It is known that the quality factors (Q) of photonic crystal nanocavities vary from sample to sample due to air-hole fabrication fluctuations. In other words, for the mass production of a cavity with a given design, we need to consider that the Q can vary significantly. So far, we have studied the sample-to-sample variation in Q for symmetric nanocavity designs, that is, nanocavity designs where the positions of the holes maintain mirror symmetry with respect to both symmetry axes of the nanocavity. Here we investigate the variation of Q for a nanocavity design in which the air-hole pattern has no mirror symmetry (a so-called asymmetric cavity design). First, an asymmetric cavity design with a Q of about 250,000 was developed by machine learning using neural networks, and then we fabricated fifty cavities with the same design. We also fabricated fifty symmetric cavities with a design Q of about 250,000 for comparison. The variation of the measured Q values of the asymmetric cavities was 39% smaller than that of the symmetric cavities. This result is consistent with simulations in which the air-hole positions and radii are randomly varied. Asymmetric nanocavity designs may be useful for mass production since the variation in Q is suppressed.

Silicon nanocavity with a quality factor of 6.7 million fabricated by a CMOS-compatible process.

Here, we report on the increase of the quality-factors of photonic crystal nanocavities fabricated by a CMOS-compatible process. We fabricated nanocavities with the same cavity design but used either a binary photomask or a phase-shift photomask in the photolithography step to assess the impact of the photomask-type on the fabrication accuracy of the air holes. We characterized 62 cavities using time-resolved measurements and the best cavity had a quality-factor of 6.65 × 106. All cavities exhibited a quality-factor larger than 2 million and the overall average was 3.25 × 106. While the estimated magnitude of the scattering loss due to the air hole variations in the 33 cavities fabricated with the phase-shift photomask was slightly lower than that in the 29 cavities fabricated with binary photomask, the phase-shift photomask did not provide a significant improvement in the fabrication accuracy. On average, the scattering loss in these samples is more than 3 times larger than that of nanocavities fabricated using electron-beam lithography, which indicates room for further improvement.

200-W short-pulse operation of photonic-crystal lasers based on simultaneous absorptive and radiative Q-switching.

Short-pulse high-peak-power lasers are crucial laser sources for various applications such as non-thermal ultrafine material processing and eye-safe high-resolution remote sensing. Realizing such operation in a single semiconductor laser chip without amplifiers or external resonators is expected to contribute to the development of compact, affordable laser sources for such applications. In this paper, we demonstrate short-pulse high-peak-power photonic-crystal surface-emitting lasers based on simultaneous absorptive and radiative Q-switching. The proposed device induces an instantaneous and simultaneous decrease in both absorptive and out-of-plane radiation losses due to saturable absorption and self-evolution of the photonic band, respectively, which results in drastic Q-switching operation of the device. Based on this concept, we experimentally demonstrate short-pulse generation with 200-W-class peak power and a pulse width of < 30 ps. In addition, via pulse compression with dispersion compensation, we achieve an even higher peak power of ∼300 W with a shorter pulse width of ∼10 ps.

Topological Unidirectional Guided Resonances Emerged from Interband Coupling.

Unidirectional guided resonances (UGRs) are optical modes in photonic crystal slabs that radiate toward one side without the need for mirrors on the other. In this Letter, we report a mechanism to realize UGRs by tuning the interband coupling effect originating from up-down symmetry breaking. We theoretically find that UGRs that reside along high-symmetric lines correspond to phase singularities of far-field radiation, depicted by phase winding numbers as a type of topological indices. We investigate the phase dislocation lines in three-dimensional parameter space and elaborate on the interplay between UGRs and non-Hermitian degeneracies accordingly. Our findings reveal the topological nature of UGRs about their generation, evolution, and annihilation in general parameter spaces, thus paving the way to new possibilities of light manipulation.

Towards optimization of photonic-crystal surface-emitting lasers via quantum annealing.

Photonic-crystal surface-emitting lasers (PCSELs), which utilize a two-dimensional (2D) optical resonance inside a photonic crystal for lasing, feature various outstanding functionalities such as single-mode high-power operation and arbitrary control of beam polarizations. Although most of the previous designs of PCSELs employ spatially uniform photonic crystals, it is expected that lasing performance can be further improved if it becomes possible to optimize the spatial distribution of photonic crystals. In this paper, we investigate the structural optimization of PCSELs via quantum annealing towards high-power, narrow-beam-divergence operation with linear polarization. The optimization of PCSELs is performed by the iteration of the following three steps: (1) time-dependent 3D coupled-wave analysis of lasing performance, (2) formulation of the lasing performance via a factorization machine, and (3) selection of optimal solution(s) via quantum annealing. By using this approach, we discover an advanced PCSEL with a non-uniform spatial distribution of the band-edge frequency and injection current, which simultaneously enables higher output power, a narrower divergence angle, and a higher linear polarization ratio than conventional uniform PCSELs. Our results potentially indicate the universal applicability of quantum annealing, which has been mainly applied to specific types of discrete optimization problems so far, for various physics and engineering problems in the field of smart manufacturing.

Dually modulated photonic crystal lasers for wide-range flash illumination.

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.

High-power CW oscillation of 1.3-µm wavelength InP-based photonic-crystal surface-emitting lasers.

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.

Sub-100-nW-threshold Raman silicon laser designed by a machine-learning method that optimizes the product of the cavity Q-factors.

Raman silicon lasers based on photonic crystal nanocavities with a threshold of several hundred microwatts for continuous-wave lasing have been realized. In particular, the threshold depends on the degree of confinement of the excitation light and the Raman scattering light in the two nanocavity modes. Here, we report lower threshold values for Raman silicon nanocavity lasers achieved by increasing the quality (Q) factors of the two cavity modes. By using an optimization method based on machine learning, we first increase the product of the two theoretical Q values by a factor of 17.0 compared to the conventional cavity. The experimental evaluation demonstrates that, on average, the actually achieved product is more than 2.5 times larger than that of the conventional cavity. The input-output characteristic of a Raman laser with a threshold of 90 nW is presented and the lowest threshold obtained in our experiments is 40 nW.

Near-field thermophotovotaic devices with surrounding non-contact reflectors for efficient photon recycling.

Near-field thermophotovoltaic (TPV) power generation has been attracting increasing attention as a promising approach for efficient conversion of heat into electricity with high output power density. Here, we numerically investigate near-field TPV devices with surrounding reflectors for efficient recycling of low-energy photons, which do not contribute to the power generation. We reveal that the conversion efficiency of a near-field TPV system can be drastically increased by introducing a pair of reflectors above and below the system, especially when the two mirrors are not in contact with the emitter and absorber. In addition, we investigate the influence of non-perfect photon recycling on the TPV efficiency and reveal that near-field TPV systems are more robust against the decrease of the reflectivity of the reflectors than the far-field TPV systems.

Detection of negatively ionized air by using a Raman silicon nanocavity laser.

The performance of a Raman silicon laser based on a high quality-factor nanocavity depends on the degree of free-carrier absorption, and this characteristic may be useful for certain applications. Here we demonstrate that laser oscillation in a Raman silicon nanocavity laser stops abruptly after an exposure to a weak flux of negatively ionized air for a few seconds. Spectral measurements reveal that the laser interruption is mainly caused by the transfer of extra electrons from the negatively ionized air molecules to the silicon nanocavity. These electrons affect the efficiency of the Raman laser by free carrier absorption. We find that the laser output gradually recovers as the extra electrons escape from the nanocavity and confirm that such a detection of ionized air is repeatable. These results show that a Raman silicon nanocavity laser can be used for the detection of ionized air with a high spatial resolution.

1.2-µm-band ultrahigh-Q photonic crystal nanocavities and their potential for Raman silicon lasers.

Nanocavity devices based on silicon that can operate in the 1.2-µm band would be beneficial for several applications. We fabricate fifteen cavities with resonance wavelengths between 1.20 and 1.23 µm. Experimental quality (Q) factors larger than one million are obtained and the average Q values are lower for shorter wavelengths. Furthermore, we observe continuous-wave operation of a Raman silicon laser with an excitation wavelength of 1.20 µm and a Raman laser wavelength of 1.28 µm. The Q values of the nanocavity modes used to confine the excitation light and the Raman scattered light are about half of those for our Raman silicon laser operating in the 1.55-µm band. Nevertheless, this device exhibits an input-output characteristic with a clear laser threshold. Finally, we consider the effect of the higher scattering probability at shorter wavelengths on the Raman laser performance in the 1.2-µm band.

Self-consistent analysis of photonic-crystal surface-emitting lasers under continuous-wave operation.

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.

Double-lattice photonic-crystal resonators enabling high-brightness semiconductor lasers with symmetric narrow-divergence beams.

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 (<0.3°) symmetric surface-emitted beam is achieved from a circular emission area of 500 µm diameter under pulsed conditions, which corresponds to a brightness of over 300 MW cm-2 sr-1. In addition, an output power up to ~7 W is obtained under continuous-wave conditions. Detailed analyses on the double-lattice structure indicate that the resonators have the potential to realize a brightness of up to 10 GW cm-2 sr-1, suggesting that compact, affordable semiconductor lasers will be able to rival existing gas and fibre/disk lasers.

Continous-wave lasing operation of 1.3-µm wavelength InP-based photonic crystal surface-emitting lasers using MOVPE regrowth.

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.

One-Chip Near-Field Thermophotovoltaic Device Integrating a Thin-Film Thermal Emitter and Photovoltaic Cell.

Thermal radiation transfer between two objects separated by a subwavelength gap (near-field thermal radiation transfer) can be orders of magnitude larger than that in free space, which is attracting increasing attention with respect to both fundamental nanoscience and its potential for high-power-density and high-efficiency conversion of heat to electricity in thermophotovoltaic (TPV) systems. However, the realization of near-field thermal radiation transfer in TPV systems involves significant challenges because it requires a subwavelength gap and large temperature difference between the emitter and the PV cell while minimizing the heat transfer that does not contribute to the photocurrent generation. To overcome these challenges, here we demonstrate a one-chip near-field TPV device consisting of a thin-film Si emitter and InGaAs PV cell with an intermediate Si substrate, which enables the suppression of the heat transfer due to sub-bandgap radiation by free carriers and surface modes. Through the one-chip integration and thermal isolation using Si process technologies, we realize a deep subwavelength gap (<150 nm) between the emitter and the intermediate substrate without using any external positioners while maintaining a large temperature difference (>700 K). Compared to the equivalent device operating in the far-field regime, we achieve 10-fold enhancement of the photocurrent in the PV cell without degrading the open-circuit voltage and fill factor, demonstrating the potential of our one-chip device for the future applications of near-field thermal radiation transfer.

Generation of optical vortex beam by surface-processed photonic-crystal surface-emitting lasers.

An optical vortex beam possesses a phase singularity that causes a null intensity at the center of the beam, and can be explained as a superposition of a phase distribution along the azimuthal direction and a plane wave. Here, we process the surface of a photonic-crystal surface-emitting laser (PCSEL) to generate an optical vortex beam. By using an eight-segmented phase plate fabricated via three chemical etching steps, a beam having null intensity is obtained. From evaluation of the beam's polarization and interference patterns, we show that the null intensity comes from the phase singularity of the optical vortex.