Photonic Crystal Surface-emitting Lasers Research Articles

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.

Static Beam Steering by Applying Metasurfaces on Photonic-Crystal Surface-Emitting Lasers

Photonic-crystal surface-emitting laser (PCSEL) holds promise for laser beam with high efficacy while retain the small divergence angle, being now all the rage as laser source in modern opto-electronic technologies to replace edge emitting lasers and vertical-cavity surface-emitting lasers (VCSELs). Arbitrary beam shaping and deflection of VCSELs with ultra-thin flat optics, aka metasurfaces, has recently demonstrated, making possibility to realize a compact system in many technologies. However, the large divergence angle and limited output power at single fundamental mode of the VCSEL limits its application. Here, we report the static steering of laser beam with high efficiency and near 1° divergence angle by shining the laser beam of the PCSEL through the metasurface array placed on top. The small divergence angle is contributed from PCSEL. On the other hand, the metasurface array offers the higher efficiency beam deflection, without considering additional collimating or focusing. This provides highly flexible platform and is capable to manipulate the laser emission through the design of the metasurfaces and the PCSELs. Moreover, the beam proprieties can be further modified for specific application through the metasurfaces. The integration of metasurface with PCSEL provides the compact and collimated laser source, promising more accurate generation of versatile laser sources on demand for automotive ultra-compact light detection and ranging (LiDAR), depth sensing, optical communication, etc.

Symmetry Criterion and Far-Field Control of Photonic-Crystal Surface-Emitting Lasers

Photonic-crystal surface-emitting lasers (PCSELs) have led to amazing results in overcoming the divergence limitation of semiconductor lasers. However, so far, the physical mechanism behind this promising control of far-field characteristics is still unclear. Here, we perform a theoretical study of the mechanism of the influence of photonic crystal (PhC) geometry on the far field of PCSELs. The perspective from group theory is adopted in our analysis. We explore the function of symmetry in the formation of the far-field pattern (FFP) and clarify the roles played by various PhC parameters in this process. Through our analysis, a symmetry criterion to design PCSELs with single-lobed narrow beams is shown, where an asymmetric in-plane PhC structure, a large vertical confinement factor, and a sufficient number of periods are required. Our results reveal the physical origin of the narrow beam of PCSELs, which can even reach 0.1° with a PhC cavity size of over 1000 μm at a lasing wavelength of 940 nm.

A Simple Method to Build High Power PCSEL Array with Isolation Pattern Design

Photonic crystal surface-emitting lasers (PCSELs) hold promising properties of both edge emitting lasers (EELs) and vertical-cavity surface-emitting lasers (VCSELs). They possess high output power while radiating light vertically, being thought of as the next generation laser source. One of the main advantages of PCSELs is their scalability of size and power, which makes them applicable to high power applications or long-distance detection. However, due to problems such as current injection and mode competition, there are limits on their dimensions. To further increase the power, the capability of two-dimensional array integration paves the way. In this work, we demonstrate a new method to fabricate PCSEL arrays by defining an isolation pattern. We also investigate the influence of aperture size and array arrangement on lasing performance.

Wide-bandgap GaN-based watt-class photonic-crystal lasers

Short-wavelength (blue-violet-to-green) lasers with high power and high beam quality are required for various applications including the machining of difficult-to-process materials and high-brightness illuminations and displays. Promising light sources for such applications are wide-bandgap GaN-based photonic-crystal surface-emitting lasers (PCSELs), which are based on two-dimensional resonance in the photonic crystal. Developments of these devices have lagged behind those of longer-wavelength GaAs-based PCSELs, because device designs for achieving robust two-dimensional resonance and a nanofabrication process that avoids introducing disorders have remained elusive for wide-bandgap GaN-based materials. Here, we address these issues and successfully realize GaN-based PCSELs with high, watt-class (>1 W) output power and a circular, single-lobed beam with a very narrow (~0.2°) divergence angle at blue wavelengths. In addition, we demonstrate continuous-wave operation with a high output power (~320 mW) and a high beam quality (M2~1). Our results will enable the use of GaN-based PCSELs in the above-mentioned applications.

Performance Analyses of Photonic-Crystal Surface-Emitting Laser: Toward High-Speed Optical Communication

This study conducts comprehensive performance analyses of a commercial photonic-crystal surface-emitting laser (PCSEL) via small-signal measurement and the bit-error-rate test. Meanwhile, the radio frequency characteristics of the PCSEL are unveiled for the first time. Compared to the vertical-cavity surface-emitting lasers, the PCSEL shows great potential for a broader optical bandwidth that is benefited from the high optical-confinement factor. A maximum bandwidth of around 2.32 GHz is experimentally observed when the PCSEL was biased at 340 mA. Moreover, a theoretical calculation was applied to shed light on the characteristics of the small-signal measurement, providing a deep insight into the corresponding intrinsic response model. The signal transmission capability of the PCSEL was investigated as well. The maximum bit rate and corresponding rise time transmitted at 500 Mbps are 1.2 Gbps and 186.16 ps, respectively. Thus, a high-speed PCSEL can be realised with a shrunk form factor, serving as a promising candidate for the next-generation light sources in high-speed optical communication.

Modulating Light Emission Performance of PCSEL via GaN HEMT Driving Circuit

In this study, a paradigm for modulating the light emission performance of photonic-crystal surface-emitting laser (PCSEL) via GaN high electron mobility transistor (HEMT) driving circuit is proposed for the first time. For light detection and ranging (LiDAR) system, a faster pulse repetition frequency with shorter pulse width can provide not only high resolution but also sufficiently precise range resolution. Hereupon, comprehensive analyses for such an integrated system are conducted with not only electro-optical responses but also the corresponding optical behaviors. The relevant electrical characteristics of the employed GaN HEMT are examined at first. Next, the integrated system on a matrix board with its corresponding circuit topology is discussed, illustrating the relevant operating principles. Thereby, sufficient systematical scrutinization for relevant light emissions is performed for both photodiode responses and the optical behaviors under different conditions, paving a holistic panorama for the LiDAR system. Thus, prospects for the next generation LiDAR system in high-power and high-speed operation can be expected.

Scaling Challenges in High Power Photonic Crystal Surface-Emitting Lasers

Since the initial proposal in 1999 by Noda <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> , photonic crystal surface emitting lasers (PCSELs) have shown to achieve large area, coherent lasing with a narrow, single mode beam. Owing to their unique orthogonal electrical/optical cavity scheme, PCSELs have emerged as one of the most promising platforms for high power diode lasers. In this paper, we report the performance trade-offs and challenges in power scaling, modal competition, and charge injection control. To address these challenges, we investigate the electrical and optical performances of PCSELs under high-power operation. Heterostructure designs with electron block layers were incorporated for efficient and balanced electrical charge injection. Symmetry breaking strategy was introduced in the photonic crystal design to engineer the in-plane momentum dependent quality factor (Q(k)) for mode discrimination and single mode operation. The impact of electrode designs was also investigated to achieve large area uniform charge injection. Simulation results of these designs indicate promising solutions and improved performances for large-area high-power and single-mode PCSEL operations.

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.

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.

General recipe to realize photonic-crystal surface-emitting lasers with 100-W-to-1-kW single-mode operation

Realization of one-chip, ultra-large-area, coherent semiconductor lasers has been one of the ultimate goals of laser physics and photonics for decades. Surface-emitting lasers with two-dimensional photonic crystal resonators, referred to as photonic-crystal surface-emitting lasers (PCSELs), are expected to show promise for this purpose. However, neither the general conditions nor the concrete photonic crystal structures to realize 100-W-to-1-kW-class single-mode operation in PCSELs have yet to be clarified. Here, we analytically derive the general conditions for ultra-large-area (3~10 mm) single-mode operation in PCSELs. By considering not only the Hermitian but also the non-Hermitian optical couplings inside PCSELs, we mathematically derive the complex eigenfrequencies of the four photonic bands around the Γ point as well as the radiation constant difference between the fundamental and higher-order modes in a finite-size device. We then reveal concrete photonic crystal structures which allow the control of both Hermitian and non-Hermitian coupling coefficients to achieve 100-W-to-1-kW-class single-mode lasing.

Progress of Photonic-Crystal Surface-Emitting Lasers: A Paradigm Shift in LiDAR Application

Nowadays, the flurry of autonomous vehicles is in full swing regarding light detection and ranging (LiDAR) and depth perception. For such visual perception, light plays an important role. We human beings recognize and distinguish surrounding details when the eye focuses light on the retina. For the LiDAR system, pulsed lasers are employed to measure the relevant range. Thus, appropriate light sources with high performance are in urgent demand. Auspiciously, a revolutionary semiconductor laser technology, namely the photonic-crystal surface-emitting laser (PCSEL), emerges over the past two decades. PCSEL exhibits not only a symmetric beam profile with narrow beam divergence but also a high-power operation with controllability. Therefore, it may be the holy grail for an ultracompact time-of-flight (ToF) LiDAR system. Hereupon, comprehensive analyses of PCSEL-relevant scientific publications and patent documents are conducted. We thereby review the development progress of PCSEL technology. Moreover, a systematic simulation is performed, providing real-time visualization of relevant point clouds with different beam divergence. PCSEL technology with unprecedented merits indeed turns a new leaf and a paradigm shift in LiDAR application is ongoing. It is believed that a lens-free and adjustment-free ultracompact apparatus in simplicity can be expected.

Investigation on GaN-Based Membrane Photonic Crystal Surface Emitting Lasers.

A GaN-based blue photonic crystal surface emitting laser (PCSEL) featured with membrane configuration was proposed and theoretically investigated. The membrane dimension, photonic crystal (PhC) material, lattice constant and thickness were studied by RCWA (Rigorous Coupled Wave Analysis), FDTD (Finite Difference Time Domain) simulations with the confinement factor and gain threshold as indicators. The membrane PCSEL’s confinement factor of active media is of 13~14% which is attributed to multi-pairs of quantum wells and efficient confinement of the mode in the membrane cavity with air claddings. The excellent confinement factor and larger Q factor of resonance mutually contribute to the lower gain threshold of the design (below 400 cm−1 for GaN-PhC with 100 nm thick top and bottom GaN layer, 40 nm hole radius and 40 nm depth). The PhC confinement factor exceeds 13% and 6% for TiO2-PhC with 80 nm and 60 nm PhC thickness and 20 nm and 40 nm distance between PhC and active media, respectively. It is around two times larger than that of GaN-PhC, which is attributed to the higher refractive index of TiO2 that pulls field distribution to the PhC layer.

Impact of Air-Hole on the Optical Performances of Epitaxially Regrown P-Side Up Photonic Crystal Surface-Emitting Lasers

Optical performances of 950 nm p-side up photonic crystal surface emitting lasers (PCSELs) with different types of air holes are numerically and experimentally investigated. Simulation results show an obvious distinction between effective index method and three dimensional finite-element method (FEM). Measured experiments including wavelength, threshold, and slope efficiency can well meet the FEM numerical predictions. It further reveals that the B mode dominates the laser action and the PCSELs accompanying with equilateral triangle and right isosceles triangle holes possess a lower threshold current and higher radiation than symmetric circle holes. The output power can be further enhanced by increasing the filling factor of air holes before the regrowth process.

Design of GaN-based photonic crystal surface emitting lasers with top TiO2 photonic crystals

We designed GaN-based blue photonic crystal surface emitting lasers (PCSELs) by incorporating a TiO2 photonic crystal (PhC) layer on top of the GaN surface. An AlGaN cladding layer was designed under the active region to form a waveguide-like cap layer. The dimension of the cap layer, the lattice constant of PhC, the hole radius, and the layer thickness were investigated to optimize the structure design in view of the PhC confinement factor and gain threshold. With a depth of 100 nm of the PhC layers, a hole radius of 40 nm, and a thickness of 100 nm of GaN layers above and below multiple quantum wells (MQWs), the PhC confinement factor of greater than 10% and the gain threshold below 400 cm−1 can be realized. Furthermore, the photonic band diagram is calculated, which shows that the designed PCSELs are operated at the Γ1 band edge with the lowest pump energy. This work demonstrates the potential of excellent properties in the future surface emitting blue laser sources.

Photonic crystal lasers: from photonic crystal surface emitting lasers (PCSELs) to hybrid external cavity lasers (HECLs) and topological PhC lasers [Invited

Photonic crystals (PhC) represent an important class of silicon photonics components employed as wavelength selective resonators to act as narrow-band mirrors in integrated lasers due to their small footprint, high surface area, and Q-factor/volume ratio that enables efficient confinement of light, required for improved performances of the laser. These properties of PhCs are key for the potential deployment of PhC based high power, energy efficient and versatile semiconductor lasers for telecom, datacom, optical sensing and biomedical applications. In this paper, we report the main advances on PhC based lasers from photonic crystal surface-emitting lasers (PCSELs) to the new hybrid external cavity laser (HECLs) configurations.

Photonic Crystal Surface Emitting Laser Operating in Pulse-Periodic Regime with Ultralow Divergence Angle

The nanosecond-level pulse-operation characteristics of photonic-crystal surface-emitting lasers (PCSELs) with ultralow divergence were investigated in detail. We demonstrate a maximum peak output power of 14 W for a current pulse width of 9 ns, which is about 28 times the saturated power under continuous wave (CW) operation. The full width at half maximum (FWHM) of the optical response pulse is about 3 ns wider than the current pulse. The maximum repetition frequency reaches 400 kHz at 10 A without significant degradation of output power while the value is 100 kHz at 40 A. Moreover, the multimode behavior of the PCSEL at a high peak current was analyzed.

Design of GaN-Based PCSEL With Temperature-Insensitive Lasing Wavelength

Considering the compensation of thermo-optic coefficients (TOC) between GaN-based cavity and TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> photonic crystal (PhC) layer, we have theoretically designed a photonic crystal surface emitting laser (PCSEL) with temperature-insensitive wavelength. The structure includes a freestanding GaN membrane with embedded multiple InGaN/GaN quantum well (QW) layer and the top TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PhC layer. The high refractive index (RI) of PhC and the freestanding membrane cavity structure cooperate to enable a stronger field-coupling in PhC, thus fulfill the athermal coupling condition. Interestingly, the wavelength temperature dependence is only 0.0015 nm/°C for such PCSEL with a 82% PhC confinement factor, and its athermal property is proportional to the field confinement factor in TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PhC layer. To illustrate, a general formula has been developed after the investigations of thermo-optic effect and thermal expansion effect.

Light Detection Functionality of Photonic-Crystal Lasers

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.

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.