Room-temperature Continuous-wave Operation Research Articles

Photonic engineering of InP towards homoepitaxial short-wavelength infrared VCSELs

Many emerging opportunities, such as three-dimensional (3D) sensing, biophotonics, and optical data links, call for vertical cavity surface-emitting lasers (VCSELs) that operate in the short-wavelength infrared (SWIR) range. In this paper, we report the use of InP distributed Bragg reflector (DBR) mirrors to overcome an impasse in wafer-level mass production of SWIR VCSELs. The DBRs were based on homoepitaxial InP structures and selectively converted through electrochemistry into quarter-wavelength stack structures of alternating nanoporous (NP) and nonporous InP layers with a record index contrast (Δn∼1.0) and near-unity reflectivity. We demonstrated VCSEL operation at both 1380 and 1550 nm from two separate structures prepared on InP substrates using NP–InP DBRs as the bottom mirror and dielectric DBRs as the top mirror. Room temperature continuous-wave (CW) operation of SWIR VCSELs was successfully achieved at both wavelengths with a threshold current density below 2kA/cm2, greater than milliwatt optical output, and a peak power conversion efficiency of 17%. Our work provides strong evidence that the decades-old challenge, in preparing an InP-compatible, high-performance DBR to support the SWIR-emitting vertical cavity, has been addressed and is poised to enable new applications.

Continuous wave room temperature operation of the 2 μ m GaSb-based photonic crystal surface emitting diode lasers

Continuous wave room temperature operation of 2 μm GaSb-based photonic-crystal surface-emitting diode lasers has been realized. The deep etched square mesa devices showed threshold current densities of 500 A/cm2 at 20 °C. The epi-side down mounted lasers generated above 10 mW of output power in the continuous wave regime and tens of milliwatts in pulses from the 200 × 200 μm2 aperture. The breakthrough in the device performance parameters was achieved thanks to a highly homogeneous air-pocket retaining epitaxial regrowth process optimized for a specifically designed antimonide diode laser heterostructure. The nanofabrication method utilizing low temperature atomic hydrogen surface cleaning yielded low disorder square lattice of droplet-shaped voids covered by uniform p-cladding layer. The laser emission spectrum as well as near/far field patterns demonstrated peculiar features presumably linked to deformation of the void shape during regrowth and formation of the array of filaments.

Room temperature continuous-wave operation of a dual-wavelength quantum cascade laser

We report on the design and fabrication of a dual-wavelength switchable quantum cascade laser (QCL) by optimizing the design of a homogeneous active region and combining superposed distributed feedback gratings. Coaxial, single-mode emissions at two different wavelengths were achieved only through adjusting the bias voltage. Room temperature continuous-wave operation with output powers of above 30 mW and 75 mW was realized for single-mode emission at 7.61 µm and 7.06 µm, respectively. The simplified fabrication process and easy wavelength control of our designed dual-wavelength QCL make it very attractive for developing miniature multi-species gas sensing systems.

Room‐temperature high‐speed mid‐infrared quantum cascade laser with π‐shape metal contact

The authors demonstrate a high-speed 8.5-μm quantum cascade laser (QCL) with room temperature continuous wave (CW) operation. The maximum output power of 141 mW is obtained at 20°C. The parasitic capacitance of the device is decreased from 36.6 to 7.1 pF by monolithic integrating a π-shape metal contact electrode. This results in an increase in the −3 dB RF modulation bandwidth from 870 MHz to 4.5 GHz compared with the conventional electrode configuration.

Promotion of Specific Single-Transverse-Mode Beam Characteristics for GaSb-Based Narrow Ridge Waveguide Lasers via Customized Parameter Design

GaSb-based single-transverse-mode narrow ridge waveguide (RW) lasers with high power and simultaneous good beam quality have broad application prospects in the mid-infrared wavelength region. Yet its design and formation have not been investigated systematically, while the beam characteristics that affect their suitability for specific applications remain rarely analyzed and optimized. The present work addresses these issues by theoretically establishing a waveguide parameter domain that generalizes the overall possible combinations of ridge widths and etch depths that support single-transverse-mode operation for GaSb-based RW lasers. These results are applied to develop two distinct and representative waveguide designs derived from two proposed major optimization routes of model gain expansion and index-guiding enhancement. The designs were evaluated experimentally based on prototype 1-mm cavity-length RW lasers in the 1950 nm wavelength range, which were fabricated with waveguides having perpendicular ridge and smooth side-walls realized through optimized dry etching conditions. The model gain expanded RW laser design with a relatively shallow-etched (i.e., 1.55 upmum) and wide ridge (i.e., 7 upmum) yielded the highest single-transverse-mode power to date of 258 mW with a narrow lateral divergence angle of 11.1^circ full width at half maximum at 800 mA under room-temperature continuous-wave operation, which offers promising prospects in pumping and coupling applications. Meanwhile, the index-guiding enhanced RW laser design with a relatively deeply etched (i.e., 2.05 upmum) and narrow ridge (i.e., 4 upmum) provided a highly stable and nearly astigmatism-free fundamental mode emission with an excellent beam quality of M^2 factor around 1.5 over the entire operating current range, which is preferable for seeding external cavity applications and complex optical systems.

Lower threshold current density of GaN-based blue laser diodes by suppressing the nonradiative recombination in a multiple quantum well.

The influence of the nonradiative recombination in a multiple quantum well of GaN-based blue laser diodes (LDs) has been are studied experimentally and theoretically by analyzing the optical and electrical properties of LDs with various thickness and indium content of quantum wells (QWs). It is found that when keeping the LD emission wavelength nearly unchanged, the LD device performance with thinner QW and higher indium content of InGaN QWs is much better than the LD with thicker QW and lower indium content, having smaller threshold current density, higher output optical power and larger slope efficiency. Typically, the threshold current density is as low as 0.69 kA/cm2, and the corresponding threshold current is only 250 mA. The lifetime is more than 10,000 hours at a fixed injection current of 1.2 A under a room-temperature continuous-wave operation. Characteristics of photoluminescence (PL) microscopy images, temperature dependent PL spectra, time-resolved PL and electroluminescence spectra demonstrate that a reduction of the nonradiative recombination centers and an improvement of homogeneity in QWs are the main reason for the performance improvement of GaN-based LD using thinner QW layers with a higher indium content in a certain range. Moreover, theoretical calculation results demonstrate that using a thinner quantum well is also helpful for improving the device performance if the change of alloy material quality is considered during the calculation.

Demonstration of room-temperature continuous-wave operation of InGaAs/AlGaAs quantum well lasers directly grown on on-axis silicon (001)

Room-temperature continuous-wave operation of InGaAs/AlGaAs quantum well lasers directly grown on on-axis silicon (001) has been demonstrated. A 420 nm thick GaAs epilayer completely free of antiphase domains was initially grown on the silicon substrate in a metal-organic chemical vapor deposition system and the other epilayers, including four sets of five-period strained-layer superlattices and the laser-structural layers, were successively grown in a molecular beam epitaxy system. The lasers were prepared as broad-stripe Fabry–Pérot ones with a stripe width of 21.5 μm and a cavity length of 1 mm. Typically, the threshold current and the corresponding threshold current density are 186.4 mA and 867 A/cm2, respectively. The lasing wavelength is around 980 nm, and the slope efficiency is 0.097 W/A with a single-facet output power of 22.5 mW at an injection current of 400 mA. This advancement makes the silicon-based monolithic optoelectronic integration relevant to quantum well lasers more promising with an enhanced feasibility.

Ultra-low threshold lasing through phase front engineering via a metallic circular aperture

Semiconductor lasers with extremely low threshold power require a combination of small volume active region with high-quality-factor cavities. For ridge lasers with highly reflective coatings, an ultra-low threshold demands significantly suppressing the diffraction loss at the facets of the laser. Here, we demonstrate that introducing a subwavelength aperture in the metallic highly reflective coating of a laser can correct the phase front, thereby counter-intuitively enhancing both its modal reflectivity and transmissivity at the same time. Theoretical and experimental results manifest a decreasing in the mirror loss by over 40% and an increasing in the transmissivity by 104. Implementing this method on a small-cavity quantum cascade laser, room-temperature continuous-wave lasing operation at 4.5 μm wavelength with an electrical consumption power of only 143 mW is achieved. Our work suggests possibilities for future portable applications and can be implemented in a broad range of optoelectronic systems.

Compact, Hybrid III-V/Silicon Vernier Laser Diode Operating From 1955–1992 nm

The 2 µm waveband is capable of enabling pervasive applications. The demonstration of the hollow-core photonic bandgap fiber and the thulium-doped fiber amplifier has highlighted the fiber propagation and amplification aspects of fiber communications, indicating its potential as an adjunct to present communication infrastructure at the O/C bands. The above is especially imperative given the current concerns with regards to the upper bandwidth limit of the single-mode fiber. Furthermore, the waveband could facilitate many more applications such as LIDAR and free-space communication. However, water absorption (OH-) is high at most of the 2 μm waveband and this will impact the optical insertion loss of applications implemented in the wavelength region. The relative low water absorption region of the waveband falls within 1950 – 2000 nm. As such, the development of a hybrid/heterogeneous III-V/silicon laser source that operates within the region is important for 2 µm silicon photonics. In this work, we demonstrate a III-V/Si hybrid tunable laser operating from 1955 - 1992 nm for the first time. Room temperature continuous wave operation is achieved with a maximum laser output power of 8.1 mW. This wavelength-tunable laser operates specifically within the low water absorption window, indicating good wavelength suitability for applications at the 2 μm waveband.

GaN-based blue laser diode with 6.0 W of output power under continuous-wave operation at room temperature

In this work, we reported the room-temperature continuous-wave operation of 6.0 W GaN-based blue laser diode (LD), and its stimulated emission wavelength is around 442 nm. The GaN-based high power blue LD is grown on a c-plane GaN substrate by metal organic chemical vapor deposition (MOCVD), and the width and length of the ridge waveguide structure are 30 and 1200 μm, respectively. The threshold current is about 400 mA, and corresponding threshold current density is 1.1 kA/cm2.

Low-Voltage, High-Brightness Silicon Micro-LEDs for CMOS Photonics

Silicon photonics realized in CMOS processes has transformed computing, communications, sensing, and imaging. Although silicon is an indirect bandgap material prohibiting efficient light generation, considerable work has been conducted in the field of silicon p-n junctions emitting broadband visible light when operating in the high-voltage reverse breakdown avalanching mode. Here, we demonstrate high-brightness near-infrared (NIR) light emission for forward-biased silicon micro-light emitting diodes (micro-LEDs) realized in an open-foundry microelectronic CMOS process-55 BCDLite-with zero modification. Under room-temperature continuous-wave operation, the external light emission intensity of over 40 mW/cm $^{{2}}$ at a central wavelength of 1020 nm is achieved for a 4- μm-diameter device at below 2.5 V. This is realized by adopting a deep vertical junction with guard ring designs that ensure carrier transport away from the device surface and material interfaces where nonradiative recombination usually dominates. Here, we also demonstrate a complete chip-to-chip communication link using only standard multimode fiber and monolithically integrated CMOS micro-LEDs and detectors.

Ultralow-Threshold Continuous-Wave Room-Temperature Crystal-Fiber/Nanoperovskite Hybrid Lasers for All-Optical Photonic Integration.

Continuous-wave (CW) room-temperature (RT) laser operation with low energy consumption is an ultimate goal for electrically driven lasers. A monolithically integrated perovskite laser in a chip-level fiber scheme is ideal. However, because of the well-recognized air and thermal instabilities of perovskites, laser action in a perovskite has mostly been limited to either pulsed or cryogenic-temperature operations. Most CW laser operations at RT have had poor durability. Here, crystal fibers that have robust and high-heat-load nature are shown to be the key to enabling the first demonstration of ultralow-threshold CW RT laser action in a compact, monolithic, and inexpensive crystal fiber/nanoperovskite hybrid architecture that is directly pumped with a 405 nm diode laser. Purcell-enhanced light-matter coupling between the atomically smooth fiber microcavity and the perovskite nanocrystallites gain medium enables a high Q (≈1500) and a high β (0.31). This 762 nm laser outperforms previously reported structures with a record-low threshold of 132nW and an optical-to-optical slope conversion efficiency of 2.93%, and it delivers a stable output for CW and RT operation. These results represent a significant advancement toward monolithic all-optical integration.

Aperture diameter dependences in GaN-based vertical-cavity surface-emitting lasers with nano-height cylindrical waveguide formed by BCl3 dry etching

We demonstrated GaN-based vertical-cavity surface emitting lasers (VCSELs) with 5–30 μm wide nano-height cylindrical waveguide formed by BCl3 etching. A 5 nm-depth etching with BCl3 showed the most efficient current blocking at the interface of the etched p++-GaN and an ITO electrode among the cases with BCl3, Ar, or O2, which could be due to not only etching damages but also diffused B atoms into the etched surface. While room-temperature continuous-wave operations of the VCSELs with the large apertures were demonstrated, maximum light output power values of the large aperture VCSELs seemed limited by nonuniform current injection and device thermal resistances.

Room-temperature continuous-wave operations of GaN-based vertical-cavity surface-emitting lasers with buried GaInN tunnel junctions

We report GaN-based vertical-cavity surface-emitting lasers (VCSELs) with buried GaInN tunnel junctions (TJs). Under room-temperature continuous-wave operation, the VCSEL with an 8 μm aperture exhibited a low operating voltage of 5.3 V at 10 kA cm−2 and a differential resistance of 110 Ω. In addition, the VCSEL with a 10 μm aperture showed a threshold current of 14.4 mA and a peak output power of 2.0 mW. An influence of an absorption loss at the GaInN TJ on laser performance was estimated using an analysis of a one-dimensional optical intensity profile based on a cross-sectional STEM image.

Sub-kHz linewidth, hybrid III-V/silicon wavelength-tunable laser diode operating at the application-rich 1647-1690 nm.

The wavelength region about of 1650 nm enables pervasive applications. Some instances include methane spectroscopy, free-space/fiber communications, LIDAR, gas sensing (i.e. C2H2, C2H4, C3H8), surgery and medical diagnostics. In this work, through the hybrid integration between an III-V optical amplifier and an extended, low-loss wavelength tunable silicon Vernier cavity, we report for the first time, a III-V/silicon hybrid wavelength-tunable laser covering the application-rich wavelength region of 1647-1690 nm. Room-temperature continuous wave operation is achieved with an output power of up to 31.1 mW, corresponding to a maximum side-mode suppression ratio of 46.01 dB. The laser is ultra-coherent, with an estimated linewidth of 0.7 kHz, characterized by integrating a 35 km-long recirculating fiber loop into the delayed self-heterodyne interferometer setup. The laser linewidth is amongst the lowest in hybrid/heterogeneous III-V/silicon lasers.

High power continuous wave operation of single mode quantum cascade lasers up to 5 W spanning λ∼3.8-8.3 µm.

In this work, we report high power continuous wave room-temperature operation single mode quantum cascade lasers in the mid-infrared spectral range from 3.8 to 8.3 µm. Single mode robustness and dynamic range are enhanced by optimizing the distributed feedback grating coupling design and the facet coatings. High power single mode operation is secured by circumventing the over-coupling issue and spatial hole burning effect. Maximum single-facet continuous-wave output power of 5.1 W and wall plug efficiency of 16.6% is achieved at room temperature. Single mode operation with a side mode suppression ratio of 30 dB and single-lobed far field with negligible beam steering is observed. The significantly increased power for single mode emission will boost the QCL applications in long-range free-space communication and remote sensing of hazardous chemicals.

Room-temperature continuous-wave operation of green vertical-cavity surface-emitting lasers with a curved mirror fabricated on {20−21} semi-polar GaN

We demonstrate a room-temperature continuous-wave operation of green vertical-cavity surface-emitting laser (VCSEL) with a 20 μm long cavity possessing a dielectric curved mirror formed over a {20−21} semi-polar gallium nitride substrate. The emission wavelength and the threshold current were 515 nm and 1.8 mA, respectively. We also confirmed that white light is generated by overlaying three prime colors of light, i.e. red, blue and green, emitted only from VCSEL.

Compact silicon photonic hybrid ring external cavity (SHREC)/InGaSb-AlGaAsSb wavelength-tunable laser diode operating from 1881-1947 nm.

In recent years, the 2 µm waveband has been gaining significant attention due to its potential in the realization of several key technologies, specifically, future long-haul optical communications near the 1.9 µm wavelength region. In this work, we present a hybrid silicon photonic wavelength-tunable diode laser with an operating range of 1881-1947 nm (66 nm) for the first time, providing good compatibility with the hollow-core photonic bandgap fiber and thulium-doped fiber amplifier. Room-temperature continuous-wave operation was achieved with a favorable on-chip output power of 28 mW. Stable single-mode lasing was observed with side-mode suppression ratio up to 35 dB. Besides the abovementioned potential applications, the demonstrated wavelength region will find critical purpose in H2O spectroscopic sensing, optical logic, signal processing as well as enabling the strong optical Kerr effect on Si.

InAs-Based Interband Cascade Lasers

We briefly review the features of interband cascade lasers (ICLs) and discuss rationales for using an InAs-based plasmon waveguide configuration for extending efficient operation of ICLs to long wavelengths. We then present our recent progress in InAs-based ICLs based on an advanced waveguide configuration. We demonstrated room temperature continuous wave operation of ICLs at wavelengths longer than 6 μm.

Design of AlGaN-based lasers with a buried tunnel junction for sub-300 nm emission

This paper discusses the design of electrically-pumped AlGaN-based in-plane lasers emitting at ∼290 nm. Our laser design utilizes strained Al0.5Ga0.5N quantum wells, and a novel polarization engineered AlGaN/InGaN/AlGaN-based tunnel junction. The low ­resistive tunnel junction is used as an intracavity contact in the device in place of the resistive p-type contact; which leads to improved hole injection and a reduced threshold voltage. Hence, room-temperature continuous-wave laser operation could be enabled. Strategies to improve the performance of the tunnel junction contact through the incorporation of low concentrations of boron in the highly-doped AlGaN tunnel junction layers as a means to increase the polarization sheet charge are also discussed.