Room-temperature Continuous-wave Lasing Research Articles

Comparison of quantum well structures for room temperature continuous wave 980 nm lasers grown on (001) Si by MOCVD

We report room temperature (RT) continuous-wave (CW) lasing of quantum well (QW) lasers grown on (001) Si substrates emitting at 980 nm. Two different QW structures, including conventional compressively strained InGaAs/GaAs QWs and strain-compensated InGaAs/GaAs/GaAsP QWs, were investigated. Photoluminescence properties and device performance of both structures on native GaAs and (001) Si substrates are discussed. By adding GaAsP barriers to the InGaAs/GaAs QWs, the lowest threshold current density of ridge waveguide edge-emitting QW lasers obtained on Si is 550 A/cm2, measured on a 10 μm × 2 mm device at RT. The working temperature of the InGaAs/GaAs/GaAsP QW lasers grown on Si can be over 95 °C in the CW mode. This work suggests a feasible approach to improve the 980 nm laser performance on Si for monolithic optoelectronic integration.

Improved performance of InGaAs/AlGaAs quantum well lasers on silicon using InAlAs trapping layers.

InGaAs/AlGaAs multiple quantum well lasers grown on silicon (001) by molecular beam epitaxy have been demonstrated. By inserting InAlAs trapping layers into AlGaAs cladding layers, misfit dislocations easily located in the active region can be effectively transferred out of the active region. For comparison, the same laser structure without the InAlAs trapping layers was also grown. All these as-grown materials were fabricated into Fabry-Perot lasers with the same cavity size of 20 × 1000 µm2. The laser with trapping layers achieved a 2.7-fold reduction in threshold current density under pulsed operation (5 µs-pulsed width, 1%-duty cycle) compared to the counterpart, and further realized a room-temperature continuous-wave lasing with a threshold current of 537 mA which corresponds to a threshold current density of 2.7 kA/cm2. When the injection current reached 1000 mA, the single-facet maximum output power and slope efficiency were 45.3 mW and 0.143 W/A, respectively. This work demonstrates significantly improved performances of InGaAs/AlGaAs quantum well lasers monolithically grown on silicon, providing a feasible solution to optimize the InGaAs quantum well structure.

Key temperature-dependent characteristics of AlGaN-based UV-C laser diode and demonstration of room-temperature continuous-wave lasing

Although the pulsed operation of AlGaN-based laser diodes at UV-C wavelengths has been confirmed in the previous studies, continuous oscillation without cooling is difficult because of the high operating voltage. In this study, the temperature dependence of key parameters was investigated and their impact on achieving continuous-wave lasing was discussed. A reduction in the threshold voltage was achieved by tapering the sides of the laser diode mesa and reducing the lateral distance between the n- and p-electrodes. As a result, continuous-wave lasing at room temperature was demonstrated at a threshold current density of 4.2 kA/cm2 and a threshold voltage of 8.7 V.

GaAs on (001) Si templates for near-infrared InP quantum dot lasers

We investigated the effects of thermal cycle annealing (TCA) at high temperatures on the defect density and morphology of GaAs epilayers grown on (001) Si substrates. Several types of TCA combined with dislocation filter layers (DFLs) were introduced to reduce the defect densities in the GaAs grown on Si substrates. Plan-view transmission electron microscopy shows that the defect density of a 2.7 μm-thick GaAs/Si template with optimized TCA and DFLs is 1.4 × 107 cm−2, lower than a 1.1 μm-thick GaAs/Si template by a factor of 40. The surface roughness of the optimized GaAs/Si template is 1.3 nm after insertion of the DFLs. Additionally, optically pumped InP quantum dot micro-disk lasers (MDLs) were fabricated on these GaAs/Si templates to evaluate the template quality. Room-temperature continuous-wave lasing of 1.5 μm-diameter MDLs was observed, with ultralow lasing thresholds ranging from 0.5 to 2 μW.

Continuous-Wave Current Injected InGaN/GaN Microdisk Laser on Si(100)

GaN-based microdisk laser on Si can be adopted as an efficient on-chip laser source for Si photonics. However, most of the reported microdisk lasers are integrated on Si(111), which is not fully compatible with cost-effective mainstream complementary metal-oxide-semiconductor (CMOS) foundries. In this study, GaN-based microdisk laser monolithically integrated on Si(100) is carefully designed and fabricated thorough wafer bonding and substrate removal. Moreover, it shows 66.5% lower junction temperature due to reduced electric injection power and thermal resistance as compared with the reported conventional GaN-based microdisk lasers grown on Si(111). The result is the room-temperature continuous-wave current-injected lasing of GaN-based microdisk laser on Si(100), showing a narrow spectral line width of about 0.16 nm and an obvious turning point in the optical output power versus injection current curve.

Broadly tunable (993-1110 nm) Yb:YLF laser.

We have investigated room-temperature continuous-wave (cw) lasing performance of Yb:YLF oscillators in detail using rod-type crystals with low Yb-doping (2%). The laser is pumped by a low-cost, high brightness, 10 W, 960 nm single-emitter multimode diode. Laser performance is acquired in both E//a and E//c configurations, using 12 different output couplers with transmission ranging from 0.015% to 70%. We have estimated the passive loss of the Yb:YLF crystal as 0.06% per cm, corresponding to an impressive crystal figure of merit above 4000. The low-doping level not only reduces the system losses but also minimizes the thermal load as the low doped crystals enable distribution of heat load in a greater volume. Using the advantages of lower loss and improved thermal behavior, we have achieved cw output power above 4 W, cw slope efficiencies up to 78%, and a record cw tuning range covering the 993-1110 nm region (117 nm). The output power performance achieved in this initial work is limited by the available pump power, and future room-temperature Yb:YLF systems have the potential to produce higher output power levels.

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.

C and L band room-temperature continuous-wave InP-based microdisk lasers grown on silicon.

Quantum-dot (QD) and quantum-dash (QDash) have been shown to be promising gain materials for lasers directly grown on Si due to their better tolerance to crystal defects and thermal stability. Here we report optically pumped InP-based InAs QDash microdisk lasers (MDLs) directly grown on on-axis (001) Si. To the best of our knowledge, this is the first demonstration of room-temperature continuous-wave lasing of a QDash MDL on Si in the C band and L band. To the best of our knowledge, the lowest threshold of around 400 µW and highest operation temperature of 323 K have been achieved. An analysis of experimental results shows that the dominant lasing wavelength of MDLs varies with the thickness and diameter of the MDLs. Our demonstration shows potential application of MDLs for multi-channel operation in densely integrated Si-photonics.

Stable room-temperature continuous-wave lasing in quasi-2D perovskite films.

Organic-inorganic lead halide quasi-two-dimensional (2D) perovskites are promising gain media for lasing applications because of their low cost, tunable colour, excellent stability and solution processability1-3. Optically pumped continuous-wave (CW) lasing is highly desired for practical applications in high-density integrated optoelectronics devices and constitutes a key step towards electrically pumped lasers4-6. However, CW lasing has not yet been realized at room temperature because of the 'lasing death' phenomenon (the abrupt termination of lasing under CW optical pumping), the cause of which remains unknown. Here we study lead halide-based quasi-2D perovskite films with different organic cations and observe that long-lived triplet excitons considerably impede population inversion during amplified spontaneous emission and optically pumped pulsed and CW lasing. Our results indicate that singlet-triplet exciton annihilation is a possible intrinsic mechanism causing lasing death. By using a distributed-feedback cavity with a high quality factor and applying triplet management strategies, we achieve stable green quasi-2D perovskite lasers under CW optical pumping in air at room temperature. We expect that our findings will pave the way to the realization of future current-injection perovskite lasers.

III/V-on-Si MQW lasers by using a novel photonic integration method of regrowth on a bonding template

Silicon photonics is becoming a mainstream data-transmission solution for next-generation data centers, high-performance computers, and many emerging applications. The inefficiency of light emission in silicon still requires the integration of a III/V laser chip or optical gain materials onto a silicon substrate. A number of integration approaches, including flip-chip bonding, molecule or polymer wafer bonding, and monolithic III/V epitaxy, have been extensively explored in the past decade. Here, we demonstrate a novel photonic integration method of epitaxial regrowth of III/V on a III/V-on-SOI bonding template to realize heterogeneous lasers on silicon. This method decouples the correlated root causes, i.e., lattice, thermal, and domain mismatches, which are all responsible for a large number of detrimental dislocations in the heteroepitaxy process. The grown multi-quantum well vertical p–i–n diode laser structure shows a significantly low dislocation density of 9.5 × 104 cm−2, two orders of magnitude lower than the state-of-the-art conventional monolithic growth on Si. This low dislocation density would eliminate defect-induced laser lifetime concerns for practical applications. The fabricated lasers show room-temperature pulsed and continuous-wave lasing at 1.31 μm, with a minimal threshold current density of 813 A/cm2. This generic concept can be applied to other material systems to provide higher integration density, more functionalities and lower total cost for photonics as well as microelectronics, MEMS, and many other applications.

Implementing a Raman silicon nanocavity laser for integrated optical circuits by using a (100) SOI wafer with a 45-degree-rotated top silicon layer

Raman silicon (Si) lasers based on high-quality-factor photonic crystal nanocavities are very compact and can operate at excitation powers of less than one microwatt. For the best performance, the nanocavity of the Raman Si laser has to be fabricated along the [100] crystal direction of the Si-on-insulator (SOI) wafer to enhance the Raman gain. On the other hand, Si photonic devices are usually fabricated along the [110] direction because crystalline Si can be easily cleaved along [110]. This rotation by 45 degrees of the nanocavity with respect to the cleavable direction can be problematic for various applications. Here we report a Raman Si nanocavity laser fabricated on a modified (100) SOI wafer in which the crystal orientation of the top Si layer is rotated in plane by 45 degrees relative to the crystal direction of the support substrate. We observe room temperature continuous wave laser oscillation with a sub-microwatt threshold and a maximum energy efficiency of 5.6%. It is found that the photonic-slab warpage induced by compressive stress is reduced in this 45°-rotated SOI wafer.

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.

Laterally confined photonic crystal surface emitting laser incorporating monolayer tungsten disulfide

With an ultimately thin active region, monolayer transition metal dichalcogenide lasers have the potential of realizing ultralow lasing threshold and power consumption. The flexibility also enables integration possibilities on unconventional substrates. Here, we report a photonic crystal surface emitting laser using monolayer tungsten disulfide as the gain medium. The cavity design utilizes a heterostructure in the photonic crystal lattice to provide lateral confinement for a high quality factor with a compact active region. Room-temperature continuous wave lasing is realized after integrating monolayer tungsten disulfide flakes onto the silicon nitride photonic crystal on a quartz substrate. Highly directional, near surface-normal emission has also been experimentally demonstrated. The work reported here demonstrates that a large-area single-mode directional laser can be realized from a monolayer gain medium, which is critical for laser scaling for on-chip integration in data and sensing applications.

Continuous wave amplified spontaneous emission in phase-stable lead halide perovskites

Sustained stimulated emission under continuous-wave (CW) excitation is a prerequisite for new semiconductor materials being developed for laser gain media. Although hybrid organic-inorganic lead-halide perovskites have attracted much attention as optical gain media, the demonstration of room-temperature CW lasing has still not been realized. Here, we present a critical step towards this goal by demonstrating CW amplified spontaneous emission (ASE) in a phase-stable perovskite at temperatures up to 120 K. The phase-stable perovskite maintains its room-temperature phase while undergoing cryogenic cooling and can potentially support CW lasing also at higher temperatures. We find the threshold level for CW ASE to be 387 W cm-2 at 80 K. These results indicate that easily-fabricated single-phase perovskite thin films can sustain CW stimulated emission, potential at higher temperatures as well, by further optimization of the material quality in order to extend the carrier lifetimes.

Low-threshold room-temperature continuous-wave optical lasing of single-crystalline perovskite in a distributed reflector microcavity.

Organic–inorganic halide perovskites have achieved remarkable success in various optoelectronic devices. A high-quality CH3NH3PbBr3 single-crystalline thin film has been directly grown in a micrometer gap between a pair of distributed reflectors with over 99.9% reflectivity, which naturally form a vertical cavity surface-emitting laser device with a single mode or several modes. The single-crystalline perovskite has an exciton lifetime of 426 ns and evidence of the exciton–photon coupling is observed. At room temperature and under continuous-wave optical pumping conditions, this device lases at a threshold of 34 mW cm−2 in the green gap. The extremely low lasing threshold suggests that polariton lasing may occur in the strongly confined optical cavity comprising the high-quality single-crystalline perovskite.

Zinc oxide clad limited area epitaxy semipolar III-nitride laser diodes.

We report continuous-wave (CW) blue semipolar (202¯1) III-nitride laser diodes (LDs) that incorporate limited area epitaxy (LAE) n-AlGaN bottom cladding with thin p-GaN and ZnO top cladding layers. LAE mitigates LD design limitations that arise from stress relaxation, while ZnO layers reduce epitaxial growth time and temperature. Numerical modeling indicates that ZnO reduces the internal loss and increases the differential efficiency of TCO clad LDs. Room temperature CW lasing was achieved at 445 nm for a ridge waveguide LD with a threshold current density of 10.4 kA/cm2, a threshold voltage of 5.8 V, and a differential resistance of 1.1 Ω.

Room-temperature continuous-wave lasing from monolayer molybdenum ditelluride integrated with a silicon nanobeam cavity.

Monolayer transition-metal dichalcogenides (TMDs) have the potential to become efficient optical-gain materials for low-energy-consumption nanolasers with the smallest gain media because of strong excitonic emission. However, until now TMD-based lasing has been realized only at low temperatures. Here we demonstrate for the first time a room-temperature laser operation in the infrared region from a monolayer of molybdenum ditelluride on a silicon photonic-crystal cavity. The observation is enabled by the unique combination of a TMD monolayer with an emission wavelength transparent to silicon, and a high-Q cavity of the silicon nanobeam. The laser is pumped by a continuous-wave excitation, with a threshold density of 6.6 W cm-2. Its linewidth is as narrow as 0.202 nm with a corresponding Q of 5,603, the largest value reported for a TMD laser. This demonstration establishes TMDs as practical materials for integrated TMD-silicon nanolasers suitable for silicon-based nanophotonic applications in silicon-transparent wavelengths.

Yellow laser performance of Dy³⁺ in co-doped Dy,Tb:LiLuF₄.

We present laser results obtained from a Dy³⁺-Tb³⁺ co-doped LiLuF₄ crystal, pumped by a blue emitting InGaN laser diode, aiming for generation of a compact 578 nm source. We exploit the yellow Dy³⁺ transition ⁴F(9/2)⇒⁶H(13/2) to generate yellow laser emission. The lifetime of the lower laser level is quenched, via energy transfer, to co-doped Tb³⁺ ions in the fluoride crystal. We report the growth technique, spectroscopic study, and room temperature continuous wave laser results in a hemispherical cavity at 574 nm, and with a highly reflective output coupler at 578 nm. A yellow laser at 578 nm is very relevant for metrological applications, in particular for pumping of the forbidden ¹S₀-³P₀ ytterbium clock transition, which is recommended as a secondary representation of the second in the international system of units.

Room temperature continuous-wave laser performance of LD pumped Er:Lu₂O₃ and Er:Y₂O₃ ceramic at 2.7 μm.

We report the demonstration of continue wave operation of diode end-pumped Er:Y2O3 and Er:Lu2O3 ceramic lasers operating at 2.7 μm at room temperature. The maximum output power of 320 mW and 611 mW was obtained from the Er:Y2O3 and Er:Lu2O3 ceramic lasers, with slope efficiency of 6.5% and 7.6%, respectively. Characteristics of Red-shift in lasing wavelength of the ceramic lasers was investigated and discussed. The study indicates that under 967 nm and 976 nm LD pumping, 15 at.% Er-doped Lu2O3 ceramic exhibit a better performance than that of Y2O3 at room temperature.

Room temperature continuous wave lasing of electrically injected GaN-based vertical cavity surface emitting lasers

Continuous wave (CW) lasing of electrically injected GaN-based vertical cavity surface emitting lasers (VCSELs) was achieved at room temperature. First, a high quality factor (Q) VCSEL-structured device with very narrow linewidth of 0.12 nm, corresponding to a Q-value of 3570 was obtained through two-step substrate transfer technique. However, poor heat dissipation ability prevented the device from lasing. Based on the high-Q resonant cavity design, we further fabricated vertical-structured VCSELs through metal bonding technique on Si substrate. CW lasing from vertical-structured VCSELs was observed with threshold current of density of 1.2 kA/cm2 and lasing linewidth of about 0.20 nm.