InGaN Laser Research Articles

An injection‐locked green InGaN diode laser

AbstractWe designed a single‐frequency green laser system that can deliver a 60‐mW frequency‐tunable single‐longitudinal‐mode laser beam, having a 6.5 MHz linewidth at around 519 nm. An external cavity diode laser can provide a wavelength‐tunable single frequency laser beam, but its output power is limited. Therefore, an injection‐locked laser was designed with a Fabry–Perot type green InGaN laser diode in an external Littrow‐type cavity as the master laser and another laser diode of the same model as the slave laser. Injection locking can effectively enhance the output power and narrow the linewidth of the master laser. We found that the injection locking occurred at longitudinal mode matching between the master laser and slave laser diodes, which is located at a specific frequency for a periodical applied current. The side peaks of the injection locking due to the transverse mode mismatching were recorded and analyzed.

Suppressed optical field and electron leakage and enhanced hole injection in InGaN laser diodes with InGaN–GaN–InGaN barriers

In this study, an InGaN laser diode (LD) with InGaN–GaN–InGaN quantum barriers was proposed and studied systematically. The energy band diagrams, stimulated recombination rate, optical field distribution, current distribution near the active region, and power–current–voltage performance curves were investigated. The simulation results suggest that the LD with InGaN–GaN–InGaN quantum barriers has better performance than the LD with conventional GaN and InGaN quantum barriers because of the properly adjusted refraction index profile and energy band diagrams, which are advantageous to both the suppressed leakage of the optical field and electrons out of and the enhanced injection of holes into the active region.

Composition-graded quantum barriers improve performance in InGaN-based laser diodes

InGaN laser diodes (LDs) with composition-graded (CG) quantum barriers (QBs) are proposed and our study of them described. In the CG QB LDs, the indium content x in the In x Ga1-x N QB was increased gradually along the growth direction. Their laser power–current–voltage performance curves, carrier concentrations, energy band diagrams, current distributions, electrostatic fields, and stimulated recombination rate in the active region were studied. It is shown that the laser power increases from 59.8 mW to 106.9 mW at 120 mA and the slope efficiency increases from 0.76 W A−1 to 1.27 W A−1, when the conventional GaN QBs are replaced by the proposed CG QBs. The simulation results indicate that LDs with CG QBs performed better than LDs with conventional GaN QBs and InGaN QBs because the suppression of electron leakage is more efficient, as is hole injection and carrier-stimulated recombination in multiple quantum wells through the appropriate formation of polarization energy band structures.

Advantages of InGaN-GaN-InGaN Delta Barriers for InGaN-Based Laser Diodes.

An InGaN laser diode with InGaN–GaN–InGaN delta barriers was designed and investigated numerically. The laser power–current–voltage performance curves, carrier concentrations, current distributions, energy band structures, and non-radiative and stimulated recombination rates in the quantum wells were characterized. The simulations indicate that an InGaN laser diode with InGaN–GaN–InGaN delta barriers has a lower turn-on current, a higher laser power, and a higher slope efficiency than those with InGaN or conventional GaN barriers. These improvements originate from modified energy bands of the laser diodes with InGaN–GaN–InGaN delta barriers, which can suppress electron leakage out of, and enhance hole injection into, the active region.

Diode-Pumped Fluorescence in Visible Range From Femtosecond Laser Inscribed Pr:LuAG Waveguides

Trivalent praseodymium (Pr3+) is the most established rare-earth ion for the direct generation of visible light. In our work, based on Pr-doped Lu3Al5O12 (LuAG) single crystal, cladding waveguides are fabricated by applying femtosecond laser inscription with different parameters. The main characteristics of the waveguides such as mode distributions, propagation losses are investigated. The investigations on confocal micro-photoluminescence enable us to illustrate femtosecond laser induced modifications in Pr:LuAG matrix. The waveguides are further pumped at a wavelength of 450 nm with an InGaN laser diode. Guided fluorescence emissions in visible range covering green, yellow-green, orange and red are obtained with a maximum slope efficiency of 4 × 10−4.

A novel yellow laser candidate: Dy3+ doped Ca3NbGa3Si2O14 crystal

In this article, a series of polycrystalline compounds of DyxCa3-xNbGa3Si2O14 (x = 0.03–0.45) were synthesized to obtain the proper Dy3+ ions doping concentration for the crystal growth. After that, a 5.58 at.% Dy:Ca3NbGa3Si2O14 (Dy:CNGS) single crystal was successfully grown by Czochralski method. Its spectral properties and Judd-Ofelt theory analysis were performed. The crystal has a relatively strong and wide absorption peak at about 453 nm with the absorption cross-section of 1.3 × 10−21 cm2 and a full width at half maximum (FWHM) of 10.2 nm. The Judd-Ofelt parameters Ω2, Ω4 and Ω6 were calculated to be 4.51, 2.93 and 0.19 × 10−20 cm2, respectively. The emission cross-section at 572 nm is 1.8 × 10−21 cm2 with a FWHM of 16.3 nm. The fluorescence time of the 4F9/2→6H13/2 transition is 0.24 ms. The results mean that Dy:CNGS crystal is a potential solid-state yellow laser material suitable for the commercial InGaN laser diode pumping.

Narrow linewidth near-UV InGaN laser diode based on external cavity fiber Bragg grating

We realize a fiber Bragg grating InGaN-based laser diode emitting at 400 nm and demonstrate its high coherency. Thanks to the fabrication of a narrowband fiber Bragg grating in the near-UV, we can reach single-mode and single-frequency regimes for the self-injection locked diode. The device exhibits 44 dB side-mode suppression ratio and mW output power. Detailed frequency noise analysis reveals sub-MHz integrated linewidth and 16 kHz intrinsic linewidth. Such a narrow linewidth laser diode in the near-UV domain with a compact and low-cost design could find applications whenever coherency and interferometric resolutions are needed.

Analysis of InGaN-Delta-InN Quantum Wells on InGaN Substrates for Red Light Emitting Diodes and Lasers

Modern multi-color RGB micro-light emitting diode (μLED) displays and digital micro-mirror laser projectors often require the use of both III-V and III-Nitride material systems for different pixel/laser colors. This is due primarily to the conventionally low efficiencies of red emitters based on the InGaN materials system, which is used to create green and blue emitters for RGB displays. The main challenges for InGaN red emitters are the quantum confined stark effect (QCSE) and the difficulty of incorporating high In-content into the active region. In this work, InGaN/InGaN/delta-InN quantum wells (QWs) on InGaN substrates are proposed and demonstrated to show significant enhancement in electron-hole wavefunction overlap ( Γe_hh ) and spontaneous emission radiative recombination rate (Rsp) in the red emission regime. Analysis of InGaN/InGaN/delta-InN QWs with InGaN barriers emitting at 630 nm was performed using a self-consistent six-band $k \cdot p$ formalism. The Γe_hh was shown to increase by more than 230% compared to an InGaN/InGaN QW emitting at 630 nm, leading to significant increases in Rsp and internal quantum efficiency ( ηIQE ). With growth of InN monolayers on InGaN now readily achievable, this novel active region design could pave the way for high-efficiency, native red-emitting InGaN LEDs and lasers.

Continuous-wave efficient cyan-blue Pr:YAlO$$_3$$ laser pumped by InGaN laser diode

We report on the first continuous-wave quasi three-level laser emitting in the cyan-blue spectral range in praseodymium-doped oxide materials, as we believe. Pr:YAlO $$_3$$ crystal, cooled to the cryogenic temperatures, was used as an active medium. To minimize resonator losses, the cavity mirrors were directly deposited on the crystal faces to form a microchip geometry. More than half-Watt of the output power at 493 nm wavelength with a slope efficiency of 26% has been obtained under 3 W InGaN laser diode pumping at 447 nm wavelength.

Surface Photochemical Corrosion as a Mechanism for Fast Degradation of InGaN UV Laser Diodes.

We studied degradation mechanisms of ultraviolet InGaN laser diodes emitting in the UVA range. Short wavelength nitride devices are subjected to much faster degradation, under the same packaging and testing conditions, than their longer wavelength counterparts. Transmission electron microscopy analysis of the degraded laser diodes showed pronounced damage to facets in the area of the active layer (waveguide, quantum wells, and electron blocking layer). Energy-dispersive X-ray spectroscopy showed that the active layers were heavily oxidized, forming a compound close in composition to Ga2O3 with proportional addition of Al in the respective area. The oxidation depth was roughly proportional to the intensity of the optical field. We propose UV-light-induced water splitting on a semiconductor surface as a mechanism of the oxidation and degradation of these devices.

High performance of a semipolar InGaN laser with a phase-shifted embedded hydrogen silsesquioxane (HSQ) grating.

Single-frequency blue laser sources are of interest for an increasing number of emerging applications but are still difficult to implement and expensive to fabricate and suffer from poor robustness. Here a novel and universal grating design to realize distributed optical feedback in visible semiconductor laser diodes (LDs) was demonstrated on a semipolar InGaN LD, and its unique effect on the laser performance was investigated. For the first time, to the best of our knowledge, a low threshold voltage, record-high power output, and ultra-narrow single-mode lasing were simultaneously obtained on the new laser structure with a thinner p-GaN layer and a third-order phase-shifted embedded dielectric grating. Under continuous-wave operation, such 450 nm lasers achieved 35 dB side-mode suppression ratio, less than 2 pm FWHM, and near 400 mW total output power at room temperature.

GaN/InAlN chirped short period superlattice as strain free top cladding for blue laser diode

This work presents the theoretical study on strain-free chirped short-period superlattices (C-SPSLs) as the top waveguide core and cladding layers for InGaN laser diodes (LDs) emitting at 450 nm. The total 530 nm thick layers of reference LD containing 100 nm p-GaN waveguide, 30 nm Al0.15Ga0.85N electron blocking layer (EBL) and Al0.065Ga0.935N cladding layers are replaced by 300 nm thick C-SPSL which contains the 33-periods of 10 monolayers (ML) GaN/ 2 ML In0.18Al0.82N, 48-periods of 6 ML GaN/2 ML In0.18Al0.82N, and 97-periods of 2 ML GaN/ 2 ML In0.18Al0.82N. The optical confinement factor of reference LD is 3.28% whereas for C-SPSL LD it is 3.42%. The better optical confinement at reduced thickness using C-SPSL is due to the high refractive index contrast. The strain in the C-SPSL is negligible. The electron leakage has reduced from 2.54 kA cm−2 to 0.166 kA cm−2 at ∼10 kA cm−2 injected current density, whereas the hole transportation has improved by 2.31 kA cm−2. The C-SPSL configuration also favors the electron blocking effect, thus EBL is not required in C-SPSL design. The light output power at a current injection of 500 mA (10 kA cm−2) is 248 mW for the new structure and ∼143 mW for reference LD. The slope efficiency is improved from ∼0.55 to ∼0.89 W A−1. Also, the dynamic resistance from I–V characteristic is ∼1.12 Ω which is lowered to ∼0.94 Ω.

Research Toward a Heterogeneously Integrated InGaN Laser on Silicon

A heterogeneously integrated InGaN laser diode (LD) on Si is proposed as a path toward visible wavelength photonic integrated circuits (PICs) on Si. Herein, InGaN films are vertically stacked on a TiO2 waveguide (WG) fabricated on a Si wafer by bonding. In the light propagation direction, it is composed of a hybrid InGaN/TiO2 section, a TiO2 WG, an adiabatic taper, and mirrors that can form a cavity. As the refractive index of GaN is well matched with that of TiO2, the optical transverse mode extends to both the GaN and TiO2 in a hybrid mode. Modes between a hybrid InGaN/TiO2 and a pure TiO2 WG can transfer with an adiabatic taper structure. The coupling loss is calculated to be less than 0.5 dB with fairly short taper length of 78 μm and tip width of 200 nm. GaN substrate removal and bonding are critical fabrication steps of this LD and PIC. The substrate removal is successfully done by photoelectrochemical etching. Although direct bonding of GaN wafers with thermal oxide on Si is successful, GaN epitaxial wafers are more difficult. An implication and remedy of this is discussed in terms of surface roughness of GaN epitaxial film.

Improving Output Power of InGaN Laser Diode Using Asymmetric In0.15Ga0.85N/In0.02Ga0.98N Multiple Quantum Wells.

Herein, the optical field distribution and electrical property improvements of the InGaN laser diode with an emission wavelength around 416 nm are theoretically investigated by adjusting the relative thickness of the first or last barrier layer in the three In0.15Ga0.85N/In0.02Ga0.98N quantum wells, which is achieved with the simulation program Crosslight. It was found that the thickness of the first or last InGaN barrier has strong effects on the threshold currents and output powers of the laser diodes. The optimal thickness of the first quantum barrier layer (FQB) and last quantum barrier layer (LQB) were found to be 225 nm and 300 nm, respectively. The thickness of LQB layer predominantly affects the output power compared to that of the FQB layer, and the highest output power achieved 3.87 times that of the reference structure (symmetric quantum well), which is attributed to reduced optical absorption loss as well as the reduced vertical electron leakage current leaking from the quantum wells to the p-type region. Our result proves that an appropriate LQB layer thickness is advantageous for achieving low threshold current and high output power lasers.

Phosphor Performance under High Intensity Excitation by InGaN Laser Diodes

Excitation of phosphors by blue (InGaN) laser diodes is a new area in solid-state lighting applications that is getting notable attention. Such laser-activated remote phosphor (LARP) configurations generate luminances exceeding the brightest high-power LEDs by factors of 2–10, while simultaneously achieving high luminous fluxes in the range of 100s–1000s lumens. To fully take advantage of the LARP approach, a strong understanding of high-intensity phosphor excitation is needed. We develop a general rate-equation model to describe intensity quenching, applicable to common Ce3+- and Eu2+-based phosphors. The model includes the potential impact of traps and recombination from the conduction band. We also simulate the nonlinear propagation of pump and converted light to allow direct comparisons to experimental data. For YAG:Ce and LuAG:Ce model systems, we show that energy-transfer (ET) upconversion to the conduction band likely constitutes the main loss mechanism in high intensity laser pumping. We also show conclusively, via steady-state, low-temperature intensity quenching measurements, that high-intensity excitation alone can be responsible for large drops in quantum efficiency, without the complicating experimental factors of thermal quenching or short-pulse excitation to minimize heating.

Realization of thin-film m-plane InGaN laser diode fabricated by epitaxial lateral overgrowth and mechanical separation from a reusable growth substrate.

A nonpolar edge emitting thin film InGaN laser diode has been separated from its native substrate by mechanical tearing with adhesive tape, combining the benefits of Epitaxial Lateral Overgrowth (ELO) and cleavability of nonpolar GaN crystal. The essence of ELO is mainly to weakening strength between native substrate and the fabricated laser device on top of it. We report a 3 mm long laser bar removed from its native GaN substrate. We confirmed edge emitting lasing operation after cleaving facets on a separated thin bar. Threshold current density of the laser was measured to be as low as 2.15 kA/cm2.

Electrical injection of a 440nm InGaN laser with lateral confinement by nanoporous-GaN.

We report observations of lateral mode confinement by a tapering nanoporous-GaN layer in the n-side cladding of a blue-emitting InGaN laser diode grown on the semipolar (202¯1¯) plane of bulk GaN. Little additional confinement occurred in the transverse direction, and the nanoporous layer did not serve as a current aperture. Nanoporous-GaN, with Si-doping of 8x1018 cm-3 and 20% porosity had a bulk resistivity of 3 Ω-cm and a thermal conductivity of 4 W/m-K, in general agreement with data reported on c-plane VCSEL structures. An excess modal loss of 19 cm-1 was found.

Alexandrite microchip lasers.

We report on a continuous-wave alexandrite (Cr3+:BeAl2O4) microchip lasers operating at 680.4 nm and 749.5 nm laser wavelengths. Microchip resonators were realized by dielectric mirrors directly deposited on the alexandrite crystal surfaces. InGaN laser diode providing up to 3.5 W of the output power at ∼445 nm wavelength was used as a pump source. More than 210 mW and 570 mW of the laser radiation have been extracted from the microchip laser systems at 680.4 nm and 749.5 nm wavelengths, respectively. The corresponding slope efficiencies related to absorbed pump power were 15 % and 39 %.

Longitudinal mode seeding in modulated InGaN laser diodes

The modulation of InGaN laser diodes is important for applications such as laser projection in cinemas as well as in virtual and augmented reality applications. Here, a modulation frequency in the 100 MHz to 1 GHz range is necessary. On this timescale, pixel crosstalk is a major issue that affects the intensity and spectral properties. We investigate the spectral and temporal dynamics of InGaN laser diodes driven with different pulse patterns. We identify longitudinal mode seeding as a mechanism between interacting pulses which are spaced by several nanoseconds: photons, remaining in the cavity after the trailing edge of the electric pulse, “seed” consequent pulses and promoting their own longitudinal mode. This leads to a changed spectral–temporal mode pattern of the consequent pulse. The long cavity ring-down time of these photons is a consequence of relatively low losses. Further investigations were performed by streak camera measurements of differently biased laser diodes. For a bias below, but close to, the threshold, the optical gain nearly compensates internal and mirror losses. Additional simulations are performed to confirm the experimental results.

Cross Sections, Transition Intensities, and Laser Generation at the 3P1 → 3H5 Transition of LiY0.3Lu0.7F4:Pr3+ Crystal

Lasing and spectroscopic properties of an LiY0.3Lu0.7F4:Pr3+ crystal grown by the Bridgman–Stockbarger method were investigated in detail. Absorption and luminescence spectra were recorded using polarized light. Lifetimes of excited states 3PJ were determined. Absorption and emission transition intensities were calculated according to f–f-transition intensity theory taking into account interconfiguration interaction. Stimulated emission transverse cross sections were determined using the Fuchtbauer–Ladenburg method. Pumping by an InGaN laser diode produced continuous-wave laser generation at 522.6 nm (3P1 → 3H5) in Pr-doped mixed fluoride crystals with a maximum output power of 0.38 W and a slope efficiency of 19%.