GaN Laser Research Articles

GaN Diode Laser Pumped Pr<SUP>3+</SUP>-Doped Solid-State Lasers

Utilizing high-power GaN laser diodes emitting light at 444nm, which have been developed for a blue light source in laser rear projection displays, high-power coherent red light at 639nm, which is also useful for laser rear projection displays, becomes available from Pr3+ doped LiYF4 lasers. In this paper, development of Pr3+: LiYF4 lasers are reviewed. In our experiment, the highest laser power of 112mW was achieved with the optical-optical conversion efficiency of 33.5%. Characteristics of this laser at elevated temperatures were also investigated for practical applications such as a laser projector.

Improvement in threshold of InGaN∕GaN quantum-well lasers by p-type modulation doping

The optical properties of modulation-doped InGaN∕GaN laser diodes are theoretically studied with the effects of electron spillover from quantum wells considered. We use a six-band model including the strain effect for calculating valence band states. The continuous subbands are treated by a dense discretization for the electrons spilling from the quantum wells. The calculation results show that the threshold current can be significantly reduced by p-type modulation doping around the wells but not by n-type doping, supposed that the layers are of a perfect quality and the impurity-induced defects are ignored. Also, the p-type modulation doping can make the threshold current more insensitive to the cavity loss compared with other cases. An optimized threshold current density can be achieved for single-quantum-well lasers by introducing p-type dopants. However, the dopant concentration is high and may be inaccessible. For double-quantum-well lasers an optimized low threshold current can be achieved with a slighter and practicable p-type doping level.

Electron spillover effects in InGaN∕GaN quantum-well lasers

The effects of electron spillover from quantum wells on the optical property of InGaN∕GaN laser diodes are theoretically studied in detail. A six-band model including strain effects is used to calculate valence band states. Continuous subbands unconfined to the quantum wells are simulated deliberately by densely discretized subbands for the spillover electrons. The calculation results show obvious differences in the radiative current densities and the gain spectra between the cases with and without considering the spillover effect. We further investigate the spillover effect on the radiative current densities and the spontaneous emission spectra, with variations in the depth and the width of quantum wells, the total loss of the cavity, and the temperature. For shallow wells, the spillover effect is particularly important. It broadens both the gain and the spontaneous emission spectra and hence deteriorates the threshold of laser diodes. Such an effect can be alleviated by employing a long cavity and a multi-quantum-well active region. The concept of the electron spillover studied in this work is not only applicable to the nitride lasers but also to other kinds of quantum-well lasers.

Interband transition matrix element and temperature dependence of the lasing threshold for GaN laser structures

By comparing experimental and theoretical radiative recombination spectra, we have determined the values of the square of the matrix element for interband optical transitions |M|2 in epitaxial GaN at different temperatures T. The data obtained were used to analyze the mechanism for the temperature dependence of the lasing threshold for a GaN laser. In the experiments, we used epitaxial layers of GaN with a wurtzite crystal structure, grown on Al2O3 (0001) substrates and excited by a focused beam from a nitrogen laser. We have shown that the spontaneous emission spectra near the lasing threshold of a GaN laser are consistent with the model of optical interband transitions not obeying a selection rule for the electron wave vector. As we have established, the parameter |M|2 practically does not vary for T = 300–470 K: 5.4·10−73 kg2·m5/sec2. Further increase in the temperature leads to an exponential fall-off in |M|2 down to 3.4·10−73 kg2·m5/sec2 at T ≈ 520 K. Such behavior of |M|2 as a function of temperature correlates with the sudden increase in the nonradiative recombination rate at T > 470 K, and may be connected with a change in the recombination mechanisms in the active layer of the GaN laser in the high temperature region.

Thermal analysis of InGaN/GaN (GaN substrate) laser diodes using transient interferometric mapping

Transient thermal behaviour and current distribution is investigated in InGaN/GaN blue lasers grown on GaN substrate. Back side transient interferometric mapping (TIM) method was applied for probing the temperature-induced phase shift due to increase in refractive index in the GaN laser ridge. The laser operates in a pulsed mode with pulse duration in the time scale from 300 ns to 2 μs. By using 2D thermal modeling, the comparison of the experimental and simulated phase shift allows to estimate a maximal temperature in the active region. The thermal mapping along the ridge reveals the inhomogeneity in the current flow attributed to substrate dopant distribution.

High-power GaN diode-pumped continuous wave Pr^3+-doped LiYF_4 laser

A cw Pr(3+):LiYF(4) laser at 639 nm pumped by a high-power GaN laser diode (444 nm) is demonstrated. The highest laser power of 112 mW is achieved with an optical-optical conversion efficiency of 33.5%. Characteristics of this laser at elevated temperatures are also investigated for practical applications such as a laser projector.

Light–Output Enhancement of Nano-Roughened GaN Laser Lift-Off Light-Emitting Diodes Formed by ICP Dry Etching

In this paper, we report the fabrication and characteristics of nano-roughened GaN laser lift-off (LLO) light-emitting diodes (LEDs) with different scale surface roughness. The surface roughness of devices was controlled by inductively coupled plasma reactive ion etching. Using this fabrication method to form nano-scaled roughness, the electrical property was almost not degraded. Furthermore, the light-output power and wall-plug efficiency of LLO LED could be both significantly enhanced about two times using this simple method

Theoretical analysis of effect of temperature on threshold parameters and field intensity in GaN material based heterostructure

Temperature dependent analysis to achieve better performance by reducing threshold current requirements and field intensity has been carried out for GaN/AlGaN heterostructure lasers. The mirror loss in the GaN cavity has been obtained as a function of temperature and cavity length. The quantum efficiency has been deduced for different values of cavity length. Dependence of recombination rate on band gap and temperature has been investigated. Threshold current density has been deduced for GaN lasers and effect of temperature on it has been investigated. The near field intensity analysis has been carried out at different temperatures for 10% aluminum mole fraction in GaN/AlGaN heterostructure lasers. Furthermore, the effective index and FWHM of near field has been investigated as a function of temperature. It has been deduced from our analysis that temperature has a dominant effect on the threshold conditions and near field intensity in the wide band gap GaN based lasers.

Experimental and theoretical study of substrate modes in (Al,In)GaN laser diodes

AbstractIn semiconductor laser diodes, high refractive index epitaxial layers can act as parasitic waveguides and cause severe losses to the mode propagating in the laser waveguide. For (Al,In)GaN laser diodes the parasitic modes are typically caused by the SiC or GaN substrate or buffer layers. Substrate modes have an impact on near‐field and far‐field of the waveguide mode, on laser threshold, and on gain spectra. We present scanning near‐field measurements of the substrate mode and demonstrate that the period of the standing wave in the GaN substrate varies over a wide range and critically depends on the effective refractive index of the waveguide mode.

Diode-pumped Pr^3+:KY_3F_10 red laser

We report the first observation to our knowledge of room-temperature continuous-wave laser operation of a Pr(3+):KY(3)F(10) single crystal at 644.5 nm, pumped by a blue GaN laser diode emitting at 446 nm. With a 2.5% transmission output coupler and a nonoptimized optical cavity, an output power of 39 mW was obtained at 644.5 nm with a laser threshold of 125 mW and a differential slope efficiency of 23%.

Laser Diode-Pumped Organic Semiconductor Lasers Utilizing Two-Dimensional Photonic Crystal Resonators

Two-dimensional photonic crystal lasers based on the small molecule organic semiconductor tris-(8-hydroxyquinoline) aluminum (Alq <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) doped with 4-Dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) are optically pumped with a conventional low-cost pulsed (In)GaN laser diode. We compare photonic crystal resonators providing first- and second-order distributed feedback and find threshold values of 1.9 and 3.2kW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. Such inorganic-organic hybrid laser systems open up a way to inexpensive, tunable, and all solid-state lasers in the full visible wavelength range

Study of the Potential Use of a 405nm GaN Laser Diode as a Surgical Tool

Study of the Potential Use of a 405nm GaN Laser Diode as a Surgical Tool

Optical gain spectra for near UV to aquamarine (Al,In)GaN laser diodes

Optical gain spectra presented for (Al,In)GaN laser diodes with lasing wavelength ranging from UV (375 nm) to aquamarine (470 nm) show a strong increase in inhomogeneous broadening, caused by Indium composition and quantum well width fluctuations which increase with Indium mole fraction. These gain spectra provides a standard data set for the calibration of microscopic many-body simulations. We demonstrate by comparison with basic simulations that the different assumptions of a global constant carrier density or of global constant quasi-Fermi levels for electrons and holes lead to a strikingly different dependency of optical gain on carrier density. For constant quasi-Fermi levels the threshold carrier density becomes insensitive to inhomogeneous broadening for realistic parameters. This is in agreement with the observation that the threshold current is nearly independent over the wavelength range from near UV to aquamarine.

Substrate Modes of (Al,In)GaN Semiconductor Laser Diodes on SiC and GaN Substrates

In semiconductor laser diodes layers with high refractive index can act as parasitic waveguides and cause severe losses to the optical mode propagating in the longitudinal direction. For (Al,In)GaN laser diodes, the parasitic modes are typically caused by the SiC or GaN substrate or buffer layers, hence the name substrate modes. A set of four different experiments shows the effect of substrate modes in the near-field (the most direct evidence of substrate modes), as side lobes in far-field, oscillations of the optical gain spectra, and as dependency of threshold current on n-cladding thickness. We derive several basic properties of the substrate modes by simple estimates. For a quantitative analysis we employ a 2-D finite element electromagnetic simulation tool. We simulate periodic variations in the cavity gain spectrum that explain the measurements in terms of absolute value and oscillation amplitude. We show that it is necessary to include the refractive index dispersion in order to get the correct period of the gain oscillations. Furthermore, we use the simulations to optimize the laser diode design with respect to substrate mode losses within the constraints given, e.g., by growth conditions

Nanometer-scale displacement sensing using self-mixing interferometry with a correlation-based signal processing technique

AbstractA self-mixing interferometer is proposed to measure nanometre-scale optical path length changes in the interferometer’s external cavity. As light source, the developed technique uses a blue emitting GaN laser diode. An external reflector, a silicon mirror, driven by a piezo nanopositioner is used to produce an interference signal which is detected with the monitor photodiode of the laser diode. Changing the optical path length of the external cavity introduces a phase difference to the interference signal. This phase difference is detected using a signal processing algorithm based on Pearson’s correlation coefficient and cubic spline interpolation techniques. The results show that the average deviation between the measured and actual displacements of the silicon mirror is 3.1 nm in the 0–110 nm displacement range. Moreover, the measured displacements follow linearly the actual displacement of the silicon mirror. Finally, the paper considers the effects produced by the temperature and current stability of the laser diode as well as dispersion effects in the external cavity of the interferometer. These reduce the sensor’s measurement accuracy especially in long-term measurements.

Focused ion beam-based fabrication of nanostructured photonic devices

This paper presents results for focused ion beam (FIB) processing of two photonic devices: 1) a GaN laser with a fourth-order grating for vertical emission, and 2) a two-dimensional (2-D) photonic crystal (PhC) structure. For the GaN laser, both L-I and I-V results are shown before and after etching, and vertical emitted power as a function of the distance along grating is shown. A finite element (FE)-based electromagnetic model is developed to support the measured results and is used to predict the optimum grating depth. For the 2-D photonic crystal, direct FIB etching is used to create a PhC in a standard InP waveguide structure. Measured and modeled transmission results are compared, and there is good agreement for band edge position. A detailed study of hole shape is presented, and this leads to the development of a multistage etching procedure involving both reactive ion etching and inductively coupled plasma etching. This results in a much improved hole shape.

Measurement of junction temperature in GaN-based laser diodes using voltage-temperature characteristics

We present a method to determine junction temperature in GaN-based laser diodes (LDs) for simple, fast, and reliable characterization of thermal properties. The large change of forward operation voltage with temperature in GaN laser diodes is advantageously used to measure junction temperature. Using this method, we compare junction temperature of LD structures with different substrates and chip mounting methods. It is found that the junction temperature can be reduced considerably by employing GaN substrates or epi-down bonding. For epi-down bonded LDs, as much as two-fold reduction in junction temperature is achieved compared to epi-up bonded ones and junction temperature rise in this case is only about 13 degrees for more than 100 mW-output power.

Displacement sensor based on optical feedback interferometry in a GaN laser diode

We describe an optical displacement sensor based on optical feedback interferometry in a blue-light emitting GaN laser diode. Also presented are preliminary results from measuring variations in the optical path length (OPL) of an external cavity (EC) in the 0- to 240-nm range. These results show that, within the specified range, the sensor follows linearly the OPL variation of the EC. Moreover, the slope between a reference and the measured OPL is 1.0003, and the average deviation from the linear slope is 5 nm in this range. Finally, we also consider the stability of the interference signal in long-term measurements.

Wide-bandwidth mode-hop-free tuning of extended-cavity GaN diode lasers

We present a new approach for extended-cavity diode-laser tuning to achieve wide mode-hop-free tuning ranges. By using a multiple piezoactuated grating mount, the cavity length and grating angle in the laser can be adjusted independently, allowing mode-hop-free tuning without the need for a mechanically optimized pivot-point mount. Furthermore, synchronized diode injection-current tuning allows diode lasers without antireflection coatings to be employed. In combination these two techniques make the construction of a cheap, efficient, and easily optimized extended-cavity diode laser possible. A theoretical analysis is presented for optimal control of piezoactuator displacements and injection current to achieve the widest possible mode-hop-free tuning ranges, and a comparison is made with measurements. The scheme is demonstrated for blue and violet GaN lasers operating at approximately 450 nm and approximately 410 nm, for which continuous tuning ranges exceeding 90 GHz have been achieved. Examples of applications of these lasers are also given.

Facet degradation of GaN heterostructure laser diodes

We investigated the degradation of cleaved facets of (Al,In)GaN laser diodes in different atmospheres. We found that operation in water-free atmospheres with sufficient oxygen shows a slow degradation. Operation in atmospheres with water vapor causes a fast degradation and an oxidation on the facet. This deposition is a permanent damage to the laser diode. If the laser diode is operated in pure nitrogen, we find a thick deposition on the facet, which shows high absorption. This deposition can be removed by either high optical output powers or by operation in atmospheres with sufficient oxygen. We also explain the influence of these coatings to the degradation behavior and see these coatings as the reason for unstable kinks in the L–I characteristics during operation.