This paper presents an extensive analysis of the degradation of InGaN-based laser diodes submitted to electrical stress. The analyzed devices, with emission in the violet spectral region, were submitted to constant current stress; the degradation process was monitored by means of electro-optical measurements, which indicated that stress induced an increase in the threshold current of the devices, ascribed to the generation of non-radiative defects. After stress, the (thick) top metallization was removed, and the optical behavior of the samples was characterized by microcathodoluminescence and micro-photoluminescence investigation. Results indicate that (i) stress induced a significant degradation of the efficiency of the devices under the ridge, i.e. in the region which is crossed by high current densities during ageing. (ii) the darkening of the ridge was detected both by micro-cathodoluminescence measurements (in which carriers are generated both in the barriers and in the quantum wells) and by micro-photoluminescence analysis with subbandgap excitation (with respect to the barriers). The experimental evidence collected within this paper demonstrates that the degradation of the laser diodes can be ascribed to an increase in the rate of non-radiative recombination within the active region of the devices, possibly due to a defect diffusion process. Hypothesis on the nature of the defects involved in the degradation process are formulated based on capacitance Deep Level Transient Spectroscopy measurements.
We demonstrate pulse periods from 0:13 to 10 ns of GaN{based ridge waveguide laser diodes with monolithically integrated absorbers in the regimes of relaxation oscillations and self{Q{switching as function of gain current and absorber voltage. We introduce a simple model for the self{Q{switching regime, describing the pulse period in terms of current injection and spontaneous emission (including Auger recombination), only. At reverse voltages larger than 35V the modal absorption exceeds 500 cm-1, which cannot be explained solely by transitions of bound states in the quantum wells. Calculations based on wavefunction overlap and quantum con ned Stark e ect (QCSE) predict a decrease of absorption at such large bias. In contrast, we show experimental ndings, proving that the absorption further increases. Due to the strong tilt of the band pro le in this regime, we take into account the Franz{Keldysh e ect in the barriers and the waveguide and discuss its possible in uence on the absorption, leading to an increased absorption at large reverse bias.
We investigate the influence of the epitaxial layer roughness on the far-field profile of the optical mode in gallium Nitride-based, c-plane ridge waveguide laser diodes. Occasionally, we observe long-range growth instabilities leading to a periodical modulation of the surface. Amplitude and period of this surface roughness is typically on the order of a few 10nm and 20 μm, respectively. Using different characterization techniques, we investigate the influence of the surface roughness on the vertical mode profile along the fast axis in the far-field, in particular the contribution of light scattering at the rough waveguide interfaces, as well as that of substrate modes.
The highly sensitive photoluminescence (PL) response of group III-Nitrides (III-N) nanowire heterostructures (NWHs) to hydrogen (H2) and oxygen (O2) allows for the realization of reliable gas detectors. For industrial real time gas monitoring applications, e.g. in the field of aerospace, a large scale laboratory setup was miniaturized by integrating electro-optical components and the NWHs within a robust micro optical system. As a result of the all optical addressing and read out the detection periphery can be completely isolated from the investigated environment which significantly increases the detection sensitivity. The optical design and fabrication techniques as well as an experimental investigation of the system performance are the main topics discussed in this paper.
We develop a monolithic picosecond laser pulse generator, based on the classical design of a group-III-nitride Fabry-Pérot laser diode with electrically separated ridge sections. We use two different multi-section design variants, with the absorber section placed either in the center or at the end of the ridge. Profiting from the very low lateral conductivity in the p-type GaN top contact layer, we implement the multi-section concept just by etching off small sections of the top metalization on the ridge. The physical mechanism underlying short pulse generation within such system, operating in the 400 - 435nm wavelength range, strongly depends both on the reverse bias applied to the absorber and the forward current in the gain section. Varying the applied reverse bias affects both the absorption and the carrier lifetime in the absorber section through changes in the QW internal field. In consequence we can distinguish between different modes of operation. For moderately long carrier lifetimes the absorber stabilizes relaxation oscillations in the GHz frequency range and self-pulsation occurs, of relatively long duration. With increasing reverse bias, and thus decreasing carrier lifetime, we observe a transition to self-Q-switching. Finally, at large enough negative bias, the carrier life time in the absorber is so short that the laser diode operates in a passive self-mode-locking regime with a repetition rate of 87 GHz and pulse duration of 2 ps for a cavity length of 540 μm.
We develop (Al, In)GaN ridge waveguide laser diodes in the violet-blue spectral region. Varying the indium content of
the InGaN quantum wells, we tailor the emission wavelength of our devices for specific applications in the range from
390 nm to 425 nm. Using different commercially available free standing GaN substrates, we adjust the epitaxial design
and optimize the processing sequence for the different kinds of substrates. In particular we focus on the ridge formation.
We compare different fabrication methods to obtain devices with ridge widths around ≤2 μm. So far, we have achieved
threshold currents around 60 mA and slope efficiencies exceeding 1 W/A.
KEYWORDS: Semiconductor lasers, Near field, Near field optics, Gallium nitride, Laser damage threshold, Cladding, Spectroscopy, Semiconductors, CCD cameras, Imaging systems
One of the most desired features of the semiconductor blue/near UV laser diodes (LDs) is the possibility to obtain
high output powers from the devices. This can be realized by means of multi emitter structures. We demonstrate
the construction of violet blue multi-quantum-well (MQW) InGaN/GaN laser mini - bars, yet quite novel system
in nitride-based devices. It consists of three laser stripes (3 μm wide), closely spaced with 40 μm pitch. The
structures were fabricated on high pressure grown, low dislocation density substrates. Under cw operation
the measured spectra demonstrate sharp, almost single line emission (FWHM around 1.43 Å at λ = 406 nm).
Measurements of the optical far field pattern revealed that when operated below threshold the device was emitting
light from all three stripes, whereas during lasing we observed the optical mode only in the area of the middle
laser stripe. The behavior of optical mode suggests the formation of the so called supermode (coherent emission
from all three devices) which was observed also in case of structures fabricated with wider ridges (of 7 μm with
20 μm pitch).
We report temperature tuning of pulsed operated InGaN LDs (5×500μm stripe,
grown on low-dislocation, high-pressure grown GaN substrates). The devices
are characterized by a rather weak temperature dependence of the threshold
current. A very broad temperature tuning range of 16nm was obtained with
increase of operation temperature by almost 200K. We were able to tune the
diode from the initial wavelength of 415nm at room temperature up to 431nm
at 201°C. After thermally cycling the device no substantial degradation was
noticed. We observed multimode emission and mode hopping with temperature
increase. At 201°C the laser's threshold current doubled and the slope efficiency
of the L-I curve dropped by 35%. These results demonstrate the potential
usage of temperature tuning of nitride-based-LDs for the atomic spectroscopy-related applications.
We have used pulsed operation, wide area InGaN laser diodes in conjunction with Littrow type external
cavity to build a tunable, single mode laser operating around 398 nm. Special coatings had been applied to the
device - antireflection coating on the output mirror and high - reflector on the back facet. The tuning range
of this device was 5.5 nm, the maximum output power reached 40mW in a single mode operation. This value
compares well with the output power of an uncoupled laser diode -170mW. The coupling between the external
cavity and the internal resonator is estimated to be around 2.5% for a waveguide dimensions of 20 x 0.3 x 500&mgr;m3.
We fabricated wide-stripe laser diodes operating between 380 and 430 nm. The threshold current density for 380 and 430 nm devices (6-7 kA/cm2) was only slightly higher than for our main stream 415 nm devices (4-6 kA/cm2). Thanks to the use of high-pressure-grown low-dislocation-density substrates we succeeded in demonstration of high power optical emission both under CW and pulse operation. For the device emitting at 415 nm we were able to demonstrate 200 mW of CW optical power (20 μm wide device) and 2.7 W under pulse current operation (peak power, 50 μm device). The main obstacle for achieving CW operation of 50 μm device was to remove the excess of heat from laser chip-diamond submount assembly.
High hydrostatic pressure can be used for wavelength tuning of semiconductor laser diodes in a wide spectral range. Coupling the laser with external grating leads to wavelength tuning within the gain spectrum (i.e. in a narrower range than with pressure) but allows for a narrow emission line and nearly continuous tuning (mode-hop free if anti-reflecting coating is applied). Here we demonstrate a combination of pressure and external-resonator tuning for the GaInNAs laser emitting at 1343 nm at ambient conditions. Using the specially designed liquid pressure cell working up to 20 kbar we shift the emission down to 1170 nm while the external grating (used in Littrow configuration) allows for fine tuning in the ~10 nm range (at each pressure).
The performance of photorefractive hybrid panels made from photoconducting polymer ane nematic liquid crystal layers are reviewed and discussed. Their properties were studied under pulsed and cw illumination. The dynamic holography experiments with pulsed laser beams give information on the charge mobility in the thin photoconducting layers. The use of panels to visualize the phase objects introducing small distortions to plane wave front (such as e.g. turbulence of air) through the well-known Zernike filtering is also shown. The advantage of such nonlinear Zernike filter over conventional one is that no precise optical adjustment is necessary and the filtering is relatively easy to control by tuning the externally applied field to the modulator and/or varying the incoming light intensity.
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