Optical Confinement Layer Research Articles

Stimulated whispering gallery modes emission from green to UVA realized by AlInGaN based quantum wells microtube

Self-rolling GaN quantum wells microtubes have been fabricated by a photoresist-assisted electrochemical etching method. Different microtubes with whispering gallery modes emitting from the green to ultraviolet range have been demonstrated. Stimulated whispery gallery mode emission from 370 to 460 nm was achieved with lasing line-width less than 0.087 nm and a threshold as low as 0.12 mJ cm−2. Such advanced optical properties can be attributed to the larger mode gain obtained by overlapping the active region with the optical confinement layers. The number of modes was greatly suppressed simultaneously leaving only basic radial modes due to the ultra-thin cavity wall.

Рабочие характеристики полупроводниковых лазеров на квантовых ямах в зависимости от ширины волноводной области

Operating characteristics of semiconductor quantum well (QW) lasers are theoretically studied in terms of the thickness of the waveguide region [optical confinement layer (OCL)]. We calculate the maximum modal gain, optical confinement factor (in QW, OCL, and cladding layers), threshold current density, electron and hole densities (in QW and OCL), internal optical loss (in QW, OCL, and cladding layers), internal differential quantum efficiency, stimulated and spontaneous recombination currents, and output optical power of the laser as functions of the OCL thickness. It is shown that up to the pump current density 50 kA/cm2 the output power of the considered lasers depends only slightly on the OCL thickness in the range of thicknesses 1.5–2.8 m. This result is important for designing high brightness lasers as broadened waveguides are used in such lasers to attain low beam divergence. At high pump current densities, the output power is shown to have a maximum as a function of the OCL thickness.

Asymmetric-waveguide, short cavity designs with a bulk active layer for high pulsed power eye-safe spectral range laser diodes

It is shown, by calculations calibrated against the authors’ recent experimental data, that an eye-safe wavelength range InGaAsP/InP high pulsed power laser design using a bulk active layer, which has a large refractive index step with respect to the optical confinement layer and is located close to the p-cladding, can provide substantial performance improvement compared to the best results achieved so far for this operating regime and wavelength. The dependence of the laser performance on the design parameters such as the thicknesses of the active layer and the waveguide, as well as the cavity length, are analysed. It is shown that the relatively thick bulk active layer in such InGaAsP/InP lasers allows the use of short cavity lengths (∼1 mm or even shorter), for achieving high pulsed power while maintaining a low p-cladding series resistance (making for high efficiency) and a narrow far field (making for high brightness). A single-asymmetry structure with the asymmetric active layer location but symmetric optical confinement layer/cladding refractive index steps gives performance only marginally inferior to that of a double-asymmetric one including asymmetric refractive index steps.

On-chip integrated photonic circuits based on two-dimensional materials and hexagonal boron nitride as the optical confinement layer

Owing to the rapid development of data communication, there is a high demand for the large-scale integration of photonic devices into broadband communication networks. For practical integration, integrated photonic circuits are ideally fabricated on silicon platforms using processes compatible with complementary metal–oxide–semiconductor technology. Two-dimensional transition metal dichalcogenides are attractive candidates as on-chip emitters and absorbers due to their direct bandgaps, compatibility with miniaturization, large exciton binding energies, anisotropic polarizations, and strong light-matter interactions. Herein, a review of the current progress in the applications of two-dimensional materials as on-chip semiconductor devices is presented, as well as their prospects of integration on the silicon photonic platform. On-chip integrated photonic circuits are proposed based on heterostructures of hexagonal boron nitride and two-dimensional materials with functions of light sources, optical modulators, and photodetectors toward high-bandwidth optical interconnects.

Evolution of light‐current characteristic shape in high‐power semiconductor quantum well lasers

The light-current characteristic (LCC) of semiconductor quantum well lasers is theoretically studied. It is discussed here that, due to internal optical absorption loss, which depends on the electron and hole densities in the optical confinement layer, (i) roll-over of the LCC occurs with increasing injection current, and, (ii) depending on the parameters of laser structures, the LCC can have two branches, i.e. the optical emission at two different output powers will be possible within a certain range of injection currents.

On the relationship between electrical and electro-optical characteristics of InAs/InP quantum dot lasers

This paper addresses the electrical and electro-optical characteristics of InAs/InP quantum dot (QD) laser diodes operating under continuous wave in the temperature range of 285 K to 353 K. Experimental data together with a model lead to an identification of the current flow mechanism in all the important drive regimes: very low bias (pre radiation regime), the exponential region of the current-voltage (I-V) curve, and in the nonlinear range where spontaneous and stimulated emission take place. The analysis discriminates among bias regions where the influences of the QD or the optical confinement layers dominate. Additionally, parameters and non-nonlinear processes under injection into the active region are described here for the first time. A high level injection (spontaneous, as well as stimulation emission regimes) results in non-linear I-V characteristics. Above the threshold, the recombination rate increases due to rapid “pinning” of the quasi-Fermi levels leading to an anomalous kink of the power exponent voltage characteristics. This manifests itself as a non-linear differential resistance increase even at elevated temperatures, up to 353 K. Capacitance voltage measurements reveal some peculiarities, namely, well defined capacitance peaks related to the QDs, hysteresis, and a low frequency negative capacitance phenomenon which enhance the laser potential functional capability.

Strong doping of the n-optical confinement layer for increasing output power of high- power pulsed laser diodes in the eye safe wavelength range

An analytical model for internal optical losses at high power in a 1.5 μm laser diode with strong n-doping in the n-side of the optical confinement layer is created. The model includes intervalence band absorption by holes supplied by both current flow and two-photon absorption (TPA), as well as the direct TPA effect. The resulting losses are compared with those in an identical structure with a weakly doped waveguide, and shown to be substantially lower, resulting in a significant improvement in the output power and efficiency in the structure with a strongly doped waveguide.

Increase in the internal optical loss with increasing pump current and the output power of quantum well lasers

The operating characteristics of semiconductor quantum-well lasers, calculated with consideration for an increase in the internal optical loss in the waveguide region with increasing pump current, are presented. The condition of global electroneutrality in the structure is used. This condition consists in that the total charge of electrons in the 2D active region (quantum well) and bulk waveguide region (optical confinement layer) is equal to the total charge of holes in these two regions. Good agreement between the calculated and experimentally determined light–current characteristics is obtained.

Theory of operating characteristics of a semiconductor quantum well laser: Inclusion of global electroneutrality in the structure

A model for calculating the operating characteristics of semiconductor quantum well (QW) lasers is presented. The model exploits the condition of global electroneutrality, which includes the charge carriers both in the two-dimensional (2D) active region (QW) and bulk waveguide region (optical confinement layer - OCL). The charge of each sign in the OCL is shown to be significantly larger than that in the QW. As a result of this, (i) the global electroneutrality condition reduces to the condition of electroneutrality in the OCL and (ii) the local electroneutrality in the QW can be strongly violated, i.e., the 2D electron and hole densities in the QW can significantly differ from each other.

Output power of a quantum dot laser: Effects of excited states

A theory of operating characteristics of quantum dot (QD) lasers is discussed in the presence of excited states in QDs. We consider three possible situations for lasing: (i) ground-state lasing only; (ii) ground-state lasing at first and then the onset of also excited-state lasing with increasing injection current; (iii) excited-state lasing only. The following characteristics are studied: occupancies of the ground-state and excited-state in QDs, free carrier density in the optical confinement layer, threshold currents for ground- and excited-state lasing, densities of photons emitted via ground- and excited-state stimulated transitions, output power, internal and external differential quantum efficiencies. Under the conditions of ground-state lasing only, the output power saturates with injection current. Under the conditions of both ground- and excited-state lasing, the output power of ground-state lasing remains pinned above the excited-state lasing threshold while the power of excited-state lasing increases. There is a kink in the light-current curve at the excited-state lasing threshold. The case of excited-state lasing only is qualitatively similar to that for single-state QDs—the role of ground-state transitions is simply reduced to increasing the threshold current.

Fundamental transverse mode selection and self-stabilization in large optical cavity diode lasers under high injection current densities

It is shown that in high-power, large optical cavity laser diodes at high injection currents, the optical losses due to nonuniform carrier accumulation in the optical confinement layer can ensure the laser operation in the fundamental transverse mode. An experimental demonstration of switching from second order mode to fundamental mode in large optical cavity lasers with current and/or temperature increase is reported and explained, with the calculated values for the switching current and temperature in good agreement with the measurements. The results experimentally prove the nonuniform nature of carrier accumulation in the confinement layer and may aid laser design for optimizing the output.

Suppression of sublinearity of light–current curve in 850 nm quantum well laser with asymmetric barrier layers

An AlGaAs/GaAs quantum well (QW) laser is fabricated with GaInP and AlGaInAs asymmetric barrier layers (ABLs) and its light–current characteristic (LCC) is compared with that of a reference conventional QW laser without ABLs. It was found that the use of the ABLs suppresses the sublinearity of the LCC at high current densities. As a result, the maximum lasing power of 9.2 W, being limited by catastrophic optical mirror damage, is achieved at a considerably lower operating current in the laser with ABLs as compared to the reference laser (12.5 against 20.2 A). The ABL effect is associated with the suppression of the parasitic recombination in the optical confinement layer, as confirmed by a decrease of the intensity of the spontaneous emission from the layer.

Strongly asymmetric waveguide laser diodes for high brightness picosecond optical pulses generation by gain switching at GHz repetition rates

Rate equations analysis, supported by travelling wave simulations, is used to show that a diode laser design with a very low (<1%) confinement factor is optimal for generating streams of high energy optical pulses for nonlinear applications by large-signal modulation with both existing and potentially improved ac current generators. An asymmetric waveguide laser design is proposed to realize this low confinement factor while simultaneously maintaining good beam properties in a single transverse mode. The rate equations model complemented by transport equations is used to quantify the effects of transport in the broad optical confinement layer(s) on the laser dynamics, and it is shown that in the proposed laser construction the detrimental effects of transport are weak.

Effect of Quantum Dots Density and temperature on Quantum Dot Laser Characteristics

Detailed theoretical study of the effects of surface density of quantum dots on the dynamics of quantum dot lasers (QDLs) is given. Temperature dependence of the threshold current density, confined level occupancy, internal loss coefficient, optical confinement layer (OCL), characteristic temperature and lowest excitation energy with the variation of internal loss constant component, root mean square of relative QD–size fluctuations, mean size of QDs and optical confinement factor are studied.

Direct and indirect capture of carriers into the lasing ground state and the light-current characteristic of quantum dot lasers

We calculate the light-current characteristic (LCC) of a quantum dot (QD) laser under the conditions of both direct and indirect capture of carriers from the optical confinement layer into the lasing ground state in QDs. We show that direct capture is a dominant process determining the ground-state LCC. Only when direct capture is slow, the role of indirect capture (capture into the QD excited state and subsequent intradot relaxation to the ground state) becomes important.

Semipolar (202¯1) GaN laser diodes operating at 388 nm grown by plasma-assisted molecular beam epitaxy

Laser diodes (LDs) were grown by plasma-assisted molecular beam epitaxy on semipolar (202¯1) GaN substrates. Metal-rich conditions provided smooth surface morphology and good structural quality as confirmed by atomic force microscopy and transmission electron microscopy studies. No stacking faults or any linear defects were formed during the growth in the InGaN/InGaN multiquantum well active region. Devices were processed with ridge-waveguide along the [1¯21¯0] direction. Mirrors were cleaved or fabricated by focused ion beam (FIB) processing. LDs operated at 388.2 nm with the threshold current density 13.2 kA/cm2 and the threshold voltage 10.8 V. The device with mirrors fabricated by FIB processing reached practically the same threshold current and slope efficiency as the one with cleaved mirrors. The authors present the beneficial role of the InGaN optical confinement layers in semipolar LDs, which can be optimized to improve the threshold current of these devices.

Narrow versus broad asymmetric waveguides for single-mode high-power laser diodes

We investigate numerically the effect of the optical confinement layer thickness on the far field properties (far field shape and input efficiency) and confinement factor of an asymmetric-waveguide high power laser diode. A strong correlation is found between the confinement and input efficiency. It is shown that the far field properties of lasers with narrow asymmetric structures tend to be superior to those of broad waveguide ones with a similar confinement factor.

Dember type voltage and nonlinear series resistance of the optical confinement layer of a high-power diode laser

An analytical model is developed for the carrier density distribution and the associated Dember type electric field/voltage in the waveguide layer of a high-power semiconductor laser for arbitrary levels of doping and injection. Nonlinear resistance of the waveguide layer is analysed; it is shown that at a very high injection level, doping the waveguide layer leads to almost negligible decrease in its effective resistance.

Carrier-Transport-Limited Modulation Bandwidth in Distributed Reflector Lasers With Wirelike Active Regions

High-speed direct modulation capability was investigated in 1.55-μm GaInAsP/InP distributed reflector (DR) lasers with wirelike active regions in terms of carrier transport from GaInAsP optical confinement layers (OCLs) to the active regions. Theoretical analysis revealed strong dependence of the modulation bandwidth on the thickness of the OCLs and width of the wirelike active regions. To confirm this prediction, two DR lasers with OCLs of different thicknesses (120 and 40 nm) were fabricated and their 3-dB bandwidths (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3 dB</sub> ) under small-signal modulation were compared. The device with the narrower OCL exhibited f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3 dB</sub> exceeding 15 GHz and clear eye opening under 25 Gb/s modulation, whereas that with the thicker OCL had f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3 dB</sub> of only 2 GHz. These results were in good agreement with the theoretical predictions.

Improvement of temperature-stability in a quantum well laser with asymmetric barrier layers

We fabricated and tested a quantum well laser with asymmetric barrier layers. Such a laser has been proposed earlier to suppress bipolar carrier population in the optical confinement layer and thus to improve temperature-stability of the threshold current. As compared to the conventional reference laser structure, our laser with asymmetric barrier layers demonstrates reduced internal optical loss, lower threshold current density at elevated temperatures, and higher characteristic temperature (143 vs. 99 K at 20 °C).