The results of studies of ring quantum-cascade lasers with surface emission due to a second-order grating formed in the top cladding layers are presented. Surface emission near 7.85 μm with a low threshold current density (3.8 kA/cm2), in comparison with ridge quantum-cascade lasers of the same cavity length is demonstrated. The results of measurements of the intensity distribution of the near and far fields at different pumping levels are presented. The estimated value of the angle of beam extraction relative to the surface normal is in the range (5.7–6.7)°.

The results of studies of 7.5–8.0 μm range surface-emitting ring quantum-cascade lasers are presented. A second-order diffraction grating with a calculated coupling coefficient of ~ 9 cm-1 is formed on the entire surface of the ring cavity by focused ion beam milling. Surface-emitting lasing at room temperature near 7.75 μm with a threshold current density of ~ 8 kA/cm2 and an outer radius of the ring cavity of 202 μm is demonstrated. The results of studying the intensity distribution in the far-field near the normal to the surface showed the presence of two maxima. It is shown that the implemented coupling coefficient is not sufficient to ensure single-mode lasing in the studied ring quantum-cascade lasers.

The possibility of realizing single-mode emission in quantum-cascade lasers due to modulation of output optical losses in a Fabry-Perot cavity is demonstrated. For the active region of the 7.5–8.0 μm spectral range, two-phonon resonance design we used thus the 50 stages and waveguide layers based on indium phosphide made it possible to realize single-mode lasing at 7.765 μm and at temperature of 292 K. Side-mode suppression ratio was about 24 dB and remained the same with an increase in the current pumping up to 1.2 of the threshold current values. The coefficient of wavelength shift with temperature (temperature tuning) in the single-mode lasing regime was 0.56 nm / K.

The possibility of fabrication of 4.6 µm spectral range quantum-cascade laser heterostructures by molecular-beam epitaxy technique with non-selective overgrowth by the metalorganic vapour-phase epitaxy is shown. The active region of the laser was formed on the basis of a heteropair of In0.67Ga0.33As/In0.36Al0.64As solid alloys. The waveguide claddings are formed by indium phosphide. The results of surface defects inspection and X-ray diffraction analysis of quantum-cascade laser heterostructures allow to conclude that the structural quality of the heterostructures is high and the estimated value of the root mean square surface roughness does not exceed 0.7 nm. Lasers with four cleaved facets exhibit lasing at room temperature with a relatively low threshold current density of the order of 1 kA /cm2.

The results of studies of quantum-cascade laser with a surface emission through a grating formed in the layers of the top cladding of the waveguide by ion beam milling are presented. The active region of the QCL heterostructure was formed based on a heteropair of In0.53Ga0.47As / Al0.48In0.52As solid alloys with two-phonon resonance design. It is shown that lasing at room temperature close to 7.9 µm is demonstrated for a laser with a ring diameter of 191 µm. The mode spacing corresponds to whispering gallery modes.

The results of studies of ring quantum-cascade laser with a surface emission with radiation output through a textured layer formed in the layers of the top cladding of the waveguide by ultrahigh vacuum ion beam milling are presented. The far-field profile shows that the radiation is output through the windows with a textured layer in the range of angles of ~ 63–75 degrees to the normal.

The near and far fields of quantum cascade lasers emitting in the spectral range near 8 µm have been studied. Lasing in the fundamental waveguide mode along the "fast" axis with a divergence of ~ 50º was observed in all samples of lasers with stripe widths of 22 and 50 µm in the entire range of pump currents. On the "slow" axis, multimode lasing and competition between the fundamental and higher waveguide modes were observed, which manifests itself for lasers with a 50 µm stripe in an increase of the radiation divergence from ~ 10º near the threshold to > 50º with a two-fold excess of the lasing threshold. The divergence along the slow axis for lasers with a stripe width of 22 µm remained unchanged over the entire range of pump currents and amounted to ~ 50º. The significant contribution of the fundamental mode to the total radiation intensity of quantum cascade lasers, being obvious from the far-field intensity distribution, cannot be established solely from the results of measurements of the near-field.

Light characteristics of narrow-stripe lasers (5.5 m) based on asymmetric AlGaAs/GaAs heterostructures are studied. It was shown that the maximum optical power achieved under continuous-wave (CW) operation is limited by thermal heating and reaches 1695 mW at a current of 2350 mA at +25°C, and the maximum efficiency reaches 54.8 %. By reducing the operating temperature to -8°C, we were able to increase the maximum power to 2 W. A peak power of 2930 mW was obtained under pulsed operation (pulse width 240 ns, amplitude 4230 mA). It is shown there is a region of an “optical dip” in the power profile with a low-efficiency lasing of a train of pulses of sub-ns duration under pulsed operation.

The studies of the spectral and dynamic characteristics of quantum-cascade lasers emitting in the long-wave infrared range are presented. It has been shown that lasers with a short resonator (~ 1 mm) generating frequency combs in a very wide spectral range. The dynamics of the the frequency comb generation mode was studied. It is shown that the intensity of the longitudinal modes of laser generation changes during the pump pulse. At the same time, simultaneous generation of all longitudinal modes of the frequency comb during flat part of the pumping pulse is observed.

A half-ring resonator design of a 7-8 μm range quantum-cascade laser with different radius values has been proposed and implemented. For a quantum-cascade laser with a radius of a half-ring resonator of 191 μm, lasing with a radiation spectrum width of 474 nm (82 cm^-1) was demonstrated at low temperatures. The FSR in such lasers was determined by the whispering gallery modes typical for ring resonators. At room temperature, the width of the lasing spectrum was 190 nm (31 cm^–1), which is caused by a decrease in gain and a possible increase in internal losses with increasing temperature. An increase in the cavity radius up to 291 μm made it possible to realize room temperature lasing with whispering gallery modes with a radiation spectrum width of 249 nm (40 cm^-1), by reducing losses on the mirrors.

An elastically balanced heterostructure of a 4.6-μm wavelength range quantum-cascade laser was grown by using molecular beam epitaxy. Heterostruсture was based on a heteropair of In0.67Ga0.33As / In0.36Al0.64As solid alloys and indium phosphide layers, which served as waveguide claddings. A high homogeneity of the layers composition and thicknesses in cascades over the wafer was proved by using X-ray diffraction for the grown heterostructure. Lasers with four cleaved facets operate at room temperature at a wavelength near 4.6 μm with a low threshold current density of 1.1 kA / cm2

AbstractSingle-mode lasing at room temperature in quantum-cascade lasers (QCLs) with arched cavity design has been demonstrated. The output optical power in single-mode lasing regime at ~7.7-μm lasing wavelength was above 6 mW with a side-mode suppression ratio of up to 25 dB. The QCL heterostructure for the arched cavities was grown by molecular-beam epitaxy (MBE) based on a heterojunction of In_0.53Ga_0.47As/Al_0.48In_0.52As solid alloys, lattice-matched with InP substrate, and InP layers performing the function of waveguide claddings.

The heterostructure of a quantum-cascade laser based on In0.53Ga0.47As/Al0.48In0.52As heteropair lattice matched with the InP was grown by molecular beam epitaxy. InP layers was used to form the optical waveguide. Room temperature lasing in the spectral range of 8 μm in the standard ridge geometry of a Fabry-Perot cavity formed by cleaved facets with peak output optical power of 0.45 W was obtained.

AbstractWe have studied the generation of frequency combs by quantum cascade lasers (QCLs) emitting in the 8-μm wavelength range. Results showed the presence of a self-pulsation regime near the lasing threshold. Further increase in the pumping current led to a sharp increase in width of the lasing spectrum, which allowed us to obtain frequency combs with a spectral width exceeding 1.5 THz. This behavior of QCLs can be explained by radiation absorption at the stripe edge that is related to the penetration of waveguide mode into unpumped regions.

AbstractThe fabrication and study of the characteristics of a lattice-matched quantum cascade laser structure on an indium-phosphide substrate, designed for a wavelength of ~4.8 μm corresponding to one of the atmospheric windows are described. The heterostructure grown by molecular-beam epitaxy consisted of thirty cascades. Lasing was experimentally observed at temperatures up to 200 K at a wavelength coinciding with the calculated one, which confirms the high heterointerface quality and high precision of the layer thicknesses and active-region doping levels.

AbstractThe fabrication and investigation of the ridge waveguide structure for a quantum cascade laser on an indium phosphide substrate intended for a wavelength of 4.8 μm, which corresponds to one of the atmospheric transparency windows, is calculated. The heterostructure is grown by molecular-beam epitaxy and consists of 30 cascades. Post-growth treatment is performed by photolithography using Russian equipment. The fabricated ridge waveguide structure allowed lasing to be attained at the required wavelength at room temperature.

AbstractWe report on room-temperature generation at the 4.8-μm wavelength in a lattice-matched structure of a quantum cascade laser (QCL) grown on indium phosphide (InP) substrate. Laser heterostructures grown by molecular beam epitaxy (MBE) comprised 30 cascades and were designed to generate at the 4.80 μm wavelength corresponding to an atmospheric transparency window. Experiments demonstrated effective lasing at temperatures from 80 to 300 K on a wavelength coinciding with the calculated value, which confirmed the high quality of interfaces, high precision of layer thicknesses, and high accuracy of active region doping.