Wavelength beam-combining of four terahertz (THz) distributed-feedback quantum-cascade lasers (QCLs) is demonstrated using low-cost THz components that include a lens carved out of a plastic ball and a mechanically fabricated blazed grating. Single-lobed beams from predominantly single-mode QCLs radiating peak power in the range of 50 − 170 m W are overlapped in the far field at frequencies ranging from 3.31 − 3.54 T H z . Collinear propagation with a maximum angular deviation of 0.3 ∘ is realized for the four beams. The total power efficiency for the focused and beam-combined radiation is as high as 25 % . This result could pave the way for future commercialization of beam-combined monolithic THz QCL arrays for multi-spectral THz sensing and spectroscopy at standoff distances.

Single-mode THz quantum-cascade lasers (QCLs) have been realized using a wide variety of techniques to obtain a combination of large power output, good beam quality with single-lobed beams, and low far-field divergence. Beam shaping using external components has not been previously exploited due to limited commercial availability of THz optical components and also the accompanying large loss from most THz optical materials. Here, we demonstrate that excellent beam characteristics could be obtained for a THz QCL by integration of a surface-emitting distributed-feedback (DFB) QCL with a simple lens within the vacuum chamber of a cryocooler. Plano-convex lenses are made from inexpensive plastic balls and integrated in proximity with a 3.4 THz DFB QCL. With appropriately chosen lens parameters, dual-lobed Airy beams are generated that autofocus into a high-intensity single-lobed beam with large focusing efficiency. A simple and general method to generate one-dimensional autofocusing Airy beams is thus demonstrated that is applicable at any wavelength. THz laser beams with high peak intensity (57 mW/mm(2) in a spot-size of 1 mm(2) ) or low-divergence (1.4 degrees x 1.8 degrees for a beam with 118 mW peak power) are realized at 62 K in a compact electrically operated Stirling cooler. A high brightness of 5.4 x 10(6) Wsr(-1) m(-2) is estimated for the focused beams by measuring the beam propagation ratios (M-2 parameters). (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Scattering due to interface-roughness (IR) and longitudinal-optical (LO) phonons are primary transport mechanisms in terahertz quantum-cascade lasers (QCLs). By choosing GaAs/Al_0.10Ga_0.90As heterostructures with short-barriers, the effect of IR scattering is mitigated, leading to low operating current-densities. A series of resonant-phonon terahertz QCLs developed over time, achieving some of the lowest threshold and peak current-densities among published terahertz QCLs with maximum operating temperatures above 100 K. The best result is obtained for a three-well 3.1 THz QCL with threshold and peak current-densities of 134 A/cm² and 208 A/cm² respectively at 53 K, and a maximum lasing temperature of 135 K. Another three-well QCL designed for broadband bidirectional operation achieved lasing in a combined frequency range of 3.1–3.7 THz operating under both positive and negative polarities, with an operating current-density range of 167–322 A/cm² at 53 K and maximum lasing temperature of 141 K or 121 K depending on the polarity of the applied bias. By showing results from QCLs developed over a period of time, here we show conclusively that short-barrier terahertz QCLs are effective in achieving low current-density operation at the cost of a reduction in peak temperature performance.

A mechanism to electrically tune the frequency of terahertz quantum cascade lasers (QCLs) is developed that allows for tuning, while the QCL is operated close to its peak bias and temperature. Two optically coupled but electrically isolated cavities are used in which the bias of a control cavity tunes the resonant-mode of the coupled QCL cavity independent of the QCL's operating bias. Approximately $4\hspace{0.17em}\text{GHz}$ electrical tuning is realized for a $3.6\hspace{0.17em}\text{THz}$ distributed-feedback QCL operating in pulsed mode at $58\hspace{0.17em}\mathrm{K}$ in a Stirling cooler. The single-mode QCL emits near-constant peak-power in the range of $5-5.3\hspace{0.17em}\text{mW}$ through the tuning range and radiates in a narrow single-lobed beam with a far-field divergence of $\sim 4°\times 11°$ . The superlattice structure of the QCL is designed to implement a low-voltage intersubband absorption transition that is detuned from that of its gain transition, the strength of which could be controlled sensitively with applied voltage utilizing resonant-tunneling injection of electrons in the absorption subband. The tuning is realized by the application of small bias voltages ($\sim 6-7$ V) and requires a narrow bias range ($\sim 1$ V, $\sim 40\hspace{0.17em}\mathrm{A}/{\text{cm}}^2$ ) to traverse across the entire tuning range, and the method should be generally applicable to all intersubband lasers including mid-infrared QCLs.

The modulation and impedance properties of a terahertz (THz) quantum cascade laser (QCL) are investigated theoretically based on a three-level rate equation model. The effect of different device parameters, namely, facet reflectivity, injection efficiency, spontaneous emission factor, and operating current, on the modulation and impedance characteristics of the QCL is analyzed in detail. The device shows a maximum modulation bandwidth (f3dB) of 21 GHz and an intrinsic impedance of 3.8 mΩ when operating under the designed conditions. The modulation bandwidth and impedance increase with the increase in facet reflectivity and injection efficiency. However, the modulation bandwidth increases but the impedance decreases with the increase in operating current. The spontaneous emission factor has no effect on both the modulation bandwidth and impedance. The theoretical model will aid in the design of THz QCLs requiring a large modulation bandwidth and the external circuit design to match the standard 50 Ω source for reducing reflections and improving the coupling efficiency.

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