High-intensity and low-divergence THz laser with 1D autofocusing symmetric Airy beams

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/mm² in a spot-size of 1 mm²) or low-divergence (1.4^∘×1.8^∘ for a beam with 118 mW peak power) are realized at 62 K in a compact electrically operated Stirling cooler. A high brightness of 5.4×10⁶ Wsr ^-1m ^-2 is estimated for the focused beams by measuring the beam propagation ratios (M ² parameters).