Continuous-wave operation of 2.7 THz photonic crystal quantum cascade lasers

Abstract

Continuous-wave laser action has been achieved in a 2.7 THz quantum cascade device operating on the modes of a two-dimensional photonic crystal resonator. The photonic structure is patterned into the device top metal contact layer. The emission is obtained from the device surface. It is single lobed and highly directional. At cryogenic temperatures the output power is ≃300 mW.Introduction: Since their inception in 2002 [1], advances in long wavelength (60-300 mm) quantum cascade laser (QCL) designs have rapidly established this technology as the leading candidate for compact semiconductor sources of coherent terahertz radiation. While improvements in the design and performance of the QCL active region (AR) have slowed in recent years, leading to a stagnation of the maximum operation temperature T max of 186 K [2], constant refinement of the applied waveguide technology is addressing other important aspects of the device performance, in particular, the quality of the output beam pattern.Currently, the most efficient solution for a terahertz QCL waveguide relies on the tranverse magnetic (TM)-polarised plasmonic modes at the upper and lower interfaces between the dielectric semiconductor AR and metallic contact layers [3,4], in so-called metal -metal waveguides. However, the consequent sub-wavelength vertical light confinement leads to highly divergent output beams from the laser facets [5]. Several strategies have been investigated to achieve more directional light output, in either edge or surface-emitting configurations, while maintaining low threshold current densities (J th ) and high T max [6][7][8]. One such approach is the photonic crystal resonator in which single frequency surface emission can be achieved by patterning the upper metallic contact layer, at the expense of generally slightly higher waveguide losses and thus lower T max compared with standard FabryPerot ridge resonators [9 -11].Continuous-wave (CW) laser operation is dependent on the dissipation of heat through the relatively high thermal resistance material of the QCL device, such that the effective temperature within the AR does not exceed T max . Thus, CW laser action may be promoted not only by improving the heat flow within the device, but also by reducing the J th of a laser, either by refining the AR energy band structure or by enhancing the overall Q factor of the resonator.In this Letter, we implement a refined PhC design which allows CW singlemode laser operation at n ¼ 2.7 THz up to a heatsink temperature of 39 K, whilst maintaining highly directional and monochromatic surface emission. The target application of these devices is local oscillators for heterodyne detection, typically for astronomic detection applications [12], where cryogenic operation and hundreds of mW of output power are acceptable performance constraints. Fig. 1 Typical emission spectra of lasers under CW operation. Measurements performed at T ¼ 20 K, with Fourier transform infrared spectrometer (resolution: 0.125 cm 21 ) and DTGS detector Inset: Surface view of ...

THz quantum cascade lasers operating on the radiative modes of a 2D photonic crystal

“…The slope efficiency is ∼74 mW∕A, and the highest recorded WPE is ∼0.13%. These output powers and WPEs are one order of magnitude higher than typically obtained from standard PhC THz lasers operating on nonradiative modes [10]. An estimate of the outcoupling efficiency defined as Q vertical ∕Q total corroborates this observation, with Q vertical of the nonradiative modes being typically a few thousand, while radiative modes exhibit Q vertical of ∼50-100.…”

“…The goal has been to improve the shape and quality of the strongly divergent emission [5,6] and the power efficiency of the devices by using the confinement and/or the dispersion properties of periodically patterned (in 1D or 2D) metal-semiconductor-metal structures. To date, a coherent single-lobed emission with reduced divergence has been obtained in edgeemitting devices, using third-order distributed feedback (DFB) gratings [7,8], and in surface-emitting devices, using 2D PhCs [9,10] and second-order DFBs [11,12]. However, efficient power extraction and, hence, wallplug efficiency (WPE) from DFB-or PhC-patterned QCLs has been elusive.…”

Photonic Engineering Technology for the Development of Terahertz Quantum Cascade Lasers

“…Nonetheless, the quasi-doughnut-shape www.advopticalmat.de or double-lobe farfield profiles are not satisfying due to the intrinsic structure symmetry and many other perturbation effects such as wire bonding, [123] contact-connecting bridge, and structure anisotropic. As shown in Figure 7i, low beam divergence of ≈10° × 10° has been achieved with a remarkable directivity of ≈20 dB, [67] which is defined as the beam brightness contrast to the background. [91] This was realized by connecting one edge of each laser with a curved waveguide coupler.…”

“…At the first attempt in THz, PhCs were used as an external reflection mirror with a narrow reflectivity band for single-mode operation of an FP cavity laser. By elaborately engineering the PhCs such as introducing absorption boundary and grading the air hole sizes to enhance the loss contrast of modes, single-mode operation was achieved with good emission characteristics [65][66][67][68] which will be elaborated in the following sections. For the former, the holes are etched into the top metallic contact layer [62,63] as shown in Figure 3a.…”

“…[121,122] In these proposals, robust single-mode operation and reduced beam divergence were reported. [63,[65][66][67] Indeed, the single-mode laser devices working at approximately second-order Bragg scattering and a series of single-lobed farfield patterns have been reported using the airhole-type PhCs with a π phase shift in the structure center. [121,122] An alternative solution is to phase-lock an array of surface-emission THz QCLs.…”