Continuous-wave terahertz quantum cascade microlaser arrays operating on various bound states in the continuum.

We report on the continuous-wave (CW) operation of 1D terahertz quantum cascade (THz QC) microlaser arrays working on various bound states in the continuum (BICs). We first created a quasi-BIC state by breaking the inversion symmetry of the microlaser array, which enables flexible control of the radiation efficiency. The optimized multi-periods array exhibits single-mode emission with the maximum output power of 21 mW (at 30 K), and the maximum operation temperature (Tcw) of 45 K. To further increase Tcw, we created a hybrid-BIC state by hybridizing a quasi-BIC generated in a few-periods array and a high-Q surface plasmon polariton mode formed in an unbiased array. The hybridization minimizes the pumping area with no obvious degradation of the threshold current density. The reduced pumping area, together with the discrete distribution of microlasers, significantly decreases the device thermal resistance. Such scheme improves the Tcw up to 79 K with a low beam divergence of 17°×17°, and the output power remains 3.4 mW at 20 K. Our work provides a novel solution to control the output power, the operating temperature, and the beam quality of THz QC lasers in CW mode.

Modeling and improving the output power of terahertz master-oscillator power-amplifier quantum cascade lasers.

A model based on carrier rate equations is proposed to evaluate the gain saturation and predict the dependence of the output power of a terahertz master-oscillator power-amplifier quantum cascade laser (THz-MOPA-QCL) on the material and structure parameters. The model reveals the design rules of the preamplifier and the power extractor to maximize the output power and the wall-plug efficiency. The correction of the model is verified by its agreement with the experiment results. The optimized MOPA devices exhibit single-mode emission at ∼ 2.6 THz with a side mode suppression ratio of 23 dB, a pulsed output power of 153 mW, a wall-plug efficiency of 0.22%, and a low divergence angle of ∼6°×16°, all measured at an operation temperature of 77 K. The model developed here is helpful for the design of MOPA devices and semiconductor optical amplifiers, in which the active region is based on intersubband transitions.

Sideband generation of coupled-cavity terahertz semiconductor lasers under active radio frequency modulation.

The radio frequency (RF) modulation is a powerful tool, which is used for generating sidebands in semiconductor lasers for active mode-locking. The two-section coupled-cavity laser geometry shows advantages over traditional Fabry-Pérot cavities in the RF modulation efficiency, because of its reduced device capacitance of short section cavity. Further, it has been widely used for active/passive mode-locking of semiconductor diode lasers. For semiconductor-based quantum cascade lasers (QCLs) emitting in the far-infrared or terahertz frequency bands, the two-section coupled-cavity configuration can strongly prevent the laser from multimode emissions. This is because of its strong mode selection (loss modulation), which the cavity geometry introduces. Here, we experimentally demonstrate that the coupled-cavity terahertz QCL can be actively modulated to generate sidebands. The RF modulation is efficient at the frequency that equals the difference frequency between the fundamental and higher order transverse modes of the laser, and its harmonics. We show for the first time that, when the laser is modulated at the second harmonic of the difference frequency, the sideband generation in coupled-cavity terahertz QCLs and the generated sidebands are equally spaced by the injected microwave frequency. Our results, which are presented here, provide a novel approach for modulating terahertz coupled-cavity lasers for active mode-locking. The coupled-cavity geometry shows advantages in generating dense modes with short cavities for potential high-resolution spectroscopy. Furthermore, the short coupled-cavity laser consumes less electrical power than Fabry-Pérot lasers that generate a similar mode spacing.

Terahertz master-oscillator power-amplifier quantum cascade laser with a grating coupler of extremely low reflectivity.

A terahertz master-oscillation power-amplifier quantum cascade laser (THz-MOPA-QCL) is demonstrated where a grating coupler is employed to efficiently extract the THz radiation. By maximizing the group velocity and eliminating the scattering of THz wave in the grating coupler, the residue reflectivity is reduced down to the order of 10-3. A buried DFB grating and a tapered preamplifier are proposed to improve the seed power and to reduce the gain saturation, respectively. The THz-MOPA-QCL exhibits single-mode emission, a single-lobed beam with a narrow divergence angle of 18° × 16°, and a pulsed output power of 136 mW at 20 K, which is 36 times that of a second-order DFB laser from the same material.