The quantum cascade laser (QCL) has been central to the development of terahertz (THz) science and technology in the last decade, thanks to its electronic and optical properties, which can be accurately tailored with quantum and photonic engineering. Control over the spatial and spectral features of THz QCL radiation has been addressed with different solutions, exploiting periodic structures such as edge-emitting 3rd-order distributed feedback lasers, plasmonic lasers, and one dimensional (1D) or two-dimensional (2D) photonic crystals. Novel resonators exploiting aperiodic photonic architectures recently emerged as a versatile approach to engineer the emission of THz QCLs, circumventing the power extraction issues of periodic resonators. Here, we report on the development of 1D and 2D THz QCL resonators providing a combination of high output power, controlled beam shaping and stable continuous wave (CW) operation. We devised a laterally corrugated wire laser with a top extraction hole array. The distinct periodicities of the lateral corrugation and the top hole array allow separate control of light feedback and extraction, optimizing the power output (42 mW in pulsed regime), slope efficiency (250 mW/A) and beam divergence (10°). Stable single-mode emission was achieved with a maximum CW power output of 6 mW. We furthermore demonstrate the first electrically pumped CW random THz lasers reaching a maximum peak power of 21 mW and a CW power of 1.7 mW with multimode emission over a 430 GHz-bandwidth. Using an external coupled cavity, a continuous tuning of 11 GHz and of 20 GHz with modehopping were demonstrated.
Recently, modelocked THz QCLs have been shown to generate 4ps pulses using monolithically integrated Gires-Tournois Interferometer (GTI) dispersion compensation schemes. However, Fourier limit pulse trains were not achieved to date that is vital to realize shorter pulses. Here we show a Fourier-limited pulse train of 3.4ps obtained from an active-modelocked QCL by exactly matching the spectral bandwidth to that of an appropriate GTI. This is despite a spectral bandwidth that is much lower than previous demonstrations.
A QCL based on a multiple stack hybrid active region was used and processed into a metal/metal waveguide. The emission frequency is centred at 3THz with a free-running bandwidth of ~0.1THz. The GTI is fabricated by etching a sub-wavelength air gap through the active region at one end of the QCL ridge. Electromagnetic simulations were performed to optimize the GTI size resulting in a 66.2µm long GTI compensating for the dispersion over a range of 0.3THz around 3.15THz. The pulse measurements are based on coherent sampling of the electric-field using electro-optic detection.
A stable 3.4ps pulse train was obtained by actively modelock the THz QCL with a microwave modulation. Each pulse shows a spectrum with 0.13THz FWHM, exactly at the Fourier-transform limit. In the frequency domain, lasing action occurs only at the off-resonance condition of the GTI as this appoints the dispersion compensated region as the most favourable range for modelocked emission.
In this work we attained the Fourier transform limit permitting the shortest demonstrated stable pulse train from a modelocked THz QCL.
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