No abstract

The gain recovery time of a bound-to-continuum terahertz frequency quantum cascade laser, operating at 1.98 THz, has been measured using broadband terahertz-pump-terahertz-probe spectroscopy. The recovery time is found to reduce as a function of current density, attaining a value of 18 ps as the laser is brought close to threshold. We attribute this reduction to improved coupling efficiency between the injector state and the upper lasing level as the active region aligns.

The effects of optical feedback (OF) in lasers have been observed since the early days of laser development. While OF can result in undesirable and unpredictable operation in laser systems, it can also cause measurable perturbations to the operating parameters, which can be harnessed for metrological purposes. In this work we exploit this ‘self-mixing’ effect to infer the emission spectrum of a semiconductor laser using a laser-feedback interferometer, in which the terminal voltage of the laser is used to coherently sample the reinjected field. We demonstrate this approach using a terahertz frequency quantum cascade laser operating in both single- and multiple-longitudinal mode regimes, and are able to resolve spectral features not reliably resolved using traditional Fourier transform spectroscopy. We also investigate quantitatively the frequency perturbation of individual laser modes under OF, and find excellent agreement with predictions of the excess phase equation central to the theory of lasers under OF.

In this work, we present a density-matrix model, which considers an infinite quantum cascade laser (QCL) and models transport via a nearest neighbor approximation. We will discuss derivation of output parameters of the model in detail and show the direct mathematical link to the semiclassical rate equation approach. This model can be extended to an arbitrary number of states in the QCL period, without a priori specification of upper and lower lasing level. Application of the model to various QCL structures is possible, including bound-to-continuum structures, which typically employ a large number of states per period. The model has been applied to a 2-THz bound-to-continuum QCL, and a very good agreement with measured V-I characteristics is obtained along with qualitative agreement with measured L-I characteristics in terms of dynamic range.

Two-dimensional spectroscopy is performed on a terahertz (THz) frequency quantum cascade laser (QCL) with two broadband THz pulses. Gain switching is used to amplify the first THz pulse and the second THz pulse is used to probe the system. Fourier transforms are taken with respect to the delay time between the two THz pulses and the sampling time of the THz probe pulse. The two-dimensional spectrum consists of three peaks at (ω = 0, ω = ω), (ω = ω, ω = ω), and (ω = 2ω, ω = ω) where ω denotes the lasing frequency. The peak at ω = 0 represents the response of the probe to the zero-frequency (rectified) component of the instantaneous intensity and can be used to measure the gain recovery.