Wideband, high-resolution terahertz spectroscopy by light-induced frequency tuning of quantum-cascade lasers

Abstract

Near-infrared optical excitation enables wideband frequency tuning of terahertz quantum-cascade lasers. In this work, we demonstrate the feasibility of the approach for molecular laser absorption spectroscopy. We present a physical model which explains the observed frequency tuning characteristics by the optical excitation of an electron-hole plasma. Due to an improved excitation configuration as compared to previous work, we observe a single-mode continuous-wave frequency coverage of as much as 40 GHz for a laser at 3.1 THz. This represents, for the same device, a tenfold improvement over the usually employed tuning by current. The method can be readily applied to a large class of devices.

Frequency and power stabilization of a terahertz quantum-cascade laser using near-infrared optical excitation

“…It is an uncoated Fabry-Pérot device, 120 µm wide and 660 µm long, and exhibits single-mode emission at 3.1 THz. The same laser was used to demonstrate molecular spectroscopy by light-induced frequency tuning [25]. The QCL is operated in a mechanical cryocooler (Ricor, model K535), and the temperature is stabilized at 45 K. The exit window of the cooler is made of high-density polyethylene (HDPE).…”

“…Since the fiber is attached to the cold finger as shown in Fig. 1(b), cooler vibrations do not have an impact on the alignment as compared to our microscope technique described in [25]. The DC current for the QCL is provided by the driver QCL1000 LAB (Wavelength Electronics).…”

“…In the case of diode laser tuning, we found that the QCL frequency shift follows a square root behavior with increasing diode laser current above threshold. Details of this method can be found in [25]. In this tuning mode, a frequency coverage of about 8 GHz [cf.…”

“…To explain the anti-correlation between frequency and output power stability in the second case, we need to discuss the impact of the control parameters on the QCL behavior. As discussed in [25], the QCL frequency and the output power depend on three control parameters: the QCL driving current, the heat sink temperature, and the diode laser excitation power. The temperature control is typically too slow for an active stabilization.…”

“…In this case, the transmittance of the modulator was controlled by modifying the graphene conductivity via electrostatic back gating [23]. In the following sections, we describe a method that allows for a simultaneous stabilization of the frequency and output power by taking advantage of the frequency and power regulation by near-infrared (NIR) excitation [24,25] in addition to frequency and power control by the modification of the driving current so that no external THz optical modulator is required.…”