Spectroscopic Study of Terahertz Generation in Mid-Infrared Quantum Cascade Lasers

Abstract

Terahertz quantum cascade laser sources based on intra-cavity difference-frequency generation are currently the only room-temperature mass-producible diode-laser-like emitters of coherent 1–6 THz radiation. Device performance has improved dramatically over the past few years to reach milliwatt-level power output and broad tuning from 1.2 to 5.9 THz, all at room-temperature. Terahertz output in these sources originates from intersubband optical nonlinearity in the laser active region. Here we report the first comprehensive spectroscopic study of the optical nonlinearity and investigate its dependence on the mid-infrared pump frequencies. Our work shows that the terahertz generation efficiency can vary by a factor of 2 or greater depending on the spectral position of the mid-infrared pumps for a fixed THz difference-frequency. We have also measured for the first time the linewidth for transitions between the lower quantum cascade laser states, which is critical for determining terahertz nonlinearity and predicting optical loss in quantum cascade laser waveguides.

Dispersion dynamics of quantum cascade lasers

“…Quantum cascade lasers (QCLs) are a semiconductor source of coherent radiation in the mid-infrared and terahertz (THz) [1], and their high powers and large nonlinearities have led to several interesting devices exploiting nonlinear effects. For example, impressive results have been obtained in generating Thz radiation from mid-infrared structures via differencefrequency generation [2][3][4], active mode locking has been achieved [5][6][7], and as of late there has been an explosion in research in using QCLs to generate frequency combs [8][9][10][11][12]. Mid-infrared difference-frequency generation can be used to generate continuous wave THz radiation without the need for cryocoolers, and frequency combs can be used to perform coherent broadband spectroscopy [13][14][15] or to perform broadband tomography [16].…”

“…Whereas the gain spectrum is made apparent by the lasing and electroluminescence spectra, dispersion characterization requires various types of broadband interferometry. In the mid-infrared, this can be done by highresolution spontaneous emission measurements [21,22] or broadband transmission measurements [2,23]; in the THz, the weak spontaneous emission necessitates the use of terahertz timedomain spectroscopy (THz-TDS) [24][25][26][27]. Though one can of course simulate the intersubband-induced dispersion via density-matrix-based approaches, the results are lacking when compared with experiment for the same reason that intersubband gain is often difficult to predict (e.g., uncertainty in the growth parameters, uncertainty in the simulation parameters).…”

Double-metal waveguide terahertz difference-frequency generation quantum cascade lasers with surface grating outcouplers

“…2(b), we expect a linear increase in the THz power output from our lasers with the waveguide length. To enable single-frequency THz emission, the lasers were placed into a dual-grating external cavity setup 36 to select the frequencies of the mid-IR pumps. The back facets of the laser bars were coated with a two-layer anti-reflection coating (YF 3 and ZnSe) to suppress mid-IR feedback from the cleaved facets.…”

“…The values of both THz power output and the mid-IR-to-THz conversion efficiency are a factor of 2 higher than that of the best-performing THz DFG-QCLs for operation around 2 THz. [36][37][38] Figure 4(d) shows the far-field profiles of the THz emission in the vertical direction as shown schematically in the figure inset. The profiles were obtained by scanning the bolometer with an approximately 1 cm aperture at a 10 cm distance from the laser facet as shown in the inset of Fig.…”

“…Our devices provide the highest peak THz power output for THz DFG-QCL sources operating below 2 THz reported to date. [36][37][38] Similar grating outcoupling schemes can be implemented in high-performance buried-heterostructure THz DFG-QCLs grown on heavily doped substrates that are expected to be able to operate continuous-wave at roomtemperature in the epi-side-up configuration, based on the reported performance of regular mid-IR QCLs.…”