Abstract
We report on the generation of frequency combs from the recently-discovered phenomenon of high-order sideband generation (HSG). A near-band gap continuous-wave (cw) laser with frequency fNIR was transmitted through an epitaxial layer containing GaAs/AlGaAs quantum wells that were driven by quasi-cw in-plane electric fields FTHz between 4 and 50 kV/cm oscillating at frequencies fTHz between 240 and 640 GHz. Frequency combs with teeth at fsideband = fNIR + nfTHz (n even) were produced, with maximum reported n > 120, corresponding to a maximum comb span > 80 THz. Comb spectra with the identical product fTHz × FTHz were found to have similar spans and shapes in most cases, as expected from the picture of HSG as a scattering-limited electron-hole recollision phenomenon. The HSG combs were used to measure the frequency and linewidth of our THz source as a demonstration of potential applications.
Highlights
In recent years, optical frequency combs (OFCs) have been used for a wide variety of applications [1, 2] such as calibration of astronomical spectrograms [3], high-resolution metrology [4], and optical communications [5]
High-order sideband generation provides a mechanism for generating optical frequency combs with bandwidths exceeding 80 THz with continuous control of the offset frequency and comb spacing
Our results demonstrate the ability to tune the comb offset frequency throughout the comb spacing by varying the NIR laser frequency near the band gap of the GaAs quantum wells (QWs) used
Summary
Optical frequency combs (OFCs) have been used for a wide variety of applications [1, 2] such as calibration of astronomical spectrograms [3], high-resolution metrology [4], and optical communications [5]. The spacing of microcomb teeth is determined by the dimensions of the micro-resonator This tooth spacing can exceed 1 THz, but is tunable only to the limited extent that the dimensions of the solid microresonator can be changed [10]. The spacing of the teeth in such OFCs is determined by the radio-frequency (RF) (typically at 10s of GHz) used to drive the modulator. As in combs generated with EOMs, the center frequency and tooth spacing of combs based on HSG are both continuously and widely tunable. The tooth spacings associated with combs based on HSG are much larger, ranging from of 100s of GHz to more than 1 THz. In HSG, a weak near-infrared (NIR) laser tuned near resonance with the exciton energy in a semiconductor is incident on a sample that is driven by a strong THz field. The electric fields required for HSG are achievable over small areas with modern sub-THz sources and careful microwave engineering, which would enable the widespread deployment of OFCs based on HSG [13]
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