Terahertz Sources Based on Metrological‐Grade Frequency Combs

Abstract

Broadband metrological-grade frequency comb (FC) synthesizers with a rich number of phase locked modes are the ideal sources for quantum sensing and quantum metrology. At terahertz (THz) frequencies, electrically pumped quantum cascade lasers (QCLs) have shown quantum-limited frequency noise operation, phase/frequency absolute referencing and self-starting FC operation, albeit over a rather restricted dynamic range, governed by the nature of the quantum gain media that entangles group velocity dispersion at the different bias points. Here, a technological approach is conceived to achieve FC operation over the entire available gain bandwidth at THz frequencies. The intracavity light intensity of a multistack QCL, inherently showing a giant Kerr nonlinearity, is altered by increasing the mirror losses of its Fabry-Perot cavity through coating the back facet with an epitaxially-grown multilayer graphene film. This enables a frequency modulated THz FC showing a proliferation of emitted modes over the entire gain bandwidth and across more than 60% of its operational range, with ≈0.18 mW per mode optical power. The QCL FC is then experimentally characterized to assess its phase coherence, reconstructing its intensity emission profile, instantaneous frequency, and electric field, thus proving its metrological nature.

Multi-Domain Modeling of Free-Running Harmonic Frequency Comb Formation in Terahertz Quantum Cascade Lasers

“…6(c). In recent years, the spectral and time domain properties of free-running THz QCLs have been extensively investigated both theoretically [42], [44], [46], [87], [88] and experimentally [8], [9], [86], [89], [90]. Selfstarting OFC emission in THz QCLs is characterized by a simultaneous frequency-and amplitude-modulated signal.…”

“…Nonlinear four-wave-mixing (FWM) processes induce efficient phase locking of the cavity modes, provided that the cavity exhibits a sufficiently low group velocity dispersion. The huge third-order nonlinearity χ (3) present in QCL gain media allows for extremely broadband OFC emission, offering the potential of full octave spanning spectra and beyond [9], [14], [15].…”

Terahertz Plasmon Polaritons in Large Area Bi2Se3 Topological Insulators

“…[54,55] This detection mechanism allows the nearfield signal back-scattered from the s-SNOM to be detected as a modulation of the voltage drop on the QCL and is inherently dependent on the phase of the back-scattered field (see Figure 3a and Experimental Section). To create the near-field hologram, the relative phase between the emitted and re-injected fields is controlled with an optical delay line, which varies the optical path length L. In these experiments, we have employed a set of different lasers: two THz-QCL frequency combs (FCs), characterized by equally spaced modes ranging from ≈2.85 to 3.45 THz (see Figure 3b, comb I [56] and Figure S1, Supporting Information, comb II), and a sequence of single mode THz-QCLs emitting at 2.0 THz, 3.0 THz, 4.3 THz, and 4.6 THz, respectively. THz QCL FCs combine multimode emission with high spectral purity and phase coherence between the modes, which is crucial to observe a multimode self-mixing signal, as recently demonstrated both theoretically and experimentally.…”

Multi-Domain Modeling of Free-Running Harmonic Frequency Comb Formation in Terahertz Quantum Cascade Lasers

“…Nonlinear four-wave-mixing (FWM) processes induce efficient phase locking of the cavity modes, provided that the cavity exhibits a sufficiently low group velocity dispersion. The huge third-order nonlinearity χ (3) present in QCL gain media allows for extremely broadband OFC emission, offering the potential of full octave spanning spectra and beyond [9], [14], [15]. Recently, OFC emission in a novel QCL operating regime was detected, featuring harmonic modes separated by multiples of the cavity roundtrip (rt) frequency.…”