Chip‐Scale Terahertz Frequency Combs through Integrated Intersubband Polariton Bleaching

Abstract

Quantum cascade lasers (QCLs) represent a fascinating accomplishment of quantum engineering and enable the direct generation of terahertz (THz) frequency radiation from an electrically biased semiconductor heterostructure. Their large spectral bandwidth, high output powers, and quantum‐limited linewidths have facilitated the realization of THz pulses by active mode‐locking and passive generation of optical frequency combs (FCs) through intracavity four‐wave‐mixing, albeit over a restricted operational regime. Here, an integrated architecture is conceived for the generation of high power (5.5–8.0 mW) THz FCs comprising an ultrafast THz polaritonic reflector, exploiting intersubband (ISB) cavity polaritons, and a broad bandwidth (2.3–3.8 THz) heterogenous THz QCL. By tuning the group‐delay‐dispersion in an integrated geometry, through the exploitation of light‐induced bleaching of the ISB‐based THz polaritons, spectral reshaping of the QCL emission and stable FC operation over an operational range up to 38%, characterized by a single and narrow (down to 700 Hz) intermode beatnote are demonstrated. This concept provides design guidelines for a new generation of compact, cost‐effective, electrically driven chip‐scale FC sources based on ultrafast polariton dynamics, paving the way toward the generation of mode‐locked THz microlasers that can strongly impact a broad range of applications in ultrafast sciences, data storage, high‐speed communication, and spectroscopy.

Self‐Induced Phase Locking of Terahertz Frequency Combs in a Phase‐Sensitive Hyperspectral Near‐Field Nanoscope

“…This could be related to the heterogeneous architecture of the active region of this THz QCL FC, as the active regions lasing at this bias produce a single comb with two distinct bands. [ 56 ] The comparison of the FTIR and beatnote shift spectra in logarithmic scale (Figure S4 , Supporting Information), show that, despite the naturally higher noise floor in the beatnote‐shift spectrum, the bandwidth of the two spectra is the same. Our experiments also show that the amplitude of the self‐mixing signal is significantly influenced by the coherence state of the laser, with the highest signal‐to‐noise ratio (SNR) corresponding to having a single powerful beatnote.…”

Terahertz Quantum Cascade Lasers as Enabling Quantum Technology

“…An alternative elegant approach relies in an external cavity configuration comprising an ultrafast THz polaritonic reflector, exploiting intersubband (ISB) cavity polaritons, and a broad bandwidth (2.3–3.8 THz) heterogeneous THz QCL. [ 85 ] The polaritonic mirror comprises a semiconductor multi‐quantum‐well (MQW) heterostructure, embedded in a ≈2 µm size double metal cavity, having a top Au grating with 16 µm period, that fix the optical coupling to the MQW. [ 85 ] A radiation polarized orthogonal to the grating lines, impinging at normal incidence, induces a fringe electric field within the MQW heterostructure that satisfies the ISB transition selection rule.…”

“…This can be unveiled in the intermode beatnote that appears single and narrow (down to 700 Hz). [ 85 ] The proposed concept has a high potential for the development of mode‐locked THz micro‐lasers exploiting saturable absorption in the tightly coupled external cavity. This promises broad application impacts in high‐speed quantum communication and ultrafast science.…”

“…[85] The polaritonic mirror comprises a semiconductor multi-quantum-well (MQW) heterostructure, embedded in a ≈2 μm size double metal cavity, having a top Au grating with 16 μm period, that fix the optical coupling to the MQW. [85] A radiation polarized orthogonal to the grating lines, impinging at normal incidence, induces a fringe electric field within the MQW heterostructure that satisfies the ISB transition selection rule. [86] By pumping the polaritonic mirror with a THz laser, a visible spectral change of the intersubband reflectivity spectrum in the 2-3 THz spectral window [86] at intensities larger than 7.8 W cm −2 is observed, with a progressive bleaching of the upper polaritonic mode.…”

Tailored nano-electronics and photonics with two-dimensional materials at terahertz frequencies

“…The former mechanism leads to a frequency modulated output, while the latter is associated with amplitude modulation. In a THz QCL frequency comb, both frequency and amplitude modulations are typically present [236,237] and act simultaneously. In the engineered coupled cavity architecture (Figure 5b) [238], the gain and absorption are spatially separated, so they do not average out to a local net gain/loss, and create a spatially dependent profile within the cavity.…”