NSF-SNSF: THz Frequency combs with Vertical-External-Cavity Surface-Emitting-Lasers

This research addresses the challenge of making terahertz semiconductor laser sources that emit electromagnetic waves with frequencies between 2 and 5 THz (i.e. wavelengths between 60 and 150 microns). More specifically, the goal is to develop new types of high-performance terahertz frequency combs – a type of laser source which emits many colors of light simultaneously, where each “color” has a frequency precisely spaced by a fixed amount from the other frequencies.

This is an international collaboration between the UCLA group, which has expertise in a unique type of external-cavity terahertz quantum-cascade (QC) laser, and a Swiss group at ETH Zürich, which has expertise in terahertz frequency comb physics and measurement. If successful, this research would result in a new terahertz source for applications in the fields of astrophysics, atmospheric science, biological and medical sciences, security screening, illicit material detection, combustion science, antiquities, waste-sorting, next-generation wireless communications, and non-destructive evaluation.

The broader impacts of the project include training graduate and undergraduate students (including international scientific exchange and visits between the two partners), as well as support recruitment and retention of a broad range of students to engineering through participation in a research project course.

The objective of this research is to demonstrate terahertz quantum-cascade (QC) metasurface laser frequency combs (FC), characterize these combs in the frequency and time-domain using novel coherent photonic techniques, and to explore the physics of comb states. This will include frequency-modulated quantum-walk combs as well as amplitude-modulated combs producing ultrafast mode-locked pulses.

In the past several years, the UCLA group has pioneered a novel configuration for terahertz QC-lasers in the vertical-external-cavity surface-emitting-laser architecture (VECSEL). The ETH Zurich group brings expertise in the development, physics, and coherent characterization of waveguide-based THz QC-laser frequency combs. The complementary expertise of both groups will be leveraged in the collaborative development of novel metasurfaces, microfabrication process, and dispersion compensation elements, as well as coherent characterization of the resulting combs for the first time, and exploration of novel frequency comb physics.

The intellectual merit lies in the unique region of parameter space made accessible by the QC-VECSEL for the study of frequency comb states. Specifically, (a) QC-laser gain material generally has a fast picosecond gain recovery time (compared to the cavity round-trip time), (b) the amplifying metasurface does not exhibit the spatial hole burning ordinarily found in a Fabry-Pérot cavity, (c) and the external cavity allows adjustment of the cavity round trip time over a large range – both shorter and longer than the gain recovery time.

Collectively, these features will allow investigation of both frequency modulated and amplitude modulated comb states, including novel comb states such as the quantum-walk comb. Practically, the QC-VECSEL exhibits many desirable features for applications, including large scalable powers, broad gain bandwidths, excellent near-diffraction limited beam patterns, the ability to readily modulate the cavity length (and thus comb tooth spacing) for tuning and stabilization, and the ability to incorporate additional cavity elements for dispersion compensation.

This collaborative U.S.-Swiss project is supported by the U.S. National Science Foundation (NSF) and the Swiss National Science Foundation (SNSF), where NSF funds the U.S. investigator and SNSF funds the partners in Switzerland.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.