High-Q resonant cavities for terahertz quantum cascade lasers.

A Campa,L Consolino,D Mazzotti,P De Natale,M Ravaro,S Bartalini,M S Vitiello

doi:10.1364/oe.23.003751

Abstract

We report on the realization and characterization of two different designs for resonant THz cavities, based on wire-grid polarizers as input/output couplers, and injected by a continuous-wave quantum cascade laser (QCL) emitting at 2.55 THz. A comparison between the measured resonators parameters and the expected theoretical values is reported. With achieved quality factor Q ≈ 2.5 × 10(5), these cavities show resonant peaks as narrow as few MHz, comparable with the typical Doppler linewidth of THz molecular transitions and slightly broader than the free-running QCL emission spectrum. The effects of the optical feedback from one cavity to the QCL are examined by using the other cavity as a frequency reference.

Highlights

Optical resonators are well-established tools commonly used in spectroscopy [1, 2]
We report on the realization and characterization of two different designs for resonant THz cavities, based on wire-grid polarizers as input/output couplers, and injected by a continuous-wave quantum cascade laser (QCL) emitting at 2.55 THz
We report on the set up of wire-grid polarizer (WGP)-based THz resonators coupled with radiation from a 2.55-THz QCL

Summary

Introduction

Optical resonators are well-established tools commonly used in spectroscopy [1, 2] They have widespread applications over the whole electromagnetic spectrum, from microwaves [3] to UV [4], while record-level optical finesse were achieved in the visible/near-IR [5], especially thanks to the advent of whispering gallery mode resonators [6]. Many chemical species have very strong rotational and rovibrational transitions in the THz range, with line-strengths much stronger than typical microwave transitions, and comparable with the strongest fundamental ro-vibrational lines lying in the mid IR For this reason, Terahertz spectrum can well represent a novel “molecular fingerprint region”, once provided that sensitivity and resolution of newly developed THz spectroscopic techniques are improved to the levels reached in other spectral regions

Methods

Discussion

Conclusion