Tuning a terahertz wire laser

Abstract

Lasers that can be continuously tuned over a broad wavelength range are essential components for sensing and spectroscopy. The frequency of a conventional tunable laser is changed similarly to a musical instrument such as the violin, whose pitch is varied by changing the length (the longitudinal component of wave vector) and the tension (the refractive index) of a string. However, such a method is difficult to implement at terahertz (THz) frequencies, where many important bio-chemical species have distinctive spectral fingerprints, because of the relatively long wavelength λ compared to the cross section w of semiconductor lasers. Continuous frequency tuning using an external-cavity grating has yet to be achieved, and changing the refractive index by temperature produces only small fractional tuning (<1%). Here we demonstrate a novel tuning mechanism that qualitatively differs from all the other methods. This mechanism is based on the unique feature of an unusual device termed “wire laser”, which is defined as a laser whose dimension in the transverse direction w is much smaller than λ. As such, a large fraction of the mode propagates outside the solid core. Placing a movable object close to the wire laser allows direct manipulation of the evanescent laser field, and consequently the lasing frequency. Based on this mechanism, continuous frequency tuning has been unambiguously demonstrated with single-mode operation free from mode hopping. Both red-shift and blue-shift tuning can be achieved from the same device by using either a dielectric or metallic movable object. In combination, a total tuning of ~137 GHz (3.6%) has been demonstrated from a single laser device at ~3.8 THz. Furthermore, these results demonstrate a mechanism for tuning wire lasers at all wavelengths.