Tunable broadband THz spectroscopy based on pulse compression in gas-filled hollow-core photonic crystal fibers

The field of terahertz (THz) spectroscopy has grown very rapidly over the last few years. Since THz spans over an ideal energy range to interact with various elementary excitations including phonons, electrons and molecular vibrations, it can be used as a powerful characterization tool in diverse scientific and industrial sectors such as materials physics, semiconductors technologies, chemistry, medicine, environment, and security [1].

To enable some of these applications, THz generation and detection schemes are continuously developed to achieve broader spectral tunability and higher detection sensitivity. One promising THz generation technique relies on optical rectification, i.e. nonlinear difference-frequency mixing between spectral components of a near-infrared (NIR) ultrashort pulse, to produce a phase-locked THz transient. This technique is usually combined with electro-optic sampling detection that allows a second NIR pulse to monitor the full oscillating electric field of the THz wave. This configuration enables time-resolved THz spectroscopy experiments which can be used to extract the complex dielectric function of various materials and to investigate ultrafast microscopic dynamics [2, 3].

Capabilities and limitations of time-resolved THz spectroscopy are typically dictated by the choice of laser source which set the spectral bandwidth and the time duration of NIR pulses. Recently, an extra-cavity pulse shaping scheme based on NIR pulse spectral broadening in a solid-core fiber and temporal pulse compression with chirped mirrors have been implemented to realize a new type of high-power THz source centered in the mid-infrared region [4]. This technique displays great potential for spectroscopy in the high-THz frequency window (∼30 THz), but lacks spectral tunability to notably resolve efficiently low-energy excitations located in the more traditional 1 to 10 THz window.

In this project, we propose to replace the extra-cavity pulse shaping module by a single gas-filled hollow-core photonic crystal fiber (HC-PCF). We will rely on pressure adjustable guiding properties of HC-PCFs and a changeable pulse duration set by the laser system to control the spectral bandwidth of transform-limited NIR pulses [5, 6]. Optical rectification of these pulses will enable an efficient scheme for THz generation and detection which can then be selectively tuned across the full THz range for sensitive spectroscopic applications. Our design also paves the way for low-cost time-resolved THz sources relying on fewer optical components.

References:

[1] M. Tonouchi, Nat. Photon. 1, 97 (2007).
[2] J.-M. Ménard et al., Nat. Commun. 5, 4648 (2014).
[3] M. Porer et al. Nat. Mater. 13, 857 (2014).
[4] I. Pupeza et al. Nat. Photon. 9, 721 (2015).
[5] P. St. J. Russell et al. Nat. Photon. 8, 278 (2014).
[6] K. F. Mak et al. Opt. Lett. 38, 3592 (2013).

Project leaders:

Jean-Michel Ménard (uOttawa), jean-michel.menard@uottawa.ca
Philip St.J. Russell (MPL), philip.russell@mpl.mpg.de