Stationary Sample Anisotropic THz Spectroscopy using Discretely Tunable THz Sources

Abstract

We demonstrate anisotropic THz spectroscopy of sucrose using newly developed compact discretely tunable THz sources for turn-key spectroscopic systems.

I. INTRODUCTION AND BACKGROUND

Simple, cost effective and compact THz systems are needed for THz applications in product quality control, security inspection and biomolecular characterization [1, 2]. Compact tunable THz sources based on optical rectification in Orientation Patterned Gallium Phosphide (OP-GAP) were previously proposed by Kozlov, et al., in which THz pulses are generated using quasi phase matched crystals where the non-linearity flips sign every coherence length. This spectroscopic approach is demonstrated here through anisotropic THz spectroscopy of sucrose crystals that provide an excellent environmentally insensitive model system to develop methods for protein and RNA characterization. One such method was recently shown to measure changes in protein structural dynamics with inhibitor binding and mutation that could provide critical information for the development of new therapeutic strategies [2]. Normalized THz spectra of sucrose previously measured using far-field THz time domain spectroscopy (TDS) [3] is shown in Figure 1a.

Figure 1.

a): Normalized sucrose spectrum using far field THz TDS. b) Power spectra of discrete OP-GaP sources. c) Normalized absorption spectra of sucrose using discrete frequency bands of OP-GaP sources

II. EXPERIMENT AND RESULTS

Here we demonstrate our overall approach, where we use OP-GaP crystals to generate ~ 1 mW terahertz. The output power is sufficiently high to use a room temperature golay cell detector. We first demonstrate that the OP-GaP crystals can be used for anisotropic spectroscopy with rotating the sample. OP-GaP crystals, produced by BAE systems [4–6], are pumped with a high power fiber laser (Fianium, Femtopower, 8 W, 1064 nm) to generate from 1 to 4 THz frequencies in 0.5 THz increments [7], the power spectra of which are shown in Figure 1b. The THz and optical beams are isolated using an off-axis parabolic mirror, that focuses the reflected THz beam at 90°, with a centered hole that disposes of the pump beam.

A 500 μm thin polished sucrose crystal is placed at the focus of the THz beam on a rotation mount. Intensity of the THz beam after passing though the sample is measured as a function of sample orientation in 5° increments using a helium-cooled bolometer. The resulting absorption spectrum of sucrose in each frequency band is shown in Figure 1c for comparison with far-field THz TDS spectra in Figure 1a. By appropriate optimization of the optics, the bolometer can be eliminated as demonstrated by figure 2, showing the same signal to noise with a room temperature golay cell for the 1.25 THz source.

Figure 2.

OPGaP 1.25 THz source transmission through sucrose single crystal showing the anisotropic absorption. The signal to noise using the room temperature Golay cell is comparable to that for the Helium cooled bolometer.

The rotation stage necessary for these measurements is cumbersome and limits rapid sample loading. To eliminate this, we use a frequency tunable quarter waveplate consisting of a wire grid polarizer positioned λ/8 from a high reflectivity mirror. The polarizer is set to 45° from the incident polarization direction, allowing half the incident intensity to pass through and be reflected by the high reflectivity mirror, recombining with the light reflected by the wire grid polarizer with a λ/4 delay. In Figure 3a we show the polarization characterization for this quarter waveplate for λ = 119 μm (2.52 THz) from a molecular gas laser (Edinburgh Instruments). Figure 3b shows the anisotropic transmission for a sucrose crystal measured by leaving the sample stationary, and simply rotating a second wire grid polarizer after the quarter wave plate.

Figure 3.

a) polarization characterization with Golay cell after the frequency tunable λ/4 plate. The output is circularly polarized allowing the anisotropy measurement on single crystal sucrose to be performed with a stationary sample . Data taken with 2.52 THz output from a molecular gas laser.

Next generation OP-GaP sources have been designed and processed with continuous tunability for use with the frequency tunable quarter waveplate to provide cost effective broadband stationary sample anisotropic terahertz spectroscopy for pharmaceutical and biomolecular characterization.