THz Anisotropy Identification using Tunable Compact Narrow Band THz Sources

Abstract

We demonstrate THz anisotropy signature determination of a protein crystal model using newly developed compact tunable narrow band THz sources for turn-key spectroscopic systems for the bio molecular community.

I. INTRODUCTION AND BACKGROUND

Spectroscopic signatures for a variety of pharmaceuticals, biomolecules and energetic materials are in the THz range [1, 2]. Simple, cost effective and compact THz systems are needed for broad application of THz to product quality control, security inspection and biomolecular characterization. Recently Kozlov and coworkers proposed compact tunable THz sources based on optical rectification in Orientation Patterned Gallium Phosphide (OP-GAP). THz pulses are generated using quasi phase matched crystals, where the sign of the non-linearity is reversed after each coherence length. In this paper we demonstrate the viability of this spectroscopic approach through anisotropic spectroscopic signature detection of molecular crystals.

Anisotropic THz spectroscopy is an efficient technique to assess the strength as well as directionality of intermolecular vibrations in macromolecules. In this the absorption of plane polarized THz light by an aligned molecular system is measured as a function of the sample orientation with respect to the polarization direction of the source. The changes in the detected anisotropy provides unique opportunities in exploring the dynamics of biomolecules, which are typically hidden from simple absorption measurements which are sensitive only to vibrational energy distributions.

Molecular crystals provide an excellent environmentally insensitive model system for developing Terahertz Microscopy methods for protein and RNA characterization. Protein anisotropic spectroscopy has recently been found to reveal changes in structural dynamics with inhibitor binding and mutation and could provide critical information for development of new therapeutic strategies [2]. Here by extending the anisotropic spectroscopy to include OP-GAP sources, we demonstrate the possibility of implementing compact and rapid THz characterization for

II. EXPERIMENTAL SETUP

OP-GaP crystals were produced at BAE systems following the methods described in the references [3–5]. The crystals were fabricated by hydride vapor phase epitaxy on quasi-phase matched templates grown by molecular beam epitaxy (MBE). The period of the domain determines the central frequency. Sources were fabricated for THz frequencies from 1.0 THz to 4 THz at 0.5THz increments.

THz pulses were generated by optical rectification in the OP-GaP crystals by pumbing with a high power fiber laser (Fianium, Femtopower, 8W, 180 fs, 80 MHz, 1064 nm).[6] The propagating THz and optical beams were isolated using an off axis parabolic mirror with a hole at the center which focused the THz beam at 90 degrees and passed the pump beam. Low pass paper filters were used to further prevent any stray generation beam from getting to the detector.

An FTIR (Bruker Vertex 70) with a helium cooled bolometer (Infrared laboratories) as detector was used to characterize the frequency of the THz beams. The collimated beams from the parabolic mirror were coupled to an external port of the FTIR. Figure 1a) shows the normalized spectra of the generating crystals recorded by the FTIR. The structure in each individual spectrum is due to atmospheric water vapor absorption.

Figure 1.

Normalized spectrum of the THz sources

The experimental setup for sample characterization followed a similar setup as the one described above. Two additional parabolic mirrors were used to create a THz beam focus and collimate it back on to the detector. An optical chopper along with a lock-in amplifier was used to detect the THz beam. Samples were affixed on a circular sample plate with 5 mm diameter central aperture and mounted on a rotation stage (Thorlabs K10CR1).

III. RESULTS AND DISCUSSION

A thin (500 um) polished sucrose crystal of which the THz absorption and anisotropy was previously characterized using our broadband Anisotropic THz Microscopy (ATM) system was used as the sample. Intensity of THz after passing though the sample was measured as a function of sample orientation at 5 degrees’ increments.

Figure 2 shows the change in transmission percentage as a function of sample orientation for three representative THz frequencies. The peak position of the intensity plots as a function of angle is an indication of directionality of the different vibrational modes with respect to the crystal orientation. On the other hand, the height of the peaks is a measure of the anisotropy. A mode with a strong anisotropy is expected to exhibit rapid change with orientation.

Figure 2 :

Anisotropic response of a c cut sucrose crystal.

This preliminary study demonstrates the feasibility of extending the ATM techniques to even narrow band sources. By tuning the periodicity of the generating crystals, we have covered a THz spectrum of up to 4 THz. Optimizing the power of the sources will further enable room temperature rapid THz characterization of molecular crystals.