Rapid and precise read-out of terahertz sensor by high-speed asynchronous optical sampling

Introduction: Frequency selective surfaces (FSS) are based on metallic resonator arrays with tailored transmittance and reflectivity properties [I]. Recently, it has been proposed in a numerical study to use THzFSS based on asymmetric split ring resonators as a sensor for detecting biomolecular sample films with a thickness of only 10 nm [2]. Because large biomolecules, e.g. DNA, exhibit a multitude of inherent vibrational modes, terahertz radiation is ideal to excite and probe these modes and to detect DNA by its binding state specific terahertz properties [3]. Applying substances to an FSS changes the resonant response of the sensor array which can be monitored in transmittance or reflectivity [4-6]. In the case of asymmetric split ring resonators there is an extraordinarily strong field enhancement in the gap region of the ring resonator [7]. Compared to classic free space radiation analysis [8, 9] this field localisation leads to a much higher sensitivity of THz-FSS based sensing reducing the required amount of sample material significantly. For diagnostic applications, however, up to 100 arrayed gene sensors need to be read out in a reasonably short time. Up to now, sensor read-out has been done with rather slow operating conventional terahertz time-domain spectrometers based on mechanical delay stages [10]. Here we present investigations on terahertz-FSS resonant frequency read-out performed with a terahertz time-domain spectrometer based on high-speed asynchronous optical sampling (ASOPS).