Abstract—A new simple time-domain THz spectrometer based on a single-shot balanced detection of the electric field is presented. It allows for the recording of the full temporal THz profile at kHz acquisition rates. Its performances are demonstrated by measuring the fast drying process of acetone on Teflon. I. INTRODUCTION AND BACKGROUND HE acquisition rate of Terahertz (THz) measurements can be considered as a decisive factor for various industrial applications. In this context, time domain techniques have been realized that avoid the mechanical delay line of the classical scanning and thus reduce significantly the acquisition time. Asynchronous and electronically controlled optical sampling with two lasers constitutes one of these alternative routes, single shot techniques based on electro-optic detection the second [1-4]. While both approaches achieve a similar acquisition rate, single-shot methods might be preferred when the system evolves on the time-scale of the acquisition rate. This is particularly the case in industrial surrounding where the distance stability between THz source, sample and THz detector is hard to be kept on the micrometer level which is necessary for the femtosecond precision in the time domain. Recently we presented single shot detection of THz fields with linear response over a large amplitude and frequency range [4]. In a first application, the electric field co- propagating with relativistic, ultrashort electron bunches was measured. The high sensitivity of the diagnostic allowed for the first time the shot to shot characterization of picosecond electron pulses via this electric field [5]. Here we apply the technique to detect THz light pulses in a single shot method. This new THz time domain spectrometer is used to monitor the fast drying process of acetone sprayed on bulk Teflon in real time. II. RESULTS As in standard THz time domain configurations, both optical probe and THz pulse are generated by the same ultrashort laser, here a Ti:Sapphire amplifier system delivering laser pulses at 780 nm with a duration of about 100 fs (figure 1). Large area optical rectification in a ZnTe crystal is used for the generation of the THz pulse [6]. After transmitting the sample the THz beam is guided in free space to the detection unit that is described in detail in [4]. Briefly, the temporally dispersed spectrum of a supercontinuum is used as time axis. By adapting the amount of dispersive material like glass in the optical path of the supercontinuum, the temporal detection window can easily be regulated without changing the beam direction. The temporal distribution of the THz field is encoded by the electro-optic effect to the supercontinuum. The wavelength dependent polarization state of the broadband probe is then analyzed for its two perpendicular contributions in polychromatic balanced detection. Besides the linear response, the detection of all probe photons enables normalization and guarantees so high stability against laser fluctuations. Ti:Sa, 100 Hz 780 nm - 100fs EO λ/4 continuum generation pulse stretcher P PBS optical spectrometer THz generation sample multichannel detector Figure 1: Scheme of the single shot THz time domain spectrometer. Polychromatic balanced detection unit for the temporally stretched supercontinuum probe with polarizer P, electro-optic detection crystal EO, quarter wave plate λ/4 and set of polarizing beam splitters PBS. To demonstrate the performance of the single shot spectrometer we have studied a dynamic process in an established application field of THz radiation. THz time domain spectroscopy is applied for scientific investigations of liquids [7,8]. Moreover, several industrial applications have been proposed and detailed as the measurement of water content for the paper or food production [9,10]. Here, based on the identical measurement principle but with the increased acquisition rate of the single shot technology, the drying process of acetone is monitored, as water a polar liquid but in contrast quickly evaporating. A dry piece of Teflon (PTFE) of 2 mm thickness was positioned vertically in the THz beam. When the acetone is sprayed onto the Teflon, the THz pulse is delayed and strongly attenuated. During the drying process the transmitted electric field recovers in amplitude and shifts back to its initial temporal position. This process was recorded at an acquisition rate of 100 Hz limited by the laser repetition rate. The inset of Figure 2 displays the temporal distribution of the THz electric field passing through the Teflon for selected times after the spraying. Thanks to the step width of 10 ms of the measurement, the temporal evolution of the electric field T U. Schmidhammer, V. De Waele, P. Jeunesse Laboratoire de Chimie Physique - ELYSE, UMR8000 CNRS-Université Paris Sud 91405 Orsay, France A New Single Shot Terahertz Time Domain Spectrometer for Rapid Product and Process Control