Jean-Louis Coutaz

A reliable method for extraction of material parameters in terahertz time-domain spectroscopy

This paper introduces a novel method that allows fast and reliable extraction of material parameters in terahertz time-domain spectroscopy. This method could be applied for most materials and requires neither simplifying assumptions nor samples of different thickness for the extraction. The presented extraction procedure operates either on truncated terahertz signals when temporal windowing is possible, or on full ones otherwise. Some experimental examples covering all practical cases are given. In particular, the extraction procedure treats the tedious case of samples for which internal reflections of the terahertz pulse slightly overlap.

Highly precise determination of optical constants and sample thickness in terahertz time-domain spectroscopy

Time-domain spectroscopy allows fast and broadband measurement of the optical constants of materials in the terahertz domain. We present a method that improves the determination of the optical constants through simultaneous determination of the sample thickness. This method could be applied to any material with moderate absorption and requires only two measurements of the temporal profile of the terahertz pulses: a reference one without the sample and one transmitted through the sample.

Influence of noise on the characterization of materials by terahertz time-domain spectroscopy

We analyze the contributions of various error sources to uncertainty in the far-infrared optical constants (refractive index and absorption coefficient) measured by terahertz (THz) time-domain spectroscopy. We focus our study on the influence of noise. This noise study is made with a thick slab of transparent material for which the THz transmitted signal exhibits temporal echoes owing to reflections in the sample. Extracting data from each of these time-windowed echoes allows us to characterize the noise sources. In THz time-domain spectroscopy experiments in which photoswitches are used as antennae, the transmitting antenna constitutes the principal noise source. The uncertainty in the far-infrared optical constants can be strongly reduced when the extraction is performed with THz echoes that have encountered many reflections in the sample.

Optical rectification at metal surfaces

We report on experimental observation of freely propagating pulses of terahertz (THz) radiation produced by optical rectification of femtosecond pulses at metal surfaces. This opens a qualitatively new way of investigation of nonlinear phenomena at metal surfaces and can be also exploited in the future for development of new THz emitters.

Electro-optic sensors for electric field measurements. II. Choice of the crystals and complete optimization of their orientation

We present a simple method to determine both direction of the electric field component to which an electro-optic sensor is sensitive and sensitivity to this component. For this purpose we introduce the concept of an electro-optic sensitivity vector. This work is done for electro-optic sensors working as amplitude modulators, as phase modulators, or as polarization state modulators. The method applies to any anisotropic crystal, regardless of crystallographic group. After defining three kinds of electro-optic sensor geometry that allow longitudinal, transversal or fully vectorial electric field probing, we make a comparative study of several commonly used electro-optic crystals. We derive the optimal orientations of the crystals with respect to the optical probe beam that lead to high-performance electro-optic sensors. We also treat isotropic crystals in the case of polarization state modulators. In contrast to anisotropic crystals that are sensitive to a unique electric field component, we show that isotropic crystals are sensitive to two orthogonal electric field components simultaneously.

Electro-optic sensors for electric field measurements. I. Theoretical comparison among different modulation techniques

We present a complete analysis of electro-optic sensors for electric field measurement based on three different modulation techniques: amplitude, phase, and polarization state modulation. We treat the most general case, considering both isotropic and anisotropic crystals and taking into account the absorption of the crystal. We derive the optimal configuration of experimental setups for the three studied modulation techniques, and we give the values of the variable physical parameters required to yield the best performance. Finally we compare the three modulation techniques and show that phase or polarization state modulations result in exactly the same performance while amplitude modulation gives slightly enhanced performance.

Study of terahertz radiation generated by optical rectification on thin gold films

Emission of terahertz (THz) radiation as a result of optical rectification of intense femtosecond laser pulses on thin gold films has been studied by time-domain THz spectroscopy. The THz amplitude was measured as a function of film thickness and incidence angle. The experiments reveal that the emitted THz field is suppressed for a thickness below 100 nm, which gives evidence of the nonlocal character of the response. The variation of incidence angle allows us to estimate the components of susceptibility tensor chi2ijk. For thicker films and near grazing incidence, the emitted THz field attains a peak value of 4 kV/cm.

Broadband THz Uncooled Antenna-Coupled Microbolometer Array—Electromagnetic Design, Simulations and Measurements

Bolometer sensors are good candidates for THz imaging thanks to their maturity and capability to sense THz waves on the whole spectrum. Starting with infrared microbolometer technology, uncooled antenna-coupled microbolometer focal plane array are being developed at CEA-LETI with the objective of offering low-cost, real-time 2D terahertz imaging sensors. One of the major challenges is studying the optical coupling mechanism of the detector in THz frequency range. In this paper, we present results of the electromagnetic design and characterization process of these focal plane arrays, concentrating on the spectral absorption .

Imaging of broadband terahertz beams using an array of antenna-coupled microbolometers operating at room temperature

We present results of 2D real-time imaging of terahertz (THz) beam generated by a photoconductive antenna driven by a femtosecond oscillator. The detector, operating at room temperature, is a 320 x 240 array of antenna-coupled microbolometers with integrated CMOS read-out electronics delivering 25 images per second. High quality images of broadband THz beams covering the 0.1-2 THz range are recorded while maintaining a signal-to-noise ratio of 10 for detected THz power as low as 25 nW. The compactness of the easy-to-use uncooled camera makes it very useful for the alignment of systems such as THz time-domain spectrometers and for the characterization of emitters, optics and other components.

Surface enhanced Raman scattering on silver grating: Optimized antennalike gain of the stokes signal of 104

Using a silver grating of variable groove depth overcoated by a thin film of copper phthalocyanine, we measured an increase of the Raman efficiency of 104 as compared to the flat case (copper phthalocyanine on a flat silver surface). This effect is due to the resonant excitation of delocalized surface plasmons. The value of 104 corresponds to an optimum groove depth of the grating which acts as a high gain directive antenna when surface plasmons resonance is achieved at the Stokes frequency.