We introduce a novel method for ultrafast selective multispectral terahertz (THz) spectroscopy, combining broadband THz pulses, Frequency Selective Surfaces (FSS), and a Schottky diode energy sensor. Traditional THz spectroscopy is costly time-consuming and expert-operated. Our system answers to these challenges by not requiring to obtaining the time trace of the electric field of the THz signal thus essentially simplifying the system. Our system efficiently identifies samples by analyzing distinct spectral signatures. Experimental results demonstrate the method's ability to distinguish samples with similar THz absorption coefficients and refractive indices, even without clear fingerprint features. Validation on paper samples with closely matched THz properties confirmed successful differentiation through data averaging and normalization. We also applied k-fold cross-validation with a neural network for multi-class classification, achieving a training accuracy of 94.5% and an average testing accuracy of 94%. This approach offers robust real-time spectroscopic identification and potential for industrial applications and predictive modelling of THz signals.
Non-linear optical phenomena, such as parametric detection and amplification, manifest themselves in materials such as lithium niobate (LN) under the influence of a powerful optical pump beam. These processes have facilitated the practical realization of femtosecond (fs) pulse sources in the visible (VIS) and near infrared (NIR) spectra. They are also central to quantum detection, promising extremely sensitive detection of low-energy photons, particularly in the terahertz (THz) frequency range. To explore this innovative detection approach, we used an intense and powerful THz source taking advantage of optical rectification in lithium niobate (LN) crystals with an inclined-pulse front-end pumping configuration. By taking advantage of the high brightness of this source, we can acquire NIR signals in real time by upconversion and broadband using a standard CCD camera. In this presentation, we will look at the technical intricacies of the source and detection methodologies, as well as our goal of achieving single THz photon detection capability in the near future, all in the context of using ytterbium lasers.
Photoconductive antennas are at the forefront of THz source technology, and the Large-Aperture Photoconductive Antenna (LAPCA) can generate intense THz pulses with peak fields surpassing 100 kV/cm. Despite the unique properties of these generated THz pulses—such as high THz field asymmetry, low central frequency around 100 GHz, and a significant ponderomotive potential—the widespread adoption of LAPCAs has been hindered by limitations in peak intensity and their fragility. In this paper, we discuss recent advancements in wide bandgap semiconductor LAPCAs featuring an interdigitated structure, facilitating the shaping of intense THz pulses with various waveforms, ranging from asymmetric quasi-half-cycle to symmetric single-cycle pulses and allowing for tunable polarization. Additionally, we explore the nonlinear interaction of these pulses with an n-doped InGaAs thin film, where we report, for the first time, high-frequency generation.
Using a ZnSe large aperture photoconductive antenna with a specific electrode structure, we demonstrate the generation of intense, sub-cycle terahertz (THz) pulses with variable elliptical polarization and peak fields above 80 kV/cm. The electrode structure is composed of six units, where three units have interdigitated horizontal electrodes, and the other three units have an interdigitated structure with vertical electrodes. The units with horizontal and vertical electrodes are positioned alternatively onto the antenna, allowing the generation of two quasi-half-cycle THz pulses with orthogonal polarization. A time delay between the two THz pulses is introduced by a phase-delay mask covering only the parts of the antenna with horizontal electrodes. By changing the mask thicknesses, we can control on demand, the polarization state of the THz pulses from linear polarization to elliptical to circular polarization over a quarter of a THz cycle.
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