Disordered materials such as glasses exhibit complex structural dynamics that are challenging to probe with conventional spectroscopies. We demonstrate that terahertz-driven four-wave mixing (FWM) at glass surfaces provides direct access to low-frequency vibrational modes and structural evolution in amorphous solids. Applied to a compositional series of PbO-silicate glasses (20-54 mol

We propose a simple method to generate a frequency-multiplexed terahertz multiple vortex beam using a spiral phase plate and a metallic mask. Using a broadband terahertz source, we experimentally demonstrate the conversion of a terahertz Gaussian beam into a frequency-multiplexed single or multiple vortex beam with topological charges ranging from 1 to 3, which is supported by simulations. This multifunctional device opens new possibilities for high-speed THz communication, information processing, and high-efficiency terahertz wavefront manipulation devices.

Hollow core circular waveguides are promising candidates for guiding terahertz waves. In this study, we numerically and experimentally investigate the coupling parameters under fixed coupling conditions for different waveguide diameters. Terahertz time-domain spectroscopy is employed for experimental of study mode propagation. Multimode behavior is observed above 0.3 THz, whereas waveguides with larger diameters exhibit reduced number of interference peaks and low single-mode loss. Efficient propagation of terahertz radiation in these waveguides may find applications in telecommunication and spectroscopy.

We recorded the hyper-Raman spectra resulting from the interaction of a near-infrared (near-IR) picosecond pulse and a terahertz (THz) ultrashort pulse at the surface of a (111) silicon sample. A simple model is proposed to analyze the evolution of the hyper-Raman spectra vs the time delay between the near-IR and THz pulses. It links the hyper-Raman spectra to the multi-phonon absorption in silicon. This approach makes it possible to demonstrate that, during carrier generation by the near-IR pulse, the two-phonon and three-phonon absorption bands are enhanced in modes involving optical phonons. This process results from the very rapid and strong population of the optical phonons induced by the photo-generated hot carriers. It occurs over a few hundreds of femtoseconds and lasts throughout the duration of the near-IR pulse.

We investigated the far-field terahertz beam profile generated from an air plasma induced by two-color femtosecond laser pulses. Under our experimental conditions (filament length shorter than the dephasing length between the two-color pulses), using electro-optic sampling in both ZnTe (0.2–2.2 THz) and GaP (0.4–6.8 THz) crystals, and ultra-broadband ABCD technique (1–17.5 THz), we determined that the THz beam exhibits a unimodal beam pattern below 4 THz and a conical one above 6 THz. This experimental finding is consistent with theoretical studies based on the unidirectional pulse propagation equation, which predict the transition of THz emission from a flat-top profile to a conical one due to the destructive interference of THz waves emitted from the plasma filament. Our results also underscore the importance of accounting for experimental artifacts, such as photo-excited losses in silicon resulting in on-axis THz absorption along with the influence of drilled mirrors, in characterizing the complex spatial and frequency-dependent behavior of two-color plasma-induced terahertz emission.

Pulsed terahertz (THz) electric fields enable various coherent THz imaging modes, such as reflection tomography, spectral imaging, and computed tomography (CT) for nondestructive inspection, quality control, and material characterization. The extension of coherent THz imaging modes to moving objects has been regarded as key to their social implementation. This Tutorial focuses on two-dimensional spatiotemporal (2D-ST) THz imaging of objects moving in one direction with constant speed as a promising means of enabling real-time coherent THz imaging. In 2D-ST THz imaging, the temporal waveform and line image of the THz pulse are simultaneously acquired without the need for mechanical scanning of the time delay and sample position using a combination of non-collinear 2D free-space electro-optic sampling with THz line-imaging optics. This 2D-ST THz imaging boosts the imaging rates of THz reflection tomography, THz spectral imaging, and THz CT to levels that are applicable to moving objects. The advanced THz reflection tomography and THz spectral imaging that result from the assistance of 2D-ST THz imaging achieve real-time line imaging of cross sections and spectral signatures, respectively. Subsequently, this enables in-line total inspection of objects moving on a translation stage or a conveyor belt. A THz CT system using real-time line projection of a THz beam is effectively applied to a 2D spectral cross section of a continuously rotating object. 2D-ST THz imaging enables the functional THz imaging of moving objects in various practical applications.

We measured the dielectric constants of alumina ceramic samples with non-destructive THz time-domain spectroscopy. Using an effective medium approximation, we demonstrated that the relative permittivity can be used to evaluate the porous alumina samples, with porosity fractions ranging from 0% to 20% per volume. This result can be used to control sample porosity, a key material parameter related to mechanical strength under compression load, hardness and wear resistance, or thermal stability.

The evolution of the THz reflectivity of a high resistivity silicon wafer upon excitation by a femtosecond pulse centered at 800 nm is studied. Upon optical excitation, the THz reflectivity of silicon is increased within less than 1 ps and it can be tailored even beyond 90 % over a broad THz frequency range. Such variation is well accounted for by the pump pulse induced carriers within a thin layer on the surface of silicon.

We demonstrate that the far-field terahertz beam generated from a Ti:Sapphire two-color laser-induced filament can exhibit a conical or a Gaussian distribution depending on the filtering experimental conditions. Using a 2D electro-optic detection, we provide the first clear evidence that the conical emission in the 0.2-2.6 THz spectral range is due to photo-induced carriers in the silicon filter typically used to block the remaining pump laser light. The low-frequency terahertz beam retrieves an almost Gaussian spatial distribution when the silicon filter is preceded by a large bandgap ceramic filter, which stops the pump beam, preventing carriers generation in the silicon filter.

We report on the conversion of an infrared vector beam into terahertz vortex beams using a < 110 >-cut ZnTe cubic crystal. First, we provide a theoretical analysis demonstrating how an infrared vector beam with the azimuthal order l can be transformed into a terahertz beam endowed with an orbital angular moment content that consists of optical vortices with topological charge +/- 2l. Experimentally, quasi-monochromatic terahertz vortex beams with topological charges +2 and -2 are produced and characterized both in amplitude and phase using real-time two-dimensional imaging of the terahertz electric field. These results enrich the terahertz vortex beam toolbox via the transfer of topological information from the infrared to terahertz domains. (C) 2018 Optical Society of America

We report on the conversion of an infrared vector beam into THz vortex beams using a ZnTe cubic crystal. We provide a theoretical analysis demonstrating how an infrared vector beam with the azimuthal order ℓ can be transformed into a THz beam endowed with an orbital angular moment content that consists of optical vortices with topological charge ±2ℓ. Experimentally, quasi-monochromatic THz vortex beams with topological charges +2 and -2 are produced and characterized both in amplitude and phase using real-time two-dimensional imaging of the terahertz electric field.

Digital holography (DH) is a technique to reconstruct the amplitude and phase images of a sample by calculating the wavefront propagation from the interference image. Although DH enables three-dimensional shape measurement based on the phase images, axial dynamic range of a single-optical-wavelength DH is limited to less than a full or half optical wavelength due to phase wrapping ambiguity. To extend the axial range over the optical wavelength, synthesized wavelength DH has been proposed. In this method, DH is performed at two different wavelengths, and then synthesized wavelengths between them are used. However, use of a single longer synthesized wavelength degrades the axial resolution because the axial dynamic range is limited by the phase noise. To extend the axial dynamic range, one has to increase the axial range while maintaining the axial resolution of sub-wavelength. One promising approach to do it is cascade linking between multiple synthetic wavelengths with different orders. In this paper, we present multicascadelinked synthetic wavelength DH using an optical-comb-referenced frequency synthesizer (OFS). OFS is a tunable external cavity laser diode phase-locked to an optical frequency comb, and is effectively used for multiple synthetic wavelengths within the range of 32 um to 1.20 m. A multiple cascade link of the phase images among an optical wavelength and 5 different synthetic wavelengths enables the shape measurement of a reflective millimeter-sized stepped surface with the axial resolution of 34 nm.

We demonstrate that terahertz generation in a two-color laser filament can exhibit conical or Gaussian far-field distribution depending on the experimental conditions. By measuring the spatial distribution of the THz electric field in the 0-3 THz spectral range, we have shown that the conical emission is likely due to photo-induced carriers in the silicon filter used to block the remaining pump laser pulses. However, the terahertz generation can retrieve a Gaussian spatial distribution by replacing the silicon filter by a ceramic one in which the photo-excitation of charge carrier does not occur.

We report on the wavefront correction of a terahertz (THz) beam using adaptive optics, which requires both a wavefront sensor that is able to sense the optical aberrations, as well as a wavefront corrector. The wavefront sensor relies on a direct 2D electro-optic imaging system composed of a ZnTe crystal and a CMOS camera. By measuring the phase variation of the THz electric field in the crystal, we were able to minimize the geometrical aberrations of the beam, thanks to the action of a deformable mirror. This phase control will open the route to THz adaptive optics in order to optimize the THz beam quality for both practical and fundamental applications.

Digital holography (DH) is a promising method for non-contact surface topography because the reconstructed phase image can visualize the nanometer unevenness in a sample. However, the axial range of this method is limited to the range of the optical wavelength due to the phase wrapping ambiguity. Although the use of two different wavelengths of light and the resulting synthetic wavelength, i.e., synthetic wavelength DH, can expand the axial range up to a few tens of microns, this method is still insufficient for practical applications. In this article, a tunable external cavity laser diode phase-locked to an optical frequency comb, namely, an optical-comb-referenced frequency synthesizer, is effectively used for multiple synthetic wavelengths within the range of 32 um to 1.20 m. A multiple cascade link of the phase images among an optical wavelength (= 1.520 um) and 5 different synthetic wavelengths (= 32.39 um, 99.98 um, 400.0 um, 1003 um, and 4021 um) enables the shape measurement of a reflective millimeter-sized stepped surface with the axial resolution of 34 nm. The axial dynamic range, defined as the ratio of the maximum axial range (= 0.60 m) to the axial resolution (= 34 nm), achieves 1.7*10^8, which is much larger than that of previous synthetic wavelength DH. Such a wide axial dynamic range capability will further expand the application field of DH for large objects with meter dimensions.

In this chapter we give a brief overview of neutron based analytical investigations applied to study archaeological ceramics, and different types of stones. Since the vast majority of archaeological objects are made of ceramics and various stones-all are of geological origin-, one of the key objectives of these studies to determine the origin of raw material. This research is called provenance research, and a wide range of neutron based methods are applicable in it. Following a very basic, user-oriented description of the methods, we introduce examples from our everyday practice. The examples are about provenance of prehistoric stone tools, about the sources of 4th-3rd c. B.C. millennium limestone idols found in the South of Portugal, as well as about the characterization of 15th-16th c. A.D. Inka pottery. A very unique application of combined neutron techniques was aimed to determine the inner content of an Eighteenth Dynasty Egyptian sealed vessel. In addition, investigations of samples from different epochs and characterization of marbles are presented.

We report on the development of a terahertz wavefront sensor able to determine the optical aberrations of a terahertz time-domain spectrometer. The system measures point-by-point the amplitude and phase of the terahertz electric field in a given plane. From this measurement, we reconstruct the terahertz wavefront and calculate its Zernike coefficients. In particular, we especially show that the focus spot of the spectrometer suffers from optical aberrations such as remaining defocus, first and second order astigmatisms, as well as spherical aberration. This opens a route to wavefront correction for improved terahertz imaging and spectroscopy.

We propose a topological beam-shaping strategy of terahertz beams using geometric phase elements made of space-variant birefringent slabs. Quasi-monochromatic terahertz vortex beams are produced and characterized both in amplitude and phase from the reconstructed real-time two-dimensional imaging of the electric field. Nonseparable superpositions of such vortex beams are also obtained and characterized by two-dimensional polarimetric analysis. These results emphasize the versatility of the spin-orbit electromagnetic toolbox to prepare on-demand structured light endowed with polarization-controlled orbital angular momentum content in the terahertz domain, which should find many uses in future terahertz technologies.