The generation and detection of terahertz and mid-infrared spectral components are observed through the second-order nonlinear processes of optical rectification, difference frequency generation, and electro-optic sampling. A broad assortment of highly-nonlinear crystals are investigated, including ZnGeP2, CdSiP2, AgGaSe2, BaGa4Se7, ZnTe, and/or GaSe. The generated and detected terahertz and mid-infrared electric fields exhibit durations ranging from <1 ps to more than tens of picoseconds, encompassing an assortment of spectral components (<1 THz to >40 THz). Additionally, the conducted measurements show both phase-matched and phase-mismatched emission and sensing. This work provides a notable collection of experimental measurements whereby second-order nonlinear effects (i.e. optical rectification, difference frequency generation, and electro-optic sampling) are used to generate and detect spectral content spanning across spectral ranges (i.e. terahertz to the mid-infrared).

We present the generation of a ‘Flying Doughnut’ terahertz pulse, exhibiting strong longitudinal magnetic field isolated from electric field. In gallium arsenide under the excitation of bichromatic azimuthally polarized cylindrical vectors pulses, we generate the transient ring currents which emit single-cycle ‘Flying Doughnut’ terahertz pulse. We measure the spatiotemporal map of the radiated electric field, which enables us to calculate space-time structure of the associated magnetic field.

We present a semiconductor-based high-field terahertz (THz) source with a spectral peak at 2.6 THz. A peak field of 303 kV/cm is produced through optical rectification of a 0.57 mJ near-infrared (NIR) generation beam. A surface-etched grating allows a tilted-pulse-front phase matching geometry, optimizing THz generation >2 THz. This configuration has the potential to yield a THz peak field around 1 MV/cm when the ultrafast NIR laser delivers 5 mJ pulses. Our scheme enables the exploration of coherent control and nonlinear effects in the region between 2 THz and 4 THz.

We present terahertz time-domain spectroscopy measurements of titanium nitride (TiN) as a function of temperature in the superconducting state. By employing a maximum-likelihood analysis method developed previously by our group, we demonstrate that the Mattis-Bardeen theory of disordered superconductivity can describe the complex frequency-dependent conductivity σ '(f) at each temperature with only one free parameter, the temperature-dependent energy gap Δ('). A conventional analysis of the same data yields comparable values of Δ(') at each temperature but without reliable estimates of the uncertainty bounds or goodness of fit. Our results provide a quantitative test of Mattis-Bardeen theory and demonstrate the advantages of our maximum-likelihood analysis framework. This study contributes towards understanding the superconducting behaviour of TiN and establishes a foundation for future work.

We demonstrate a high-sensitivity room-temperature detection scheme for terahertz (THz) radiation based on parametric frequency upconversion. The upconverted photons generated by the mixing of a THz pulse with a near-infrared (NIR) pulse in a (110)-oriented GaP crystal are spectrally resolved using a monochromator and a commercial single-photon detector sensitive in the NIR. We detect THz pulses with energies as low as 1.4 zJ (or about 1.5 photons per pulse) at a frequency of 2 THz when averaged over only 50k pulses.

We show evidence that a systematic error is present in nearly all the reported evidence for photoinduced superconductivity and describe how it may create the appearance of photoinduced superconductivity where none exists. The error results from a saturation nonlinearity that distorts the photoconductivity depth profile at high pump intensities, and by extension distorts the photoconductivity spectrum. Similar errors may emerge in other pump-probe spectroscopy measurements, and we discuss how to correct for them.

Chemistry involves structural dynamics which transform chemical structures from one form to another. However, out of the vast milieu of quantum vibrations that exist in a molecule, it boils down to a few key motions that drive the system across the transition state, and it is the anharmonicity at the transition state that leads to reduced dimensionality. We performed ultrafast spectroscopy to unearth the concept of dimensionality reduction and the effect of anharmonicity in the prototypical ring-closing and spin-crossover reaction.

In the realm of ultrafast physics and femtochemistry, understanding photochemical bond-breaking processes requires a holistic approach. Traditionally, the dynamics of valence electrons and atomic movements were studied separately using distinct techniques [1-3]. However, a comprehensive understanding necessitates their simultaneous observation [4]. In this pursuit, we investigate the laser-induced dissociation dynamics of bromine molecules through a pump-probe experiment employing the Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) technique. Bromine, with its simple diatomic nature and observable spectroscopic transitions, serves as an invaluable model for studying bond-breaking processes. We demonstrate the simultaneous observation of valence electrons and atoms during a photodissociation process by analyzing correlated photoelectron and photoion momentum spectra. Normalized differential (ND) plots of photoelectron momentum distributions enable us to visualize the evolution of molecular orbitals and the separation of atoms in single ionization and Coulomb explosion processes, respectively. Furthermore, we resolve vibrational wave packet dynamics in Br2+ states with a resolution of only a few wavenumbers. Our experimental results are substantiated by a theoretical simulation based on a semiclassical two-step model specifically tailored for molecules. This concerted investigation promises a deeper comprehension of ultrafast molecular transformations, bridging the gap between electronic and structural dynamics in the realm of photochemical reactions.