Enhanced Transmission at the Zeroth-Order Mode of a Terahertz Fabry-Perot Cavity.

A planar Fabry-Perot cavity with intermirror spacing of d ≪ λ is explored for its "zero-order mode" terahertz transmission. The enhanced transmission observed as d → 0 indicates that such cavities satisfy the resonance conditions across a broad terahertz bandwidth. The experimental signatures from this elusive, "technically challenging" regime are evidenced using time-domain terahertz spectroscopy and are complemented by numerical calculations. The results raise intriguing possibilities for terahertz field modulation and pave new paths for strong coupling of multiple transition frequencies simultaneously.

Enhanced molecular orientation via NIR-delay-THz scheme: Experimental results at room temperature.

Light-induced orientation of gas phase molecules is a long-pursued goal in physics and chemistry. Here, we experimentally demonstrate a six-fold increase in the terahertz-induced orientation of iodomethane (CH3I) molecules at room temperature, provided by rotational pre-excitation with a moderately intense near-IR pulse. The paper highlights the underlying interference of multiple coherent transition pathways within the rotational coherence manifold and is analyzed accordingly. Our experimental and theoretical results provide desirable and practical means for all-optical experiments on oriented molecular ensembles.

Molecular orientation echoes via concerted terahertz and near-IR excitations.

A new and efficient method for orientation echo spectroscopy is presented and realized experimentally. The excitation scheme utilizes concerted rotational excitations by both ultrashort terahertz and near-IR pulses and its all-optical detection is enabled by the molecular orientation-induced second harmonic method [J. Phys. Chem. A126, 3732 (2022)10.1021/acs.jpca.2c03237]. This method provides practical means for orientation echo spectroscopy of gas phase molecules and highlights the intriguing underlying physics of coherent rotational dynamics induced by judiciously-orchestrated interactions with both resonant (terahertz) and nonresonant (NIR) fields.

Molecular Orientation-Induced Second-Harmonic Generation: Deciphering Different Contributions Apart.

We demonstrate and explore an all-optical technique for direct monitoring of the orientation dynamics in gas-phase molecular ensembles. The technique termed "MOISH" utilizes the transiently lifted inversion symmetry of polar gas media and provides a sensitive and spatially localized probing of the second-harmonic generation signal that is directly correlated with the orientation of the gas. Our experimental results reveal selective electronic and nuclear dynamical contributions to the overall nonlinear optical signal and decipher them apart using the "reporter gas" approach. "MOISH" provides new crucial means for implementing advanced coherent rotational control via concerted excitation by both terahertz and optical fields.

Low-Power Laser Ignition of an Antenna-Type Secondary Energetic Copper Complex: Synthesis, Characterization, Evaluation, and Ignition Mechanism Studies.

In recent years, development of new energetic compounds and formulations, suitable for ignition with relatively low-power lasers, is a highly active and competitive field of research. The main goal of these efforts is focused on achieving and providing much safer solutions for various detonator and initiator systems. In this work, we prepared, characterized, and studied thermal and ignition properties of a new laser-ignitable compound, based on the 5,6-bis(ethylnitroamino)-N'2,N'3-dihydroxypyrazine-2,3-bis(carboximidamide) (DS3) proligand. This new energetic proligand was prepared in three steps, starting with 5,6-bis(ethylamino)-pyrazine-2,3-dicarbonitrile. Crystallography studies of the DS3-derived Cu(II) complex (DS4) revealed a unique stacked antenna-type structure of the latter compound. DS4 has an exothermal temperature of 154.5 °C and was calculated to exhibit a velocity of detonation of 6.36 km·s-1 and a detonation pressure of 15.21 GPa. DS4 showed properties of a secondary explosive, having sensitivity to impact, friction, and electrostatic discharge of 8 J, 360 N, and 12 mJ, respectively. In order to study the mechanism of ignition by a laser (using a diode laser, 915 nm), we conducted a set of experiments that enabled us to characterize a photothermal ignition mechanism. Furthermore, we found that a single pulse, with a time duration of 1 ms and with a total energy of 4.6 mJ, was sufficient for achieving a consistent and full ignition of DS4. Dual-pulse experiments, with variable time intervals between the laser pulses, showed that DS4 undergoes ignition via a photothermal mechanism. Finally, calculating the chemical mechanism of the formation of the complex DS4 and modeling its anhydrous and hydrated crystal structures (density functional theory calculations using Gaussian and HASEM software) allowed us to pinpoint a more precise location of water molecules in experimental crystallographic data. These results suggest that DS4 has potential for further development to a higher technology readiness level and for integration into small-size safe detonator systems as for many civil, aerospace, and defense applications.

Iris-assisted terahertz field-induced second-harmonic generation in air.

Terahertz field-induced second-harmonic generation (TFISH) is a technique for optical detection of broadband THz fields. We show that by placing an iris at the interaction volume of the THz and optical fields, the TFISH signal increases by several tenfold in atmospheric air. The iris-assisted TFISH amplification is characterized at varying air pressures and probe intensities and provides an elegant platform for studying nonlinear phase matching in the gas phase.

Differential chopping controller with high-resolution and accuracy using digital signal processor.

This work presents a digital mixer that receives two signals at different frequencies and produces their difference or their sum. Unlike analog circuits that are limited in their ability to separate between close frequencies and require an additional filtering step, we present a digital signal processor-based circuit with excellent frequency separation capabilities. Since software is used to calculate the output function, the obtained output signal is practically unlimited.

Strong coupling of collective intermolecular vibrations in organic materials at terahertz frequencies.

Several years ago, strong coupling between electronic molecular transitions and photonic structures was shown to modify the electronic landscape of the molecules and affect their chemical behavior. Since then, this concept has evolved into a new field known as polaritonic chemistry. An important ingredient in the progress of this field was the demonstration of strong coupling with intra-molecular vibrations, which enabled the modification of processes occurring at the electronic ground-state. Here we demonstrate strong coupling with collective, inter-molecular vibrations occurring in organic materials in the low-terahertz region ([Formula: see text]1012 Hz). Using a cavity filled with α-lactose molecules, we measure the temporal evolution and observe coherent Rabi oscillations, corresponding to a splitting of 68 GHz. These results take strong coupling into a new class of materials and processes, including skeletal polymer motions, protein dynamics, metal organic frameworks and other materials, in which collective, spatially extended degrees of freedom participate in the dynamics.

Generation of spatiotemporally tailored terahertz wavepackets by nonlinear metasurfaces.

The past two decades have witnessed an ever-growing number of emerging applications that utilize terahertz (THz) waves, ranging from advanced biomedical imaging, through novel security applications, fast wireless communications, and new abilities to study and control matter in all of its phases. The development and deployment of these emerging technologies is however held back, due to a substantial lack of simple methods for efficient generation, detection and manipulation of THz waves. Recently it was shown that uniform nonlinear metasurfaces can efficiently generate broadband single-cycle THz pulses. Here we show that judicious engineering of the single-emitters that comprise the metasurface, enables to obtain unprecedented control of the spatiotemporal properties of the emitted THz wavepackets. We specifically demonstrate generation of propagating spatiotemporal quadrupole and few-cycles THz pulses with engineered angular dispersion. Our results place nonlinear metasurfaces as a new promising tool for generating application-tailored THz fields with controlled spatial and temporal characteristics.

The effects of sample conductivity on the efficacy of dynamic nuclear polarization for sensitivity enhancement in solid state NMR spectroscopy.

In recent years dynamic nuclear polarization (DNP) has greatly expanded the range of materials systems that can be studied by solid state NMR spectroscopy. To date, the majority of systems studied by DNP were insulating materials including organic and inorganic solids. However, many technologically-relevant materials used in energy conversion and storage systems are electrically conductive to some extent or are employed as composites containing conductive additives. Such materials introduce challenges in their study by DNP-NMR which include microwave absorption and sample heating that were not thoroughly investigated so far. Here we examine several commercial carbon allotropes, commonly employed as electrodes or conductive additives, and consider their effect on the extent of solvent polarization achieved in DNP from nitroxide biradicals. We then address the effect of sample conductivity systematically by studying a series of carbons with increasing electrical conductivity prepared via glucose carbonization. THz spectroscopy measurements are used to determine the extent of µw absorption. Our results show that while the DNP performance significantly drops in samples containing the highly conductive carbons, sufficient signal enhancement can still be achieved with some compromise on conductivity. Furthermore, we show that the deleterious effect of conductive additives on DNP enhancements can be partially overcome through pulse-DNP experiments.

Contact-free conductivity probing of metal nanowire films using THz reflection spectroscopy.

We utilize time-domain Terahertz (THz) reflectivity measurements for characterizing the surface conductivity of Polyethylene-terephthalate coated with nanowire (NW) films to form novel transparent electrodes (TE). We find good correspondence between the film conductivity and the THz-field reflectivity that provide uniquely desirable means for non-destructive, contactless conductivity measurements of large area NW-based-TEs. We demonstrate the robustness of THz reflectivity measurements to deviations invoked on NW film composition and film uniformity. The dependence of THz reflectivity on area NW coverage follows an anisotropic effective medium model for the dielectric constant.

Echo Spectroscopy in Multilevel Quantum-Mechanical Rotors.

We study the dynamics of rotational echoes in gas phase molecular ensembles and their dependence on the delay and intensity of the excitation pulses. We explore the unique dynamics of alignment echoes that arise from the multilevel nature of the molecular rotors and impose severe difficulties in utilizing echo responses for rotational spectroscopy. We show experimentally and theoretically that judicious control of both the delay and intensity of the second pulse enables multilevel "rotational echo spectroscopy." The proposed methodology paves the way to rotational spectroscopy in high-density gas samples.

Rotational Echoes: Rephasing of Centrifugal Distortion in Laser-Induced Molecular Alignment.

We study and demonstrate the rephasing property of the echo response in a multilevel rotational system of iodomethane via long time-resolved optical birefringence measurements. The strong centrifugal distortion of iodomethane is utilized as a dephasing mechanism imprinted on the echo signal and is shown to rephase throughout its evolution. The dependence of the echo signal amplitude on the driving pulses' intensities is theoretically and experimentally explored. The analogy to Hahn's spin echoes is discussed, and a quantum-mechanical version of Hahn's track runners is provided for the case of multilevel rotational system.

Coherent Radiative Decay of Molecular Rotations: A Comparative Study of Terahertz-Oriented versus Optically Aligned Molecular Ensembles.

The decay of field-free rotational dynamics is experimentally studied by two complementary methods: laser-induced molecular alignment and terahertz-field-induced molecular orientation. A comparison between the decay rates of different molecular species at various gas pressures reveals that oriented molecular ensembles decay faster than aligned ensembles. The discrepancy in decay rates is attributed to the coherent radiation emitted by the transiently oriented ensembles and is absent from aligned molecules. The experimental results reveal the dramatic contribution of coherent radiative emission to the observed decay of rotational dynamics and underline a general phenomenon expected whenever field-free coherent dipole oscillations are induced.

Nonlinear two-dimensional terahertz photon echo and rotational spectroscopy in the gas phase.

Ultrafast 2D spectroscopy uses correlated multiple light-matter interactions for retrieving dynamic features that may otherwise be hidden under the linear spectrum; its extension to the terahertz regime of the electromagnetic spectrum, where a rich variety of material degrees of freedom reside, remains an experimental challenge. We report a demonstration of ultrafast 2D terahertz spectroscopy of gas-phase molecular rotors at room temperature. Using time-delayed terahertz pulse pairs, we observe photon echoes and other nonlinear signals resulting from molecular dipole orientation induced by multiple terahertz field-dipole interactions. The nonlinear time domain orientation signals are mapped into the frequency domain in 2D rotational spectra that reveal J-state-resolved nonlinear rotational dynamics. The approach enables direct observation of correlated rotational transitions and may reveal rotational coupling and relaxation pathways in the ground electronic and vibrational state.

Rotational Control of Asymmetric Molecules: Dipole- versus Polarizability-Driven Rotational Dynamics.

We experimentally study the optical- and terahertz-induced rotational dynamics of asymmetric molecules in the gas phase. Terahertz and optical fields are identified as two distinct control handles over asymmetric molecules, as they couple to the rotational degrees of freedom via the molecular dipole and polarizability selectively. The distinction between those two rotational handles is highlighted by different types of quantum revivals observed in long-duration (>100 ps) field-free rotational evolution. The experimental results are in excellent agreement with random phase wave function (RPWF) simulations [Phys. Rev. A 91, 063420 (2015)] and provide verification of the RPWF as an efficient method for calculating asymmetric molecular dynamics at ambient temperatures, where exact calculation methods are practically not feasible. Our observations and analysis pave the way for orchestrated excitations by both optical and terahertz fields as complementary rotational handles that enable a plethora of new possibilities in three-dimensional rotational control of asymmetric molecules.

Commensurate two-quantum coherences induced by time-delayed THz fields.

The interaction of carbonyl sulfide dipolar gas molecules with two time-delayed, single-cycle THz pulses is shown both experimentally and theoretically to induce two-quantum rotational coherences that are significantly enhanced with respect to those induced by one THz pulse, depending on the relative delay of the pulses. The underlying phenomenon is quite general in that it can occur even after a single THz pulse if more than one molecular species is present, since the free induction decay emitted by one species (demonstrated here by atmospheric water vapor) can provide the second field interaction for the other.

Molecular orientation and alignment by intense single-cycle THz pulses.

Intense single-cycle THz pulses resonantly interacting with molecular rotations are shown to induce field-free orientation and alignment under ambient conditions. We calculate and measure the degree of both orientation and alignment induced by the THz field in an OCS gas sample, and correlate between the two observables. The data presents the first observation of THz-induced molecular alignment in the gas phase.

A novel experimental method for the local mechanical testing of human coronal dentin.

OBJECTIVES: The small volume of human dentin available for sample preparation and the local variations in its microstructure present a real challenge in the determination of their mechanical properties. The main purpose of the present study was to develop a new procedure for the preparation and mechanical testing of small-scale specimens of biomaterials such as dentin, so as to probe local mechanical properties as a function of microstructure. METHODS: Ultra short laser pulses were used to mill a block of dentin into an array of 16 microm size dentin pillars. These could then be individually tested in compression with an instrumented nanoindenter fitted with a 30 microm wide flat punch. RESULTS: The laser-based pillar preparation procedure proved effective and reliable. Data was produced for the mechanical properties of a first set of dry dentin micro-pillars. SIGNIFICANCE: This novel experimental approach enables the preparation and compression of micron-scale samples with well-defined microstructure. For dentin, this means samples containing a relatively small number of well-defined parallel tubules, with a distinct orientation relative to the applied load. The ability to isolate the separate effects of microstructural parameters on the mechanical properties is of major significance for future substantiation of theoretical models.

Selective alignment of molecular spin isomers.

We experimentally demonstrate field-free, spin-selective alignment of ortho- and para molecular spin isomers at room temperature. By means of two nonresonant, strong, properly delayed femtosecond pulses within a four wave mixing arrangement, we observed selective alignment for homonuclear diatomics composed of spin 1/2 (15N) or spin 1 (14N) atoms. The achieved selective control of the isomers' angular distribution and rotational excitation may find applications to analysis, enrichment, and actual physical separation of molecular spin modifications.