An innovative detection technique for capillary electrophoresis: Localized terahertz emission-time domain spectroscopy.

Terahertz (THz) time-domain spectroscopy (TDS) is a recently emerging analysis method which can provide unique information on molecular vibration and rotation induced by inter/intra-molecular interactions. Although the application of THz-TDS to high-performance microscale separation methods like capillary electrophoresis (CE) has been anticipated, it has been hindered due to the diffraction limit of THz wave (typically, hundreds µm). In order to realize CE-THz-TDS, in this study, we placed a narrow open-tubular capillary on the surface of a GaAs semiconductor substrate as a "localized" THz-emitter. By focusing femtosecond pulsed laser beams at the surface of a gallium arsenide (GaAs) substrate closest to the capillary, THz waves were locally generated to pass through the capillary, so that THz absorbance spectra were obtained from the capillary which has narrower inner diameter than the diffraction limit. As a typical result from acetic acid analysis in the CE-THz-TDS platform, information on the refractive index and extinction coefficient was obtained, which showed non-linear and linear concentration dependence, respectively, similar to conventional THz-TDS using large liquid cells. Finally, CE-THz-TDS analysis of several carboxylic acids was demonstrated. Two acids were successfully separated and detected with THz-TDS, where their electrophoretic mobility values were estimated as close to those obtained with conventional contactless conductivity detection. Our proposed CE-THz-TDS showed the potential for the systematic analysis of inter/intra-molecular weak interactions like hydrogen bonds, which are unable to obtain with conventional detectors.

Transition from Diffusive to Superdiffusive Transport in Carbon Nanotube Networks via Nematic Order Control.

The one-dimensional confinement of quasiparticles in individual carbon nanotubes (CNTs) leads to extremely anisotropic electronic and optical properties. In a macroscopic ensemble of randomly oriented CNTs, this anisotropy disappears together with other properties that make them attractive for certain device applications. The question however remains if not only anisotropy but also other types of behaviors are suppressed by disorder. Here, we compare the dynamics of quasiparticles under strong electric fields in aligned and random CNT networks using a combination of terahertz emission and photocurrent experiments and out-of-equilibrium numerical simulations. We find that the degree of alignment strongly influences the excited quasiparticles' dynamics, rerouting the thermalization pathways. This is, in particular, evidenced in the high-energy, high-momentum electronic population (probed through the formation of low energy excitons via exciton impact ionization) and the transport regime evolving from diffusive to superdiffusive.

Terahertz Spectroscopy Tracks Proteolysis by a Joint Analysis of Absorptance and Debye Model.

Terahertz waves have attracted great attention in biomolecule research because of the fact that they cover the range of energy levels of weak interactions, skeleton vibrations, and dipole rotations during inter- and intramolecular interactions in biomacromolecules. In this study, we validated the feasibility of employing terahertz time-domain spectroscopy (THz-TDS) for the nondestructive and label-free monitoring of protein digestion. The acid protease, pepsin, was used at its optimal pH to hydrolyze bovine serum albumin. Correspondingly, the control group experiment was also conducted by adjusting the pH value to inactivate pepsin. The progress of these two experiments was tracked by a compact commercial THz-TDS for 1 h. On one hand, the reaction-time-dependent absorption coefficient was calculated, and a direct absorption coefficient analysis was completed. The results indicate that protein hydrolysis can be easily monitored over time by focusing on the variation tendency of the absorption coefficient from a macroscopic perspective. On the other hand, we explored the use of the Debye model to analyze the dielectric properties of the solution during protein hydrolysis. The results of the Debye analysis prove that it is possible to investigate in detail the microscopic dynamics of biomacromolecule solutions at the molecular level by THz-TDS. Our research examined the process of protein hydrolysis by a combination of absorption spectra and Debye analysis and demonstrated that terahertz spectroscopy is a powerful technology for the investigation of biomolecular reactions, with potential applications in variety of fields.

Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication.

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.

Distributed source model for the full-wave electromagnetic simulation of nonlinear terahertz generation.

The process of terahertz generation through optical rectification in a nonlinear crystal is modeled using discretized equivalent current sources. The equivalent terahertz sources are distributed in the active volume and computed based on a separately modeled near-infrared pump beam. This approach can be used to define an appropriate excitation for full-wave electromagnetic numerical simulations of the generated terahertz radiation. This enables predictive modeling of the near-field interactions of the terahertz beam with micro-structured samples, e.g. in a near-field time-resolved microscopy system. The distributed source model is described in detail, and an implementation in a particular full-wave simulation tool is presented. The numerical results are then validated through a series of measurements on square apertures. The general principle can be applied to other nonlinear processes with possible implementation in any full-wave numerical electromagnetic solver.

Scanning laser terahertz near-field imaging system.

We have proposed and developed a scanning laser terahertz (THz) near-field imaging system using a 1.56 µm femtosecond fiber laser for high spatial resolution and high-speed measurement. To obtain the two-dimensional (2D) THz images of samples, the laser pulses are scanned over a 2D THz emitter plate [DASC: 4'-dimenthylamino-N-methyl-4- stilbazolium p-chlorobenzenesulfonate] by a galvano meter. In this system, THz wave pulses locally generated at the laser irradiation spots transmit through the sample set on the emitter, and the amplitude of the transmitted THz wave pulse is detected by using a typical THz time-domain spectroscopy (THz-TDS) technique. Using this system, we have succeeded in obtaining THz transmission images of a triangle shaped metal sheet of millimeter-size and a human hair sample with a spatial resolution of sub-wavelength order up to ~27 µm (~λTHz/28) at an imaging speed of about 47 seconds/image for 512 x 512 pixels.

Sub-diffraction thin-film sensing with planar terahertz metamaterials.

Planar metamaterials consisting of subwavelength resonators have been recently proposed for thin dielectric film sensing in the terahertz frequency range. Although the thickness of the dielectric film can be very small compared with the wavelength, the required area of sensed material is still determined by the diffraction-limited spot size of the terahertz beam excitation. In this article, terahertz near-field sensing is utilized to reduce the spot size. By positioning the metamaterial sensing platform close to the sub-diffraction terahertz source, the number of excited resonators, and hence minimal film area, are significantly reduced. As an additional advantage, a reduction in the number of excited resonators decreases the inter-cell coupling strength, and consequently the resonance Q factor is remarkably increased. The experimental results show that the resonance Q factor is improved by more than a factor of two compared to the far-field measurement. Moreover, for a film with a thickness of λ/375 the minimal area can be as small as 0.2λ × 0.2λ. The success of this work provides a platform for future metamaterial-based sensors for biomolecular detection.