Dmitry S. Bulgarevich

Microscopic solvent structure of subcritical and supercritical methanol from ultraviolet/visible absorption and fluorescence spectroscopies

Ultraviolet/visible absorption and fluorescence spectroscopies at different temperatures and pressures were applied to investigate the microscopic solvent structures of subcritical and supercritical methanol using 4-nitroanisole, ethyl-(4-dimethylamino)benzoate, Reichardt’s dye, and anthracene as the probe molecules. It was found that at temperatures higher than 150 °C the long winding chains of sequentially hydrogen-bonded methanol molecules were probably broken, but the small hydrogen-bonded aggregates possibly existed in methanol even at higher temperature. It was also found that the solvation process of the anthracene molecule in the S0-ground state obeyed the Langmuir adsorption model. However, in the case of fluorescence measurements in supercritical methanol, we detected deviations from the simple Langmuir adsorption model. These deviations were explained in terms of preferential solvation of the solvent molecules around photoexcited anthracene. Judging from the experimental results, it was conclud...

Microscopic solvent structure of supercritical carbon dioxide and its mixtures with methanol in the cybotactic region of the solute molecule

UV/VIS absorption spectroscopy was used to estimate the microscopic solvent polarities of supercritical carbon dioxide alone and its mixtures with methanol. It was found that the local composition of methanol in the cybotactic region of 4-nitroanisole was larger than the corresponding bulk concentration. With increasing pressure the methanol molecules were pressed out of the solvation shell of 4-nitroanisole by the carbon dioxide molecules. From the solubility measurements the solvation numbers of the carbon dioxide molecules around 4-nitroanisole were calculated as a function of fluid density.

Highly-efficient aperture array terahertz band-pass filtering

Microfabricated narrow band-pass filters were developed for terahertz spectral region. Their features were over 100% extraordinary optical transmission and invariance for the strength and peak positions on filters illumination side, rotation angle, and incident light polarization.

A matter of symmetry: terahertz polarization detection properties of a multi-contact photoconductive antenna evaluated by a response matrix analysis

While terahertz time domain spectroscopy (THz-TDS) is a well-established technique, polarization sensitive measurements are challenging due to the need of broadband polarization devices. Here, we characterize our recently introduced multi-contact photoconductive detector antenna with a response matrix analysis. We show that the lead lines attached to electrodes reduce the antenna symmetry and thereby influence the properties of the response matrices. With a wire grid polarizer, we simulate a sample influencing the polarization angle and the intensity of the incident THz pulse. Evaluating the measurements with the response matrix analysis, our results show a well agreement of the adjusted and measured polarization angles and intensities over a frequency range from 0.25 to 0.8 THz.

A polarization-sensitive 4-contact detector for terahertz time-domain spectroscopy

A light polarization angle-sensitive photoconductive detector for terahertz time-domain spectroscopy is computer-modeled, microfabricated, and tested. The experimental results show good agreement with the linear angular response for an ideal detector. The detectors frequency, angular, and crosstalk responses are discussed in the context of theoretical and experimental considerations.

Linear Dichroism Detection and Analysis in Terahertz Spectral Range

Two simple experimental techniques of quantitative linear dichroism analysis were developed with Jones calculus for polarization-sensitive terahertz time-domain spectroscopy. Both of them involve the optical measurements at only three angles of rotation for sample or polarizer setup. Such non-destructive evaluation techniques could be a useful addition for quality control of polymer or composite materials. As an example, the production-induced linear dichroism in polytetrafluoroethylene was detected, analyzed, and interpreted as a result of the well-aligned macromolecular arrangement within optical volume probed by terahertz beam.

Gigahertz time-domain spectroscopy and imaging for non-destructive materials research and evaluation.

By using optical sampling with repetition frequency modulation of pump/probe laser pulses on photoconductive emitter/detector antennas, the high-speed time/frequency domain gigahertz imaging is reported due to the absence of opto-mechanical delay line in this optical scheme. The clear contrast for a 3-cm wide metal plate, which was placed behind a 5-cm thick concrete block, was observed with a 1 × 1 mm image pixilation. On average, it took only ~0.75 s per pixel/waveform acquisition/assignment with a 675 ps time-domain window. This could become a valuable non-destructive evaluation technique in gigahertz spectral range with all benefits of time-domain spectroscopy.

Polarization-variable emitter for terahertz time-domain spectroscopy

We report on the progress in the development of linear polarization-variable multi-electrode emitters for terahertz time-domain spectroscopy. The results on its microfabrication, the finite element method modeling of appropriate bias distribution between electrodes, the finite-difference time-domain simulated spectral output, and actual experimental testing are presented. The rotation of the emitted terahertz field with linear polarization on an angle multiple of 45° can be achieved by synchronized bias and single polarizer rotations.

Pattern recognition with machine learning on optical microscopy images of typical metallurgical microstructures

For advanced materials characterization, a novel and extremely effective approach of pattern recognition in optical microscopic images of steels is demonstrated. It is based on fast Random Forest statistical algorithm of machine learning for reliable and automated segmentation of typical steel microstructures. Their percentage and location areas excellently agreed between machine learning and manual examination results. The accurate microstructure pattern recognition/segmentation technique in combination with other suitable mathematical methods of image processing and analysis can help to handle the large volumes of image data in a short time for quality control and for the quest of new steels with desirable properties.

External magnetic field distribution mapping using terahertz emission from indium antimonide

We demonstrate and report the feasibility of utilizing terahertz (THz) surface emission from semiconductors as a mapping tool for magnetic field distribution. Using a standard THz time-domain spectroscopy setup, the THz emission of indium antimonide (InSb) was systematically measured at several different points of an external magnetic field. The initial results suggest promising directions in developing a practical THz emission-based magnetic field mapping technique for non-destructive electromagnetic imaging applications.