Zeren Li

Fast blood flow monitoring in deep tissues with real-time software correlators

We introduce, validate and demonstrate a new software correlator for high-speed measurement of blood flow in deep tissues based on diffuse correlation spectroscopy (DCS). The software correlator scheme employs standard PC-based data acquisition boards to measure temporal intensity autocorrelation functions continuously at 50 - 100 Hz, the fastest blood flow measurements reported with DCS to date. The data streams, obtained in vivo for typical source-detector separations of 2.5 cm, easily resolve pulsatile heart-beat fluctuations in blood flow which were previously considered to be noise. We employ the device to separate tissue blood flow from tissue absorption/scattering dynamics and thereby show that the origin of the pulsatile DCS signal is primarily flow, and we monitor cerebral autoregulation dynamics in healthy volunteers more accurately than with traditional instrumentation as a result of increased data acquisition rates. Finally, we characterize measurement signal-to-noise ratio and identify count rate and averaging parameters needed for optimal performance.

Identify paraffin-embedded brain glioma using terahertz pulsed spectroscopy

The refractive indices, absorption coefficients and complex dielectric constants spectra of paraffin-embedded brain glioma and normal brain tissues have been measured by a terahertz time domain spectroscopy (THz-TDS) system in the range of 0.2 – 2.0 THz. The spectral differences between glioma and normal brain tissues were obtained. Our results indicate that, compared with normal tissue, glioma had higher refractive index, absorption coefficient, and dielectric constant. Based on these results, the suitable frequency components for different methods of glioma imaging (intensity imaging, coherent imaging and terahertz pulsed imaging) are analyzed.

Near-perfect terahertz wave amplitude modulation enabled by impedance matching in VO2 thin films

We present a terahertz (THz) amplitude modulation method with near perfect E-field amplitude modulation depths that is based on impedance matching in VO2 thin films during the thermally induced insulator-metal transition (IMT). It was observed that the impedance matching-induced THz amplitude modulation was sensitive to the resistance switching characteristics of the VO2 thin films. By designing the VO2 thin films to have four orders of magnitude of change in resistance during the IMT, we experimentally achieved an E-field amplitude modulation depth of 94.5% (intensity modulation depth of 99.7%) between the insulator phase of VO2 and the impedance matching state, and an E-field amplitude modulation depth of 97.6% (intensity modulation depth of 99.94%) between the impedance matching state and the metallic phase of VO2 at 0.5 THz. The experimental results were consistent with the results of simulations based on the transmission matrix model.

Characterizing the hydration state of L-threonine in solution using terahertz time-domain attenuated total reflection spectroscopy

The hydration of biomolecules is closely related to the dynamic process of their functional expression, therefore, characterizing hydration phenomena is a subject of keen interest. However, direct measurements on the global hydration state of biomolecules couldn’t have been acquired using traditional techniques such as thermodynamics, ultrasound, microwave spectroscopy or viscosity, etc. In order to realize global hydration characterization of amino acid such as L-threonine, terahertz time-domain attenuated total reflectance spectroscopy (THz-TDS-ATR) was adopted in this paper. By measuring the complex permittivity of L-threonine solutions with various concentrations in the THz region, the hydration state and its concentration dependence were obtained, indicating that the number of hydrous water decreased with the increase of concentration. The hydration number was evaluated to be 17.8 when the molar concentration of L-threonine was 0.34 mol/L, and dropped to 13.2 when the molar concentration increased to 0.84 mol/L, when global hydration was taken into account. According to the proposed direct measurements, it is believed that the THz-TDS-ATR technique is a powerful tool for studying the picosecond molecular dynamics of amino acid solutions.

Label-free monitoring of cell death induced by oxidative stress in living human cells using terahertz ATR spectroscopy

We demonstrated that attenuated total reflectance terahertz time-domain spectroscopy (ATR THz-TDS) is able to monitor oxidative stress response of living human cells, which is proven in this work that it is an efficient non-invasive, label-free, real-time and in-situ monitoring of cell death. Furthermore, the dielectric constant and dielectric loss of cultured living human breast epithelial cells, and along with their evolution under oxidative stress response induced by high concentration of H2O2, were quantitatively determined in the work. Our observation and results were finally confirmed using standard fluorescence-labeled flow cytometry measurements and visible fluorescence imaging.

High-peak-power terahertz sources pumped by high power laser and single-shot measurement of terahertz temporal waveform

Optical rectification of laser pulses in LiNbO3 crystal pumped by high power laser is one of the most powerful way to generate this high-peak-power terahertz pulses. It enhances the laser-terahertz transform efficiency by tilted-pulse-front pumping(TPFP) to fulfill phase match in the LiNbO3 crystal. However, comprehensive theoretical analysis is still lack. In this work, a detailed theoretical model to investigate the THz generation efficiency by using nonlinear susceptibility tensor of LiNbO3 crystal was presented. Based on femtosecond laser system, a setup to generate high-peak-power terahertz pulses and a time domain spectroscopy system are established. The property of generated terahertz pulses was analyzed by using terahertz camera and THz time domain system. We also realized the single-shot measurement of terahertz temporal waveform by using this terahertz source.

Polarization dependent terahertz generation efficiency by optical rectification in LiNbO3

Optical rectification of laser pulses in LiNbO3 by tilted-pulse-front pumping (TPFP) is a powerful way to generate terahertz(THz) pulses. However, comprehensive theoretical analysis is still lack. In this work, we first established and presented a detailed theoretical model for TPFP scheme, which then was used to analyze the pump beam polarization dependent terahertz pulses generated by this scheme. The results indicate that one can change the polarization state of the terahertz pulse by changing the pump beam polarization. A scheme using tilted-pulse-front pumping was also set up, and the generated terahertz pulses have maximal conversion efficiency when the pump beam electric field vector is parallel to the crystal axis, which is consistent with theoretical model.

The efficiency and polarization of the terahertz pulses generated by tilted-pulse-front pumping scheme

Optical retification of laser pulses in LiNbO3 by tilted-pulse-front pumping(TPFP) is a powerful way to generate terahertz(THz) pulses. Detailed theoretical analysis of the TPFP THz pulses generation efficiency and polarization with different pump beam polarization was presented. The THz generation efficiency for various of pump beam polarization angle and THz pulse polarization were also researched. The results predict that the magnitude of the THz electric field is maximized when the pump beam electric field vector is parallel to the LiNbO3 crystal axis.