Yuejin Zhao

Non-contact detection of oxygen saturation based on visible light imaging device using ambient light

A method that remotely measures blood oxygen saturation through two cameras under regular lighting is proposed and experimentally demonstrated. Two narrow-band filters with their visible wavelength of 660nm and 520nm are mounted to two cameras respectively, which are then used to capture two photoplethysmographic (PPG) from the subject simultaneously. The data gathered from this system, including both blood oxygen saturation and heart rate, is compared to the output of a traditional figure blood volume pulse (BVP) senor that was employed on the subject at the same time. Result of the comparison showed that the data from the new, non-contact system is consistent and comparable with the BVP senor. Compared to other camera-based measuring method, which requires additional close-up lighting, this new method is achievable under regular lighting condition, therefore more stable and easier to implement. This is the first demonstration of an accurate video-based method for non-contact oxygen saturation measurements by using ambient light with their respective visible wavelength of 660nm and 520nm which is free from interference of the light in other bands.

Polarization sensitive terahertz time-domain spectroscopy for birefringent materials

We present a polarization sensitive terahertz detection method which is able to measure both orthogonal components of the terahertz electric field. It allows for the study of polarization dependent properties of materials. Azimuthal angle dependent transmittance in 0.2–2.6 THz frequency region for crystalline quartz is measured. Polarized terahertz transmission spectroscopy shows that birefringence can result in transmission minima. This work suggests that polarization sensitive detection is effective for removing fake absorption features caused by material birefringence.

Terahertz wave reference-free phase imaging for identification of explosives

We present terahertz reference-free phase imaging for identification of three explosive materials (HMX, RDX, and DNT. We propose a feature extraction technique to locate the spectral position of an unknown material’s absorption lines without using the reference signal. The samples are identified by their absorption peaks extracted from the negative first-order derivative of the sample signal phase divided by the frequency at each pixel. This technique will greatly benefit the future development of standoff distance, large size focal-plane terahertz imaging system.

Broadband terahertz metamaterial absorber based on sectional asymmetric structures.

We suggest and demonstrate the concept and design of sectional asymmetric structures which can manipulate the metamaterial absorber’s working bandwidth with maintaining the other inherent advantages. As an example, a broadband terahertz perfect absorber is designed to confirm its effectiveness. The absorber’s each cell integrates four sectional asymmetric rings, and the entire structure composed of Au and Si3N4 is only 1.9 μm thick. The simulation results show the bandwidth with absorptivity being larger than 90% is extended by about 2.8 times comparing with the conventional square ring absorber. The composable small cell, ultra-thin, and broadband absorption with polarization and incident angle insensitivity will make the absorber suitable for the applications of focal plane array terahertz imaging.

A phase feature extraction technique for terahertz reflection spectroscopy

We present a feature extraction technique for identification of explosive and biological materials using terahertz reflection time-domain spectroscopy (RTDS). The absorption signatures of the materials are extracted directly from the second-order derivative of the phase of the sample beam with respect to frequency. This technique provides a straightforward and fast solution to solve the phase-retrieval problem in RTDS and will benefit the future development of a standoff, large-size focal-plane terahertz sensing and imaging system.

Terahertz multiwavelength phase imaging without 2π ambiguity

We present a terahertz phase imaging method with multiwavelengths. This novel approach can image an object with a larger optical length compared with using the largest wavelength in the terahertz spectrum and does not involve the usual phase unwrapping in the detection of phase discontinuity. Furthermore, this technique can effectively reduce the noise background. We present two examples to demonstrate the validity of this new method. Both measurements show that multiwavelength phase imaging is a straightforward and efficient phase data processing method in terahertz imaging applications.

Reflective gradient metasurfaces for polarization-independent light focusing at normal or oblique incidence

Reflective gradient metasurfaces are reported as flat, ultra-thin light focusers using a cross-resonator array with spatially varied geometric parameters atop a continuous gold ground plane spaced by a layer of SiO2. The sub-wavelength cross-shaped building element offers polarization-independent performance and full 2π phase tuning range by varying its width and length, which is explained by an analytical model based on harmonically oscillating dipole antenna. With a radial phase gradient, a metasurface is demonstrated to function as a parabolic reflector at 1.47 μm wavelength with the measured efficiency of 44%. In addition, by elaborately engineering the planar distribution of different building elements, another two focusing reflectors are designed and experimentally verified to anomalously reflect and concentrate light along normal direction but with oblique incident angles of 30° and 60°, respectively.

Performance enhancement of uncooled infrared focal plane array by integrating metamaterial absorber

This letter presents an infrared (IR) focal plane array (FPA) with metamaterial absorber (MMA) integrated to enhance its performance. A glass substrate, on which arrays of bimaterial cantilevers are fabricated as the thermal-sensitive pixels by a polyimide surface sacrificial process, is employed to allow the optical readout from the back side of the substrate. Whereas the IR wave radiates onto the FPA from the front side, which consequently avoids the energy loss caused by the silicon substrate compared with the previous works. This structure also facilitates the integration of MMA by introducing a layer of periodic square resonators atop the SiNx structural layer to form a metal/dielectric/metal stack with the gold mirror functioning as the ground plane. A comparative experiment was carried out on the FPAs that use MMA and ordinary SiNx as the absorbers, respectively. The performance improvement was verified by the evaluation of the absorbers as well as the imaging results of both FPAs.

Optical readout method based on a narrow-strip filter for microcantilever array sensing.

An effective optical readout approach based on a narrow-strip filter is presented to detect bends of a bimaterial microcantilever focal plane array, by which light intensity of the image plane (CCD image sensor plane) can be increased and its uniformity on the image plane effectively enhanced. It reduces the noise equivalent temperature difference of the microcantilever focal plane array IR imaging system and improves uniformity of the IR images. A comparative experiment is designed to verify effectiveness. The experimental results show that the proposed method has advantages of preferable effect.

The role of the nanospine in the nanocomb arrays for surface enhanced Raman scattering

We have investigated the surface enhanced Raman scattering (SERS) from Au nanocombs and nanorods under different excitation conditions. The SERS intensity from nanocombs is always larger than that from nanorods, but the polarized SERS dependence is similar for the two nanostructures. These results agree quantitatively well with the local E-field calculations, and the nanospine in the nanocomb increases the local E-field over all surfaces of the nanocomb structure. The combination of experimental and numerical results predicts that the Raman enhancement at 633 nm excitation is estimated to be from seven to eight times that at 785 nm excitation.