Yanning Li

An Integrated Laser-Induced Piezoelectric/Differential Confocal Surface Acoustic Wave System for Measurement of Thin Film Young’s Modulus

The present paper presents the design and development results of a system setup for measuring Youngs modulus of thin films by laser-induced surface acoustic waves based on the integration of two detection methods, namely, piezoelectric transducer detection and differential confocal detection, which may be used for conducting consecutive or simultaneous measurements. After demonstrating the capabilities of each detection approach, it is shown how, depending on a wider range of applications, sample materials and measurement environments, the developed integrated system inherits and harnesses the main characteristics of its detection channels, resulting in an more practical and flexible equipment for determining Youngs modulus than traditional nanoindentation equipment, and also suitable for cross-validation purposes.

Simulation study of melanoma detection in human skin tissues by laser-generated surface acoustic waves.

Abstract. Air pollution has been correlated to an increasing number of cases of human skin diseases in recent years. However, the investigation of human skin tissues has received only limited attention, to the point that there are not yet satisfactory modern detection technologies to accurately, noninvasively, and rapidly diagnose human skin at epidermis and dermis levels. In order to detect and analyze severe skin diseases such as melanoma, a finite element method (FEM) simulation study of the application of the laser-generated surface acoustic wave (LSAW) technique is developed. A three-layer human skin model is built, where LSAW’s are generated and propagated, and their effects in the skin medium with melanoma are analyzed. Frequency domain analysis is used as a main tool to investigate such issues as minimum detectable size of melanoma, filtering spectra from noise and from computational irregularities, as well as on how the FEM model meshing size and computational capabilities influence the accuracy of the results. Based on the aforementioned aspects, the analysis of the signals under the scrutiny of the phase velocity dispersion curve is verified to be a reliable, a sensitive, and a promising approach for detecting and characterizing melanoma in human skin.

A Four-Quadrant PVDF Transducer for Surface Acoustic Wave Detection

In this paper, a polyvinylidene fluoride (PVDF) piezoelectric transducer was developed to detect laser-induced surface acoustic waves in a SiO2-thin film–Si-substrate structure. In order to solve the problems related to, firstly, the position of the probe, and secondly, the fact that signals at different points cannot be detected simultaneously during the detection process, a four-quadrant surface acoustic wave PVDF transducer was designed and constructed for the purpose of detecting surface acoustic waves excited by a pulse laser line source. The experimental results of the four-quadrant piezoelectric detection in comparison with the commercial nanoindentation technology were consistent, the relative error is 0.56%, and the system eliminates the piezoelectric surface wave detection direction deviation errors, improves the accuracy of the testing system by 1.30%, achieving the acquisition at the same time at different testing positions of the sample.

Optimization for liquid crystal variable retarder-based spectroscopic polarization measurements

We present an approach for improving liquid crystal variable retarder (LCVR)-based spectroscopic polarization measurements. As deduced mathematically, the transfer coefficients from the random intensity noise to the signal noise are functions of modulation parameters of the LCVR, i.e., modulation range (MR) and initial retardation. Simulations allow more details about the roles of two parameters. A broad MR reduces effectively the values of the coefficients and leads to a better signal quality. However, as the MR narrows, initial retardation begins to influence the signal quality. To obtain a high-quality spectrum, a recommended solution is to settle the MR more than π at each wavelength. This treatment has two advantages: non-sinusoidal modulation becomes possible and the modulations do not average to zero. Moreover, it weakens the interference of non-uniform intensity distribution in wavelengths of the signal spectrum. These conclusions are proven in experiments. Further, this approach is valid for other polarimeters and ellipsometers based on LCVRs.

Retardation correction for photoelastic modulator-based multichannel reflectance difference spectroscopy

The wavelength dependence of the retardation induced by a photoelastic modulator (PEM) is a central issue in multichannel modulator-based spectroscopic ellipsometry and reflectance difference spectroscopy (RDS), where the optical signal is detected simultaneously at different wavelengths. Here we present a refined analysis of the modulator crystals retardation and its effect on the signal quality. Two retardation correction schemes that take into account the actual wavelength dependence of the stress-optic coefficient are introduced. It is demonstrated experimentally that both methods provide a better correction than the procedure currently used in multichannel RDS. We define quality factors to evaluate the actual performance of the multichannel detection system as compared with a wavelength adaptive single-channel experiment. These quality factors thus provide a useful guideline for choosing the appropriate PEM retardation or reference wavelength in a multichannel experiment.

Normal-incidence reflectance difference spectroscopy based on a liquid crystal variable retarder

We propose liquid crystal variable retarder-based reflectance difference spectroscopy for normal-incidence measurements. Principles, instrumentation, data collection and reduction, and calibration procedures are provided. The signal noise is better than 10-3, and the spectral range is from 1.6 to 2.4 eV with 346 photon energy channels. As a demonstration, reflectance difference signals of a multilayer pentacene film on poly (ethylene terephthalate) (PET) film are presented with different polarization azimuths. The characteristic peaks at 1.8 and 1.97 eV, corresponding to the Davydov splitting of pentacene crystal, are observed, which indicate well-ordered in-plane anisotropic structure of pentacene crystal film on PET. Thanks to normal incidence, this design is immune to adjusting the optical structure for the measurements with different working distances, and the objective lens is easily integrated to realize microarea measurements.

Real-time monitoring of 2D semiconductor film growth with optical spectroscopy

Real-time monitoring of the growth is essential for synthesizing high quality two dimensional (2D) transition-metal dichalcogenides with precisely controlled thickness. Here, we report the first real time in situ optical spectroscopic study on the molecular beam epitaxy of atomically thin molybdenum diselenide (MoSe2) films on sapphire substrates using differential reflectance spectroscopy. The characteristic optical spectrum of MoSe2 monolayer is clearly distinct from that of bilayer allowing a precise control of the film thickness during the growth. Furthermore, the evolution of the characteristic differential reflectance spectrum of the MoSe2 thin film as a function of the thickness sheds light on the details of the growth process. Our result demonstrates the importance and the great potential of the real time in situ optical spectroscopy for the realization of controlled growth of 2D semiconductor materials.

Simulation on the detection of subsurface cracks by the surface acoustic wave

The laser-acoustic method is an effective tool for important material properties test. In the present paper, the surface acoustic wave (SAW) is applied in the nondestructive detection of subsurface cracks in metal. The study focuses on the dispersion of surface acoustic wave propagating in the layer with cracks. The finite element method (FEM) is employed by establishing a series of subsurface cracks models and a more real-like displacement load function is proposed to excite the surface acoustic wave. The subsurface cracks models are divided into 3 groups in depth and 3 groups in the separation distance to analyze the propagation properties of surface acoustic wave and influences of subsurface cracks on the phase velocity. The relation of phase velocity with the dimension of cracks in the frequency domain is investigated in the simulation. It is found that the finite element results fit very well with the surface acoustic wave theory and the dispersion curves in the frequency domain are very suitable to evaluate the properties of subsurface cracks in metal.

Study on measurement interaction force on living cell by atomic force microscopy

A theoretical and experimental investigation of the force-distance relation in the case between a pyramidal tip and a cell is presented. The shape of the tip in use consists of a truncated pyramid covered by a spherical cap. The interaction force is computed the total interaction between macroscopic bodies. The experimental method is depicted in detail. The experimental results are comparison with the theory. The mechanical properties of the cell can be determined accurately and spatially well resolved from interaction force of tip-cell in AFM.