Li-Ping Yu

Paired circularly polarized heterodyne ellipsometer

We develop a paired circularly polarized heterodyne ellipsometer (PCPHE), in which a heterodyne interferometer based on a two-frequency circularly polarized laser beam is set up. It belongs to an amplitude-sensitive ellipsometer that is able to provide not only a wider dynamic range of polarization modulation frequency but also a higher detection sensitivity than that of a conventional photometric ellipsometer. A real-time and precise measurement of ellipsometric parameters, which demonstrated an accuracy of less than 1 nm on thickness measurement of SiO(2) thin film deposited on silicon substrate, can be applied with the PCPHE.

Determination of retardation parameters of multiple-order wave plate using a phase-sensitive heterodyne ellipsometer

To characterize the linear birefringence of a multiple-order wave plate (MWP), an oblique incidence is one of the methods available. Multiple reflections in the MWP are produced, and oscillations in the phase retardation measurement versus the oblique incident angle are then measured. Therefore, an antireflection coated MWP is required to avoid oscillation of the phase retardation measurement. In this study, we set up a phase-sensitive heterodyne ellipsometer to measure the phase retardations of an uncoated MWP versus the oblique incident angle, which was scanned in the x-z plane and y-z plane independently. Thus, the effect on multiple reflections by the MWP is reduced by means of subtracting the two measured phase retardations from each other. As a result, a highly sensitive and accurate measurement of retardation parameters (RPs), which includes the refractive indices of the extraordinary ray n(e) and ordinary ray n(o), is obtained by this method. On measurement, a sensitivity (n(e),n(o)) of 10(-6) was achieved by this experiment setup. At the same time, the spatial shifting of the P and S waves emerging from the MWP introduced a deviation between experimental results and the theoretical calculation.

Characterization of a nematic PALC at large oblique incidence angles

Compared with conventional photometric methods of measuring cell parameters, including the cell gap and the pretilt angle of a nematic parallel-aligned liquid crystal (PALC) using multiple wavelengths at normal incidence, this research proposes the use of a phase-sensitive interferometric ellipsometer to determine cell parameters precisely based on a single wavelength at large oblique incidence angles. The advantage of this method is that it detects the phase difference using an optical heterodyne interferometer in which a common phase noise rejection mode is provided. Thus, there is a high signal-to-noise ratio (SNR) on the phase measurement. In addition, a range of large oblique incidence angles on the PALC is used so that a high sensitivity measurement of the cell parameters is obtained experimentally. During the measurements, the multiple reflections and spatial shifting effect of the emerging extraordinary ray (E-ray) and ordinary ray (O-ray) from the PALC at large oblique incidence angles are able to be reduced effectively by the use of retro-reflected geometry in the interferometer. The experimental results verify that the sensitivities for the cell gap and pretilt angle measurements are 0.3 nm and 0.01 degrees , respectively.

Two-dimensional cell parameters of twisted nematic liquid crystal with an amplitude-sensitive heterodyne ellipsometer

To be compared with the wavelength modulation technique for measuring two-dimensional (2D) cell parameters of a twisted nematic liquid crystal (TN-LC), we propose an amplitude-sensitive heterodyne ellipsometer (ASHE) of a single wavelength that is able to characterize TN-LC in 2D quantitatively. A quarter-wave plate (QWP) is rotated continuously in this setup to modulate the polarization state of the incident laser beam to obtain the amplitude ratio of the S and P waves versus the rotation angle of the QWP. Thus the cell parameters, including the twisted angle Phi, untwisted phase retardation Gamma, rubbing direction angle alpha, and cell gap d, of a TN-LC cell are obtained simultaneously by best fitting the detected amplitude ratio with a prediction based on the transfer matrix of TN-LC cell. 2D distributions of (Phi,Gamma,alpha,d) are then obtained either by scanning the TN-LC cell or by using a CCD camera for high-speed measurement. In this experiment, the stability of the amplitude-ratio measurement of the proposed ASHE was 0.5%. The goal is to integrate the rotating elliptical wave plate with the TN-LC cell in a heterodyne ellipsometer to obtain cell parameters at amplitude sensitivity. This increases not only the sensitivity of the measurement but also the possibility of extending the 2D distribution of cell parameters in real time.

POLARIZATION-SENSITIVE OPTICAL COHERENCE TOMOGRAPHY USING A MODIFIED BALANCE DETECTOR

In conventional polarization-sensitive optical coherence tomography (PS-OCT), phase retardation is obtained by the amplitude of P and S polarization only, and the fast axis angle is obtained by the phase difference in P and S polarizations via Hilbert transformation. In this paper, we proposed a modified PS-OCT setup in which the phase retardation and fast axis angle are simply expressed as the function of the amplitude of P and S polarization and their differential signal. Due to the common-path feature between the two channels of P and S polarization, the fluctuation in the measurement of phase retardation and fast axis angle caused by excess noise and phase noise from the laser source can be reduced by the differential signal of P and S polarization via a modified balance detector. Thus, the signal of phase retardation and fast angle axis in the deep layer of a porcine sample can be improved.

Zeeman laser scanning confocal microscope and its ability on reduction of specimen-induced spherical aberration

The spherical aberration induced by refractive-index mismatch results in the degradation on the quality of sectioning images in conventional confocal laser scanning microscope (CLSM). In this research, we have derived the theory of image formation in a Zeeman laser scanning confocal microscope (ZLSCM) and conducted experiments in order to verify the ability of reducing spherical aberration in ZLSCM. A Zeeman laser is used as the light source and produces the linearly polarized photon-pairs (LPPP) laser beam. With the features of common-path propagation of LPPP and optical heterodyne detection, ZLSCM shows the ability of reducing the specimen-induced spherical aberration and improving the axial resolution simultaneously.

Measurement of diffuse photon-pairs density wave in a multiple-scattering medium

As a continuation of the previously developed theory of a diffuse photon-pairs density wave (DPPDW) [Appl. Opt.44, 1416-1425 (2005)APOPAI0003-693510.1364/AO.44.001416], this research experimentally studies and verifies the DPPDW theory in a heterogeneous multiple-scattering medium. The DPPDW is generated by collecting the scattered linear polarized photon pairs (LPPPs) in the multiple-scattering medium. Theoretically, the common-path propagation of LPPPs not only provides common phase noise rejection mode but also performs coherence technique via heterodyne detection. In addition, the polarization gating and spatial coherence gating of LPPPs would suppress the severe scattered photon in the multiple-scattering medium. In the experiment, the amplitude and phase wavefronts of DPPDWs, which are distorted by a small object embedded in a homogeneous multiple-scattering medium, are measured in one dimension or two dimensions by scanning the source detector pair. The measured distortion of DPPDW wavefronts are detected precisely and are consistent with the theoretical calculation of DPPDW. It implies an improvement on the detection sensitivity of a small object compared with the conventional diffuse photon density wave (DPDW).

Glucose detection in a highly scattering medium with diffuse photon-pair density wave

We propose a novel optical method for glucose measurement based on diffuse photon-pair density wave (DPPDW) in a multiple scattering medium (MSM) where the light scattering of photon-pair is induced by refractive index mismatch between scatters and phantom solution. Experimentally, the DPPDW propagates in MSM via a two-frequency laser (TFL) beam wherein highly correlated pairs of linear polarized photons are generated. The reduced scattering coefficient μ2s′ and absorption coefficient μ2a of DPPDW are measured simultaneously in terms of the amplitude and phase measurements of the detected heterodyne signal under arrangement at different distances between the source and detection fibers in MSM. The results show that the sensitivity of glucose detection via glucose-induced change of reduced scattering coefficient (δμ2s′) is 0.049%mM−1 in a 1% intralipid solution. In addition, the linear range of δμ2s′ vs glucose concentration implies that this DPPDW method can be used to monitor glucose concentration continuously and noninvasively subcutaneously.

Measurement of the surface effect of a small scattering object in a highly scattering medium by use of diffuse photon-pairs density wave

The surface effect close to the boundary of a small light-scattering object in a highly scattering medium is experimentally demonstrated. This is the first attempt to measure the surface effect of a small spherical scattering object in 1% intralipid solution by use of developed diffuse photon-pairs density wave (DPPDW) in terms of the amplitude and phase detection. Theoretically, the surface effect of a small scattering object in turbid media is localized close to the boundary according to the perturbation theory, concerning an inhomogeneous distribution of the diffusion coefficient in the frequency-domain diffusion equation. Hence, an improvement of the spatial resolution of the image via an inverse algorithm, which relates to detection sensitivity of localization to the boundary of the image object in a multiple scattering medium, is anticipated. In this study, we demonstrate that DPPDW is able to sense the surface effect of a 2-mm spherical scattering object in 1% intralipid solution, with high sensitivity. Subsequently, an improvement of spatial resolution of imaging in turbid media by using DPPDW in comparison with conventional diffuse photon density wave (DPDW) using inverse algorithm is discussed.

Two-Dimensional Distributions of Five Cell Parameters of Twisted Nematic Liquid Crystal Device by Numerical Photometric Ellipsometer

A novel numerical photometric ellipsometer (NPE) under the condition of normal incidence and using a single wavelength laser beam is proposed wherein a continuously rotated quarter wave plate (QWP) at constant speed is required. To precisely measure the cell parameters of a twisted nematic liquid crystal device (TNLCD), the calibration of the QWP is necessary before measurement in this method. All five cell parameters of the TNLCD, namely, twisted angle, cell gap, rubbing angle, pretilt angle, and phase retardation, are obtained by least-square fitting of the measurement data versus the rotation angle of the QWP with the calculated intensity ratio of p- and s-polarization components according to our previously developed theory. The NPE can be extended to the two-dimensional distributions of all five cell parameters by using charge-coupled device (CCD) cameras. Experimentally, the results demonstrate the ability of the NPE to characterize the five cell parameters of the TNLCD in two dimensions with high accuracy and good repeatability.