Shaw Wei Kok

Improve performance of scanning probe microscopy by balancing tuning fork prongs

This paper presents an approach for improving the Q-factor of tuning fork probe used in scanning probe microscopes. The improvement is achieved by balancing the fork prongs with extra mass attachment. An analytical model is proposed to characterize the Q-factor of a tuning fork probe with respect to the attachment of extra mass on the tuning fork prongs, and based on the model, the Q-factors of the unbalanced and balanced tuning fork probes are derived and compared. Experimental results showed that the model fits well the experimental data and the approach can improve the Q-factor by more than a factor of three. The effectiveness of the approach is further demonstrated by applying the balanced probe on an atomic force microscope to obtain improved topographic images.

Near-field ellipsometry for thin film characterization.

A near-field ellipsometry method is presented for nano-scale thin film characterization. The technique fuses the topographic and ellipso-metric optical measurements that are simultaneously obtained by a scanning near-field optical microscopy (SNOM). It is shown that the proposed near-field ellipsometry is able to attain nano-scale lateral resolution and correct artifacts in characterization. The effectiveness of the proposed method is verified by simulation and experimental studies.

Reflection-based near-field ellipsometry for thin film characterization

This paper presents a near-field ellipsometry method for nano-scale thin film characterization. The method is based on a reflection configuration of near-field optical detection. In the method, a new set of ellipsometry equations is established by taking into consideration the near-field sample-probe interaction and the topography of the thin film. Experimental and simulation results have shown that the proposed near-field ellipsometry is able to attain precise thin film characterization with nano-scale lateral resolution.

Artifact removal by intrinsic harmonics of tuning fork probe for scanning near-field optical microscopy.

This paper presents a new method to reduce the topographical artifacts in scanning near-field optical microscopy (SNOM) images. The method uses the harmonics caused intrinsically by the nonlinearity in the oscillation of the SNOM probe even when the probe is working in a normal condition without extra excitation. Using these intrinsic harmonics, the gradient of the received SNOM signal with respect to the probe motion is obtained. Then, taking advantage of a SNOM capable of simultaneously obtaining both the topographical and optical signals, topographical artifacts are calculated from the product of the gradient and the topographical signal, and then removed from the received SNOM signal. The effectiveness of the proposed method is demonstrated experimentally.

An improved dynamic model of tuning fork probe for scanning probe microscopy

This paper presents a two coupled oscillators model to describe the dynamics of a tuning fork with a probe attached. The two coupled oscillators are unbalanced only in their effective masses and the damping ratios. By applying a frequency domain system identification approach in experimental investigation of various probe attachment cases, a good accuracy of the model is demonstrated. The effectiveness of the model is further demonstrated in quantitative analysis of the noise performance and the sensitivity of force sensing with a tuning fork probe. Compared with existing models, the proposed model can more accurately characterize the dynamics of a tuning fork probe.

Nano-Imaging Collagen by Atomic Force, Near-Field and Nonlinear Microscope

As the most abundant protein in the human body, collagen has a very important role in vast numbers of bio-medical applications. The unique second order nonlinear properties of fibrillar collagen make it a very important index in nonlinear optical imaging based disease diagnosis of the brain, skin, liver, colon, kidney, bone, heart and other organs in the human body. The second-order nonlinear susceptibility of collagen has been explored at the macroscopic level and was explained as a volume-averaged molecular hyperpolarizability. However, details about the origin of optical second harmonic signals from collagen fibrils at the molecular level are still not clear. Such information is necessary for accurate interpolation of bio-information from nonlinear optical imaging techniques. The later has shown great potential in collagen based disease diagnosis methodologies. In this paper, we report our work using an atomic force microscope (AFM), near field (SNOM) and nonlinear laser scanning microscope (NLSM) to study the structure of collagen fibrils and other pro-collagen structures.

Shear-force atomic force microscope by using the second resonance regime of tuning fork probe

An imaging scheme of shear-force atomic force microscope is proposed by exploiting the second resonance regime of the tuning fork probe. Theoretical analysis and experimental results demonstrate that the imaging scheme can deliver better sensitivity and higher resolution of topographic imaging.

Loss Measurement of Photonic Integrated Waveguides by Scanning Near-field Optical Microscopy

A method of using scanning near-field optical microscopy to measure the loss of embedded optical waveguides is presented. The method gives accurate measurement of propagation loss and other parameters characterizing the manufacturing quality of waveguides.

Monotonicity of Likelihood Ratio of MAP Correlation Detection for OCDMA Communication

In this paper, the monotonicity of the likelihood ratio used in the optimal correlation detection is established for optical code division multiple-access (OCDMA). The monotonicity property not only guarantees the optimality of the detection, it also enables us to calculate the bit-error-rate bounds for OCDMA in the presence of uncertain a priori probabilities.

EDFA Gain Clamping with a Self-Tuning All-Optical Feedback Loop

Using automatic control theory, this paper presents a scheme of using a self-tuning all-optical feedback loop to clamp the gain of erbium-doped fiber amplifiers while the relaxation oscillation in gain clamping is reduced as specified.