Mark C. Pitter

Full-field heterodyne interference microscope with spatially incoherent illumination

A heterodyne interference microscope arrangement for full-field imaging is described. The reference and object beams are formed with highly correlated, time-varying laser speckle patterns. The speckle illumination confers a confocal transfer function to the system, and by temporal averaging, the coherence noise that often degrades coherent full-field microscope images is suppressed. The microscope described is similar to a Linnik-type microscope and allows the use of high-numerical-aperture objective lenses, but the temporal coherence of the illumination permits the use of a low-power achromatic doublet in the reference arm. The use of a doublet simplifies alignment of the microscope and can reduce the cost. Preliminary results are presented that demonstrate full-field surface height precision of 1 nm rms.

Wide-field surface plasmon microscopy with solid immersion excitation

This letter shows how an aplanatic solid immersion lens system may be used for high-resolution wide-field surface plasmon microscopy. The lens configuration means that no fluid couplant is required; it has the additional advantage that very high refractive indices may be used to reduce the incident angle at which surface plasmons are excited. A transmission spatial light modulator is incorporated conjugate with the back focal plane; this allows the image contrast to be controlled in a precise and flexible manner. Its use also provides definitive demonstration that the image contrast arises from surface plasmon excitation.

Subpixel microscopic deformation analysis using correlation and artificial neural networks

Microscopic deformation analysis has been performed using digital image correlation and artificial neural networks (ANNs). Cross-correlations of small image regions before and after deformation contain a peak, the position of which indicates the displacement to pixel accuracy. Subpixel resolution has been achieved here by nonintegral pixel shifting and by training ANNs to estimate the fractional part of the displacement. Results from displaced and thermally stressed microelectronic devices indicate these techniques can achieve comparable accuracies to other subpixel techniques and that the use of ANNs can facilitate very fast analysis without knowledge of the analytical form of the image correlation function.

Resolution in structured illumination microscopy: a probabilistic approach

Structured illumination can be employed to extend the lateral resolution of wide-field fluorescence microscopy. Since a structured illumination microscopy image is reconstructed from a series of several acquired images, we develop a modified formulation of the imaging response of the microscope and a probabilistic analysis to assess the resolution performance. We use this model to compare the fluorescence imaging performance of structured illumination techniques to confocal microscopy. Specifically, we examine the trade-off between achievable lateral resolution and signal-to-noise ratio when photon shot noise is dominant. We conclude that for a given photon budget, structured illumination invariably achieves better lateral resolution than confocal microscopy.

Remote photoacoustic detection of liquid contamination of a surface

A method for the remote detection and identification of liquid chemicals at ranges of tens of meters is presented. The technique uses pulsed indirect photoacoustic spectroscopy in the 10-microm wavelength region. Enhanced sensitivity is brought about by three main system developments: (1) increased laser-pulse energy (150 microJ/pulse), leading to increased strength of the generated photoacoustic signal; (2) increased microphone sensitivity and improved directionality by the use of a 60-cm-diameter parabolic dish; and (3) signal processing that allows improved discrimination of the signal from noise levels through prior knowledge of the pulse shape and pulse-repetition frequency. The practical aspects of applying the technique in a field environment are briefly examined, and possible applications of this technique are discussed.

Wide-field high-resolution structured illumination solid immersion fluorescence microscopy.

The use of aplanatic solid immersion lenses (ASILs) made of high-refractive-index optical materials provides a route to wide-field high-resolution optical microscopy. Structured illumination microscopy (SIM) can double the spatial bandwidth of a microscope to also achieve high-resolution imaging. We investigate the combination of ASILs and SIM in fluorescence microscopy, which we call structured illumination solid immersion fluorescence microscopy (SISIM), to pursue a microscopic system with very large NA and high lateral resolution. We demonstrate that the combination can produce a wide-field high-resolution microscopic system with bandwidth corresponding to an NA of 3. Future developments of the SISIM system to make it achieve even higher resolution are proposed.

Surface-plasmon microscopy with a two-piece solid immersion lens: bright and dark fields.

We report bright-field and dark-field surface-plasmon imaging using a modified solid immersion lens and a commercial objective of moderate NA in the epi configuration. The contrast and resolution are extremely good, giving well-resolved images of protein monolayers both in air and in water. We also describe a two-part solid immersion lens that allows the sample to be moved without degrading the image quality in any observable way. The merits of the two-part lens are discussed and compared to commercially available microscope objectives. Finally, we introduce a simple Greens function model that illustrates the key features of both bright-field and dark-field surface-plasmon imaging.

Multichannel, time-resolved picosecond laser ultrasound imaging and spectroscopy with custom complementary metal-oxide-semiconductor detector

This paper presents a multichannel, time-resolved picosecond laser ultrasound system that uses a custom complementary metal-oxide-semiconductor linear array detector. This novel sensor allows parallel phase-sensitive detection of very low contrast modulated signals with performance in each channel comparable to that of a discrete photodiode and a lock-in amplifier. Application of the instrument is demonstrated by parallelizing spatial measurements to produce two-dimensional thickness maps on a layered sample, and spectroscopic parallelization is demonstrated by presenting the measured Brillouin oscillations from a gallium arsenide wafer. This paper demonstrates the significant advantages of our approach to pump probe systems, especially picosecond ultrasonics.

Rapid photoreflectance spectroscopy for strained silicon metrology

We present an improved photoreflectance (PR) spectroscopy technique upon the prior art in providing a rapid acquisition method of the PR spectrum in a simultaneous and multiplexed manner. Rapid PR (RPR) application is the on-line monitoring of strained silicon. Shrinkage in the silicon bandgap is measured and converted to strain, using theoretical models. Experimental RPR results are in good correlation with Raman spectroscopy.

Focus errors and their correction in microscopic deformation analysis using correlation

Subpixel digital image correlation has been applied to microscope images to analyze surface deformation. Nonintegral pixel shifting and successive approximation are used to calculate the subpixel element of the sample displacement without introducing systematic interpolation errors. Although in-plane displacement precision of better than 2% of a pixel, or < 15 nm at x10 magnification, is shown to be achievable, the use of even moderate numerical aperture microscope objectives render the technique sensitive to errors or variations in sample focusing. The magnitude of this effect is determined experimentally and a focus compensation method is described and demonstrated.