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.

Use of artificial neural networks on optical track width measurements.

We have demonstrated recently that, by using an ultrastable optical interferometer together with artificial neural networks (ANNs), track widths down to 60 nm can be measured with a 0.3 NA objective lens. We investigate the effective conditions for training ANNs.

Spatially resolved acoustic spectroscopy for fast noncontact imaging of material microstructure.

We have developed a noncontact and nondestructive technique that uses laser-generated and detected surface acoustic waves to rapidly determine the local acoustic velocity, in order to map the microstructure of multi-grained materials. Optical fringes excite surface waves at a fixed frequency, and the generation efficiency is determined by how closely the fringe spacing matches the acoustic wavelength in the excitation region. Images of titanium alloys are presented, acquired using the technique.

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.

Surface acoustic wavefront sensor using custom optics.

We have designed and had manufactured a custom surface acoustic wavefront sensor, using a standard CMOS process. Ultrasound propagating along the surface of an object perturbs the reflection of incident laser light, which has been focused onto the object using a cylindrical lens. These high-frequency angular perturbations of reflected light relate to the amplitude and phase of the ultrasound along a line on the surface of the object, and thus correspond to the acoustic wavefront.

Optimisation using measured Green's function for improving spatial coherence in acoustic measurements.

Aberrating materials can degrade acoustic measurements by distorting the acoustic wavefront and causing acoustic speckle (as opposed to speckle noise which is a manifestation of coherent backscatter). The amplitude and phase fluctuations associated with acoustic speckle can introduce considerable measurement uncertainty which is difficult to deal with. This paper demonstrates a new technique which optimises the spatial distribution of the generation of the ultrasound to compensate for the aberration.