Allan Rosencwaig

Theory of the photoacoustic effect with solids

When chopped light impinges on a solid in an enclosed cell, an acoustic signal is produced within the cell. This effect is the basis of a new spectroscopic technique for the study of solid and semisolid matter. A quantitative derivation is presented for the acoustic signal in a photoacoustic cell in terms of the optical, thermal, and geometric parameters of the system. The theory predicts the dependence of the signal on the absorption coefficient of the solid, thereby giving a theoretical foundation for the technique of photoacoustic spectroscopy. In particular, the theory accounts for the experimental observation that with this technique optical absorption spectra can be obtained for materials that are optically opaque.

Thermal-Wave Imaging.

Thermal features on and beneath the surface of a sample can be detected and imaged with a thermal-wave microscope. Various methodologies for the excitation and detection of thermal waves are discussed, and several applications, primarily in microelectronics, are presented.

Photoacoustic Spectroscopy of Biological Materials

A new technique for performing optical spectroscopy on solids has been developed. Photoacoustic spectra of cytochrome c and hemoglobin show how this technique can be used to obtain information about optical absorptions and subsequent de-excitations in solid biological materials, particularly those which cannot readily be studied by conventional means.

Photoacoustic Effect with Solids: A Theoretical Treatment

Chopped light impinging on a solid sample in an enclosed cell produces an acoustic signal within the cell. A derivation for the acoustic pressure supports the experimental observation that optical absorption spectra may be obtained from the acoustic signal even when the sample is completely opaque to transmitted light. The complete solution for the temperature has the form A(x) + B(x)ejw in each medium, neglecting transients. Applying the boundary conditions of temperature and heat flux continuity at the sample surfaces (x = 0 and x = -1) and an ambient temperature To at the cell walls (x = -I -lb and x = lg), we can obtain the full temperature distribution throughout the cell. In particular, the a-c component of the gas temperature distribution is given by a=c(x, t) Oeg4x+it where 0 represents the complex amplitude of the time-dependent temperature of the solid sample at the solid-gas boundary (x = 0). The explicit expression for 0 is where y is the ratio of specific heats, P0 and VO are the ambient pressure and volume, respectively, and -6 V is the incremental volume. Thus we get

A photoacoustic study of newborn rat stratum corneum

We have applied photoacoustic spectroscopy, a new spectroscopic technique, to the study of intact stratum corneum, a translucent membrane with high ultraviolet light scattering, and have obtained clear spectra in the 220-450 nm region. The photoacoustic technique has been used to study the effects of hydration and chemical alteration of the stratum corneum. Newborn rat stratum corneum shows marked spectral changes during the initial 60 h maturation period. These changes are attributed partly to the environmental and conformational changes occurring within the stratum corneum matrix during this time period. We also postulate the possibility of a novel molecular change within the keratin protein to account for our results.

Noncubic Garnet Anisotropy from Growth‐Induced Pair Ordering

A growth‐induced pair anisotropy model is presented to account for the pronounced noncubic anisotropies recently studied in flux‐grown magnetic garnets. In this model the noncubic anisotropy arises from the presence of a relatively small amount of growth‐induced short‐range pair ordering. The resulting pair anisotropy consisting in general of both dipolar and pseudodipolar terms is shown to have many of the features of the observed anisotropy. In particular we perform a detailed calculation of the anisotropy energy under the {110} and {112} growth facets of the garnets when such short‐range pair ordering occurs between ions of the rare‐earth sublattice and those of the iron sublattices. Good agreement is obtained with presently available experimental data.

Domain‐Wall Energies in Orthoferrites

Domain‐wall energies and effective wall thicknesses are derived for the rare‐earth orthoferrites. The geometric factors arising from the orthorhombic symmetry make the Bloch walls more energetic and thicker than the Neel walls.

Magnetic bubble analog simulator

A mechanical simulator of field‐access magnetic bubble devices has been constructed and tested. Bubbles, represented by permanently magnetized cylinders, 0.1 in. in height and 0.15–0.30 in. in diameter, move through oil in response to poles induced in 1/32 in. mu‐metal circuit features by a rotating in‐plane magnetic field. By strobing the motion of a bubble under a 0.5‐in. disk, an equivalent coercivity of 1 Oe and mobility 0.5 in./sec Oe were inferred. Inertia and parasitic drag are estimated, and it is concluded that the viscous damping model with coercivity is valid for the analog device. A T‐bar corner is shown which was designed on the simulator and has been tested with real bubbles up to the expected frequency limit.