Spatial resolution of thermal wave microscopes

Abstract

It is demonstrated theoretically and confirmed experimentally that the intrinsic spatial resolution of a thermal wave microscope in the extreme near field limit is independent of thermal wavelength and is determined by the depth of the thermal scatterer beneath the surface of the specimen.

Contrast Mechanisms in the Thermoacoustic Microscope

“…In one of these experiments [13] the scatterer has a continuously varying aepth. In this experiment the observed resolution is two or three oraers of magnitude better than the acoustic wavelength and deteriorates linearly with depth, as predicted by the extreme-near-field theory [12]. Another recent experiment using such a microscope has been carried out on an aluminum bicrystal having a nearly vertical grain boundary [15].…”

“…The inverse power of the distance which appears in the Green's function is assumed to De slowly varying in both of these limits. However, in the situation which we described aoove, with a small source which can be brought close to the scatterer, the variation of the phase factor is dominated by that of the inverse power of the distance, and the scattering becomes quasi-static (extreme-near-field 1 imit) [12]. In this limit the incident energy is effectively localized to a region of the scatterer whose dimensions are comparable to the size of the source or the source-scatterer distance, whichever is larger.…”

Applications of Phase Microscopy to Photothermal Spectroscopy

“…By operat ing in a differentia1 mode one may map spectroscopic absorption differences in the ne ar fie1d 1i mit [20], thereby obtaining resolution greater than either the therma1 or IR wave1engths emp10yed. By scanning the samp1e stage, a map of absorption may be made at a number of wave1engths.…”

Picosecond Transient Thermoreflectance: Time-Resolved Studies of Thin Film Thermal Transport

“…If we assume a Gaussian shaped optical pulse of duration r, and intensity 1, the source term in Eq. (3) becomes 2 P(z,t) = I(l_R)aRe-az e-(t/r) (4) where R and a are the metal reflectivity and absorbtivity, respectively. Substituting this heating source term into Eq.…”

“…This technique uses short laser pulses to heat the sample surface, followed by a timeresolved measurement of the black-body radiat ion to determine the surface cooling rate. Another method, photothermal deflectometry, has been used to determine thermal properties by measuring the optically-induced thermoelastic deformation of the surface or the heating induced refractive index gradient in a gas above the surface [3,4]. More recently, Rosencwaig et al [5] have shown that a modulated thermoreflectance measurement is an equally sensitive method which can yield the same informat ion as the deflection techniques.…”