Andreas Mandelis

Theory of photopyroelectric spectroscopy of solids

Light absorption by a solid material and conversion of part, or all, of the optical energy into heat due to nonradiative deexcitation processes within the solid can give rise to an electrical signal in a pyroelectric thin film in contact with the sample. This effect forms the basis of a new spectroscopic technique for the study of condensed phase matter. A one‐dimensional theory is presented, which describes the dependence of the pyroelectric signal on the optical, thermal, and geometric parameters of the solid/pyroelectric system. Specifically, the theory examines the conditions under which the photopyroelectric signal exhibits a linear dependence on the optical absorption coefficient of the solid. Thus a theoretical basis for the technique of photopyroelectric spectroscopy is established. Qualitative comparisons between predictions of the theory and preliminary experimental observations are used to test the applicability of the theory to experimental configurations of practical interest.

Solid‐state sensors for trace hydrogen gas detection

This paper reviews the development, history, theoretical basis, and experimental performance of solid‐state hydrogen detectors under flow‐through conditions available to date such as pyroelectric, piezoelectric, fiber optic, and electrochemical devices. Semiconductor hydrogen detectors will only be reviewed briefly, as excellent reviews on this subject already exist. In view of the fact that almost all the devices that will be discussed later in this paper use Pd as a hydrogen trap, we devote a subsection to examining the role of palladium as a catalyst as well as some of the characteristics of the Pd‐H2 system. Non‐solid‐state hydrogen sensors, such as the flame ionization detector are not the object of this review. A useful feature of this review is a comparison of operating characteristics of each device in a general table in Sec. VII. In that section a general discussion is presented, including a critical comparison of the capabilities and parameters of various solid‐state hydrogen sensors in the form...

Thermal‐wave resonator cavity

A thermal‐wave resonant cavity was constructed using a thin aluminum foil wall as the intensity‐modulated‐laser‐beam induced oscillator source opposite a pyroelectric polyvilidene fluoride wall acting as a signal transducer and cavity standing‐wave‐equivalent generator. It was shown that scanning the frequency of oscillation produces the fundamental and higher overtone resonant extrema albeit with increasingly attenuated amplitude—a characteristic of thermal‐wave behavior. Experimentally, scanning the cavity length produced a sharp lock‐in in‐phase resonance with simple linewidth dependencies on oscillation (chopping) frequency and intracavity gas thermal diffusivity. The thermal diffusivity of air at 294 K was measured with three significant figure accuracy: 0.211±0.004 cm2/s. The novel resonator can be used as a high‐resolution thermophysical property sensor of gaseous ambients.

Frequency-domain photopyroelectric spectroscopy of condensed phases (PPES): A new, simple and powerful spectroscopic technique

Abstract A new, powerful, and experimentally very convenient spectroscopic technique is described. The technique uses pyroelectric polyvinylidene difluoride (PVDF) thin film detectors in contact with solid or liquid samples. The instrumental and spectral characteristics of photopyroelectric spectroscopy (PPES) are presented, and its advantages over conventional photoacoustic spectroscopy (PAS) are discussed.

Phase measurements in the frequency domain photoacoustic spectroscopy of solids

Photoacoustic spectroscopy provides information about both the amplitude and phase of the response of a system to an optical excitation process. This paper presents a theoretical model of photoacoustic processes in the frequency domain which includes the relaxation time of the radiationless deexcitations and a two‐layer absorbing system. Emphasis is placed on the effect of these conditions on the phase of the photoacoustic signal and the utility of this measurement in evaluating material parameters. Circumstances under which the phase may be used to measure the optical absorption coefficient of the solid and the nonradiative relaxation times are defined. The value of the phase measurement in the study of surface films is discussed.

Thermal-wave resonator cavity design and measurements of the thermal diffusivity of liquids

A liquid-ambient-compatible thermal wave resonant cavity (TWRC) has been constructed for the measurement of the thermal diffusivity of liquids. The thermal diffusivities of distilled water, glycerol, ethylene glycol, and olive oil were determined at room temperature (25 °C), with four-significant-figure precision as follows: (0.1445±0.0002)×10−2 cm2/s (distilled water); (0.0922±0.0002)×10−2 cm2/s (glycerol); (0.0918±0.0002)×10−2 cm2/s (ethylene glycol); and (0.0881±0.0004)×10−2 cm2/s (olive oil). The liquid-state TWRC sensor was found to be highly sensitive to various mixtures of methanol and salt in distilled water with sensitivity limits 0.5% (v/v) and 0.03% (w/v), respectively. The use of the TWRC to measure gas evolution from liquids and its potential for environmental applications has also been demonstrated.

Absolute optical absorption coefficient measurements using transverse photothermal deflection spectroscopy

Information about the optical absorption coefficient of solid materials in contact with a fluid phase can be obtained from photothermal deflection (PDS) measurements using both the signal amplitude and phase channels of the PDS response of a system to an optical excitation. This paper presents a theoretical model of photothermal processes in the transverse (TPDS) experimental configuration. The theory is used to determine the dependence of both signal channels on the optical absorption coefficient of the solid material and to define absorption coefficient ranges within which TPDS can be used as a spectroscopic technique. A method concerning the use of the combined amplitude and phase data for the absolute measurement of the absorption coefficient is presented for the experimentally important thermally thick limit.

Frequency-domain photothermoacoustics: Alternative imaging modality of biological tissues

Frequency-domain photothermoacoustic (FD-PTA) imaging of biological tissues is presented and compared with the conventional time-domain methodology. We demonstrate that tissue imaging can be performed with high axial resolution without the necessity to employ short-pulse and high peak-power laser systems to generate acoustic transients. The presented analysis shows that depth information in the FD-PTA method can be recovered by using linear frequency-modulated (chirped) optical excitation and frequency-domain signal processing algorithms. The signal-to-noise ratio can be increased significantly using correlation processing, which can compensate for the small amplitude of acoustic waves typical to the periodic excitation mode. Additionally, narrow-band signal demodulation enables depth-specific and confocal tissue imaging using the optically induced photothermoacoustic effect. Application of the FD-PTA is demonstrated in experiments with turbid phantoms and ex vivo tissue specimens.

Thermophotonic radar imaging: An emissivity-normalized modality with advantages over phase lock-in thermography

One major problem of frequency-domain photothermal radiometry, or alternatively in two-dimensional lock-in thermography, is the compromise one has to make between dynamic range (probing depth) and depth resolution. The thermal-wave radar incorporates chirped excitation through matched filtering to maintain good resolution and depth range inside a sample. This letter experimentally demonstrates the advantages of chirped modulation and introduces a thermophotonic modality of thermal-wave radar based on an emissivity-normalized, higher-dynamic-range contrast parameter known as cross-correlation phase. Finally, comparisons made on a biological (dental) sample show potential applications of the method.

Time‐domain photoacoustic spectroscopy of solids

Most conventional photoacoustic spectroscopy of solids has employed a periodically modulated light source. The availability of high‐intensity pulsed light sources of short pulse duration makes possible the study of the time response of a photoacoustic system in which the solid is excited by a single optical pulse. A one‐dimensional theoretical model is presented in which the time dependence of the photoacoustic response is evaluated for systems of variable optical absorption coefficient and sample thickness. The analysis is restricted to the case for which nonradiative relaxation processes occur instantaneously on the time scale of the measurement. Typical response curves for optically excited systems are presented and interpreted in terms of physical processes occurring in the cell.