Total internal reflection photoacoustic detection spectroscopy

Techniques and Challenges for Characterizing Metal Thin Films with Applications in Photonics

“…The portion of the field that penetrates through the film may be absorbed by chromophores in the medium beyond the film, which in turn incites an ultrasonic pressure wave through the optical absorption and resultant rapid thermoelastic expansion of the absorber. This acoustic energy may then be detected by a suitable ultrasonic transducer, which yields information about the film's optical and geometric properties, including refractive index and thickness [73]. The peak-to-peak magnitude of the photoacoustic pressure wave is directly related to the film thickness, its refractive index and the incident angle of the laser beam.…”

Evanescent Field Based Photoacoustics: Optical Property Evaluation at Surfaces

“…This family includes, but is not limited to, total internal reflection photoacoustic spectroscopy (TIRPAS), optical tunneling photoacoustic spectroscopy (OTPAS), and surface plasmon resonance photoacoustic spectroscopy (SPRPAS). The interested reader should refer to the following references for derivations of the equations used for TIRPAS 5,6,18,23,25,26,[33][34][35][43][44][45][46][47] , PAS/TIRPAS refractometry 18 , and OTPAS 6 . In each case, the photoacoustic effect is generated through a different excitation mechanism than simple transmittance through a prism; for instance, in TIRPAS, the light is evanescently coupled through a prism/substrate/sample interface into the chromophores (which could include the sample material itself, or guest molecules within the sample), whereas in SPRPAS, the primary mode of excitation is instead through the absorption of a surface plasmon, which is a secondary E-M wave created when the energy of the evanescent field is transferred into the electron cloud of a metal layer deposited on the prism surface.…”

“…That being said, a new set of techniques, known as evanescent field-based photoacoustics (EFPA) 5,6,15,18, as shown in Figure 1, has the potential to estimate material properties at the nanoscale in a consolidated set of experiments. EFPA encompasses the sub-techniques of total internal reflection photoacoustic spectroscopy (TIRPAS) 23,25,26,[33][34][35][43][44][45] , photoacoustic spectroscopy/total internal reflection photoacoustic spectroscopy refractometry (PAS/TIRPAS refractometry) /C p where α is the volume thermal expansion coefficient, v s is the speed of sound in the medium, and C p is the heat capacity at constant pressure, H 0 is the radiant exposure of the laser beam, c is the speed of sound in the excited medium, x is length, and t is time. The magnitude of the resulting acoustic wave relies directly upon the optical absorption coefficient of the material, µ a , which is the inverse of the optical penetration depth, δ, which is in turn a measure of the distance the light travels until it decays to 1/e of its initial optical intensity.…”

“…With such a small optical penetration depth, the evanescent field is able to interact with the environment very close to the interface of the two materials, and well below the optical and acoustic diffraction limits. The optical properties of materials or particles within this range may perturb the field or otherwise alter its generation, which interaction can be detected by a variety of methods 3,5,6,10,15,17,18,21,23,[25][26][27][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][84][85][86][87][88][89][90][91][92][93][94][95] .…”

“…More recently, previous investigations have shown that increased sensitivity and utility are possible with modern polyvinylidene fluoride (PVDF) ultrasonic detectors and q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers. Specifically, nanosecond-pulsed Nd:YAG lasers result in a 10 6 fold increase in the peak power, which enables EFPA techniques to become useful tools for evaluating the optical properties of a variety of materials and interfaces 5,6,15,18,[21][22][23][24][25][26][27][28][29][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]84,96 . Additionally, previous work has further shown the capability of such techniques to determine structural information about materials at an interface, which was previously never achievable with traditional photoacoustic spectroscopy (PAS) technologies due to their relatively large penetration depth 53,55,57,59,61,62,69,73,75,80,81 .…”