Photo Acoustic Effect And its usage for spectroscopy

Photo-Acoustic effect principle Modulated Light Local Heating Absorption: I a = αI 0 Thermal Expansion Pressure Wave = Sound wave Absorption

Discovery: A.G.Bell (1880) Sun Light Rotating disc Modulated light Thin Disk Sound

Example applications of PA effect Characterization of solid materials Separation between different gases and measuring gas concentration Glucose level monitoring in blood Imaging of blood vessels Concentration of textile dyes Concentration of soot particles in diesel engines

PA effect – wave equation Acoustic velocity => temporal delay Volumetric Thermal Expansion = Change in volume with Temperature: Specific heat = Heat energy required to increase temperature by  T Absorption coefficient [cm -1 ]: Amount of light energy absorbed in the sample. Also affects the light beam itself

Light beam considerations for PA experiment PA amplitude depends on light intensity derivative -> light should be modulated\pulsed Pulsed light is preferred:  Low average intensity (can use very short pulses)  High derivative of CW beam requires high frequency – attenuated by most liquids Pulse requirement:  Short rise time -> higher derivative  Short pulses – lower average energy (eye-safety)  Narrow spatial beam

Light Beam properties Temporal Spatial – high absorption Semi-spherical wave Spatial – low absorption Semi-cylindrical wave

Pressure wave propagation FDM simulation results High absorption: α=11.6 [cm -1 ] No reflections, no dispersion or medium attenuation

Pressure wave propagation FDM simulation results Low absorption: α=0.9 [cm -1 ] No reflections, no dispersion or medium attenuation

Effect of absorption coefficient Pressure wave at fixed location (FDM simulation)

Effect of spatial beam diameter

Additional considerations For FDM simulation Reflections from cuevette and pressure wave generation in cuevette Dispersion of acoustic wave Medium MTF PZT’s spatial response Scattering effects

Pros & Cons for PA usage for spectroscopy in the eye Pros:  Doesn’t depend on transmission or reflection of the light beam – can work with opaque materials, higher immunity to scattering effects  May work in various wavelengths  Signal depends on various characteristics of medium in addition to absorption (heat capacity, acoustic velocity) that may be used to improve detection  Depends on light intensity derivative – may be used with short pulses that have low average intensity  Option to work in wavelength where good optical broad- band detectors are hard to find

Pros & Cons for PA usage for spectroscopy in the eye Cons:  Requires to find specific wavelength in which there’s a good separation between signal of target proteins and other proteins\solvent  Requires a high-quality light source: pulses with very short rise time