Photo Acoustic Effect And its usage for spectroscopy.
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Transcript of Photo Acoustic Effect And its usage for spectroscopy.
Photo Acoustic EffectAnd its usage for spectroscopy
Photo-Acoustic effect principleModulated Light
Local Heating
Absorption: Ia = αI0
Thermal Expansion
Pressure Wave = Sound wave
VC
IT
p
a
))((Pressure 2 Tvac
)(0 TVV
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
))((1 2
2
2
I
tCP
tv pac
Acoustic velocity => temporal delay
Volumetric Thermal Expansion =Change in volume with Temperature :
T
V
V
1
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 absorptionSemi-spherical
wave
Spatial – low absorption
Semi-cylindrical wave
Pressure wave propagationFDM simulation results
High absorption: α=11.6 [cm-1]
No reflections, no dispersion or medium attenuation
Pressure wave propagationFDM simulation results
Low absorption: α=0.9 [cm-1]
No reflections, no dispersion or medium attenuation
Effect of absorption coefficientPressure wave at fixed location (FDM simulation)
Effect of spatial beam diameter
Additional considerationsFor 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