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