MadSci Network: Engineering

Re: action of photons on a piezoelectric crystal

Date: Mon Jan 24 11:15:02 2005
Posted By: Kenneth Beck, Senior Research Scientist, Chemistry and Physics of Complex Systems, Pacific Northwest National Laboratory
Area of science: Engineering
ID: 1098962075.Eg

Dear Philippe,

The explicit answer to your qestion is: No.

I'm assuming I won't bore you by examining why, in fact, piezoeletric 
cryatals and material act like excellent detectors for electromagnetic 
radiation - although it is not "radiation pressure" which they sense.

When I was a graduate student in the last century we had a problem of 
measuring the spatial profile of our infrared CO2 (carbon dioxide) 
laser.  Our postdoc and I came up with the idea of using piezoelectric 
crystals made from modified barium titanate (PZT) and piezoelectric 
materials, such as poly-vinyledene di-fluoride (PVDF) - a plastic.  Our 
reasoning flowed from the need for a material that could withstand high 
irradiation intensities and still respond with a linear signal output.  
That is, the measured signal had to be proportional to the laser pulse 
energy even at 10 Joules/cm^2 fluence.  Piezoelectrics to the rescue!

So, why do they work? It is not the piezoelectric effect that's active in 
raidation detection, but the second-order pyroelectric effects common to 
all piezoelectric materials. They are permanently polarized and acts as a 
pyroelectric detector because any change in the temperature difference 
between both the surfaces (front-to-back) generates corresponding change 
in polarization and as a result a voltage signal across them. 

A pyroelectric calorimeter has been developed using PVDF foil to measure 
the energy of x-rays emitted from laser produced plasma which provides 
two types of signal in pyroelectric calorimeter mode. One of the signal 
has fast response which is produced due to thermo-elastic effect in foil 
whereas other one has slow response which is actual time integrated 
calorimetric signal. However, both the signals are proportional to the 
absorbed x-ray energy - that is they have a linear response. 

It's been found that 100% absorption of the x-rays occurs on aluminum-
coated detectors.  Sensitivity is increase if a gold coating is used, 
instead.  A black coated PVDF detector will enhance the piezoelectric 
sensitivity for spectral range UV, VIS,IR. 

Hope this helps!

---* Dr. Ken Beck

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