Re: Thermal Waves ?

[Brian Whatcott <betwys@DIRECTVINTERNET.COM>]

At 07:44 AM 11/23/02, you wrote:
> What is "second sound?"
> ...
> A sample is slowly scanned by a focused beam of laser light
> of small power (not to burn anything at a focus). The beam
> intensity is modulated by a mechanical chopper; that is how
> the frequency can be chosen. Sound waves, produced at the
> irradiated spot, are picked by a piezoelectric microphone.
>
> The signal is so small (microvolts) that a lock-in amplifier is
> needed. The output voltage V, produced by the amplifier,
> changes as the light beam goes from one spot on the surface
> to another. The V(y,z) happens to be a good map of the
> invisible structure below the shiny surface of a Si chip. The
> n and p doped regions were recognizable in the illustration
> as well as mechanical cracks and other defects. The speaker
> said that instruments of that kind have been used in industry
> since 1970's.
> Ludwik Kowalski

This technique is reminiscent of the acoustic microscope,
 which can use signal sources as high as 100 MHz to illuminate
opaque specimens at high resolution, and whose signal does
not decay so rapidly as a thermal pulse would.

In this connection, it always seemed a shame to me, that an
underground house would need to be so deep in order to have
the day's heating arrive at the coldest hour of night, that the
temperature would already have stabilized to the unvarying
seasonal average, the diurnal variation subsided to the
noise level.

Brian Whatcott
  Altus OK                      Eureka!

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.