Nonlinear optics and phase sensitive detection
Tue Aug 1 08:47:20 BST 2006
In less than a month I start work on a new project in Australia,
working in [Dave Kielpinski's atomic cooling
group] at Griffith University.
One aspect of the project is to use high repetition rate pulsed
mode-locked lasers to cool atomic hydrogen. This relies on a couple
of tricks, one being the phase coherence of the frequency comb (see
[quant-ph/0306099]), and the
other being the fact that nonlinear optics is a lot more efficient for
pulsed lasers than for continuous wave lasers of the same average
power. (This is for simple materials, periodically poled materials in
a waveguide configuration can also give good conversion efficiency for
CW lasers. However these materials are tailored for a specific
frequency, may not be as flexible, require precise temperature
stabilization, may not work at the wavelengths we want, are more
expensive, less readily available, etc. Look again in five years time when
they are in more general use.)
If of order unity frequency conversion of pulses is possible then it becomes possible to think about an interferometric setup to probe sample properties. The setup consists of two nonlinear crystals with the sample under investigation between them. Assume for the sake of argument that the sample does not respond to the pump wavelength, so that we can let the pump pulse pass through the sample (a more complicated setup using dichroics to separate the pump and probe pulses is possible if this is not the case). The probe pulse produced by the nonlinear crystal passes through the sample and undergoes phase and amplitude changes. It then hits the second nonlinear crystal together with the pump pulse and is reconverted to the pump wavelength, imprinting the sample information onto the pump.
The point of doing this is that the pump wavelength may be more suited to manipulation be conventional optics than the probe, for example a visible pump producing a VUV or X-ray probe pulse, or entangled photons. Rather than inventing new techniqes for manipulating short wavelength optical signals, instead work out how to efficiently convert the short wavelength back to the original wavelength and do the signal processing then.
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