[time-nuts] optically excite a quartz crystal?

Andy Bardagjy andybardagjy at gmail.com
Mon Apr 21 12:06:04 EDT 2014


When this question was first posed, AOM's first jumped to my mind. An AOM (sometimes AOD) is an Acousto-Optic Modulator that works by setting up an acoustic wave in a crystal. When a laser is directed through (or reflected by) an AOM, it is deflected. 

One way to think about this is the crystal lattice deforms with the period of the acoustic wave. The lattice deformations form a grating of regions of varying indices of refraction which produce varying phase delay. This steers the beam just as a grating would. 

Another way of thinking about it is the 'phonon's' momentum and the photon's momentum add producing a deflection (like two billiard balls colliding). This might seem impossible, but the math works out exactly..

Another nice thing is, if you recall, the EM field produced in the far-field of a coherent EM wave impinging on a grating is the Fourier Transform of the grating itself. In something like a CD, the grating has hard edges, producing lots of harmonic content. AOMs look like sine waves, most of the energy ends up in the deflected beam. 

AOMs are often constructed using quartz crystals because they are fairly broadband (optically) and piezoelectric. The piezoelectric properties make it easy to set up the acoustic wave in the crystal.

The key difference is, AOMs are typically designed to absorb as little power as possible (power absorption is their primary failure mechanism). Depending on your laser, you might want to find an AOM that is less transparent in your excitation regime.

That said, you can get a lot of energy out of lasers these days. Megawatt pulses are not impossible with something like a Kerr-lens mode locked laser. This results in extremely high electric fields (MW pulse in 0.1 mm^2).

Andy Bardagjy
bardagjy.com

On Apr 21, 2014, at 6:47 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> 
> On 04/21/2014 03:18 PM, Attila Kinali wrote:
>> On Mon, 21 Apr 2014 14:54:12 +0200
>> Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
>> 
>>>> They used a 10mW HeNe laser, modulated with 1kHz to 1MHz on
>>>> various quartz cuts (X+5°, DT, AT) and could measure oscillations
>>>> of the quartz using metal electrodes.
>>>> The mechanism of exitation was photothermal
>>> 
>>> 10 mW laser is reasonable.
>>> 
>>> What levels of signal where they getting?
>> 
>> 6.2mV, with an real laser incident power of 5mW (the AOM "ate" half of the power)
> 
> Cool. Today we use semiconductor lasers that we can modulate directly, so no need for the AOM on the NeHe laser.
> 
> Cheers,
> Magnus
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