Re: Second harmonic generation Q?

On Sep 12, 9:52 pm, Dave Bell <db...@xxxxxxxxxxxxxxxxxxxx> wrote:
Greetings--

The effect does exist at lower intensities, but the conversion rate
drops with the input intensity. So at ordinary (non-laser)
intensities, the effect is unmeasureably small.

With my students I do a demonstration in which an input power of
perhaps 20 milliWatts of infrared light passes through a material, and
some (small fraction) of it gets frequency-doubled, producing perhaps
2 microWatts of output blue light.

That conversion efficiency is 10^-4, or 0.01% conversion.

If I could use 200 mW input, 10 x as much, the conversion efficiency
would in principle rise 10 x too, giving a 0.10% conversion
efficiency. And 0.1% of 200 milliWatts is 200 microWatts of output --
much better.

So you see there's a real premium on high input power; this is
sometime achieved by using pulses of very short duration, which
exhibit peak power very much higher than their average. There are
other ways to achieve higher efficiency, too, but none is very simple.

So there's no threshold to the process, but it's very hard (in a
single-pass geometry) to get a decent conversion efficiency; even the
paltry numbers listed above apply only to light of one polarization,
and one (optimum) wavelength, and for one of the materials with
highest known conversion efficiency.

Hoping that's not discouraging--
D. Van Baak

It might be discouraging, but it's very interesting!
What sort of mechanism drives this, so the conversion rate is more or
less proportional to the input power?
(Too bad the proportionality doesn't extend to ridiculous limits!
20 mW => 1e-4
200 mW => 1e-3
2 W => 1e-2
20 W => 1e-1
200 W => 1
2 kW = 10X !!)

Dave

The output electric field Eout goes (at low enough input levels) as
the square of the input electric field, Ein^2, so the output power
Pout also goes as the square of the input power, Pin^2. If you write
Pout = k Pin^2, you can get for 'conversion efficiency' the ratio
Pout/Pin = k Pin,
showing that conversion efficiency rises with power.

The square-law relationship is appropriate to a view of the process in
which two input low-energy photons have to pool their energy to make
one output high-energy photon.

Of course this mechanism saturates as the conversion efficiency
approaches 1! and that is the motivation for putting doubling
crystals into laser cavities, or external cavities, to give a higher
conversion efficiency in the stronger light fields inside the
cavities.

Your familiar hand-held 532-nm green laser pointer has its output
light born by intracavity frequency-doubling of 1064 nm laser
radiation; the doubling crystal is likely KTP, and the laser is Nd:YAG
or Nd:YALO, itself diode-laser pumped at about 808 nm.

Cheers, D. Van Baak

.

Relevant Pages

  • Re: Second harmonic generation Q?
    ... exhibit peak power very much higher than their average. ... single-pass geometry) to get a decent conversion efficiency; ... and for one of the materials with ... Your familiar hand-held 532-nm green laser pointer has its output ...
    (sci.optics)
  • Re: Second harmonic generation Q?
    ... The effect does exist at lower intensities, ... If I could use 200 mW input, 10 x as much, the conversion efficiency ... So you see there's a real premium on high input power; ... and for one of the materials with ...
    (sci.optics)