[Laser] Laser Digest, Vol 87, Issue 7

[Dave]

> Date: Sun, 18 Mar 2012 14:34:34 -0700 (PDT)
> From: Tim Toast<toasty256 at yahoo.com>
> Subject: Re: [Laser] Strange effect
> To: laser at mailman.qth.net
> Message-ID:
> 	<1332106474.81544.YahooMailClassic at web37903.mail.mud.yahoo.com>
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>
> Thanks for all the info Yves.
> Maybe this means there's a new band we can use here. For use
> with short light pulses at less than about 100 Hz rates and maybe
> linear modulation with low frequencies under 100 Hz (PSK etc)
> 10 microns is in a good atmospheric window for long range use.
> Eye-safe too at the lower powers (I THINK).

Hmm, interesting.  I wonder if there are any low-band-gap energy 
semiconductors
that would respond directly to such radiation?  I know that Lead Sulfide 
is a
fairly low-band-gap energy semiconductor (band-gap energy of about .37 eV,
compared to 1.107 eV for Silicon, and .67 eV for Germanium).  There are
even a few others that are even lower.

If I'm running the numbers correctly (E=.37 eV, c=3E8 m/s, 
h=4.135667516E-15 eV*s),
then a band gap energy of .37 eV corresponds to a wavelength of about 3.353
micrometers (33530 Angstroms).  Thus, using a Lead Sulfide detector as a 
photodiode
(or, even as a PIN diode?) should give you sensitivity down to 3.353 
microns.  That's
not quite down to 10 microns, although it's getting pretty close.

10 microns would be a band-gap energy of .124 eV, which is getting quite 
low.
The only thing I show with a band-gap energy that low is the alpha phase 
of Tin (.08 eV),
and I don't know if anyone has tried making a semiconductor out of that 
(Note that
there very well may be other materials with band-gap energies that low; 
it's just
that I haven't come across them in the table I've compiled.).  There are 
a couple
of candidates that come close, though, such as Copper Iron Selenide 
(CuFeSe2) at
.16 eV, Cadmium Arsenide (Cd3As2) at .14 eV, Cadmium Tin Arsenide 
(CdSnAs2) at
.26 eV, Indium Antimonide (InSb) at .17-.18 eV, Mercury Selenide (HgSe) 
at 0-.061 eV!,
Mercury Telluride (HgTe) at .2-.3025 eV (at 4K), and Lead Selenide 
(PbSe) at .26 eV.
Of course, I don't know if there's any work being done on producing 
semiconductors/
photodiodes/PIN diodes/etc., out of such materials.  But, there's some 
information
to get someone started if they want to do some research.  :-)

Anyway, using a low-band-gap semiconductor for such wavelengths would allow
you to run full data rates, although it very well may require cryogenic 
cooling to
minimize leakage to an acceptable level.  Maybe.

> As for transmitters in this band - CO2 stands out. People have
> been home building CO2 lasers for quite a while now. There are
> several kinds including CW and TEA pulse types. Lots of room
> for experimentation and not out of the question for Home building
> of all but the detector itself.

And, perhaps even the detector itself!  :-)

> I don't know of any natural sources of 10 micron pulses except
> for maybe lightning or astronomical sources. This band should
> be nearly completely free of manmade light pollution. There would
> be lots of heat sources but mostly CW from heated objects and the
> Sun.

I think you're mostly correct on that, although you may get quite a
bit of 120 Hz hum from incandescent lights, which can emit at quite
low frequencies.

Dave