Re: Is it possible to IR modulate a UV carrier wave?

Greetings:

Background
Ultraviolet lasers are being developed for use in photolithography for the 
development and manufacturing of the next several generations of integrated 
circuits (ICs) where each chip will have about one billion transistors. Each 
of these chips will have lines and features with nanometer (nm) dimensions. 
Current IC technology has features about 0.1 micrometers (100 nm) in size. 
It is interesting to note that one nanometer is about two atoms in length! 
We are currently in the process of moving from microelectronics to 
nanoelectronics and as we approach circuits with dimensions of a few dozen 
atoms, we must develop new circuit concepts based in quantum mechanics in 
place of current semiconductor electronics.  Instead of working with 
electronic bits these circuits will operate with Q-bits.

For machining, cutting, welding, drilling etc. the minimum spot size 
(feature size) that a laser beam can be focussed down to is about one 
wavelength in diameter. The most common high-powered lasers currently are 
carbon dioxide lasers which operate at infrared (IR) wavelengths near 10 
micrometers (10,000 nm) and a variety of lasers that operate in the IR near 
1 micrometer (1000nm) wavelengths. Ultraviolet (UV) wavelength lasers 
operate at wavelengths less 400nm and currently produce relatively low power 
when compared to the high power IR lasers.

Question
Your question asks if UV lasers can be modulated with IR energy to produce 
more heat than current UV lasers can generate.

Answer 
(Reference: Amnon Yariv, "Introduction to Optical Electronics", Holt, 
Rinehart and Winston, 1971.)

All of the techniques used at radio frequencies have been demonstrated at 
optical frequencies provided similar devices are available at optical 
wavelengths. In the radio frequency (RF) portion of the electromagnetic 
spectrum, where electronic tubes and transistors operate with amplification 
(gain), it is possible to modulate a radio frequency oscillator with high 
powered audio signals that produce a composite spectrum of energy around the 
RF carrier frequency with a higher energy density than a single frequency 
oscillator can produce alone. Often noise modulation is used to produce 
composite waveforms with the greatest uniform energy density.

Unfortunately current optical modulators are loss modulators, that is they 
remove energy from the optical carrier frequency to generate complex 
modulated waveforms. The greater the information bandwidth of the modulation 
the greater is the total loss of energy from the optical carrier frequency. 
We are beginning to develop optical amplifiers with gain, such as erbium 
fiber doped amplifiers (ERDFA) that operate in the IR at 1550 nm at powers 
of a few watts. However, we are a along way from generating greater UV 
energy by the modulation process. We will need optical amplifiers first 
(lasers).

Frequency multiplication is a technique which shows much greater potential 
for upconverting optical energy to smaller wavelengths. In this technique we 
use crystals with nonlinear optical properties to double the optical 
frequency (half the wavelength). Recently I purchased a green laser pointer 
that has a 1064 nm infrared laser diode that is frequency doubled in a 
neodymium crystal doped with YV04  to 532 nm , a green wavelength. Frequency 
doubling from the IR has been reported to be more than 50% efficient. To 
double to UV wavelengths we would need to start with a high powered blue 
laser such as the argon ion laser operating at 480 nm doubled to 240nm. 

Unfortunately we run into a new problem if we use the doubling  technique to 
generate UV energy. IR photons and longer visible wavelengths photons 
(red,orange) vibrate molecules (heat them) until the bonds are broken and 
the molecules fly apart. UV photon energy is great enough to produce 
ionizing radiation and the photons can tear atoms apart.  This is why UV 
energy is carcinogenic and so dangerous to living cells. This ionizing 
process also darkens optically transparent doubler and modulator crystals in 
a process called "optical damage". Optical damage is caused by impurity 
atoms being ionized by optical photons and is very difficult to 
overcome. It will take a great deal of materials research to grow 
nonlinear crystals that can operate at blue and UV wavelengths without 
optical damage. 

Today, our best hope to generate power in the UV is to develop laser 
oscillators and laser amplifiers working at wavelengths less than 400 nm. 
Beyond that we may move to X-ray lasers and focussed ion and electron beams 
for lithography and nanomachining.

Best regards, your Mad Scientist
Adrian Popa