A common LED emitter, Gallium Arsenide, apparently has a wavelength of about 760 nm. Then on the receiving end, there is a filtering circuit to eliminate ambient IR and to demodulate the message. Can we modulate the IR emitter signal to obtain different frequencies? Materials have different absorption properties that correspond to different frequencies of the IR emitted. So, if we modulate the output of the IR emitter, will the modulation be enough to trigger the different materials' absorption. Or do we need to use a different LED (other than Gallium Arsenide for example)?
A common LED emitter, gallium arsenide, apparently has a frequency of about 760 nm.
Figure 1. 760 nm is off the end of the visible red range.
OK so far.
Then on the receiving end, there is a filtering circuit to eliminate ambient IR and to demodulate the message.
Not quite. The filter is to eliminate ambient light which may overload the sensor. If it is to let through the signal IR then it must let through the ambient IR too. A good quality band-pass filter might be able to narrow the accepted IR just to the wavelenghts of interest and this will help.
The filter does not demodulate the message. That is done by the receiver circuitry. The filter is just "sunglasses for the receiver".
Can we modulate the IR emitter signal to obtain different frequencies?
You can modulate the IR signal on and off at various frequencies as vini_i has explained in his/her answer. Note that this is not changing the frequency of the IR light.
Figure 2. You can see this modulation with your phone's camera which is sensitive to infrared. With a half-decent camera the blinking of the LED will be visible. The blinking is the modulation.
Notice that with the above experiment that if the modulation was to change the wavelength of the IR then the colour would change. It doesn't.
Materials have different absorption properties that correspond to different frequencies of the IR emitted. So, if we modulate the output of the IR emitter, will the modulation be enough to trigger the different materials' absorption.
If you were able to modify the IR wavelength then yes, but since you can't the answer is no.
Or do we need to use a different LED (other than gallium arsenide for example)?
LEDs aren't tunable other than at manufacture although some change in colour is possible when they are heated. You can try this yourself with a visible light LED. Connect it to a bench PSU, start with the current limit set to 20 mA and then slowly crank it up until it dies. On the way you should see a shift in the colour as well as an increased light output.
Tunable IR sources are available for laboratory work such as obtaining IR signature spectra for various materials such as plastic films. These sweep through the far IR spectrum and produce graphs showing the absorption or transmittance spectrum for a sample. These can then be compared with reference data to establish the likely plastic composition as each type will have a particular signature. This in turn can be used to determine the quality or purity of the plastic.
Note that element names from the periodic table are always written lowercase while the first letter of their symbol is capitalised. I fixed this in the quotes from your post.
The frequency of 760nm if the LED cannot be changed without replacing it with another one.
The modulation you speak of is the act of turning the LED on and off rapidly. One way is to use segments of these on/off pulses to represent zeroes and ones. The demodulation is turning the pulses back into zeroes and ones. This act does not change the frequency of the LED.
Now I suspect you may be referring to a tunable laser used in laser absorption spectroscopy. The wavelength of laser diodes can be optically converted.
A single wavelength laser diode (e.g. green 530 nm) can be converted to wavelengths from UV throughout the entire IR band.
An example is this tunable, visible and near IR light source that uses a single 532 nm laser diode. It converts to wavelengths of 450 – 650 nm & 900 – 1300 nm.
Another example of wavelength conversion is a white LED that converts 450 nm blue up to red wavelengths using phosphors.
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A 760 nm wavelength LED emits photons that are oscillating at a frequency of 394 Thz.
It is not an IR LED but rather emits the color of "far red" which can be called "near IR".
Far-red light is light at the extreme red end of the visible spectrum, just before infra-red light. Usually regarded as the region between 710 and 850 nm wavelength, -- Wikipedia Far-Red
760 nm is not invisible but does not excite human image capturing receptors of the retina nearly as much as shorter wavelengths (higher frequency).
Where green 555 nm has an average human "perceived brightness level" (photopic luminous efficacy) of 100,000, 760 nm far red has a photopic luminous efficacy of 6.
-- source: Relative Sensitivity Curve for the C.I.E. Standard Observer
Higher optical frequency photons carry more energy than lower frequency photons.
The energy level (frequency) of a photon exiting an LED is determined by the energy level of the LED's band gap. It takes more (or less) electrical energy for electrons to attempt to cross the band gap from the anode to cathode depending on the length of the band gap and its impurities (dopants).
As the electrons are attempting to cross the band gap many of the electrons are transformed into photons. The energy level of the bandgap is what puts the "spin" on the photon's oscillations.
While the bandgap energy level is mostly a function of the materials (e.g. silicon, germanium and dopants) its energy is also influenced by temperature, amount of current, and modulation (or lack there of).
LED drivers can change the average current by changing the amplitude of the current or by using modulation (PWM). The preferable method of dimming an LED is by PWM rather than decreasing the average current because PWM does not alter the chromaticity where changing the current amplitude will alter chromaticity. Modulation does not directly change the wavelength but using modulation will preserve the chromaticity by keeping the current amplitude constant. -- source: OSRAM App Note Dimming LEDs
Therefore if you currently dim the LED using PWM you could minimally change the wavelength by not using PWM and rather by decreasing the current.
Examples of changes in chromaticity due to current and temperature:
Materials have different absorption properties that correspond to different frequencies of the IR emitted.
Material absorption does not affect an LED's wavelength. Absorption will affect the internal quantum efficiency of the LED (number of photons emitted from LED vs. number of photons emitted from the LED's quantum well).
will the modulation be enough to trigger the different materials' absorption.
The material absorption is constant. The phase of the photons oscillation is what changes absorption, reflection, and transmittance.
The image below is from Richard Feynman's lecture on quantum electo-dynamics and the theory of photons. The phase of the photon's oscillation is represented by the direction of the arrow's angle (highlighted).