When imaging sound waves in a crystal, the sound wave train causes strains at defective points along its path. Electrons trapped near the defects change spin state, (and) these charges emit photons.
So it is possible, with the used of crystals and sound waves to generate photon pulse. If the process of strain and photon emissions is repeatable (given due relaxation time), then a sonic pulse of certain frequency will give rise to a photon pulse of the same frequencies.
Maybe a carrier frequency derived from the atomic distances in the crystal that keeps the lattice in a 'energized' state will help with such sonic photon emissions. Superimposed on the carrier/base frequency will be the desired photon emission frequency.
This way there can photon pulse train of a particular frequency without LEDs.
