Using a Variable Probe to Measure Forward and Backward Propagating Phonon-Polaritons

A photon is a particle of light. A phonon is a vibration within a crystal. Photons and phonons can couple with each other to form a quasi-particle called a phonon-polariton. The dispersion curve of a particular phonon-polariton shows how the frequency of the polariton depends on wavevector. The crystal LiNbO3 has a phonon-polariton that oscillates at frequencies between 0 and 4 THz depending on the wavevector. We hypothesized that with a broadband THz pulse (with frequencies between 0.5 and 5 THz) we can excite all along the dispersion curve for this phonon-polariton in LiNbO3. However, with our a typical 800 nm probe pulse, we could only see one clear peak at ~3.7 THz. The wavevector of the probe pulse determines which frequencies it “sees”. We prove that we are indeed exciting all along the phonon-polariton curve with our THz pulse by ranging the probe wavelength between 1250 nm and 1550 nm. This allows us to probe different regions of the phonon-polariton dispersion curve, experimentally mapping parts of the curve. We experimentally measure both the forward and backward propagating wavevectors of the phonon-polariton. Our measured values match up well with the known LiNbO3 phonon-polariton dispersion curve.