Fourier Domain Rotational Anisotropy Second Harmonic Generation

We describe a novel scheme of detecting rotational anisotropy second harmonic generation (RA-SHG) signals using a lock-in amplifier referenced to a fast scanning RA-SHG apparatus. The method directly measures the n^th harmonics of the scanning frequency corresponding to SHG signal components of C_n symmetry that appear in a Fourier series expansion of a general RA-SHG signal. We will present measurements on a GaAs test sample to compare point-by-point averaging with the lock-in based method. We will discuss how, when divided by the C_∞ signal component, the lock-in detected data allow for both self-referenced determination of ratios of C_n components of up to 1 part in 10⁴ and significantly more sensitive measurement of the relative amount of different C_n components when compared with conventional methods.