Raman spectra were acquired using a Horiba LabRAM HR Evolution microscope (Horiba Scientific). Laser light from a 532 nm Nd:YAG laser was focused onto samples using an infinity-corrected achromatic 50× long-working objective (0.5 NA, LMPLFLN50X, Olympus) with the average power of ∼ 10 -11 mW. The scattered light was collected by the objective in a 180 • backscattering geometry. The scattered light was focused into a spectrometer and dispersed using a 1800 gr/mm grating. For low-frequency (< 500 cm -1 ) spectral measurements, two volume Bragg gratings (OptiGrate) were installed before the spectrometer to reject Rayleigh light. We experimentally verified that our low-frequency measurements can reliably measure down to 6 cm -1 (Figure ). To confirm that there was no interference from our sample substrate in the low-frequency region, we measured a spectrum of aluminum foil (Figure ). Spectra were imaged using a Synapse EM CCD camera (1600X200-FV, Horiba Scientific). Spectral acquisition times were 20 s per spectrum. Between 27 -128 spectra were taken per sample depending on signalto-noise. Our spectral acquisition parameters were chosen to maximize signal-to-noise without incurring potential photodamage to the sample.