Responsivity setup: Light from a globar (SLS203L, Thorlabs) was focused with a CaF2 lens, and modulated with a chopper (MC2000B-EC, Thorlabs), onto a monochromator (SpectraPro HRS-300, Princeton Instruments) equipped with two gratings (150 G/mm, blaze wavelength of 0.8 µm; 150G/mm, blaze wavelength of 2 µm). Higher frequency orders were filtered out with longpass filters (780 nm, 1000 nm, 1500 nm, Ge wafer). Ten time-traces each one second long were recorded and the resulting power spectral densities were averaged. The 50 Hz frequency was manually removed from the frequency spectra.