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Background-free quasi-steady-state photoinduced absorption spectroscopy by dual optical modulation

We present a simple and inexpensive means to measure background-free photoinduced absorption spectra by implementing a dual optical modulation scheme. Our objective is to measure quasi-steady-state absorption spectra of long-lived photoexcitations, such as polarons and triplet excitons in organic semiconductors, when the photoluminescence of the sample is strong compared to the photoinduced absorption signal. In our instrument, we modulate a continuous-wave laser at a frequency $f_{pu}$ with a mechanical chopper before exciting the material. We measure the fractional change of transmission of the sample with a tuneable monochromatic probe source modulated at a frequency $f_{pr}$, and with a digital lockin amplifier at a reference frequency $f_{ref} = f_{pu} + f_{pr}$. We generate the reference sinusoidal wave for the lockin amplifier using a simple home-built electronic circuit that generates their sum-frequency signal. This scheme allows measurement of the fractional change of transmittance of the sample induced by the pump laser at a frequency $f_{pu} + f_{pr}$ without parasitic signal at a frequency $f_{pu}$, such as photoluminescence of the sample or pump-laser scatter. We demonstrate the power of this approach by measuring cleanly the photoinduced absorption spectrum of a highly luminescent semiconductor polymer, poly(9,9-dioctylfluorene-co-benzothiadiazole), at 10\,K. We compare this measurement favourably with that obtained by simple modulation of the pump laser after subtraction of the dominant photoluminescence background.