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We present a simple and inexpensive means to measure background-freephotoinduced absorption spectra by implementing a dual optical modulationscheme. Our objective is to measure quasi-steady-state absorption spectra oflong-lived photoexcitations, such as polarons and triplet excitons in organicsemiconductors, when the photoluminescence of the sample is strong compared tothe photoinduced absorption signal. In our instrument, we modulate acontinuous-wave laser at a frequency $f_{pu}$ with a mechanical chopper beforeexciting the material. We measure the fractional change of transmission of thesample 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 lockinamplifier using a simple home-built electronic circuit that generates theirsum-frequency signal. This scheme allows measurement of the fractional changeof 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 asphotoluminescence of the sample or pump-laser scatter. We demonstrate the powerof this approach by measuring cleanly the photoinduced absorption spectrum of ahighly luminescent semiconductor polymer,poly(9,9-dioctylfluorene-co-benzothiadiazole), at 10\,K. We compare thismeasurement favourably with that obtained by simple modulation of the pumplaser after subtraction of the dominant photoluminescence background.

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