Experimental determination of optical absorption of materials, predominantly revealing electronic bandgap and excitonic effects, requires proper sample preparation for optimal reflectivity and transparency. In some cases these conditions are hard to meet, such as in amorphous materials, powders, or other structures exhibiting intense light scattering. When illuminated with amplitude-modulated light beam, the investigated semiconducting material excitations undergo radiative and nonradiative recombination, with the latter releasing heat. Periodic changes of sample-gas temperature interface generate pressure fluctuations, detected as acoustic waves. That effect gave rise to a method called photoacoustic spectroscopy (PAS), a classic representative of photothermal methods.
So far, our research succeeded in revealing optical absorption by means of PAS in semiconducting nanowires grown on opaque structures, two-dimensional materials, inorganic perovskites, and highly emissive colloidal nanoplatelets.
Representative papers
Exciton Binding Energy of Two-Dimensional Highly Luminescent Colloidal Nanostructures Determined from Combined Optical and Photoacoustic Spectroscopies, The Journal of Physical Chemistry Letters 10 (12), 3459 (2019)
Optical Properties of In_2xGa_2–2xO₃ Nanowires Revealed by Photoacoustic Spectroscopy, ACS Applied Materials & Interfaces 11 (21), 19260 (2019)
Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals, Scientific Reports 7 (1), 15365 (2017)
Photoacoustic spectroscopy of absorption edge for GaAsBi/GaAs nanowires grown on Si substrate, Applied Physics Letters 109 (18), 182106 (2016)

Supercontinuum light source, often called the white laser, provides a collimated beam containing a broad electromagnetic spectrum, most often including visible and near-infrared ranges. It can be utilized for various optical spectroscopy experiments, including PAS, with the advantage lying in high power density and imminent compatibility with microscopic systems.
Representative paper
Supercontinuum source as a probing beam in photoreflectance and photoacoustic spectroscopy, Measurement 146, 879 (2019)