Frequency domain interferometry for measuring ultrafast refractive index modulation and surface deformation

ABSTRACT Ultrafast optical spectroscopy delivers unparalleled insights into the dynamic response of photoactive materials, including semiconducting, photonic and phase-change materials. The most applied experimental tool – transient absorption spectroscopy – derives signals from induced changes in the intensity of transmitted light, assumed to relate to the imaginary part of the refractive index. However, the entire complex refractive index of materials changes in the excited state; changes in the real part of the refractive index can have significant effects on transient absorption spectra and the function of optical devices. In this review, we introduce an emerging ultrafast spectroscopy method – frequency domain interferometry. This simple adaptation of transient absorption spectroscopy provides a model-independent means of spectrally resolving photoinduced changes in a materials refractive index. After introducing the theory and implementation of the method, we describe several case studies, including the optical response of metal-halide perovskites and phase modulators, and surface displacement of phase-change materials. Finally, we describe recent and future improvements that can enhance the time-resolution and signal sensitivity of this technique. The advances and applications highlighted in this review demonstrate the potential of the method to become a standard part of the ultrafast spectroscopy toolbox for characterising optoelectronic and photonic materials and devices. GRAPHICAL ABSTRACT