Often known as “nanoscale earthquakes”, surface acoustic waves (SAWs) are widely used in electronics, optics, microfluidics and biological fields, applied in medical diagnostics to mobile communication and quantum computing [1]. Typically SAWs are generated on a piezoelectric substrate by applying a sinusoidal voltage signal to comb-like metal electrodes, called as interdigital transducers (IDTs), following which strain is produced in the substrate via inverse-piezoelectric effect, leading to internal stress and propagation of SAW. Commonly, symmetric IDTs produce a single frequency SAW propagating perpendicular to the aperture of the IDT. For so-called tapered IDTs (TIDTs) the periodicity is tuned along the aperture of the transducers. This variation increases the bandwidth and encodes the spatial coordinate along the aperture in the frequency of the SAW. This position-frequency encoding can be employed for position sensing of mass loading or local variation of the conductivity of a material [2, 3]. As a result, SAWs can uniquely probe the presence of a material completely contact-free, detecting on the change in its transmission coefficient, ΔS21.
Here, we present SAW tomography of the photophysical properties of two different types of halide perovskite (CsPbBr3 and CsPb(IxBr1-x)3) nanowires (NWs). These NWs are drop-casted along the propagation path of delay lines formed by TIDTs. We begin by demonstrating position-dependent SAW-NW interaction by measuring the photoluminescence (PL) for different frequencies of the TIDT passband. In these experiments, we observe the expected quenching of PL of halide NWs in presence of SAW [4] exclusively at positions where SAW is propagating for the selected frequency. Next, we move on to photoconductivity tomography and measure the change of conductivity upon photogeneration via ΔS21. Using a blue laser, both the NWs absorb radiation and the corresponding ΔS21 with respect to frequency along the aperture width of TIDT maps areas covered by the NWs. These tomograms agree well with the optical microscope images and the change of ΔS21 due to mass loading by the NWs. Finally, when using a defocused red laser, only CsPb(IxBr1-x)3 NWs absorb and the corresponding ΔS21 probes exclusively the change of photoconductivity in these NWs. Our approach can be extended to fully-fledged spectrally resolved tomography on different materials harnessing wavelength-dependent light absorption and using a spectrally filtered white-light source.
References
[1] P. Delsing et al., J. Phys. D: Appl. Phys. 52, 353001 (2019).
[2] A. Müller et al., NATO Science Series, vol 233. Springer, Dordrecht (2006).
[3] M. Streibl et el., Appl. Phys. Lett. 75, 4139−4141 (1999).
[4] L. Janker et al., Nano Lett., 19, 8701−8707 (2019).