In a conventional ultrafast acoustic experiment, coherent acoustic phonons are generated in the form of a picosecond strain pulse and detected by monitoring strain-induced changes in the optical properties of the studied medium. This experimental scheme has been used over the years to investigate a wide range of phenomena and seems to have little room for significant enhancement. Nevertheless, in our study, we demonstrate how to achieve ultimate detection sensitivity within this scheme, employing a layer with a narrow polariton resonance as a phonon detector that allows detection of strain pulses with amplitudes well below lattice thermal fluctuations [1].
The key element of the performed experiment is the periodic structure grown on a GaAs substrate and consisting of 30 GaAs quantum wells (QWs) of 17.5-nm thickness separated by 8-nm (Ga,Al)As barriers. Each QW hosts a narrow exciton resonance centred at ħωX = 1.5307 eV. Photons with the energy ħω ≈ ħωX and excitons in the QWs form a coupled state referred to as an exciton-polariton, and the entire structure can be treated as an effective medium with a narrow optical resonance at ħω = 1.5307 eV and strong permittivity dispersion within the resonance spectral width. The strain-induced shift of ħωX through the deformation potential mechanism results in significant changes of the refractive index and, thus, modulation of reflectivity.
In the validating experiment performed in a conventional pump-probe scheme, the 150-fs pump laser pulses excited a 100-nm-thick Al film on the backside of the GaAs substrate. Ultrafast thermal expansion of the film generated a strain pulse, which travelled through the substrate toward the polaritonic layer, where it was monitored employing the probe pulses tuned to the spectral position of the polariton resonance. We measured the transient reflectivity, which possesses periodic oscillations due to the dynamical interference of the probe pulse reflected at the structure’s front surface and the propagating strain pulse. This signal, often referred to as time-domain Brillouin scattering, remained well detectable at the pump fluences down to 10 nJ/cm2 and the corresponding strain pulse amplitude of 10-9. The increase of the Al film temperature at this fluence was just 0.1 K, resulting in its thermal expansion of 100 attometres, which is 4 orders of magnitude less than atomic thermal motion at T = 10 K. We have also proved the large dynamical range of the polaritonic detection and examined how the transient reflectivity signal depends on the probe pulse fluence and its spectral detuning from the polariton resonance [2].
[1] M. Karzel et al., Nat. Mater. (2025), doi: 0.1038/s41563-025-02229-3.
[2] M. Karzel et al., ACS Photon. 11, 5147 (2024).