Polarization-Sensitive Super-Resolution Phononic Reconstruction of Nanostructures

Abstract

In this paper, we show for the first time the polarization-sensitive super-resolution phononic reconstruction of multiple nanostructures in a liquid environment by overcoming the diffraction limit of the optical system (1 μm). By using time-resolved pump-probe spectroscopy, we measure the acoustic signature of nanospheres and nanorods at different polarizations. This enables the size, position, and orientation characterization of multiple nanoparticles in a single point spread function with the precision of 5 nm, 3 nm, and 1.4°, respectively. Unlike electron microscopy where a high vacuum environment is needed for imaging, this technique performs measurements in liquids at ambient pressure, ideal to study the insights of living specimens. This is a potential path toward super-resolution phononic imaging where the acoustic signatures of multiple nanostructures could act as an alternative to fluorescent labels. In this context, phonons also offer the opportunity to extract information about the mechanical properties of the surrounding medium as well as access to subsurface features.

Figure 1

(a) Simple schematic of the experimental setup. (b) Magnified area of the sample stage with input and output circular polarized light, which is modified by a half-wave plate (HWP) and polarizing beam splitter (PBS) (inset). This allows simultaneous measurement of the horizontal (H) and vertical (V) axis at the sample plane, as well as their rotation with the HWP (ϕ_Sample = 2ϕ_HWP). (c) Diagram of the optical point spread function (PSF) with a sphere and a rod vibrating. (d) Simulated mechanical response of a sphere (single frequency) and a rod (extensional and breathing mode frequencies) with linear polarized light. Here, it can be seen that the detected rod vibrations can be turned on and off, which allows the orientation characterization.

Figure 2

The extinction cross-section for two rods (112 × 40 and 145 × 50 nm) when the light polarization is along the length (on) or along the width (off). The probe laser wavelength is 780 nm.

Figure 3

The phononic reconstruction method. (a) Overlay between SEM and the frequency map where a single frequency is measured on a sphere (green), whereas two modes are obtained on a rod (red + blue = purple). (b) Phononic reconstruction of the size, shape, orientation, and localization with the SEM overlaid. (c) Angle characterization plots where the sphere is insensitive to linear light polarization and the rod matches with the real orientation maximum signal at 107°. (d–f) Time and frequency traces from each optical point spread function.

Figure 4

Optical, super-resolution phononic reconstruction and SEM images of multiple nanostructures inside the same optical point spread function.