Abstract
Imaging the optical properties of individual nanosystems beyond fluorescence can provide a wealth of information. However, the minute signals for absorption and dispersion are challenging to observe, and only specialized techniques requiring sophisticated noise rejection are available. Here we use signal enhancement in a high-finesse scanning optical microcavity to demonstrate ultra-sensitive imaging. Harnessing multiple interactions of probe light with a sample within an optical resonator, we achieve a 1,700-fold signal enhancement compared with diffraction-limited microscopy. We demonstrate quantitative imaging of the extinction cross-section of gold nanoparticles with a sensitivity less than 1 nm2; we show a method to improve the spatial resolution potentially below the diffraction limit by using higher order cavity modes, and we present measurements of the birefringence and extinction contrast of gold nanorods. The demonstrated simultaneous enhancement of absorptive and dispersive signals promises intriguing potential for optical studies of nanomaterials, molecules and biological nanosystems.
Highlights
Imaging the optical properties of individual nanosystems beyond fluorescence can provide a wealth of information
We report three sets of measurements that demonstrate the potential of cavity enhancement for ultrasensitive imaging: first, we image gold nanoparticles with superior sensitivity and quantitatively evaluate their extinction crosssections
We study 40-nm gold nanoparticles as a reference system whose extinction cross-section can be calculated
Summary
Imaging the optical properties of individual nanosystems beyond fluorescence can provide a wealth of information. To achieve the required sensitivity, the imaging techniques demonstrated to date are carefully optimized to measure one single quantity by reducing measurement noise, implementing noise rejection techniques, and by signal averaging[1] This has enabled the imaging of weak sample absorption, for example, by photothermal microscopy[2,3] or direct absorption spectroscopy[4,5], as well as imaging of dispersive objects by interferometric scattering[6,7]. We report on a versatile approach that combines cavity enhancement with high-resolution imaging and provides high sensitivity for both sample absorption and dispersion simultaneously It is based on an open-access optical microcavity[15,16,17,18,19] made of two highly reflective mirrors, which permits imaging a sample by raster-scanning it through a microscopic cavity mode. We make use of the simultaneous enhancement of absorptive and dispersive signals and demonstrate measurements of the polarization dependent extinction and polarizability of gold nanorods, providing a way for the detailled characterization of the polarizability tensor of a nanoscale sample
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