Scanning probe technologies offer unparalleled insights into nanoscale light-matter interactions but are often hindered by the complex interplay of concurrent light-induced effects. To overcome this limitation, we have recently developed a technique known as decoupled optical force nanoscopy (Dofn). Dofn uniquely separates light-induced forces—such as optical gradient, photoacoustic, and photothermal effects—by employing a designated modulation and demodulation of the phases of measured optical forces. In my presentation, I will discuss how Dofn enables the precise probing of nanoscale chirality emerging from nanostructures, providing new avenues for understanding and manipulating light-matter interactions at the nanoscale.
KEYWORDS: Ultrasonography, Photoacoustic imaging, Microscopy, Data acquisition, Photoacoustic spectroscopy, In vivo imaging, Algorithm development, Temporal resolution, Super resolution, Oncology
Photoacoustic (PA) imaging can map the physiological conditions of tissue and track the biodistribution of contrast agents; ultrasound localization microscopy (ULM) with microbubbles provides deep-tissue super-resolution blood vessel images and blood velocity maps. When combined, the dual imaging technique will become a powerful tool with diverse applications. However, longer acquisition time of ULM poses a challenge. To address this, we propose sparsity constraint optimization to accelerate ULM and develop an interleaved PA/fast ULM imaging sequences for the dual imaging. We demonstrate 3D and time-lapse dual imaging with a commonly used linear array imaging system in a mouse model.
Scanning probe technology is advantageous in visualizing nanoscale light-matter interactions, however, this technology is limited by the slow raster scanning speed. In this talk, I will discuss how to visualize these ultrafast interactions beyond the conventional scanning speed. We develop a technique called decoupled optical force nanoscopy that can differentiate optical forces with different origins.
The signal-to-noise ratio (SNR) is the main figure of merit that assesses the quality of magnetic resonance imaging (MRI). Enhancing SNR oftentimes involves enhancing the magnetic field intensities of the constant homogenous magnetic field from the main coil, or the oscillating magnetic field from the radio frequency (RF) coil. Here we show that the SNR of MRI can also be improved by enhancing the coupling between the imaging subject and the receiving coil during the signal reception. We will provide a theoretical design of an ultrathin metasurface with micrometer thickness and high flexibility. Our metasurface can enhance the SNR by up to 28 times in the region of interest; at the same time, it is designed to minimally disturb the excitation fields by less than 1.6%, thus maintaining the uniformity of the excitation, important to achieve a high-quality MR image with minimal artifacts.
KEYWORDS: Plasmonics, Super resolution microscopy, Atomic force microscopy, Molecules, Visualization, Macromolecules, Proteins, Optical tweezers, Resonance energy transfer, Fluorescence resonance energy transfer
In this talk, I will discuss how light can be sculpted with engineered nanostructures to enhance chiral light-matter interactions. With these nanostructures, we have developed optical force nanoscopes to visualize and quantify molecular chirality with high sensitivity and resolution. Specifically, we have designed and developed a cavity-enhanced atomic force microscope to image chiral optical forces with nanometer spatial resolution and piconewton force sensitivity. We use this technique to measure the chirality of DNA molecules, on the order of few tens of molecules. These studies provide a foundation for new sensing and imaging techniques at the single molecular to cellular level in-situ and in real time.
Enantiomer separation is a critical step in many chemical syntheses, particularly for pharmaceuticals, but prevailing chemical methods remain inefficient. Here, we introduce an optical technique to sort chiral specimens using coaxial plasmonic apertures. These apertures are composed of a deeply subwavelength dielectric channel embedded in silver (or gold) and can stably trap sub-20-nm dielectric specimens. Using both full-field simulations and analytic calculations, we first show that selective trapping of enantiomers can be achieved with circularly polarized illumination and proper index-matching of the immersed liquid with the particles being trapped. Opposite enantiomers experience distinct trapping forces in both sign and magnitude: one is trapped in a deep potential well while the other is repelled with a potential barrier. These potentials maintain opposite signs across a range of chiral polarizabilities and enantiomer-aperture separations. We also demonstrate how atomic force microscopy can be used to directly probe the near field optical forces from our coaxial nano-aperture. Our measurement reveals the spatial distribution of the optical near-field forces on a nanometer-sized dielectric specimen. To directly visualize the enantio-selective optical forces, we pattern silicon AFM-probes with chiral patterns. Our near-field force mapping indicates a differentiable force in the piconewton range on the chiral probes, exerted by our coaxial aperture with circularly polarized illumination. Our theoretical and experimental demonstrations indicate that the interaction of chiral light and chiral specimens can be mediated by achiral plasmonic apertures, providing a possible route toward all-optical enantiopure syntheses.
Recent ideas involving plasmonic metamaterials have been put forward to enhance the overall bandwidth of operation of
quarter-wave plates for circular polarization detection. The proposed metamaterial geometries are inherently complex to
realize and difficult to scale beyond the near-infrared frequencies. Here, we show how proper stacks of lithographically
printed plasmonic metasurfaces with simple patterns may provide large extinction ratios for the detection of circular
polarization, combined with broadband operation and simplicity of design and realization. In this paper, we will discuss
some physical insights into the modeling of these structures, fundamental advantages and some limitations of this
approach.
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