Browsing by Author "Naidu, Gopal Narmada"

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    Chiral Plasmonic Pinwheels Exhibit Orientation-Independent Linear Differential Scattering under Asymmetric Illumination
    (American Chemical Society, 2023) McCarthy, Lauren A.; Verma, Ojasvi; Naidu, Gopal Narmada; Bursi, Luca; Alabastri, Alessandro; Nordlander, Peter; Link, Stephan
    Plasmonic nanoantennas have considerably stronger polarization-dependent optical properties than their molecular counterparts, inspiring photonic platforms for enhancing molecular dichroism and providing fundamental insight into light-matter interactions. One such insight is that even achiral nanoparticles can yield strong optical activity when they are asymmetrically illuminated from a single oblique angle instead of evenly illuminated. This effect, called extrinsic chirality, results from the overall chirality of the experimental geometry and strongly depends on the orientation of the incident light. Although extrinsic chirality has been well-characterized, an analogous effect involving linear polarization sensitivity has not yet been discussed. In this study, we investigate the differential scattering of rotationally symmetric chiral plasmonic pinwheels when asymmetrically irradiated with linearly polarized light. Despite their high rotational symmetry, we observe substantial linear differential scattering that is maintained over all pinwheel orientations. We demonstrate that this orientation-independent linear differential scattering arises from the broken mirror and rotational symmetries of our overall experimental geometry. Our results underscore the necessity of considering both the rotational symmetry of the nanoantenna and the experimental setup, including illumination direction and angle, when performing plasmon-enhanced chiroptical characterizations. Our results demonstrate spectroscopic signatures of an effect analogous to extrinsic chirality for linear polarizations.
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    Numerical Modeling of Nanoparticle-Assisted Photothermal Therapy: Understanding Photon Propagation, Thermal Ablation, and Strategies for Precise Cancer Treatment Optimization
    (2023-12-01) Naidu, Gopal Narmada; Nordlander, Peter
    Nanoparticle-assisted photothermal therapy (NAPTT) is an innovative approach to cancer treatment that employs nanoparticles (NPs) to selectively elevate tumor cell temperatures through near-infrared (NIR) light absorption, thereby minimizing damage to healthy tissues. To enhance the effectiveness of NAPTT, it is crucial to develop numerical methods that accurately predict the behavior of light and heat dissipation within tissue. In this study, we have developed a theoretical model based on the finite element method (FEM) that provides valuable insights into photon propagation within nanoparticle-embedded tissue and the subsequent thermal ablation process. The model takes into account light scattering and absorption by both tissue and NPs, leading to heat generation and dissipation. We validated the model by comparing its predictions with experimental results using tissue phantoms, demonstrating excellent agreement. Furthermore, we explain the NP concentration-dependent thermal response as a consequence of two competing processes: increased light absorption and back-scattering with NP concentration. For the first time, we incorporated temperature-dependent tissue properties into our simulations, resulting in a temperature increase of up to 20%.To maximize treatment efficacy, we conducted a comprehensive analysis of different treatment parameters and propose strategies that provide precise control for more efficient and targeted treatment. In summary, our study advances NAPTT through the development of a robust numerical model validated by experimental data. It offers valuable insights into the physical processes underlying photothermal therapies and presents strategies for optimizing cancer treatment.
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    Theoretical Characterization of the Optical Properties of Plasmonic Nanostructures
    (2022-08-22) Naidu, Gopal Narmada; Nordlander , Peter
    Since Faraday’s investigation of colloidal gold in the mid-1800s, the optical properties of metal nanoparticles have long been of interest albeit their nanoscale dimensions and presence of surface plasmon resonances. More, recently, new lithographic techniques as well as improvements to classical wet chemistry methods have made it possible to synthesize metal nanoparticles with a wide range of sizes, shapes and dielectric environments, enabling more intriguing optical properties such as circular dichroism response by chiral structures, hot spots by close gap between nanoparticles, geometry-dependent resonance wavelengths, and Fano resonances. Optical investigative methods in comparison to electrical or mechanical methods take advantage of observing these characteristics of nanomaterials without significantly modifying or permanently damaging them due to their noncontact and noninvasive probing nature. In this thesis, we work together with our experimental collaborators who synthesize these nanostructures and utilize state-of-the-art spectroscopic techniques to measure a range of optical properties for various practical applications. We develop theoretical methods and models using advanced numerical simulations and modelling to elucidate and explain the mechanisms behind these observed unique optical features. In the first part of the thesis, we use Finite Difference Time Domain (FDTD) method to characterize the optical properties of selectively faceted aluminum nanocrystals synthesized using a novel dual-catalyst strategy under single particle dark-field spectroscopic excitation. We show clear agreement between the experimental and simulated spectra and further use charge distributions along with multipole analysis to understand the mechanism behind the observed scattering spectra. In the second chapter, using the Finite Element Method (FEM) we investigate the differential scattering of electron-beam lithographically fabricated rotationally symmetric chiral plasmonic pinwheels when they are asymmetrically irradiated with linearly polarized light. We demonstrate orientation-independent linear differential scattering that arises due to the broken mirror and rotational symmetry of our overall experiment geometry. On the whole, this thesis demonstrates the adept use of theoretical methods and models to provide intuitive insights into complex nanostructure properties and experimental conditions.