Rithvik R. Gutha

Undamped ultrafast pulsation of plasmonic fields via coherent exciton-plasmon coupling.

We show that under certain conditions the plasmonic field of a hybrid system consisting of a metallic nanoparticle and a semiconductor quantum dot can be converted into ultrashort stationary pulses with temporal widths as short as 300 ps. This happens as this system interacts with an infrared and visible laser fields, both with time-independent amplitudes. These fields generate quantum coherence via simultaneous interband and intersubband transitions of the quantum dot, forcing the polarization dephasing rate of the quantum dot to become negative during the plasmon pulses. This makes the amplitudes of such pulses time-independent (undamped), indicating total suppression of quantum decoherence of the quantum dot. These results suggest that hybrid quantum dot-metallic nanoparticle systems can act as undamped coherent-plasmonic oscillators.

Turning on plasmonic lattice modes in metallic nanoantenna arrays via silicon thin films

We study control of optical coupling of plasmon resonances in metallic nanoantenna arrays using ultrathin layers of silicon. This technique allows one to establish and tune plasmonic lattice modes of such arrays, demonstrating a controlled transformation from the localized surface plasmon resonances of individual nanoantennas to their optimized collective lattice modes. Depending on the polarization and incident angle of light, our results support two different types of the silicon-induced plasmonic lattice resonances. For s-polarization these resonances follow the Rayleigh anomaly, while for p-polarization an increase in the incident angle makes the lattice resonances significantly narrower and slightly blueshifted.

Biological sensing and control of emission dynamics of quantum dot bioconjugates using arrays of long metallic nanorods

We study biological sensing using plasmonic and photonic-plasmonic resonances of arrays of ultralong metallic nanorods and analyze the impact of these resonances on emission dynamics of quantum dot bioconjugates. We demonstrate that the LSPRs and plasmonic lattice modes of such array can be used to detect a single self-assembled monolayer of alkanethiol at the visible (550 nm) and near infrared (770 nm) range with well resolved shifts. We study adsorption of streptavidin-quantum dot conjugates to this monolayer, demonstrating that formation of nearly two dimensional arrays of quantum dots with limited emission blinking can lead to extra well-defined wavelength shifts in these modes. Using spectrally-resolved lifetime measurements we study the emission dynamics of such quantum dot bioconjugates within their monodispersed size distribution. We show that, despite their close vicinity to the nanorods, the rate of energy transfer from these quantum dots to nanorods is rather weak, while the plasmon field enhancement can be strong. Our results reveal that the nanorods present a strongly wavelength or size-dependent non-radiative decay channel to the quantum dot bioconjugates.

Ultrahigh refractive index sensitivity and tunable polarization switching via infrared plasmonic lattice modes

We demonstrate tunable polarization-dependent infrared plasmonic lattice modes in the range of 1 to 1.7 μm in arrays of large gold nanodisks with a rectangle lattice structure. We show that when these arrays are exposed to air, their main mode appears around 1 μm. Under this condition, addition of chemicals leads to significant wavelength shifts in this mode, offering a refractive index sensitivity of about 795 nm/RIU (refractive index unit). Our results show that this process is accompanied by excitation of a sharp peak associated with an infrared lattice mode at about 1.62 μm, suggesting an abrupt refractive-index switching of the collective modes of the arrays. By depositing ultrathin layers of Si, we show that the wavelength of the 1 μm mode can be shifted, covering the whole telecom band ranges. We demonstrate that this can lead to tunable narrow- and wide-band polarization switching of the collective modes of the arrays within this range with a high extinction ratio.

Polarization switching from plasmonic lattice mode to multipolar localized surface plasmon resonances in arrays of large nanoantennas

We experimentally investigate plasmonic lattice modes of gold nanoantenna arrays that occur in asymmetric structures containing a silica substrate and either air or a thin layer of a high-index dielectric. Very distinct polarization switching is observed in the nanoantenna arrays wherein by rotating the incident light polarization by ninety degrees, the array can exhibit either a plasmonic lattice mode or a multipolar localized surface plasmon resonance of varying nature. A large range of nanoantenna lengths are studied, and since the length of the nanoantennas dictates the multipolar localized surface plasmon resonance, we find that the characteristics of the polarization switching are affected accordingly. We also investigate how the thin layer of the high-index dielectric on top of the nanoantenna arrays, in conjunction with varying nanoantenna length, impacts the generation of plasmonic lattice modes and the polarization switching in the arrays. The high-index dielectric is found to assist in the gene...

Metallic nanoparticle shape and size effects on aluminum oxide-induced enhancement of exciton-plasmon coupling and quantum dot emission

We investigate the shape and size effects of gold metallic nanoparticles on the enhancement of exciton-plasmon coupling and emission of semiconductor quantum dots induced via the simultaneous impact of metal-oxide and plasmonic effects. This enhancement occurs when metallic nanoparticle arrays are separated from the quantum dots by a layered thin film consisting of a high index dielectric material (silicon) and aluminum oxide. Our results show that adding the aluminum oxide layer can increase the degree of polarization of quantum dot emission induced by metallic nanorods by nearly two times, when these nanorods have large aspect ratios. We show when the aspect ratio of these nanorods is reduced to half, the aluminum oxide loses its impact, leading to no improvement in the degree of polarization. These results suggest that a silicon/aluminum oxide layer can significantly enhance exciton-plasmon coupling when quantum dots are in the vicinity of metallic nanoantennas with high aspect ratios.

Control of spontaneous emission of quantum dots using correlated effects of metal oxides and dielectric materials

We study the emission dynamics of semiconductor quantum dots in the presence of the correlated impact of metal oxides and dielectric materials. For this we used layered material structures consisting of a base substrate, a dielectric layer, and an ultrathin layer of a metal oxide. After depositing colloidal CdSe/ZnS quantum dots on the top of the metal oxide, we used spectral and time-resolved techniques to show that, depending on the type and thickness of the dielectric material, the metal oxide can characteristically change the interplay between intrinsic excitons, defect states, and the environment, offering new material properties. Our results show that aluminum oxide, in particular, can strongly change the impact of amorphous silicon on the emission dynamics of quantum dots by balancing the intrinsic near band emission and fast trapping of carriers. In such a system the silicon/aluminum oxide charge barrier can lead to large variation of the radiative lifetime of quantum dots and control of the photo-ejection rate of electrons in quantum dots. The results provide unique techniques to investigate and modify physical properties of dielectrics and manage optical and electrical properties of quantum dots.

Semiconductor quantum dot super-emitters: spontaneous emission enhancement combined with suppression of defect environment using metal-oxide plasmonic metafilms

We demonstrate that a metal-oxide plasmonic metafilm consisting of a Si/Al oxide junction in the vicinity of a thin gold layer can quarantine excitons in colloidal semiconductor quantum dots against their defect environments. This process happens while the plasmon fields of the gold layer enhance spontaneous emission decay rates of the quantum dots. We study the emission dynamics of such quantum dots when the distance between the Si/Al oxide junction and the gold thin layer is varied. The results show that for distances less than a critical value the lifetime of the quantum dots can be elongated while they experience intense plasmon fields. This suggests that the metal-oxide metafilm can keep photo-excited electrons in the cores of the quantum dots, suppressing their migration to the surface defect sites. This leads to suppression of Auger recombination, offering quantum dot super-emitters with emission that is enhanced not only by the plasmon fields (Purcell effect), but also by strong suppression of the non-radiative decay caused by the defect sites.

Ultrahigh refractive index sensitivity via lattice-induced meta-dipole modes in flat metallic nanoantenna arrays

We investigate control of plasmonic-photonic coupling in flat metallic nanoantenna arrays. We demonstrate that when the nanoantennas are packed together along their short axis (transverse lattice constant) and the incident light polarization is along their long axis, they can support lattice-induced plasmonic resonance coupled to a super-photonic mode that densely fills the superstrate volume. Our results show that at a certain wavelength, this resonance joins the plasmonic tip modes of the nanoantennas, forming meta-dipole modes. These modes have field profiles similar to those of the natural plasmonic dipole modes of individual nanoantennas, but they occur at much shorter wavelengths and offer a very high bulk refractive index sensitivity (925 ± 12 nm/RIU). We show that with an increase in the transverse lattice constant, such a sensitivity decreases as the meta-dipole modes disappear. Under this condition, the refractive index sensitivity supported by natural modes of the nanoantennas increases, as the plasmonic edge mode suppression caused by charge rearrangement decreases.We investigate control of plasmonic-photonic coupling in flat metallic nanoantenna arrays. We demonstrate that when the nanoantennas are packed together along their short axis (transverse lattice constant) and the incident light polarization is along their long axis, they can support lattice-induced plasmonic resonance coupled to a super-photonic mode that densely fills the superstrate volume. Our results show that at a certain wavelength, this resonance joins the plasmonic tip modes of the nanoantennas, forming meta-dipole modes. These modes have field profiles similar to those of the natural plasmonic dipole modes of individual nanoantennas, but they occur at much shorter wavelengths and offer a very high bulk refractive index sensitivity (925 ± 12 nm/RIU). We show that with an increase in the transverse lattice constant, such a sensitivity decreases as the meta-dipole modes disappear. Under this condition, the refractive index sensitivity supported by natural modes of the nanoantennas increases, as the...

Biological sensing using hybridization phase of plasmonic resonances with photonic lattice modes in arrays of gold nanoantennas

We study biological sensing using the hybridization phase of localized surface plasmon resonances (LSPRs) with diffraction modes (photonic lattice modes) in arrays of gold nanoantennas. We map the degree of the hybridization process using an embedding dielectric material (Si), identifying the critical thicknesses wherein the optical responses of the arrays are mainly governed by pure LSPRs (insignificant hybridization), Fano-type coupling of LSPRs with diffraction orders (hybridization state), and their intermediate state (hybridization phase). The results show that hybridization phase can occur with slight change in the refractive index (RI), leading to sudden reduction of the linewidth of the main spectral feature of the arrays by about one order of magnitude while it is shifted nearly 140 nm. These processes, which offer significant improvement in RI sensitivity and figure of merit, are utilized to detect monolayers of biological molecules and streptavidin-conjugated semiconductor quantum dots with sensitivities far higher than pure LSPRs. We further explore how these sensors can be used based on the uncoupled LSPRs by changing the polarization of the incident light.