arXiv subjects

Alexandra Boltasseva

Publications and source records attributed to Alexandra Boltasseva.

At least 73 records · Page 4Linked to original sources

arXiv2019

Photonic topological phase transition on demand

On-demand, switchable phase transitions between topologically non-trivial and trivial photonic states are demonstrated. Specifically, it is shown that integration of a 2D array of coupled ring resonators within a thermal heater array enables unparalleled control over topological protection of photonic modes. Importantly, auxiliary control over spatial phase modulation opens up a way to guide topologically protected edge modes along generated virtual boundaries. The proposed approach can lead to practical realizations of topological phase transitions in many photonic applications, including topologically protected photonic memory/logic devices, robust optical modulators, and switches.

physics.optics↗

arXiv2019

Deterministic integration of single nitrogen-vacancy centers into nanopatch antennas

Quantum emitters coupled to plasmonic nanoantennas produce single photons at unprecedentedly high rates in ambient conditions. This enhancement of quantum emitters' radiation rate is based on the existence of optical modes with highly sub-diffraction volumes supported by plasmonic gap nanoantennas. Nanoantennas with gap sizes on the order of few nanometers have been typically produced using various self-assembly or random assembly techniques. Yet, the difficulty of controllably fabricate nanoantennas with the smallest mode sizes coupled to pre-characterized single emitters until now has remained a serious issue plaguing the development of quantum plasmonic devices. We demonstrate the transfer of nanodiamonds with single nitrogen-vacancy (NV) centers to an epitaxial silver substrate and their subsequent deterministic coupling to plasmonic gap nanoantennas. Through fine control of the assembled nanoantenna geometry, a dramatic shortening of the NV fluorescence lifetime was achieved. We furthermore show that by preselecting NV centers exhibiting a photostable spin contrast, a coherent spin dynamics can be measured in the coupled configuration. The demonstrated approach opens unique applications of plasmon-enhanced quantum emitters for integrated quantum information and sensing devices.

quant-ph↗

arXiv2018

Formation of Bound States in the Continuum in Hybrid Plasmonic-Photonic Systems

A bound state in the continuum (BIC) is a localized state of an open structure with access to radiation channels, yet it remains highly confined with, in theory, infinite lifetime and quality factor. There have been many realizations of such exceptional states in dielectric systems without loss. However, realizing BICs in systems with losses such as plasmonics remains a challenge. In this Letter, we explore the possibility of realizing BICs in a hybrid plasmonic-photonic structure consisting of a plasmonic grating coupled to a dielectric optical waveguide. The plasmonic-photonic system supports two distinct groups of BICs: symmetry protected BICs at the Γ-point and off-Γ Friedrich-Wintgen BICs. The photonic waveguide modes are strongly coupled to the gap plasmons in the grating leading to an avoided crossing behavior with a high value of Rabi splitting of 150 meV. Moreover, we show that the strong coupling significantly alters the band diagram of the hybrid system revealing opportunities for supporting stopped light at an off-Γ wide angular span.

physics.optics↗

arXiv2018

Accelerating Light with Metasurfaces

It has been recently shown that especially engineered light beams have the remarkable ability to propagate along curved trajectories in vacuum. Current methods for generating accelerating beams use phase modulators and lenses leading to length scales on the order of tens of centimeters or larger. This poses constraints and severely limits their applicability inside materials. Here, we accelerate light inside glass using a metasurface consisting of plasmonic nanoantennas. Highly-bending beams with radii of curvature on the order of a hundred microns were generated, and the imaged intensities agree well with theory. Our approach for generating accelerating beams allows for their integration into on-chip photonic systems.

physics.optics↗

arXiv2018

Hybrid Plasmonic Bullseye Antennas for Efficient Photon Collection

We propose highly efficient hybrid plasmonic bullseye antennas for collecting photon emission from nm-sized quantum emitters. In our approach, the emitter radiation is coupled to surface plasmon polaritons that are consequently converted into highly directional out-of-plane emission. The proposed configuration consists of a high-index titania bullseye grating separated from a planar silver film by a thin low-index silica spacer layer. Such hybrid systems are theoretically capable of directing 85 % of the dipole emission into a 0.9 NA objective, while featuring a spectrally narrow-band tunable decay rate enhancement of close to 20 at the design wavelength. Hybrid antenna structures were fabricated by standard electron-beam lithography without the use of lossy adhesion layers that might be detrimental to antenna performance. The fabricated antennas remained undamaged at saturation laser powers exhibiting stable operation. For experimental characterization of the antenna properties, a fluorescent nanodiamond containing multiple nitrogen vacancy centers (NV-center) was deterministically placed in the bullseye center, using an atomic force microscope. Probing the NV-center fluorescence we demonstrate resonantly enhanced, highly directional emission at the design wavelength of 670 nm, whose characteristics are in excellent agreement with our numerical simulations.

physics.optics↗

arXiv2017

Controlling the plasmonic properties of ultrathin TiN films at the atomic level

By combining first principles theoretical calculations and experimental optical and structural characterization such as spectroscopic ellipsometry, X-ray spectroscopy, and electron microscopy, we study the dielectric permittivity and plasmonic properties of ultrathin TiN films at an atomistic level. Our results indicate a remarkably persistent metallic character of ultrathin TiN films and a progressive red shift of the plasmon energy as the thickness of the film is reduced. The microscopic origin of this trend is interpreted in terms of the characteristic two-band electronic structure of the system. Surface oxidation and substrate strain are also investigated to explain the deviation of the optical properties from the ideal case. This paves the way to the realization of ultrathin TiN films with tailorable and tunable plasmonic properties in the visible range for applications in ultrathin metasurfaces and flexible optoelectronic devices.

cond-mat.mes-hall↗

arXiv2017

Ultrabright room-temperature single-photon emission from nanodiamond nitrogen-vacancy centers with sub-nanosecond excited-state lifetime

Ultrafast emission rates obtained from quantum emitters coupled to plasmonic nanoantennas have recently opened fundamentally new possibilities in quantum information and sensing applications. Plasmonic nanoantennas greatly improve the brightness of quantum emitters by dramatically shortening their fluorescence lifetimes. Gap plasmonic nanocavities that support strongly confined modes are of particular interest for such applications. We demonstrate single-photon emission from nitrogen-vacancy (NV) centers in nanodiamonds coupled to nanosized gap plasmonic cavities with internal mode volumes about 10 000 times smaller than the cubic vacuum wavelength. The resulting structures features sub-nanosecond NV excited-state lifetimes and detected photon rates up to 50 million counts per second. Analysis of the fluorescence saturation allows the extraction of the multi-order excitation rate enhancement provided by the nanoantenna. Efficiency analysis shows that the NV center is producing up to 0.25 billion photons per second in the far-field.

physics.optics↗

arXiv2017

Ultrathin and Multicolor Optical Cavities with Embedded Metasurfaces

Over the past years, photonic metasurfaces have demonstrated their remarkable and diverse capabilities for achieving advanced control over light propagation by confining electromagnetic radiation within the deeply subwavelength thickness of these artificial films. Here, we demonstrate that metasurfaces also offer new unparalleled capabilities for decreasing the overall dimensions of integrated optical components and systems. We propose an original approach of embedding a metasurface inside one of the most fundamental optical elements -an optical cavity- to drastically scale-down the thickness of the optical cavity. We apply this methodology to design and implement a metasurfaces-based nano-cavity where the Fabry-Perot interferometric principle has been modified to reduce the cavity thickness below the conventional half-wavelength minimum. In addition, the nano-cavities with embedded metasurfaces can support independently tunable resonances at multiple bands. As a proof-of-concept, using nano-structured metasurfaces within 100-nm nano-cavities, we experimentally demonstrate high spatial resolution color filtering and spectral imaging. The proposed approach can be extrapolated to compact integrated optical systems on-a-chip such as VCSEL, high-resolution spatial light modulators, imaging spectroscopy systems, and bio-sensors.

physics.optics↗

arXiv2017

Optical time reversal from time-dependent Epsilon-Near-Zero media

Materials with a spatially uniform but temporally varying optical response have applications ranging from magnetic field-free optical isolators to fundamental studies of quantum field theories. However, these effects typically become relevant only for time-variations oscillating at optical frequencies, thus presenting a significant hurdle that severely limits the realisation of such conditions. Here we present a thin-film material with a permittivity that pulsates (uniformly in space) at optical frequencies and realises a time-reversing medium of the form originally proposed by Pendry [Science 322, 71 (2008)]. We use an optically pumped, 500 nm thick film of epsilon-near-zero (ENZ) material based on Al-doped zinc oxide (AZO). An incident probe beam is both negatively refracted and time-reversed through a reflected phase-conjugated beam. As a result of the high nonlinearity and the refractive index that is close to zero, the ENZ film leads to time reversed beams (simultaneous negative refraction and phase conjugation) with near-unit efficiency and greater-than-unit internal conversion efficiency. The ENZ platform therefore presents the time-reversal features required e.g. for efficient subwavelength imaging, all-optical isolators and fundamental quantum field theory studies.

physics.optics↗

arXiv2017

Surface-plasmon opto-magnetic field enhancement for all-optical magnetization switching

The demand for faster magnetization switching speeds and lower energy consumption has driven the field of spintronics in recent years. The magnetic tunnel junction is the most developed spintronic memory device in which the magnetization of the storage layer is switched by spin-transfer-torque or spin-orbit torque interactions. Whereas these novel spin-torque interactions exemplify the potential of electron-spin-based devices and memory, the switching speed is limited to the ns regime by the precessional motion of the magnetization. All-optical magnetization switching, based on the inverse Faraday effect, has been shown to be an attractive method for achieving magnetization switching at ps speeds. Successful magnetization reversal in thin films has been demonstrated by using circularly polarized light. However, a method for all-optical switching of on-chip nanomagnets in high density memory modules has not been described. In this work we propose to use plasmonics, with CMOS compatible plasmonic materials, to achieve on-chip magnetization reversal in nanomagnets. Plasmonics allows light to be confined in dimensions much smaller than the diffraction limit of light. This in turn, yields higher localized electromagnetic field intensities. In this work, through simulations, we show that using localized surface plasmon resonances, it is possible to couple light to nanomagnets and achieve significantly higher opto-magnetic field values in comparison to free space light excitation.

physics.optics↗

arXiv2017

Electron spin contrast of Purcell-enhanced nitrogen-vacancy ensembles in nanodiamonds

Nitrogen-vacancy centers in diamond allow for coherent spin state manipulation at room temperature, which could bring dramatic advances to nanoscale sensing and quantum information technology. We introduce a novel method for the optical measurement of the spin contrast in dense nitrogen-vacancy (NV) ensembles. This method brings a new insight into the interplay between the spin contrast and fluorescence lifetime. We show that for improving the spin readout sensitivity in NV ensembles, one should aim at modifying the far field radiation pattern rather than enhancing the emission rate.

cond-mat.mes-hall↗

arXiv2017

Temperature-dependent optical properties of plasmonic titanium nitride thin films

Due to their exceptional plasmonic properties, noble metals such as gold and silver have been the materials of choice for the demonstration of various plasmonic and nanophotonic phenomena. However, noble metals' softness, lack of tailorability and low melting point along with challenges in thin film fabrication and device integration have prevented the realization of real-life plasmonic devices.In the recent years, titanium nitride (TiN) has emerged as a promising plasmonic material with good metallic and refractory (high temperature stable) properties. The refractory nature of TiN could enable practical plasmonic devices operating at elevated temperatures for energy conversion and harsh-environment industries such as gas and oil. Here we report on the temperature dependent dielectric functions of TiN thin films of varying thicknesses in the technologically relevant visible and near-infrared wavelength range from 330 nm to 2000 nm for temperatures up to 900 0C using in-situ high temperature ellipsometry. Our findings show that the complex dielectric function of TiN at elevated temperatures deviates from the optical parameters at room temperature, indicating degradation in plasmonic properties both in the real and imaginary parts of the dielectric constant. However, quite strikingly, the relative changes of the optical properties of TiN are significantly smaller compared to its noble metal counterparts. Using simulations, we demonstrate that incorporating the temperature-induced deviations into the numerical models leads to significant differences in the optical responses of high temperature nanophotonic systems. These studies hold the key for accurate modeling of high temperature TiN based optical elements and nanophotonic systems for energy conversion, harsh-environment sensors and heat-assisted applications.

cond-mat.mtrl-sci↗

arXiv2016

Material platforms for integrated quantum photonics

On-chip integration of quantum optical systems could be a major factor enabling photonic quantum technologies. Unlike the case of electronics, where the essential device is a transistor and the dominant material is silicon, the toolbox of elementary devices required for both classical and quantum photonic integrated circuits is vast. Therefore, many material platforms are being examined to host the future quantum photonic computers and network nodes. We discuss the pros and cons of several platforms for realizing various elementary devices, compare the current degrees of integration achieved in each platform and review several composite platform approaches.

physics.optics↗

arXiv2016

Lasing Action with Gold Nanorod Hyperbolic Metamaterials

Coherent nanoscale photon sources are of paramount importance to achieving all-optical communication. Several nanolasers smaller than the diffraction limit have been theoretically proposed and experimentally demonstrated using plasmonic cavities to confine optical fields. Such compact cavities exhibit large Purcell factors, thereby enhancing spontaneous emission, which feeds into the lasing mode. However, most plasmonic nanolasers reported so far have employed resonant nanostructures and therefore had the lasing restricted to the proximity of the resonance wavelength. Here, we report on an approach based on gold nanorod hyperbolic metamaterials for lasing. Hyperbolic metamaterials provide broadband Purcell enhancement due to large photonic density of optical states, while also supporting surface plasmon modes to deliver optical feedback for lasing due to nonlocal effects in nanorod media. We experimentally demonstrate the advantage of hyperbolic metamaterials in achieving lasing action by its comparison with that obtained in a metamaterial with elliptic dispersion. The conclusions from the experimental results are supported with numerical simulations comparing the Purcell factors and surface plasmon modes for the metamaterials with different dispersions. We show that although the metamaterials of both types support lasing, emission with hyperbolic samples is about twice as strong with 35% lower threshold vs. the elliptic ones. Hence, hyperbolic metamaterials can serve as a convenient platform of choice for nanoscale coherent photon sources in a broad wavelength range.

physics.optics↗

arXiv2016

Enhancing Hot Carrier Collection for Solar Water Splitting with Plasmonic Titanium Nitride

The use of hot electrons generated from the decay of surface plasmons is a new paradigm to increase the conversion yield in solar energy technologies. Titanium nitride (TiN) is an emerging plasmonic ceramic that offers compatibility with CMOS technology, corrosion resistance, as well as mechanical strength and durability thus outperforming noble metals (i.e., Au, Ag) in terms of cost, mechanical, chemical and thermal stability. Here, we show that plasmonic TiN nanoparticles produce 25% higher photocurrent enhancement than Au nanoparticles decorated on TiO2 nanowires for photoelectrochemical water splitting. Our results highlight that TiN offers superior performance in hot carrier generation due to enhanced absorption efficiency, increased electron mean free path, and the ability to form an Ohmic junction with TiO2, thus enabling an extremely efficient electron collection not achievable with plasmonic Au nanoparticles. Our findings show that transition metal nitrides enable practical plasmonic devices with enhanced performance for solar energy conversion.

cond-mat.mtrl-sci↗

arXiv2016

Temperature-dependent optical properties of gold thin films

Understanding the temperature dependence of the optical properties of thin metal films is critical for designing practical devices for high temperature applications in a variety of research areas, including plasmonics and near-field radiative heat transfer. Even though the optical properties of bulk metals at elevated temperatures have been studied, the temperature-dependent data for thin metal films, with thicknesses ranging from few tens to few hundreds of nanometers, is largely missing. In this work we report on the optical constants of single- and polycrystalline gold thin films at elevated temperatures in the wavelength range from 370 to 2000 nm. Our results show that while the real part of the dielectric function changes marginally with increasing temperature, the imaginary part changes drastically. For 200-nm-thick single- and polycrystalline gold films the imaginary part of the dielectric function at 500 0C becomes nearly twice larger than that at room temperature. In contrast, in thinner films (50-nm and 30-nm) the imaginary part can show either increasing or decreasing behavior within the same temperature range and eventually at 500 0C it becomes nearly 3-4 times larger than that at room temperature. The increase in the imaginary part at elevated temperatures significantly reduces the surface plasmon polariton propagation length and the quality factor of the localized surface plasmon resonance for a spherical particle. We provide experiment-fitted models to describe the temperature-dependent gold dielectric function as a sum of one Drude and two Critical Point oscillators. These causal analytical models could enable accurate multiphysics modelling of gold-based nanophotonic and plasmonic elements in both frequency and time domains.

cond-mat.mtrl-sci↗

arXiv2015

Quasi-coherent thermal emitter based on refractory plasmonic materials

The thermal emission of refractory plasmonic metamaterial - a titanium nitride 1D grating - is studied at high operating temperature (540 °C). By choosing a refractory material, we fabricate thermal gratings with high brightness that are emitting mid-infrared radiation centered around 3 $μ$m. We demonstrate experimentally that the thermal excitation of plasmon-polariton on the surface of the grating produces a well-collimated beam with a spatial coherence length of 32λ (angular divergence of 1.8°) which is quasi-monochromatic with a full width at half maximum of 70 nm. These experimental results show good agreement with a numerical model based on a two-dimensional full-wave analysis in frequency domain.

physics.optics↗

arXiv2015

Plasmon Resonance in Multilayer Graphene Nanoribbons

Plasmon resonance in nanopatterned single layer graphene nanoribbon (SL-GNR), double layer graphene nanoribbon (DL-GNR) and triple layer graphene nanoribbon (TL-GNR) structures is studied both experimentally and by numerical simulations. We use 'realistic' graphene samples in our experiments to identify the key bottle necks in both experiments and theoretical models. The existence of electrical tunable plasmons in such stacked multilayer GNRs was first experimentally verified by infrared microscopy. We find that the strength of the plasmonic resonance increases in DL-GNR when compared to SL-GNRs. However, we do not find a further such increase in TL-GNRs compared to DL-GNRs. We carried out systematic full wave simulations using finite element technique to validate and fit experimental results, and extract the carrier scattering rate as a fitting parameter. The numerical simulations show remarkable agreement with experiments for unpatterned SLG sheet, and a qualitative agreement for patterned graphene sheet. We believe that further improvements such as introducing a bandgap into the numerical model could lead to a better quantitative agreement of numerical simulations with experiments. We also note that such advanced modeling would first require better quality graphene samples and accurate measurements.

physics.optics↗