arXiv subjects

Alexander O. Govorov

Publications and source records attributed to Alexander O. Govorov.

At least 37 records · Page 2Linked to original sources

arXiv2017

Understanding Hot-Electron Generation and Plasmon Relaxation in Metal Nanocrystals: Quantum and Classical Mechanisms

Generation of energetic (hot) electrons is an intrinsic property of any plasmonic nanostructure under illumination. Simultaneously, a striking advantage of metal nanocrystals over semiconductors lies in their very large absorption cross sections. Therefore, metal nanostructures with strong and tailored plasmonic resonances are very attractive for photocatalytic applications. However, the central questions regarding plasmonic hot electrons are how to quantify and extract the optically-excited energetic electrons in a nanocrystal. We develop a theory describing the generation rates and the energy-distributions of hot electrons in nanocrystals with various geometries. In our theory, hot electrons are generated owing to surfaces and hot spots. The formalism predicts that large optically-excited nanocrystals show the excitation of mostly low-energy Drude electrons, whereas plasmons in small nanocrystals involve mostly hot electrons. The energy distributions of electrons in an optically-excited nanocrystal show how the quantum many-body state in small particles evolves towards the classical state described by the Drude model when increasing nanocrystal size. We show that the rate of surface decay of plasmons in nanocrystals is directly related to the rate of generation of hot electrons. Based on a detailed many-body theory involving kinetic coefficients, we formulate a simple scheme describing the plasmon's dephasing. In most nanocrystals, the main decay mechanism of a plasmon is the Drude friction-like process and the secondary path comes from generation of hot electrons due to surfaces and electromagnetic hot spots. This latter path strongly depends on the size, shape and material of the nanocrystal, correspondingly affecting its efficiency of hot-electron production. The results in the paper can be used to guide the design of plasmonic nanomaterials for photochemistry and photodetectors.

cond-mat.mes-hall↗

arXiv2017

What's so Hot about Electrons in Metal Nanoparticles?

Metal nanoparticles are excellent light absorbers. The absorption processes create highly excited electron-hole pairs and recently there has been interest in harnessing these hot charge carriers for photocatalysis and solar energy conversion applications. The goal of this Perspectives article is to describe the dynamics and energy distribution of the charge carriers produced by photon absorption, and the implications for the photocatalysis mechanism. We will also discuss how spectroscopy can be used to provide insight into the coupling between plasmons and molecular resonances. In particular, the analysis shows that the choice of material and shape of the nanocrystal can play a crucial role in hot electron generation and coupling between plasmons and molecular transitions. The detection and even calculation of many-body hot-electron processes in the plasmonic systems with continuous spectra of electrons and short lifetimes are challenging, but at the same time very interesting from the point of view of both potential applications and fundamental physics. We propose that developing an understanding of these processes will provide a pathway for improving the efficiency of plasmon-induced photocatalysis.

cond-mat.mes-hall↗

arXiv2017

Aluminum Nanoparticles with Hot Spots for Plasmon-Induced Circular Dichroism of Chiral Molecules in the UV Spectral Interval

Plasmonic nanocrystals with hot spots are able to localize optical energy in small spaces. In such physical systems, near-field interactions between molecules and plasmons can become especially strong. This paper considers the case of a nanoparticle dimer and a chiral biomolecule. In our model, a chiral molecule is placed in the gap between two plasmonic nanoparticles, where the electromagnetic hot spot occurs. Since many important biomolecules have optical transitions in the UV, we consider the case of Aluminum nanoparticles, as they offer strong electromagnetic enhancements in the blue and UV spectral intervals. Our calculations show that the complex composed of a chiral molecule and an Al-dimer exhibits strong CD signals in the plasmonic spectral region. In contrast to the standard Au- and Ag-nanocrystals, the Al system may have a much better spectral overlap between the typical biomolecule's optical transitions and the nanocrystals' plasmonic band. Overall, we found that Al nanocrystals used as CD antennas exhibit unique properties as compared to other commonly studied plasmonic and dielectric materials. The plasmonic systems investigated in this study can be potentially used for sensing chirality of biomolecules, which is of interest in applications such as drug development.

cond-mat.mes-hall↗

arXiv2017

Hot spot-mediated non-dissipative and ultrafast plasmon passage

Plasmonic nanoparticles hold great promise as photon handling elements and as channels for coherent transfer of energy and information in future all-optical computing devices. Coherent energy oscillations between two spatially separated plasmonic entities via a virtual middle state exemplify electron-based population transfer, but their realization requires precise nanoscale positioning of heterogeneous particles. Here, we show the assembly and optical analysis of a triple particle system consisting of two gold nanoparticles with an inter-spaced silver island. We observe strong plasmonic coupling between the spatially separated gold particles mediated by the connecting silver particle with almost no dissipation of energy. As the excitation energy of the silver island exceeds that of the gold particles, only quasi-occupation of the silver transfer channel is possible. We describe this effect both with exact classical electrodynamic modeling and qualitative quantum-mechanical calculations. We identify the formation of strong hot spots between all particles as the main mechanism for the loss-less coupling and thus coherent ultra-fast energy transfer between the remote partners. Our findings could prove useful for quantum gate operations, but also for classical charge and information transfer processes.

cond-mat.mes-hall↗

arXiv2017

Simple and Complex Metafluids and Metastructures with Sharp Spectral Features in a Broad Extinction Spectrum: Particle-Particle Interactions and Testing the Limits of the Beer-Lambert Law

Metallic nanocrystals (NCs) are useful instruments for light manipulation around the visible spectrum. As their plasmonic resonances depend heavily on the NC geometry, modern fabrication techniques afford a great degree of control over their optical responses. We take advantage of this fact to create optical filters in the visible-near IR. Our systems show an extinction spectrum that covers a wide range of wavelengths (UV to mid-IR), while featuring a narrow transparency band around a wavelength of choice. We achieve this by carefully selecting the geometries of a collection of NCs with narrow resonances that cover densely the spectrum from UV to mid-IR except for the frequencies targeted for transmission. This fundamental design can be executed in different kinds of systems, including a solution of colloidal metal NCs (metafluids), a structured planar metasurface or a combination of both. Along with the theory, we report experimental results, showing metasurface realizations of the system, and we discuss the strengths and weaknesses of these different approaches, paying particular attention to particle-particle interaction and to what extent it hinders the intended objective by shifting and modifying the profile of the planned resonances through the hybridization of their plasmonic modes. We have found that the Beer-Lambert law is very robust overall and is violated only upon aggregation or in configurations with nearly-touching NCs. This striking property favors the creation of metafluids with a narrow transparency window, which are investigated here.

cond-mat.mes-hall↗

arXiv2016

Plasmonic Nanostars with Hot Spots for Efficient Generation of Hot Electrons under Solar Illumination

Nanostars (NSTs) are spiky nanocrystals with plasmonic hot spots. In this study, we show that strong electromagnetic fields localized in the nanostar tips are able to generate large numbers of energetic (hot) electrons, which can be used for photochemistry. To compute plasmonic nanocrystals with complex shapes, we develop a quantum approach based on the effect of surface generation of hot electrons. We then apply this approach to nanostars, nanorods and nanospheres. We found that that the plasmonic nanostars with multiple hot spots have the best characteristics for optical generation of hot electrons compared to the cases of nanorods and nanospheres. Generation of hot electrons is a quantum effect and appears due to the optical transitions near the surfaces of nanocrystals. The quantum properties of nanocrystals are strongly size- and material-dependent. In particular, the silver nanocrystals significantly overcome the case of gold for the quantum rates of hot-electron generation. Another important factor is the size of a nanocrystal. Small nanocrystals are more efficient for the hot-electron generation since they exhibit stronger quantum surface effects. The results of this study can useful for designing novel material systems for solar photocatalytic applications.

cond-mat.mes-hall↗

arXiv2016

Localization of Temperature Using Plasmonic Hot Spots in Metal Nanostructures: The Nano-Optical Antenna Approach and Fano Effect

It is challenging to strongly localize temperature in small volumes because heat transfer is a diffusive process. Here we show how to overcome this limitation using electrodynamic hot spots and interference effects in the regime of continuous-wave (CW) excitation. We introduce a set of figures of merit for the localization of temperature and for the efficiency of the plasmonic photo-thermal effect. Our calculations show that the temperature localization in a trimer nanoparticle assembly is a complex function of the geometry and sizes. Large nanoparticles in the trimer play the role of the nano-optical antenna whereas the small nanoparticle in the plasmonic hot spot acts as a nano-heater. Under the peculiar conditions, the temperature increase inside a nanoparticle trimer can be localized in a hot spot region at the small heater nanoparticle and, in this way, a thermal hot spot can be realized. However, the overall power efficiency of temperature generation in this trimer is much smaller than that of a single nanoparticle. We can overcome the latter disadvantage by using a trimer with a nanorod. In the trimer assembly composed of a nanorod and two spherical nanoparticles, we observe a strong plasmonic Fano effect that leads to the concertation of optical energy dissipation in the small heater nanorod. Therefore, the power-absorption efficiency of temperature generation in the nanorod-based assembly greatly increases due to the strong plasmonic Fano effect. The Fano heater incorporating a small nanorod in the hot spot has obviously the best performance compared to both single nanocrystals and a nanoparticle trimer. The principles of heat localization described here can be potentially used for thermal photo-catalysis, energy conversion and bio-related applications.

cond-mat.mes-hall↗

arXiv2014

Plasmonic Metastructures and Nanocomposites with a Narrow Transparency Window in a Broad Extinction Spectrum

We propose and describe plasmonic nanomaterials with unique optical properties. These nanostructured materials strongly attenuate light in a broad wavelength interval ranged from 400 nm to 5 um but exhibit a narrow transparency window centered at a given wavelength. The main elements are nanorods and nano-crosses of variable sizes. The nanomaterial can be designed as a solution, nanocomposite film, metastructure or aerosol. The principle of the formation of the transparency window in the abroad extinction spectrum is based on the narrow lines of longitudinal plasmons of single nanorods and nanorod complexes. To realize the spectrum with a transmission window, we design a nanocomposite material as a mixture of nanorods of different sizes. Simultaneously, we exclude nanorods of certain length from the nanorod ensemble. The width of the plasmonic transparency window is determined by the intrinsic and radiative broadenings of the nanocrystal plasmons. We also describe the effect of narrowing of the transparency window with increasing the concentration of nanocrystals. Two well-established technologies can be used to fabricate such nano- and metamaterials, the colloidal synthesis and lithography. Nanocomposites proposed here can be used as optical materials and coatings for shielding of electromagnetic radiation in a wide spectral interval with a simultaneous possibility of communication using a narrow transparency window.

cond-mat.mes-hall↗

arXiv2013

Theory of photo-injection of hot plasmonic carriers in metal-semiconductor nanostructures

We investigate theoretically the effect of injection of plasmonic carriers from an optically-excited metal nanocrystal to a semiconductor contact or to attached molecules. The distributions of optically-excited hot carriers are dramatically different in metal nanocrystals with large and small sizes. In large nanocrystals, most carriers have very small energies and the hot carrier distribution resembles the case of a plasmon wave in bulk. In nanocrystals smaller than 20nm, the carrier distribution extends to larger energies and occupies the whole region E_{F}<E<omega. The physical reason for the above behaviors is non-conservation of momentum in a nanocrystal. Because of the above properties, nanocrystals of small sizes are most suitable for designing of opto-electronic and photosynthetic devices based on injection of plasmonic electrons and holes. The central parameter of the problem is DeltaN=omega/q_{L}*v_{F}, where q_{L} is the momentum transfer and v_{F} is the Fermi velocity. In gold nanocrystals, when DeltaN<7, the high-energy hot-electron generation is efficient. For larger parameters DeltaN, the number of high-energy electrons is greatly reduced. Another important factor is the polarization of the exciting light. For efficient excitation of carriers with high energies, the electric-field polarization vector should be perpendicular to a prism-like nanoantenna (slab or platelet) with a small width ~ 10-20 nm. We also show the relation between our theory for injection in plasmonic nanocrystals and the Fowler theory of injection from a bulk metal. Along with a prism geometry (or platelet geometry), we consider cubes. The results can be applied to design both purely solid-state opto-electronic devices and systems for photo-catalysis and solar conversion.

cond-mat.mes-hall↗

arXiv2011

DNA-based Self-Assembly of Chiral Plasmonic Nanostructures with Tailored Optical Response

Surface plasmon resonances generated in metallic nanostructures can be utilized to tailor electromagnetic fields. The precise spatial arrangement of such structures can result in surprising optical properties that are not found in any naturally occurring material. Here, the designed activity emerges from collective effects of singular components equipped with limited individual functionality. Top-down fabrication of plasmonic materials with a predesigned optical response in the visible range by conventional lithographic methods has remained challenging due to their limited resolution, the complexity of scaling, and the difficulty to extend these techniques to three-dimensional architectures. Molecular self-assembly provides an alternative route to create such materials which is not bound by the above limitations. We demonstrate how the DNA origami method can be used to produce plasmonic materials with a tailored optical response at visible wavelengths. Harnessing the assembly power of 3D DNA origami, we arranged metal nanoparticles with a spatial accuracy of 2 nm into nanoscale helices. The helical structures assemble in solution in a massively parallel fashion and with near quantitative yields. As a designed optical response, we generated giant circular dichroism and optical rotary dispersion in the visible range that originates from the collective plasmon-plasmon interactions within the nanohelices. We also show that the optical response can be tuned through the visible spectrum by changing the composition of the metal nanoparticles. The observed effects are independent of the direction of the incident light and can be switched by design between left- and right-handed orientation. Our work demonstrates the production of complex bulk materials from precisely designed nanoscopic assemblies and highlights the potential of DNA self-assembly for the fabrication of plasmonic nanostructures.

cond-mat.mes-hall↗

arXiv2008

Optical properties of coupled metal-semiconductor and metal-molecule nanocrystal complexes: the role of multipole effects

We investigate theoretically the effects of interaction between an optical dipole (semiconductor quantum dot or molecule) and metal nanoparticles. The calculated absorption spectra of hybrid structures demonstrate strong effects of interference coming from the exciton-plasmon coupling. In particular, the absorption spectra acquire characteristic asymmetric lineshapes and strong anti-resonances. We present here an exact solution of the problem beyond the dipole approximation and find that the multipole treatment of the interaction is crucial for the understanding of strongly-interacting exciton-plasmon nano-systems. Interestingly, the visibility of the exciton resonance becomes greatly enhanced for small inter-particle distances due to the interference phenomenon, multipole effects, and electromagnetic enhancement. We find that the destructive interference is particularly strong. Using our exact theory, we show that the interference effects can be observed experimentally even in the exciting systems at room temperature.

cond-mat.mes-hall↗

arXiv2007

Theory of plasmon-enhanced Foerster energy transfer in optically-excited semiconductor and metal nanoparticles

We describe the process of Foerster transfer between semiconductor nanoparticles in the presence of a metal subsystem (metal nanocrystals). In the presence of metal nanocrystals, the Foerster process can become faster and more long-range. The enhancement of Foerster transfer occurs due to the effect of plasmon-assisted amplification of electric fields inside the nanoscale assembly. Simultaneously, metal nanocrystals lead to an increase of energy losses during the Foerster transfer process. We derive convenient equations for the energy transfer rates, photoluminescence intensities, and energy dissipation rates in the please of plasmon resonances. Because of strong dissipation due to the metal, an experimental observation of plasmon-enhanced Foerster transfer requires special conditions. As possible experimental methods, we consider cw- and time-resolved photoluminescence studies and describe the conditions to observe plasmon-enhanced transfer. In particular, we show that the photoluminescence spectra should be carefully analyzed since the plasmon-enhanced Foerster effect can appear together with strong exciton energy dissipation. Our results can be applied to a variety of experimental nanoscale systems.

cond-mat.mes-hall↗

arXiv2006

Semiconductor-metal nanoparticle molecules: hybrid excitons and non-linear Fano effect

Modern nanotechnology opens the possibility of combining nanocrystals of various materials with very different characteristics in one superstructure. The resultant superstructure may provide new physical properties not encountered in homogeneous systems. Here we study theoretically the optical properties of hybrid molecules composed of semiconductor and metal nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly coupled and demonstrate novel properties. At low incident light intensity, the exciton peak in the absorption spectrum is broadened and shifted due to incoherent and coherent interactions between metal and semiconductor nanoparticles. At high light intensity, the absorption spectrum demonstrates a surprising, strongly asymmetric shape. This shape originates from the coherent inter-nanoparticle Coulomb interaction and can be viewed as a non-linear Fano effect which is quite different from the usual linear Fano resonance.

cond-mat.other↗

arXiv2006

Electronic states in a magnetic quantum-dot molecule: phase transitions and spontaneous symmetry breaking

We show that a double quantum-dot system made of diluted magnetic semiconductor behaves unlike usual molecules. In a semiconductor double quantum dot or in a diatomic molecule, the ground state of a single carrier is described by a symmetric orbital. In a magnetic material molecule, new ground states with broken symmetry can appear due the competition between the tunnelling and magnetic polaron energy. With decreasing temperature, the ground state changes from the normal symmetric state to a state with spontaneously broken symmetry. Interestingly, the symmetry of a magnetic molecule is recovered at very low temperatures. A magnetic double quantum dot with broken-symmetry phases can be used a voltage-controlled nanoscale memory cell.

cond-mat.other↗

arXiv2005

Magnetic semiconductor artificial atom with many particles: Thomas-Fermi model and ferromagnetic phases

Many-particle electron states in semiconductor quantum dots with carrier-mediated ferromagnetism are studied theoretically within the self-consistent Boltzmann equation formalism. Depending on the conditions, a quantum dot may contain there phases: partially spin-polarized ferromagnetic, fully spin-polarized ferromagnetic, and paramagnetic phases. The physical properties of many-body ferromagnetic confined systems come from the competing carrier-mediated ferromagnetic and Coulomb interactions. The magnetic phases in gated quantum dots with holes can be controlled by the voltage or via optical methods.

cond-mat.mes-hall↗

arXiv2005

Voltage-tunable ferromagnetism in semimagnetic quantum dots with few particles: magnetic polarons and electrical capacitance

Magnetic semiconductor quantum dots with a few carriers represent an interesting model system where ferromagnetic interactions can be tuned by voltage. By designing the geometry of a doped quantum dot, one can tailor the anisotropic quantum states of magnetic polarons. The strong anisotropy of magnetic polaron states in disk-like quantum dots with holes comes from the spin splitting in the valence band. The binding energy and spontaneous magnetization of quantum dots oscillate with the number of particles and reflect the shell structure. Due to the Coulomb interaction, the maximum binding energy and spin polarization of magnetic polarons occur in the regime of Hund's rule when the total spin of holes in a quantum dot is maximum. With increasing number of particles in a quantum dot and for certain orbital configurations, the ferromagnetic state becomes especially stable or may have broken symmetry. In quantum dots with a strong ferromagnetic interaction, the ground state can undergo a transition from a magnetic to a nonmagnetic state with increasing temperature or decreasing exchange interaction. The characteristic temperature and fluctuations of magnetic polarons depend on the binding energy and degeneracy of the shell. The capacitance spectra of magnetic quantum dots with few particles reveal the formation of polaron states.

cond-mat.mes-hall↗

arXiv2005

Spin-Forster transfer in optically excited quantum dots

The mechanisms of energy and spin transfer in quantum dot pairs coupled via the Coulomb interaction are studied. Exciton transfer can be resonant or phonon-assisted. In both cases, the transfer rates strongly depend on the resonance conditions. The spin selection rules in the transfer process come from the exchange and spin-orbit interactions. The character of energy dissipation in spin transfer is different than that in the traditional spin currents. The spin-dependent photon cross-correlation functions reflect the exciton transfer process. In addition, a mathematical method to calculate Förster transfer in crystalline nanostructures beyond the dipole-dipole approximation is described.

cond-mat.mes-hall↗

arXiv2004

Impurity-enhanced Aharonov-Bohm effect in neutral quantum-ring magnetoexcitons

We study the role of impurity scattering on the photoluminescence (PL) emission of polarized magnetoexcitons. We consider systems where both the electron and hole are confined on a ring structure (quantum rings) as well as on a type-II quantum dot. Despite their neutral character, excitons exhibit strong modulation of energy and oscillator strength in the presence of magnetic fields. Scattering impurities enhance the PL intensity on otherwise "dark" magnetic field windows and non-zero PL emission appears for a wide magnetic field range even at zero temperature. For higher temperatures, impurity-induced anticrossings on the excitonic spectrum lead to unexpected peaks and valleys on the PL intensity as function of magnetic field. Such behavior is absent on ideal systems and can account for prominent features in recent experimental results.

cond-mat.mes-hall↗