Coupling optical transitions to a single mode of an optical cavity can to enable generation of indistinguishable single photons, nonlinear-optical applications, quantum transduction, and control of chemical pathways. For all of these applications, coupling strengths need to be large compared to decoherence rates of the emitter. I will discuss progress towards this goal for various quantum emitters, including semiconductor nanocrystals (quantum dots), defects in silicon, and organic molecules. I will emphasize in particular the use of plasmonic nanocavities, which can have mode volumes well below the diffraction limit, and thus can provide coupling strengths than enable quantum photonics at room temperature.
Coupling optical or vibrational transitions to a single mode of an optical cavity has the potential to enable nonlinear-optical applications, quantum transduction, and control of chemical pathways. However, previous experiments have been limited to cryogenic temperatures. By coupling single colloidal quantum dots to plasmonic nanocavities, we have demonstrated induced transparency and strong coupling at room temperature. Second-harmonic-generation experiments on single gold nanorods strongly coupled to a monolayer transition metal dichalcogenide point towards strong nonlinear-optical effects in these systems.
Two common assumptions for simple molecular liquids are that they exhibit a Newtonian response and the no-slip boundary condition at solid-liquid interfaces. By making ultrafast optical measurements of vibrating metal nanoparticles in simple liquids, we have shown that these assumptions can break down on nanometer length scales and on the picosecond time scales that are characteristic of nanoscale motion. Our measurements quantitatively validate a Maxwell model for viscoelasticity in simple, compressible liquids, and provide a measurement of single-nanometer-scale slip lengths at the nanoparticle-liquid interface.
At the nanoscale, simple liquids are expected to exhibit complex fluid-mechanical behavior, including viscoelasticity and violation of the no-slip boundary condition. We have observed these phenomena experimentally by optically exciting and probing the vibrations of metal nanoparticles in liquids.
Strong quantum-mechanical coupling between single emitters and cavity modes enables quantum transduction between photons and excitations in the solid state. However, previous experiments have been limited to cryogenic temperatures. By coupling single colloidal quantum dots to plasmonic nanocavities, we have demonstrated strong coupling at room temperature for single solid-state emitters.
Strong quantum-mechanical coupling between single emitters and plasmonic nanocavities has the potential to enable new classical and quantum photonic technologies at room temperature. Through a new technique of tip-enhanced strong coupling, we have demonstrated the ability to systematically study and dynamically control strong coupling between plasmons and single quantum dots at room temperature.
Acoustic oscillations of metal nanoparticles can be used to study the properties of liquids at GHz frequencies and nanometer length scales. We use time-resolved spectroscopy to probe the dynamics of the metal nanoparticle oscillations utilizing a pump-probe technique. The incident pump laser pulse heats the nanoparticles leading to expansion and impulsive excitation of vibrations of the nanoparticles. The oscillations produce shifts in the plasmon resonance, which are monitored by measuring the change in absorption of a second weak broadband probe pulse. In our experiment, we immersed a sample of highly monodisperse gold bipyramids in water-glycerol mixtures from which we determined the damping resulting from the structure-liquid interactions. Performing these measurements over a range of temperatures provides a means to vary the fluid properties of a given water-glycerol mixture. Viscous damping could account for the measured results at low glycerol concentrations and sufficiently high temperatures but failed to describe the damping for high glycerol concentrations and sufficiently low temperatures. Accounting for the viscoelastic nature of the liquid mixtures mostly resolved the discrepancies, but consistently overestimated the degree of damping. Ultimately, allowing for a finite slip length produced good agreement with the measured damping rates. Our results show that standard assumptions in the fluid mechanics of simple liquids – a purely viscous response and the no-slip boundary condition – must be revisited at short length scales and fast time scales.
Achieving room-temperature quantum-mechanical strong coupling, or vacuum Rabi splitting, between a single emitter and a plasmon resonance has been a longstanding goal. Recently, two peaks have been observed in the scattering spectra of plasmonic metal nanostructures coupled to single molecules and single quantum dots, and this was taken as evidence of strong coupling. However, a two-peak scattering structure can also arise at intermediate coupling strengths, below the strong-coupling threshold, due to Fano interference between the plasmon and emitter dipole.
We unambiguously distinguish between intermediate and strong coupling by measuring both the scattering spectra and the photoluminescence spectra of coupled plasmon-emitter structures. Specifically, we couple single colloidal quantum dots to a plasmon resonator by placing them in the gap between a gold nanoparticle and a silver film. We observe weak, intermediate, and strong coupling in these hybrid metal-semiconductor structures at room temperature, depending on the detailed nanoscale structure of the metal nanoparticle.
These structures have the potential to serve as ultrafast, low-power plasmonic modulators on the nanoscale. Both induced transparency and strong coupling can be canceled by absorbing a photon in the quantum dot, leading to a strong change in extinction at the quantum dot transition frequency. Since only a single photon must be absorbed by the QD for this to happen, the energy needed for modulation has the potential to be extremely low, and the structure has the potential to enable all-optical quantum information processing.
Although the enhanced near fields produced by plasmon resonances in metal nanoparticles can enable strong local light-matter interactions, the strong dissipation of plasmon resonances would seem to be incompatible with quantum-mechanical phenomena such as entanglement. Counter to this intuition, the dissipation can in fact lead to the production of transient entanglement between the occupation states of dots coupled to a common plasmonic system. Building on previous results that showed entanglement between pairs of dots, we scale to larger systems of two, three, and more closely spaced dots. Moreover, we show that tuning the degree of coupling between each dot and the common plasmonic nanostructure enables entanglement to be created in the case where the entire system begins in its ground state and is excited by a single laser pulse. Entanglement is achieved without the need for the dots to be individually addressable, and without the need for controlled quantum gates, postselective measurements, or engineering of the dissipative environment. Through analytical solutions and numerical simulation of a model Hamiltonian based on a cavity-quantum-electrodynamics approach, we determine system configurations that maximize pairwise entanglement among the quantum dots, illustrating in principle the potential for true “quantum plasmonics.
Optical measurements of acoustic oscillations in metal nanoparticles provide a sensitive probe into the mechanical properties of materials at GHz frequencies and nanometer length scales. In these experiments, an incident pump laser heats the nanoparticles, leading to their expansion and the excitation of mechanical vibrations. The vibrations produce oscillations in the plasmon resonance frequency of the nanoparticles, which are monitored by measuring the change in transmission through the sample of a second, probe laser pulse. By making these measurements on a highly monodisperse sample of bipyramidal gold nanoparticles, we were able to determine both the frequency and the decay rate of the vibrations. Measurements on nanoparticles in different solvents made it possible to determine the portion of damping and the vibrational frequency shift that are due to coupling to the surrounding liquid environment. Viscous damping could account for results at low viscosities, but significant discrepancies were observed for higher viscosities. The discrepancies were ultimately resolved by accounting for the viscoelastic nature of the surrounding liquids. For more viscous liquids, relaxation times are higher, and thus more of the vibrational energy is stored as elastic energy in the surrounding liquid. This reduces damping, and the restoring force provided by the stored energy increases the vibrational frequency, the opposite of what would occur for an ordinary Newtonian fluid. These measurements demonstrate that metal nanoparticles can serve as nanoscale rheometers, with the picosecond response times required to reveal viscoelastic effects in conventional liquids.
Quantum wells (QWs) are thin semiconductor layers than confine electrons and holes in one dimension. They are widely used for optoelectronic devices, particularly semiconductor lasers, but have so far been produced using expensive epitaxial crystal-growth techniques. This has motivated research into the use of colloidal semiconductor nanocrystals, which can be synthesized chemically at low cost, and can be processed in the solution phase. However, initial demonstrations of optical gain from colloidal nanocrystals involved high thresholds.
Recently, colloidal synthesis methods have been developed for the production of thin, atomically flat semiconductor nanocrystals, known as nanoplatelets (NPLs). We investigated relaxation of high-energy carriers in colloidal CdSe NPLs, and found that the relaxation is characteristic of a QW system. Carrier cooling and relaxation on time scales from picoseconds to hundreds of picoseconds are dominated by Auger-type exciton-exciton interactions. The picosecond-scale cooling of hot carriers is much faster than the exciton recombination rate, as required for use of these NPLs as optical gain and lasing materials.
We therefore investigated amplified spontaneous emission using close-packed films of NPLs. We observed thresholds that were more than 4 times lower than the best reported value for colloidal nanocrystals. Moreover, gain in these films is 4 times higher than gain reported for other colloidal nanocrystals, and saturates at pump fluences more than two orders of magnitude above the ASE threshold. We attribute this exceptional performance to large optical cross-sections, relatively slow Auger recombination rates, and narrow ensemble emission linewidths.
We demonstrate three-dimensional optical trapping and orientation of individual Au nanorods, Au/Ag core/shell
nanorods, and Au bipyramids in solution, using the longitudinal surface-plasmon resonance to enhance optical forces.
Laser light that is detuned slightly to the long-wavelength side of the resonance traps individual and multiple particles
for up to 20 minutes; by contrast, light detuned to the short-wavelength side repels rods from the laser focus. Under
stable-trapping conditions, the trapping time of individual particles depends exponentially on laser power, in agreement
with a Kramers escape process. Trapped particles have their long axes aligned with the trapping-laser polarization, as
evidenced by a suppression of rotational diffusion about the short axis. When multiple particles are trapped
simultaneously, evidence of interparticle interactions is observed, including a nonlinearly increasing two-photon
fluorescence intensity, increasing fluorescence fluctuations, and changing fluorescence profiles as the trapped particle
number increases.
Colloidal growth of plasmonic nanostructures may present some advantages such as shape control at the nm
scale with atomic smoothness of the surfaces and possibly reduced damping. We show that the seed-mediated
growth of gold nanostructures is strongly dependent on the gold seed nanocrystal structure. Starting with
gold seed solutions prepared such that they are either single crystalline or multiply twinned, growth yields
either nanorods with good control over the aspect ratio (~10%) or elongated bipyramidal nanoparticles. The
nanorods are single crystalline while the gold bipyramids are penta-fold-twinned. The gold bipyramids are
also strikingly monodisperse in shape with the sharpest ensemble surface plasmon resonance reported so far.
Silver can be coated onto the gold nanostructures leading to a large blue-shift of the longitudinal plasmon
resonance. Surprisingly, even a thin silver layer introduces much additional damping explained as scattering at the
Au/Ag interface. Silver can be converted to silver sulphide yielding a large red-shift. The metal-semiconductor
composite materials may present interesting nonlinear optical properties which are being currently investigated.
Finally, the nonlinear optical scattering from individual Au nanorods was measured under excitation by ultrafast
laser pulses on resonance with their longitudinal plasmon mode. Surprisingly, the ultrafast nonlinearity can be
attributed entirely to the heating of conduction electrons and does not exhibit any response associated with
coherent plasmon oscillation. This indicates an unanticipated damping of strongly driven plasmons.
We have measured nonlinear scattering from plasmons in individual Au nanorods and have correlated second-harmonic
activity of Ag nanoparticles and clusters to morphology. The measurements reveal novel ultrafast nonlinear phenomena
related to electron confinement. Surprisingly, the coherent plasmon response is suppressed relative to the hot electron
response indicating enhanced plasmon dephasing. In a parallel set of studies we demonstrate nanometer scale
localization of the nonlinear optical response of single nanoparticles and aggregates and correlate this with their
morphology. Position markers are fabricated on an optical and electron-transparent substrate (Si3N4 thin film) that
allows optical measurements and transmission electron microscopy (TEM) imaging of the identical nanoparticles or
aggregates. The second harmonic (SH) activity optical image of individual Ag nanostructures is registered with the
TEM image. Centroid localization of the optical signals allows correlation with better than 25 nm precision. This is
sufficient to determine the origin of optical "hot spots" within multi-particle aggregates.
We demonstrate three-dimensional optical trapping and orientation of individual Au nanorods in solution, taking advantage of the longitudinal surface-plasmon resonance to enhance optical forces. Stable trapping is achieved using laser light that is detuned slightly to the long-wavelength side of the resonance; by contrast, light detuned to the short-wavelength side repels rods from the laser focus. Under stable-trapping conditions, the trapping time depends exponentially on laser power, in agreement with a Kramers escape process. Trapped rods have their long axes aligned with the trapping-laser polarization, as evidenced by a suppression of rotational diffusion about the short axis. The ability to trap and orient individual metal nanoparticles may find important application in assembly of functional structures, sorting of nanoparticles according to their shape, and development of novel microscopy techniques.
Excitation of plasmons in a metal nanoparticle leads to localization of electromagnetic fields within the particle, which is expected to result in strong optical nonlinearities. We study ultrafast nonlinearities in optical scattering from single gold nanorods under resonant excitation at the plasmon frequency, and observe changes of as much as 20% in the scattering cross section over the 20-fs laser pulse duration. Unexpectedly, the magnitude of the ultrafast nonlinearity is the same as that due to heating of conduction electrons in the metal.
A high efficiency, triggered single photon source with applications to quantum communications is discussed. The sources is formed from an InAs-based quantum dot located in the center of a micropost cavity formed from GaAs, with top and bottom GaAs/AlAs distributive Bragg reflector pairs, and lateral processing. When pumped above band into the semiconductor host, correlation measurements show a reduction in the two-photon probability to 0.14, compared to unity for a Poisson source. The external efficiency of this structure is 0.24.
In experiments conducted nearly 20 years ago, the spontaneous emission from single atoms was modified using electromagnetic cavities. In a condensed matter analogy to a single atom, we demonstrate that the spontaneous emission from an isolated InAs quantum dot can be modified as well. The single quantum dot spontaneous emission is coupled with high efficiency to a single, polarization-degenerate cavity mode using a compact, semiconductor resonator structure. The quantum dot is embedded in a planar epitaxial microcavity, which is processed into a post of submicron diameter. The single quantum dot spontaneous emission lifetime is reduced from the noncavity value of 1.3 ns to 280 ps, resulting in a single-mode spontaneous emission coupling efficiency of 78%. It is believed that this structure will be useful in triggered photons sources for quantum cryptography.
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