Mark I. Stockman

Nanoplasmonics: past, present, and glimpse into future

A review of nanoplasmonics is given. This includes fundamentals, nanolocalization of optical energy and hot spots, ultrafast nanoplasmonics and control of the spatiotemporal nanolocalization of optical fields, and quantum nanoplasmonics (spaser and gain-assisted plasmonics). This article reviews both fundamental theoretical ideas in nanoplasmonics and selected experimental developments. It is designed both for specialists in the field and general physics readership.

Nanoplasmonics: The physics behind the applications

The field of nanoplasmonics is young but rich in phenomena that have inspired practical uses in physics, biomedicine, environmental monitoring, and national security.

Optical-field-induced current in dielectrics

The time it takes to switch on and off electric current determines the rate at which signals can be processed and sampled in modern information technology. Field-effect transistors are able to control currents at frequencies of the order of or higher than 100 gigahertz, but electric interconnects may hamper progress towards reaching the terahertz (1012 hertz) range. All-optical injection of currents through interfering photoexcitation pathways or photoconductive switching of terahertz transients has made it possible to control electric current on a subpicosecond timescale in semiconductors. Insulators have been deemed unsuitable for both methods, because of the need for either ultraviolet light or strong fields, which induce slow damage or ultrafast breakdown, respectively. Here we report the feasibility of electric signal manipulation in a dielectric. A few-cycle optical waveform reversibly increases—free from breakdown—the a.c. conductivity of amorphous silicon dioxide (fused silica) by more than 18 orders of magnitude within 1 femtosecond, allowing electric currents to be driven, directed and switched by the instantaneous light field. Our work opens the way to extending electronic signal processing and high-speed metrology into the petahertz (1015 hertz) domain.

Controlling dielectrics with the electric field of light

The control of the electric and optical properties of semiconductors with microwave fields forms the basis of modern electronics, information processing and optical communications. The extension of such control to optical frequencies calls for wideband materials such as dielectrics, which require strong electric fields to alter their physical properties. Few-cycle laser pulses permit damage-free exposure of dielectrics to electric fields of several volts per ångström and significant modifications in their electronic system. Fields of such strength and temporal confinement can turn a dielectric from an insulating state to a conducting state within the optical period. However, to extend electric signal control and processing to light frequencies depends on the feasibility of reversing these effects approximately as fast as they can be induced. Here we study the underlying electron processes with sub-femtosecond solid-state spectroscopy, which reveals the feasibility of manipulating the electronic structure and electric polarizability of a dielectric reversibly with the electric field of light. We irradiate a dielectric (fused silica) with a waveform-controlled near-infrared few-cycle light field of several volts per angström and probe changes in extreme-ultraviolet absorptivity and near-infrared reflectivity on a timescale of approximately a hundred attoseconds to a few femtoseconds. The field-induced changes follow, in a highly nonlinear fashion, the turn-on and turn-off behaviour of the driving field, in agreement with the predictions of a quantum mechanical model. The ultrafast reversibility of the effects implies that the physical properties of a dielectric can be controlled with the electric field of light, offering the potential for petahertz-bandwidth signal manipulation.

Spaser Action, Loss Compensation, and Stability in Plasmonic Systems with Gain

We demonstrate that the conditions of spaser generation and the full loss compensation in a dense resonant plasmonic-gain medium (metamaterial) are identical. Consequently, attempting the full compensation or overcompensation of losses by gain will lead to instability and a transition to a spaser state. This will limit (clamp) the inversion and lead to the limitation on the maximum loss compensation achievable. The criterion of the loss overcompensation, leading to the instability and spasing, is given in an analytical and universal (independent from systems geometry) form.

Toward Full Spatiotemporal Control on the Nanoscale

We introduce an approach to implement full coherent control on nanometer length scales. It is based on spatiotemporal modulation of the surface plasmon polariton (SPP) fields at the thick edge of a nanowedge. The SPP wavepackets propagating toward the sharp edge of this nanowedge are compressed and adiabatically concentrated at a nanofocus, forming an ultrashort pulse of local fields. The profile of the focused waveform as a function of time and one spatial dimension is completely coherently controlled.

Optimized nonadiabatic nanofocusing of plasmons by tapered metal rods

Using rigorous numerical methods of analysis, this paper investigates nonadiabatic nanofocusing in tapered nanorods with the major emphasis on structural optimization for achieving maximal possible local field enhancement. Simple analytical equations for the determination of the optimal length of the tapered rod are presented and discussed. It is also shown that for the considered structures, optimal taper angle and optimal length of the rod only very weakly depend on the radius of curvature of the rounded tip of the rod. Contrary to this, enhancement of the local electric field at the rounded tip strongly increases with decreasing radius of the tip. Comparison of the numerical results with the adiabatic theory of nanofocusing results in accurate verification of the applicability conditions for adiabatic approximation in tapered nanorods.

Generation of Traveling Surface Plasmon Waves by Free-Electron Impact

The injection of a beam of free 50 keV electrons into an unstructured gold surface creates a highly localized source of traveling surface plasmons with spectra centered below the surface plasmon resonance frequency. The plasmons were detected by a controlled decoupling into light with a grating at a distance from the excitation point. The dominant contribution to the plasmon generation appears to come from the recombination of d-band holes created by the electron beam excitation.

Electromagnetic Theory of SERS

Surface-enhanced Raman scattering (SERS) is one of the strongest and most enigmatic effects in physics and optics, see the excellent review [1]. It was discovered approximately thirty years ago [2, 3, 4]. The SERS is manifested as an enhancement by many orders of magnitude of the intensity of Raman radiation by molecules bound to nanorough metal surfaces and nanostructured metal systems such as colloidal clusters of noble metals. Theories of SERS that immediately followed its discovery considered molecules bound to metal spheroids [5,6,7,8]. Hemispheroids on flat surfaces was a model emulating rough surfaces [9]. A result of general interest of this paper is a formula expressing the SERS intensity enhancement for a molecule bound at a position r0 in terms of local field E(r) at this location,

Nanoscience: Dark-hot resonances

The resonant behaviour of clusters of gold nanoparticles has been tuned by gradually bringing the particles together. The approach could have many applications, including chemical and biological sensing.