Coherent Surface Plasmon Research Articles

Enhancement and control of a coherent localized surface plasmon through pump-pulse engineering

Enhancement and control of a coherent localized surface plasmon through pump-pulse engineering

A comparative study of coherent and incoherent drives in a four-level quantum dot–based spaser

Abstract In this article, we theoretically investigate a spaser (surface plasmon amplification by stimulated emission of radiation) comprising a spherical silver nanoparticle surrounded by a four-level gain medium of quantum dots. The spaser system is pumped coherently and incoherently with the same excitation rate, and the characteristics of the resultant coherent localized surface plasmon (LSP) mode are compared for the two pumping scenarios. We provide a detailed analytical expression for the steady state and demonstrate that the incoherent pump is more suitable for the continuous spaser mode. The reason is better understood by studying the temporal evolution of the number of LSPs N n , where the LSP oscillation starts earlier for an incoherent drive and relaxes to a steady state with a large value of N n . At a large pump rate, the spaser curve shows saturation. In addition, we have found that the resonance peak of the spaser field is independent of coherent and incoherent pumping, whereas the peak amplitude of the field depends on the pump rate.

Coherent surface plasmon polariton amplification via free-electron pumping

Surface plasmonics with its unique confinement of light1,2 is expected to be a cornerstone for future compact radiation sources and integrated photonics devices. The energy transfer between light and matter is a defining aspect that underlies recent studies on optical surface-wave-mediated spontaneous emissions3-5. However, coherent stimulated emission of free electrons, which is essential for free-electron light sources, and its dynamical amplification process remain to be disclosed in a clear, unambiguous and calibrated manner. Here we present the coherent amplification of terahertz surface plasmon polaritons via free-electron-stimulated emission: a femtosecond optical pulse creates an in-phase free-electron pulse with an initial terahertz surface wave, and their ensuing interactions intensify the terahertz surface wave coherently. The underlying dynamics of the amplification, including a twofold redshift in the radiation frequency over a one-millimetre interaction length, are resolved as electromagnetic-field-profile evolutions using an optical pump-probe method. By extending the approach to a properly phase-matched electron bunch, our theoretical analysis predicts a super-radiant surface-wave growth, which lays the ground for a stimulated surface-wave light source and may facilitate capable means for matter manipulation, especially in the terahertz band.

Interactions between Plasmonic Nanoantennas and Vortex Beams.

The orbital angular momentum (OAM) of light offers a new degree of freedom for light-matter interactions, yet how to control such interactions with this physical dimension remains open. Here, by developing a numerical method enabling optical OAM simulations, we provide insights into complex plasmon behaviors with the physical dimension of OAM, and we prove in theory that plasmonic nanostructures can function as efficient antennas to intercept and directionally reradiate the power of OAM beams. The interplay between optical OAM and spin angular momentum (SAM) generates novel particle polarizations and radiations, which were inaccessible before. For arrayed nanoparticles, coherent surface plasmons with specific phase retardations determined by OAM of the beams enable directional power radiations, making a phased array antenna. These findings expand our knowledge of nanoplasmonics in the OAM area and are promising for quantum information processing and dynamic sensing of ultraweak biosignals.

Coherent Surface Plasmon Hole Burning via Spontaneously Generated Coherence.

Surface plasmon (SP)—induced spectral hole burning (SHB) at the silver-dielectric interface is investigated theoretically. We notice a typical lamb dip at a selective frequency, which abruptly reduces the absorption spectrum of the surface plasmons polaritons (SPP). Introducing the spontaneous generated coherence (SGC) in the atomic medium, the slope of dispersion becomes normal. Additionally, slow SPP propagation is also noticed at the interface. The spectral hole burning dip is enhanced with the SGC effect and can be modified and controlled with the frequency and intensity of the driving fields. The SPP propagation length at the hole-burning region is greatly enhanced under the effect of SGC. A propagation length of the order of 600 µm is achieved for the modes, which is a remarkable result. The enhancement of plasmon hole burning under SGC will find significant applications in sensing technology, optical communication, optical tweezers and nano-photonics.

Compact Plasmonic Distributed-Feedback Lasers as Dark Sources of Surface Plasmon Polaritons.

Plasmonic modes in optical cavities can be amplified through stimulated emission. Using this effect, plasmonic lasers can potentially provide chip-integrated sources of coherent surface plasmon polaritons (SPPs). However, while plasmonic lasers have been experimentally demonstrated, they have not generated propagating plasmons as their primary output signal. Instead, plasmonic lasers typically involve significant emission of free-space photons that are intentionally outcoupled from the cavity by Bragg diffraction or that leak from reflector edges due to uncontrolled scattering. Here, we report a simple cavity design that allows for straightforward extraction of the lasing mode as SPPs while minimizing photon leakage. We achieve plasmonic lasing in 10-μm-long distributed-feedback cavities consisting of a Ag surface periodically patterned with ridges coated by a thin layer of colloidal semiconductor nanoplatelets as the gain material. The diffraction to free-space photons from cavities designed with second-order feedback allows a direct experimental examination of the lasing-mode profile in real- and momentum-space, in good agreement with coupled-wave theory. In contrast, we demonstrate that first-order-feedback cavities remain "dark" above the lasing threshold and the output signal leaves the cavity as propagating SPPs, highlighting the potential of such lasers as on-chip sources of plasmons.

Coherent amplification and inversion less lasing of surface plasmon polaritons in a negative index metamaterial with a resonant atomic medium

Surface plasmon polaritons (SPPs) lasing requires population inversion, it is inefficient and possesses poor spectral properties. We develop an inversion-less concept for a quantum plasmonic waveguide that exploits unidirectional superradiant SPP (SSPP) emission of radiation to produce intense coherent surface plasmon beams. Our scheme includes a resonantly driven cold atomic medium in a lossless dielectric situated above an ultra-low loss negative index metamaterial (NIMM) layer. We propose generating unidirectional superradiant radiation of the plasmonic field within an atomic medium and a NIMM layer interface and achieve amplified SPPs by introducing phase-match between the superradiant SPP wave and coupled laser fields. We also establish a parametric resonance between the weak modulated plasmonic field and the collective oscillations of the atomic ensemble, thereby suppressing decoherence of the stably amplified directional polaritonic mode. Our method incorporates the quantum gain of the atomic medium to obtain sufficient conditions for coherent amplification of superradiant SPP waves, and we explore this method to quantum dynamics of the atomic medium being coupled with the weak polaritonic waves. Our waveguide configuration acts as a surface plasmon laser and quantum plasmonic transistor and opens prospects for designing controllable nano-scale lasers for quantum and nano-photonic applications.

Partially coherent surface plasmon polariton vortex fields

We introduce a class of partially coherent surface plasmon polariton (SPP) fields carrying optical vortices, generated through a judicious superposition of planar SPPs with a prescribed initial phase distribution and arbitrary correlations at a metal-air interface. We explore the global degree of coherence, energy densities, energy flows, orbital and spin angular momenta, and the polarization states of such partially coherent SPP vortex (SPPV) fields in terms of their coherent mode representation. The salient physical characteristics of these SPPV fields can be widely tuned by controlling the constituent SPP correlations, tailoring such SPPV fields to desired applications in statistical nanoplasmonics.

Coherent Vibrational Spectroscopy of Electrochemical Interfaces with Plasmonic Nanogratings.

The fundamental understanding of electrochemistry urges accurate knowledge of all interfacial properties at the molecular level, but the retrieval of such information is a real challenge. Optical spectroscopies facilitated by surface plasmon enhancement can shed light on this field, yet past studies relied on either highly inhomogeneous "hot spots" or planar plasmon modes with limited enhancement. Here we report an in situ sum-frequency vibrational spectroscopy scheme using plasmonic nanogratings, which enable strong, coherent surface plasmon excitation even on planar electrodes. With two classical reactions, the gold oxidation and pyridine adsorption in water, we demonstrate the realization of coherent vibrational spectroscopy in the strong absorption region, revealing the polar orientation and ordering of interfacial species that are crucial toward the mechanistic understanding of electrochemical phenomena.

Tunable spherical graphene surface plasmon amplification by stimulated emission of radiation

A structure is suggested for spasing. The presented surface plasmon amplification by stimulated emission of Radiation (SPASER) is made up of a graphene nanosphere, which supports localized surface plasmon modes, and a quantum dot array, acting as a gain medium. The gain medium is pumped by an external light source. Since all the plasmons are carried on a graphene platform, the structure features coherent surface plasmons with high confinement and large life time. All the structures are analyzed theoretically using full quantum mechanical description. The main advantage of the proposed SPASER is its simple tuning capability of changing graphene’s Fermi level, which is performed by either chemical doping in the manufacturing time or electrostatic gating. We suggest the utilization of the proposed SPASER for exciting coherent long-range surface plasmons on a graphene sheet. The near field of the SPASER couples to the surface plasmons on graphene sheet and compensates the large momentum mismatch between surface plasmons and photons.

Scattering of partially coherent surface plasmon polariton fields by metallic nanostripe

BackgroundSurface plasmon polaritons (SPPs) are electromagnetic waves that propagate at a metal-dielectric interface. Until recently, monochromatic, fully coherent SPPs have mainly been considered.MethodsWe investigate by numerical simulations the generation and properties of polychromatic, partially coherent SPPs and their scattering from a nanostripe. We use both in-house and commercial codes.ResultsA standing SPP field is created in Kretschmann’s setup between the excitation point and the nanostripe. From the scattered far-field spectrum, all spatiotemporal coherence properties of the SPP field pattern can be deduced. Illustrative examples of such coherence variations are analyzed.ConclusionsPlasmonic nanostructures produce strong confined SPP fields of widely controllable coherence.

Doped silicon quantum dots as sources of coherent surface plasmons

In the present work, we propose using a doped silicon quantum dot (Si-QD) as a source of coherent surface plasmons (SPASER). The possibility of spasing in single Si-QD is investigated theoretically utilizing full quantum mechanical treatment. We show that spasing can take place in doped Si-QDs whenever the quality factor of a plasmon mode exceeds some minimum value. The minimum value depends on the size and doping concentration of Si-QDs. It can be used to design an optimum structure as SPASER in silicon technologies. The condition on the quality factor is translated to a condition for radius and it is shown that for a given localized surface plasmon (LSP) mode, the radius should be less than the critical value. This value only depends on the mode index. The required relations for design purposes are derived and, as an example of feasibility of the approach, a SPASER is designed for mid infrared. Moreover, we propose a more applicable device by arranging an array of doped Si-QDs on top of a graphene layer. Interaction between the surface plasmon polariton (SPP) modes of graphene and LSP modes of Si-QDs causes outcoupling of an intense ultra narrow beam of coherent SPPs to be used in real probable applications.

Coherent surface plasmon amplification through the dissipative instability of 2D direct current

Abstract We propose an original concept for on-chip excitation and amplification of surface plasmon polaritons. Our approach, named nanoresotron, utilizes the collective effect of dissipative instability of a 2D direct current flowing in vicinity of a metal surface. The instability arises through the excitation of self-consistent plasma oscillations and results in the creation of a pair of collective surface electromagnetic modes in addition to conventional plasmon resonances. We derive the dispersion equations for these modes using self-consistent solutions of Maxwell’s and 2D hydrodynamics equations. We find that the phase velocities of these new collective modes are close to the drift velocity of 2D electrons. We demonstrate that the slow mode is amplified while the fast mode exhibits absorption. Estimates indicate that very high gain are attainable, which makes the nanoresotron a promising scheme to electrically excite and regenerate surface plasmon polaritons.

Toward broadband, dynamic structuring of a complex plasmonic field.

The ability to tailor a coherent surface plasmon polariton (SPP) field is an important step toward many new opportunities for a broad range of nanophotonic applications. Previously, both scanning a converging SPP spot and designing SPP profiles using an ensemble of spots have been demonstrated. SPPs, however, are normally excited by intense, coherent light sources, that is, lasers. Hence, interference between adjacent spots is inevitable and will affect the overall SPP field distributions. We report a reconfigurable and wavelength-independent platform for generating a tailored two-dimensional (2D) SPP field distribution by considering the coherent field as a whole rather than as individual spots. With this new approach, the inherent constraints in a 2D coherent field distribution are revealed. Our design approach works not only for SPP waves but also for other 2D wave systems such as surface acoustic waves.

Partially coherent axiconic surface plasmon polariton fields

We introduce a class of structured polychromatic surface electromagnetic fields, reminiscent of conventional optical axicon fields, through a judicious superposition of partially correlated surface plasmon polaritons. We show that such partially coherent axiconic surface plasmon polariton fields are structurally stable and statistically highly versatile with regard to spectral density, polarization state, energy flow, and degree of coherence. These fields can be created by plasmon coherence engineering and may prove instrumental broadly in surface physics and in various nanophotonics applications.

Coherent pseudo-mode representation of partially coherent surface plasmon polaritons.

We develop a general coherent (pseudo)-mode representation of partially coherent surface plasmon polaritons in the Kretschmann excitation geometry. We obtain explicit analytical results for coherent pseudo-modes of narrowband SPPs with Gaussian spectra and spectral correlations. We also evaluate the global degree of coherence of such SPPs which quantifies their overall coherence state within a given spatial domain. Not only will the developed coherent pseudo-mode representation facilitate SPP coherence state characterization, but it will also substantially reduce the computational complexity of the studies of partially coherent SPP interactions with nanoparticles.

Imaging the dark emission of spasers.

Spasers are a new class of laser devices with cavity sizes free from optical diffraction limit. They are an emergent tool for various applications, including biochemical sensing, superresolution imaging, and on-chip optical communication. According to its original definition, a spaser is a coherent surface plasmon amplifier that does not necessarily generate a radiative photon output. However, to date, spasers have only been studied with scattered photons, and their intrinsic surface plasmon emission is a "dark" emission that is yet to be revealed because of its evanescent nature. We directly image the surface plasmon emission of spasers in spatial, momentum, and frequency spaces simultaneously. We demonstrate a nanowire spaser with a coupling efficiency to plasmonic modes of 74%. This coupling efficiency can approach 100% in theory when the diameter of the nanowire becomes smaller than 50 nm. Our results provide clear evidence of the surface plasmon amplifier nature of spasers and will pave the way for their various applications.

Anisotropy‐assisted non‐scattering coherent absorption of surface plasmon‐polaritons

The ability to control propagation of electromagnetic guided modes lies at the heart of integrated nanophotonics. Surface plasmon‐polaritons are a class of guided modes which can be employed in integrated optical systems. Here, we present a theoretical design of a coherent surface plasmon absorber which can perfectly harvest energy of coherently incident surface plasmons without parasitic scattering into free space modes. Excitation of free space modes which usually accompanies scattering of a surface plasmon by an interface boundary is avoided due to specially tailored anisotropy of the absorber. The concept of coherent SPP absorber is analyzed numerically for spatially non‐uniform and finite‐size structures. We believe that our results will be important for the development of integrated nanoplasmonic systems.

Numerically exact solution of the many emitter–cavity laser problem: Application to the fully quantized spaser emission

A numerically exact solution to the many emitter -- cavity problem as an open many body system is presented. The solution gives access to the full, nonperturbative density matrix and thus the full quantum statistics and quantum correlations. The numerical effort scales with the third power in the number of emitters. Notably the solution requires none of the common approximations like good/bad cavity limit. As a first application the recently discussed concept of coherent surface plasmon amplification -- spaser -- is addressed: A spaser consists of a plasmonic nanostructure that is driven by a set of quantum emitters. In the context of laser theory it is a laser in the (very) bad cavity limit with an extremely high light matter interaction strength. The method allows us to answer the question of spasing with a fully quantized theory.

Chiral near fields generated from plasmonic optical lattices

Plasmonic fields are usually considered nonchiral because of the transverse magnetic polarization of surface plasmon modes. We, however, show here that an optical lattice created from the intersection of two coherent surface plasmons propagating on a smooth metal film can generate optical chirality in the interfering near field. We reveal in particular the emergence of plasmonic potentials relevant to the generation of near-field chiral forces. This draws promising perspectives for performing enantiomeric separation schemes within the near field.