Toward single-molecule detection with sensors based on propagating surface plasmons

Surface plasmon resonance (SPR) sensors are known to be able to detect very low surface concentrations of (bio)molecules on macroscopic areas. To explore the potential of SPR biosensors to achieve single-molecule detection, we have minimized the read-out area (to ～64 μm²) by employing a sensor system based on spectroscopy of surface plasmons generated on a diffractive structure via a microscope objective and light collection through a small aperture. This approach allows for decreasing the number of detected molecules by 3 orders of magnitude compared to state-of-the-art SPR sensors. A protein monolayer has been shown to produce a response of 5000 times the baseline noise, suggesting that as few as ～500 proteins could be detected by the sensor.     

Interaction between localized and propagating surface plasmons: Ag fine particles on Al surface

Samples consisting of Ag fine particles 4nm in size placed near to an Al surface were prepared by depositing Ag-SiO₂ composite films by a rf cosputtering method onto vacuum-evaporated Al films. Angle-scan attenuated total reflection (ATR) measurements were performed over a wide wavelength range (λ=350-630nm) covering the resonance region (λ-410nm) of localized surface plasmon (LSP) in the Ag particles. It was found that the existence of Ag fine particles near to the Al surface led to a deformation of the dispersion curve of the surface plasmon polariton (SPP) propagating on the Al surface, in particular, in the LSP resonance region. The interaction between LSP and SPP was found to become much stronger as the distance between the particles and the surface decreases. The theoretical calculations based on the effective medium theory, which gave an effective dielectric function of the Ag-SiO₂ composite film, reproduced the deformation of the dispersion curve of the SPP. The deformation of the dispersion observed experimentally was qualitatively explained by the behavior of the effective dielectric function of the Ag-SiO₂ composite film.     

Life-time of surface plasmons in small metal particles

Existing theories concerning the life-time of surface plasmons in small metal particles fail to explain recent experimental data for silver. Therefore, two of these theories were critically re-examined, and some numerical errors were found. The semi-classical approach by Lozovik et al. yields, after correction, physically reasonable results, which are nearly identical with theoretical results by Kreibig. The corrected RPA-result by Lushnikov et al. is in quantitative agreement with recent experimental data for silver.     

Modified method of surface plasmons in metal superlattices

We present a modified method to solve the surface plasmons(SPs) of semi-infinite metal/dielectric superlattices and predicted new SP modes in physics. We find that four dispersion-equation sets and all possible SP modes are determined by them. Our analysis and numerical calculations indicate that besides the SP mode obtained in the original theory, the other two SP modes are predicted, which have either a positive group velocity or a negative group velocity. We also point out the possible defect in the previous theoretical method in accordance to the linear algebra principle.     

Observation of a new type of THz resonance of surface plasmons propagating on metal-film hole arrays

Highly conducting metal-film subwavelength hole arrays, lithographically fabricated on high-resistivity silicon wafers in optical contact with thick silicon plates, have been characterized by terahertz time-domain spectroscopy with subpicosecond resolution and over a frequency range from 0.5 to 3 THz with 5 GHz resolution. A well-defined ringing structure extending to more than 250 psec is observed on the trailing edge of the transmitted THz pulse. In the frequency domain this ringing structure corresponds to a new type of extremely sharp resonant line structure between the fundamental surface plasmon modes of the hole array. A simple theoretical model is presented and shows good agreement with the experimental data.     

Red shift of surface plasmons in small metal particles

It is shown that the experimentally found red shift of the surface plasmon resonance in a small metal particle is a direct measure of the first moment of the induced surface charge.     

Terahertz-pulse emission through laser excitation of surface plasmons in a metal grating

The second-order processes of optical-rectification and photoconduction are well known and widely used to produce ultrafast electromagnetic pulses in the terahertz frequency domain. We present a new form of rectification that relies on the excitation of surface plasmons in metal films deposited on a shallow grating. Multiphoton ionization and ponderomotive acceleration of electrons in the enhanced evanescent field of the surface plasmons results in a femtosecond current surge and emission of terahertz electromagnetic radiation. Using gold, this rectification process is third or higher-order in the incident field.     

Resonant coupling of surface plasmons to radiation modes by use of dielectric gratings

Efficient outcoupling of surface-plasma waves to radiation modes by use of dielectric diffraction gratings on a flat metallic surface is discussed. The dielectric gratings, which have a surface-relief structure with only several tens of nanometers in peak-to-trough height on a flat metal surface, can efficiently extract radiation modes propagating in free space from the surface-plasmon modes. An outcoupling efficiency of 50% is estimated with the rigorous coupled-wave diffraction theory, and it is confirmed by the experiment.     

Remote excitation and remote detection of a single quantum dot using propagating surface plasmons on silver nanowire

Using propagating surface plasmons(SPs) on a silver nanowire(NW), we demonstrate that a focused laser light at the end of the silver NW can excite a single quantum dot(QD) microns away from the excitation spot. The QD–NW interaction allows the excited QD convert part of its energy into propagating SPs, which then can be detected at remote sites.Simultaneous multi-QD remote excitation and remote detection can also be realized. Furthermore, the tight confinement of the propagating SPs around the NW surface enables the selective excitation of QDs very close in space, which cannot be realized under the conventional excitation condition. This remote excitation and remote detection approach may find applications in optical imaging and the sensing of chemical and biological systems.     

Optical generation of terahertz plasmons on comb-shaped surface of metal

The possibilities of a recently proposed (Opt. Express 17, 9323 (2009)) method for generating terahertz surface plasmons on a microstructured (comb-shaped) metal surface using a nonlinear polarization pulse that moves with a superluminal velocity and is induced by an ultrashort laser pulse in a strip of electrooptic material deposited on the surface are theoretically studied. For an arbitrary direction of motion of the nonlinear source along the comb-shaped surface, fields of excited terahertz plasmons and the angular spectral distribution of the radiated energy are calculated. It is shown that the spectral and energy characteristics of plasmons can be efficiently controlled by varying the direction of motion of the source. Conditions (parameters of the comb-shaped structure, direction and velocity of motion of the source) that ensure the maximal efficiency of the optical-to-terahertz conversion are found. The developed method of terahertz generation is promising for surface terahertz spectroscopy.     

Surface plasmons in a free-electron metal

The dispersion and damping of the surface plasmons are calculated for a free-electron model of Al. At large wave-vector the frequency of the surface plasmons approaches that of the bulk plasmons. Their damping remains quite small up to a wave-vector close to the bulk plasmon cut-off. The change in zero-point energy of the plasmons is calculated, and it gives a large positive contribution to the surface energy.     

Acoustic plasmons in a two-band metal

The properties of acoustic plasmon excitations and the conditions for their existence in a metal with two overlapping bands one of which is narrow (d-band) and the other broad (s-band) are discussed. Model calculations in which the d-electrons are treated in the tight-binding approximation and the s-electrons are assumed to be free are presented. The prospects for finding well defined acoustic plasmons in a real system are examined. The example of the V₃Ga crystal is considered in some detail.     

Total resonant absorption of light by plasmons on the nanoporous surface of a metal

We have calculated light absorption spectra of planar metal surfaces with a two-dimensional lattice of spherical nanovoids just beneath the surface. It is shown that nearly total absorption of light occurs at the plasma resonance in a void lattice in the visible range when the intervoid spacing and the void deepening into the metal are thinner than the skin depth, which ensures optimal coupling of void plasmons to external light. We conclude that the absorption and local-field properties of this type of nanoporous metal surface can be effectively tuned through nanoengineering of the spherical pores and that they constitute a very attractive system for various applications in future submicron light technology.     

Surface plasmons propagating parallel to the grooves of a large amplitude grating

The dispersion relation is obtained for a surface plasmon propagating parallel to the grooves of a large amplitude grating. Two variants of the Rayleigh method arc used: the first requires the evaluation of the zeros of a determinant; the second yields the dispersion relation in terms of the solution of a matrix eigenvalue problem. The dispersion relations are solved numerically for a sinusoidal and for a symmetric sawtooth grating profile, yielding in each case an infinity of discrete branches.     

The effect of gain and absorption on surface plasmons in metal nanoparticles

The compensation of loss in metal by gain in interfacing dielectric has been demonstrated in a mixture of aggregated silver nanoparticles and rhodamine 6G dye. An increase of the quality factor of surface plasmon (SP) resonance was evidenced by the sixfold enhancement of Rayleigh scattering. The compensation of plasmonic losses with gain enables a host of new applications for metallic nanostructures, including low- or no-loss negative-index metamaterials. We have also predicted and experimentally observed a suppression of SP resonance in metallic nanoparticles embedded in dielectric host with absorption. PACS 61.46.Df; 73.20.Mf; 78.67.Bf     

Edge plasmons on a non planar surface

We give an integral formulation for the problem of finding the dispersion relation for electrostatic modes localized around the edge of a dielectric wedge. This formulation is valid for any surface profile. We apply it to the case of two semi-infinite planes matched with a cylindrical surface.     

Resonant tunneling mediated by resonant emission of intersubband plasmons

We study tunneling through resonant tunneling diodes (RTD) with very long emitter drift regions (up to 2 microm). In such diodes, charge accumulation occurs near the double barrier on the emitter side, in a self-induced potential pocket. This leads to a substantial enhancement of the wave function overlap between states of the pocket and the RTD, and, consequently, to increased off-resonant current mediated by various scattering processes. For RTD with the longest drift region (2 microm), an additional strong current peak is observed between the first and the second resonant peaks. We attribute this pronounced feature to the intersubband transitions mediated by resonant emission of intersubband plasmons.     

Excitation of surface plasmons in metal particles by fast electrons and x rays

The cross sections for excitation of surface plasmons by fast electrons and x rays are calculated within the random phase approximation for a spherical metal particle. The results are compared with those obtained within the hydrodynamic model of a free electron gas.