Coupled Plasmon Modes Research Articles

Identification of surface optical phonon in wurtzite InN epitaxial thin films by coherent phonon spectroscopy

Coherent phonon spectroscopy of wurtzite InN epitaxial thin films was carried out with time-resolved second-harmonic generation technique. In addition to the bulk A1(LO) phonon and A1(LO)–plasmon coupling modes, a phonon mode at 543cm−1 was identified and characterized. It is found that this phonon mode is localized in the topmost few atomic layers and sensitive to surface modification. This vibration mode is described as the opposite motion between the In and N atoms along the c axis. This phonon mode is assigned to the surface optical phonon of InN.

Coupled plasmon modes and their localization in graded plasmonic chains

Plasmonic waves occur in the subwavelength scale with transverse confinement below the diffraction limit. In this work, we report results of longitudinal localization–delocalization transitions of coupled plasmon modes in graded chains of metallic nanodots. Two graded models are studied: graded index of refraction in the host medium and incremental spacing between the nanoparticles. The coupled plasmon modes in these graded systems exhibit strong localization, showing a tunable passband in finite size systems. These localized modes survive in presence of weak loss in the nanodots. To understand the localization mechanism, we construct equivalent systems of one-dimensional coupled harmonic oscillators, whose coupling strength or masses are gradually varied from one end to the other, with additional on-site potentials. Confining and transmitting electromagnetic energy in these structures may pave new way for many fruitful applications in plasmonics.

Dispersion and transitions of dipolar plasmon modes in graded plasmonic waveguides

Coupled plasmon modes are studied in graded plasmonic waveguides, which are periodic chains of metallic nanoparticles embedded in a host with gradually varying refractive indices. The authors identify three types of localized modes called “light,” “heavy,” and “light-heavy” plasmonic gradons outside the passband, according to various localizations. The authors also demonstrate different transitions among extended and localized modes when the interparticle separation d is smaller than a critical dc, whereas the three types of localized modes occur for d>dc, with no extended modes. The transitions can be explained with phase diagrams constructed for the lossless metallic systems.

Plasmonics: Metal Nanostructures for Subwavelength Photonic Devices

This paper reviews recent progress toward the creation of a nanophotonic framework for confining and guiding electromagnetic energy at visible and near-infrared frequencies using surface plasmon excitations sustained by metallic nanostructures. Prominent geometries such as metal/insulator/metal-gap waveguides are assessed in terms of light confinement and the energy attenuation of the guided modes, as well as metal-nanoparticle waveguides that guide light via near-field coupling of localized particle or gap plasmon modes for deep subwavelength confinement. The effective mode volume concept of dielectric optics is then applied to plasmonic nanoresonators, which allows a comparison with established dielectric microcavities and demonstrates the deep subwavelength confinement achievable in metallic nanocavities. Lastly, a solution to the coupling problem of surface plasmon-polariton modes to the outside world is presented in the form of a fiber-accessible metal-nanoparticle plasmon waveguide with experimentally demonstrated power transfer up to 75% at lambda=1.5 mum

On the mixed nature of the 740 cm −1 band in wurtzite GaN films: A polarized Raman scattering investigation

A detailed study of the polarized Raman scattering of wurtzite GaN films is presented, focusing on the nature of the band centered at 740 cm −1 observed in the X( Z, Z) X configuration. The origin of this band is ascribed to the mixed contribution of the A 1 and E 1 longitudinal phonon modes coupled with the free carrier excitation. The spectral profile of the 740 cm −1 Raman band has been successfully reconstructed through a linear combination of the A 1–E 1 longitudinal phonon plasmon-coupled modes, leading to a free carrier concentration in good agreement with Hall effect measurements.

Optical switching in graded plasmonic waveguides

A mechanism of longitudinal confinement of optical energy via coupled plasmon modes is proposed in chains of noble metal nanoparticles embedded in a graded dielectric medium, which is analogous to the confinement of electrons in semiconductor quantum wells. In these systems, one can control the transmission of optical energy by varying the graded refractive index of the host medium or the separation between the nanoparticles to realize the photonic analog of electronic transistors. Possible passband tunability by nanoparticle spacing and modulation of the refractive index in the host medium have been presented explicitly and compared favorably with numerical calculations.

Improved Resolution of Epitaxial Thin Film Doping Using FTIR Reflectance Spectroscopy

Room temperature Fourier Transform Infrared Reflection Spectroscopy (FTIR) was used to investigate the thickness and Free Carrier Concentration (FCC) of heavily and lightly doped 4H and 6H-SiC epitaxial films. Multiple epitaxial layer stacks typical of lateral devices such as the MESFET were grown on 6H-SiC semi-insulating substrates. The estimation of thickness and FCC of the n-channel epi layer is improved by studying the Longitudinal Optical Phonon Plasmon Coupled Modes (LPP). A modelbased analysis of the experimental reflectance spectra from these samples is performed using a dielectric function that accounts for the phonon-photon coupling and plasmonphoton coupling. The value of the LPP+ mode frequency estimated from the reflectance spectrum in the range 600-1200 cm-1 is observed to increase in direct correlation with the electron free-carrier concentration.

Coherent longitudinal optical phonon and plasmon coupling in the near-surface region of InN

Coherent phonon spectroscopy of a high-quality InN epitaxial layer is carried out using time-resolved second-harmonic generation. A coherent longitudinal optical phonon and plasmon coupling mode only at 447cm−1 can be resolved in the spectrum. Its frequency shows no dependence on the photoinjected carrier density up to 1.5×1019cm−3. This phenomenon is attributed to the hybridization of a coherent A1(LO) phonon with the intrinsic cold plasma accumulated in the near-surface region of InN, where the plasma density could reach on the order of 1020cm−3, much higher than the bulk carrier concentration 1×1018cm−3 determined by Hall effect measurement.

Optical properties of epitaxial lateral overgrowth GaN structures studied by Raman and cathodoluminescence spectroscopies

The properties of epitaxial lateral overgrowth (ELO) GaN layers were studied by means of cathodoluminescence (CL), micro-Raman spectroscopies, and transmission electron microscopy (TEM). CL shows a strong enhancement of the luminescence emission in the ELO regions, where TEM showed the absence of dislocations. The CL enhancement observed is mostly due to the yellow luminescence (YL) band, which contrasts with the good crystal quality observed in the ELO regions by TEM and Raman spectroscopy. Local CL spectra in the ELO regions showed a different behavior of the near band edge emission in relation to the buffer layers and central part of the vertical growth region. Donor-acceptor pair recombination is enhanced in the ELO regions. The Raman spectra indicate a low strain level in the ELO regions. Plasmon-coupled modes are not observed in these areas, indicating the absence of free carriers. CL and Raman observations could thus be explained in terms of the good crystalline quality of the ELO regions, in which the incorporation of impurities is electrically compensated by the formation of deep acceptors, probably VGa, responsible as well for the enhancement of the YL band.

Influence of electron–phonon scattering on optical absorption in ZnSe

We investigate the influence of quantum kinetic effects connected with the scattering of carriers with plasmons and LO-phonons on the excitonic absorption in ZnSe within a linear response theory. Within our approach the quantum-kinetic effects are reflected in a dependence of the many-body effects on the energy. In contrast to earlier treatments we use a unified description of the longitudinal dielectric function, including coupled plasmon and LO-phonon modes. We present a way to handle the well known polaron contributions in order to provide the correct excitonic features at low excitation. We find, that after elimination of the polaron contributions the phonons only weakly influence the interband self-energy for small densities at about 1014 − 1015 cm−3 in comparison to the plasmons. With increasing density structures around the LO-phonon-energy ± ωLO appear. These are more pronounced inside the band, but increasingly affect the region of excitonic absorption too. Surprisingly there is a stronger coupling of plasmon and phonon modes as expected from the pure dispersions.

Coherent longitudinal optical phonon and plasmon coupling in GaAs

The coupling between femtosecond laser-excited coherent LO phonons and plasmon in GaAs was investigated in real time via two-pump time-resolved second-harmonic generation. The coherent LO phonon–plasmon coupling mode dominated the lattice oscillation when the photoexcited plasma density exceeded 3×1018/cm3. Its oscillation intensity showed sinusoidal dependence on the separation time between the two pumps and a phase shift from the initial LO phonon oscillation. These observations reveal the formation time of photoexcited plasmon and its coherent coupling with LO phonon in femtosecond time scale.

Coupled two-layer plasmon modes induced in a single quantum well by in-plane magnetic fields

We present a complete investigation of the collective excitations in quantum wells subject to in-plane magnetic fields $\mathbf{B},$ based on the random phase approximation and self-consistently calculated ground states in the Hartree approximation. We find that, for a symmetric quantum well of width w, high in-plane $\mathbf{B}$ fields ${(l}_{B}\ensuremath{\ll}w)$ can, besides the usual optical intrasubband plasmons (OP's), give rise to an additional branch of undamped acoustic plasmons (AP's), which does not exist in a single well without $\mathbf{B}.$ In low $\mathbf{B}$ fields ${(l}_{B}\ensuremath{\gg}w),$ the magnetic-field broken symmetry of electron wave functions causes an anticrossing feature of the plasmon dispersions even in the symmetric well. In intermediate $\mathbf{B}$ fields ${(l}_{B}\ensuremath{\lesssim}w),$ an unusual anisotropy effect occurs: the number of the plasmon branches depends on the angle between the magnetic field and the in-plane plasmon wave vector $\mathbf{q}.$ For $\mathbf{B}\ensuremath{\Vert}\mathbf{q},$ there exist two branches of plasmons (OP's and AP's), whereas only one (OP) exists for $\mathbf{B}\ensuremath{\perp}\mathbf{q}.$

Cathodoluminescence and microRaman study of GaN ELO structures

ABSTRACTEpitaxial Lateral Overgrowth (ELO) is an effective method to reduce dislocations in heterostructures with large lattice mismatch. This method has been widely used to improve the quality of GaN layers. We present herein a study of the properties of ELO GaN layers grown by HVPE from an MOVPE GaN buffer layer. Cathodoluminescence shows a strong enhancement of the luminescence emission in the ELO regions, where TEM has proven the absence of dislocations. Local cathodoluminescence spectra show that this enhancement is mostly due to the yellow luminescence band. Besides Donor-Acceptor Pair recombination bands are observed in the near band gap spectral range. Raman data show that the ELO layers present a good crystalline quality. The Raman spectra did not reveal the presence of free carriers in concentration high enough to allow the presence of Longitudinal Optic Phonon Plasmon Coupled modes. The results are discussed in terms of the incorporation of impurities in the ELO layers together with the formation of compensating deep acceptors, probably VGa.

Plasmonics-A Route to Nanoscale Optical Devices

The further integration of optical devices will require the fabrication of waveguides for electromagnetic energy below the diffraction limit of light. We investigate the possibility of using arrays of closely spaced metal nanoparticles for this purpose. Coupling between adjacent particles sets up coupled plasmon modes that give rise to coherent propagation of energy along the array. A point dipole analysis predicts group velocities of energy transport that exceed 0.1c along straight arrays and shows that energy transmission and switching through chain networks such as corners (see Figure) and tee structures is possible at high efficiencies. Radiation losses into the far field are expected to be negligible due to the near-field nature of the coupling, and resistive heating leads to transmission losses of about 6 dB/μm for gold and silver particles. We analyze macroscopic analogues operating in the microwave regime consisting of closely spaced metal rods by experiments and full field electrodynamic simulations. The guiding structures show a high confinement of the electromagnetic energy and allow for highly variable geometries and switching. Also, we have fabricated gold nanoparticle arrays using electron beam lithography and atomic force microscopy manipulation. These plasmon waveguides and switches could be the smallest devices with optical functionality.

Evidence of free carrier concentration gradient along the c-axis for undoped GaN single crystals

Results of measurements of infrared reflectivity and micro-Raman scattering on the undoped GaN high pressure grown single crystals are reported. These crystals have usually a high electron concentration due to unintentional doping by oxygen. We show, by the shift of the plasma edge (infrared reflectivity measurements), that the free electron concentration is always higher on the (0 0 0 1 ̄ )N face of the GaN single crystal than on the (0 0 0 1)Ga face. In order to determine the profile of the free carrier concentration, we performed transverse micro-Raman scattering measurements along the (0 0 0 1) c-axis of the crystal with spatial resolution of 1 μm. Micro-Raman experiments give a quantitative information on the free carrier concentration via the longitudinal optical phonon–plasmon (LPP) coupling modes. Thus, by studying the behavior of the LPP mode along the c-axis, we found the presence of a gradient of free electrons. We suppose that this gradient of electrons is due to the gradient of the main electron donor, in undoped GaN single crystals, i.e. oxygen impurity. We propose a growth model which explains qualitatively the incorporation of oxygen during the growth of GaN crystal under high pressure of nitrogen.

Inelastic light scattering spectra due to coupled plasmon modes inparallel quantum wires

The effects of tunnelling on the collective charge-density excitations in twocoupled parallel quantum wires and their inelastic light scattering spectraare studied theoretically. The evolution of the collective excitation spectrais presented as a function of the tunnelling strength by altering the symmetryof the two quantum wires. We find that, for two symmetric quantum wires in aresonant tunnelling regime, the plasmon modes are similar to the intrasubbandand intersubband plasmon modes. However, in the non-resonant tunnellingcondition, the mode mixing occurs. The inelastic light scattering spectrum dueto the mixed plasmon modes are discussed in detail.

Electromagnetic energy transfer and switching in nanoparticle chain arrays below the diffraction limit

Electromagnetic energy transfer in plasmon wires consisting of chains of closely spaced metal nanoparticles can occur below the diffraction limit by means of coupled plasmon modes. Coherent propagation with group velocities that exceed 0.1 c is possible in straight wires and around sharp corners (bending radius much less than wavelength of visible light). Energy transmission through chain networks is possible at high efficiencies and is a strong function of the frequency and polarization direction of the plasmon mode. Although these structures exhibit transmission losses due to heating of about 3 dB/500 nm, they have optical functionality that cannot be obtained in other ways at a length scale \ensuremath{\ll}1 \ensuremath{\mu}m.

Detection of terahertz radiation from longitudinal optical phonon–plasmon coupling modes in InSb film using an ultrabroadband photoconductive antenna

Terahertz radiation from longitudinal optical (LO) phonon–plasmon coupling modes in InSb films is observed using an ultrafast photoconductive antenna detector. We demonstrate a response frequency of up to 7 THz for a low-temperature-grown GaAs-based photoconductive antenna gated with 25 fs laser pulses. It is found that the emission frequencies of the coupling modes are dependent only on the residual carrier density, not on the excitation carrier density. It is also found that the LO phonon–plasmon oscillations in semiconductors can serve as an efficient THz source.

Coupled Plasmon Modes in an Ordered Hexagonal Monolayer of Metal Nanoparticles: A Direct Observation

We report on the experimental observation of STM-induced photon emission in ultrahigh vacuum on a network of 4-nm silver spheres. The spheres are covered by a dielectric, electrically insulating, organic layer and deposited on Au(111). The bias-dependent spatial distribution of the photon emission rates reveals the electric-field distribution of the different coupled plasmon modes in this model.

Study of coherent LO phonon–plasmon-coupled modes by double-pulse excitation

The coherent LO phonon–plasmon-coupled mode in n-GaAs has been studied by using a double-pulse excitation. When the irradiation power of the second pump pulse in the double-pulse excitation is increased, the lower branch (L_ mode) of the LO phonon–plasmon-coupled mode generated by the second pump pulse are shifted to the higher-frequency side than that generated by the first pump pulse. When the separation time between the two pulse components of the double-pulse excitation is varied, the amplitude of the L_ mode after the second pump pulse changes dramatically. These results indicate that the first coherent L_ mode generated by the first pump pulse changes to a new state by the second pump pulse and influences the amplitude of the new coherent L_ mode generated by the second pump pulse.