Modulation Of Surface Plasmon Polaritons Research Articles

Azimuthally Modulating Surface Plasmon Polaritons

Abstract We suggest a scheme for the lossless excitation of surface plasmon polaritons (SPPs) at a metal-dielectric interface using a Laguerre Gaussian (LG) beam. We consider a three-layer structure consisting of a top transparent medium, a middle metal thin film, and a dielectric bottom layer. The dielectric medium contains three-level atoms exhibiting electromagnetically induced transparency (EIT). By applying a control field, a LG beam, and a microwave field, we induce azimuthally varying permittivity, which leads to the azimuthal modulation of SPPs. The propagation length of SPPs is increased when excited by a LG beam carrying optical angular momentum (OAM). Moreover, the group velocity of SPPs can be effectively controlled by adjusting the OAM and azimuthal angle of the LG beam, allowing for both slow and fast propagation of SPPs.

Enhanced magnetic modulation of surface plasmon polaritons on hyperbolic metasurfaces.

In this Letter we demonstrate a fundamentally new, to the best of our knowledge, concept to enhance the magnetic modulation of the surface plasmon polaritons (SPPs) by using hybrid magneto-plasmonic structures consisting of hyperbolic plasmonic metasurfaces and magnetic dielectric substrates. Our results show that the magnetic modulation of SPPs in the proposed structures can be an order of magnitude stronger than in the hybrid metal-ferromagnet multilayer structures conventionally used in active magneto-plasmonics. We believe that this effect will allow for the further miniaturization of magneto-plasmonic devices.

Spatiotemporal Fourier transform with femtosecond pulses for on-chip devices

On-chip manipulation of the spatiotemporal characteristics of optical signals is important in the transmission and processing of information. However, the simultaneous modulation of on-chip optical pulses, both spatially at the nano-scale and temporally over ultra-fast intervals, is challenging. Here, we propose a spatiotemporal Fourier transform method for on-chip control of the propagation of femtosecond optical pulses and verify this method employing surface plasmon polariton (SPP) pulses on metal surface. An analytical model is built for the method and proved by numerical simulations. By varying space- and frequency-dependent parameters, we demonstrate that the traditional SPP focal spot may be bent into a ring shape, and that the direction of propagation of a curved SPP-Airy beam may be reversed at certain moments to create an S-shaped path. Compared with conventional spatial modulation of SPPs, this method offers potentially a variety of extraordinary effects in SPP modulation especially associated with the temporal domain, thereby providing a new platform for on-chip spatiotemporal manipulation of optical pulses with applications including ultrafast on-chip photonic information processing, ultrafast pulse/beam shaping, and optical computing.

THz pulse train generation through ultrafast development of surface plasmon-polariton modulation instability

Propagation of high-intensity electromagnetic waves in a waveguide structure could initiate nonlinear effects resulting in drastic changes of their spatial and temporal characteristics. We study the modulation instability effect induced by propagation of surface plasmon polaritons in a silver thin-film waveguide. The nonlinear Schrodinger equation for propagating surface plasmon wave is obtained. It is shown numerically that the modulation instability effect can give rise to ultrafast spatial redistribution and longitudinal localization of surface plasmon-polariton wave energy in subwavelength scale. The dependence of plasmon wave dispersion and nonlinear characteristics on metal film thickness is considered. We demonstrate that the use of films with the thickness varying along the waveguide length allows reduction of the generated pulse width and increase of frequency comb bandwidth. The proposed technique is promising for design of ultra-compact (tens of nm) optical generators delivering pulse trains with the repetition rate higher than 1 THz.

All-Optical Graphene-Based Modulation of Surface Plasmon Polaritons via Modulation Instability for Secure Optical Communication

All-Optical Graphene-Based Modulation of Surface Plasmon Polaritons via Modulation Instability for Secure Optical Communication

Plasmonic electro-optic modulator based on degenerate semiconductor interfaces

Abstract We present a semiconductor-based optoelectronic switch based on active modulation of surface plasmon polaritons (SPPs) at lattice-matched indium gallium arsenide (In0.53Ga0.47As) degenerately doped pn++ junctions. The experimental device, which we refer to as a surface plasmon polariton diode (SPPD), is characterized electrically and optically, showing far-field reflectivity modulation for mid-IR wavelengths. Self-consistent electro-optic multiphysics simulations of the device’s electrical and electromagnetic response have been performed to estimate bias-dependent modulation and switching times. The numerical model shows a strong agreement with the experimental results, validating the claim of excitation and modulation of SPPs at the junction, thus potentially providing a new pathway toward fast optoelectronic devices.

Polarization-based dynamic manipulation of Bessel-like surface plasmon polaritons beam.

Taking advantage of the phase modulation of surface plasmon polaritons (SPPs) generated by subwavelength slits, we elaborately designed an axicon-shaped slit array to dynamically manipulate the profiles of Bessel-like SPPs beams by changing the polarization of the incident light. As the polarization direction of linearly polarized light changes from horizontal to vertical, the profile of SPPs beam will evolve from the zeroth-order Bessel function to the first-order Bessel function. For circularly polarized light, the geometry Pancharatnam-Berry (PB) phase is introduced and can lead to the transverse shift of the main lobe of SPPs beam. The evolution of the beam profile and the distance of the transverse shift are theoretically illustrated by analyzing the phase distribution of SPPs. The proposed versatile approach suggests charming applications including on-chip communications, polarization-controlled particle manipulation and the design of polarization-based dynamic SPPs devices.

Universal switching of plasmonic signals using optical resonator modes

We propose and investigate, both experimentally and theoretically, a novel mechanism for switching and modulating plasmonic signals based on a Fano interference process, which arises from the coupling between a narrow-band optical Fabry–Pérot cavity and a surface plasmon polariton (SPP) source. The SPP wave emitted from the cavity is actively modulated in the vicinity of the cavity resonances by altering the cavity Q-factor and/or resonant frequencies. We experimentally demonstrate dynamic SPP modulation both by mechanical control of the cavity length and all-optically by harnessing the ultrafast nonlinearity of the Au mirrors that form the cavity. An electro-optical modulation scheme is also proposed and numerically illustrated. Dynamic operation of the switch via mechanical means yields a modulation in the SPP coupling efficiency of ~80%, while the all-optical control provides an ultrafast modulation with an efficiency of 30% at a rate of ~0.6 THz. The experimental observations are supported by both analytical and numerical calculations of the mechanical, all-optical and electro-optical modulation methods.

Near-field focus steering along arbitrary trajectory via multi-lined distributed nanoslits.

The modulation of near-field signals has recently attracted considerable interest because of demands for the development of nano-scale optical devices that are capable of overcoming the diffraction limit of light. In this paper, we propose a new type of tuneable plasmonic lens that permits the foci of surface plasmon polariton (SPP) signals to be continuously steered by adjusting the input polarization state. The proposed structure consists of multi-lined nanoslit arrays, in which each array is tilted at a different angle to provide polarization sensitivity and the nanoslit size is adjusted to balance the relative amplitudes of the excited SPPs from each line. The nanoslits of each line are designed to focus SPPs at different positions; hence, the SPP focal length can be tuned by modifying the incident polarization state. Unlike in previously reported studies, our method enables plasmonic foci to be continuously varied with a smooth change in the incident linear polarization state. The proposed structures provide a novel degree of freedom in the multiplexing of near fields. Such characteristics are expected to enable the realization of active SPP modulation that can be applied in near-field imaging, optical tweezing systems, and integrated nano-devices.

Magnetic field control and wavelength tunability of SPP excitations using Al2O3/SiO2/Fe structures

Here, we show the high wavelength tunability and magnetic field modulation of surface plasmon polaritons (SPPs) of a waveguide mode that Double-layer Dielectrics and Ferromagnetic Metal, Al2O3/SiO2/Fe, trilayer structures exhibit when excited in the Otto configuration of attenuated total reflection setup. First by modeling, and then experimentally, we demonstrate that it is possible to tune the wavelength at which the angular dependent reflectance of these structures reaches its absolute minimum by simply adjusting the SiO2 intermediate dielectric layer thickness. This precise wavelength corresponds to the cut-off condition of SPPs' waveguide mode supported by the proposed structure, and it can be then switched between two values upon magnetization reversal of the Fe layer. In this specific situation, a large enhancement of the transverse magneto-optical effect is also obtained.

Design of hybrid structure for fast and deep surface plasmon polariton modulation.

The electric and optical performance of different surface plasmon polariton (SPP) electric modulation structures have been investigated by comparing the response speed and modulation figures of merit (FoM). To overcome the capacitance limitation and improve the response speed, we proposed a novel silver-graphene-dielectric-graphene-semiconductor vertical structure. Semiconductor nano-waveguide is introduced to help reduce ohmic loss in silver waveguide and reflect the leaked optical field back, enhancing the modulation depth. Through optimization, a device with estimated modulation FoM of more than 70% and hundreds of GHz response speed and 3 dB bandwidth is designed, which may bring great improvement to previous optical modulators.

Non-reciprocal Phase Shift and Mode Modulation in Dynamic Graphene Waveguides

We propose a graphene Ramsey interferometer to study the non-reciprocal phase shift and modulation of surface plasmon polaritons (SPPs). The Ramsey interferometer is constructed by a spatially separated graphene pair that can support two SPP modes. By dynamically modulating the surface conductivity of graphene, the SPP modes will undergo uncompleted cross conversion and an additional phase shift. When adopting two modulation regions in the interferometer, the converted mode will jump back with a nonreciprocal phase shift which can be controlled by the modulation length or phase. The SPP mode of phase shift will interfere with the throughout unconverted one, resulting in arbitrary power arrangement between the two SPP modes. The study may find great applications in plasmonic isolators, phase detectors, and one-way mode converters.

Surface plasmon resonance modulation in nanopatterned Au gratings by the insulator-metal transition in vanadium dioxide films.

Correlated experimental and simulation studies on the modulation of Surface Plasmon Polaritons (SPP) in Au/VO2 bilayers are presented. The modification of the SPP wave vector by the thermally-induced insulator-to-metal phase transition (IMT) in VO2 was investigated by measuring the optical reflectivity of the sample. Reflectivity changes are observed for VO2 when transitioning between the insulating and metallic states, enabling modulation of the SPP in the Au layer by the thermally induced IMT in the VO2 layer. Since the IMT can also be optically induced using ultrafast laser pulses, we postulate the viability of SPP ultrafast modulation for sensing or control.

Controlling the surface plasmon excitation efficiency using dielectric magneto-optical cavity

The diffraction of a plane wave by a magneto-optical cavity located on a metal interface is investigated theoretically. We show that the excitation of localized eigenmodes of the cavity allows one to efficiently excite the surface plasmon polariton (SPP). We exploit the polarization and symmetry properties of the cavity modes to propose an efficient approach for controlling the SPP intensity through an external magnetic field. The presented theoretical predictions are in good agreement with the rigorous computations based on the generalized Lorentz reciprocity theorem and aperiodic Fourier modal method. The magnetization-induced relative intensity variation of the excited SPP demonstrated in numerical computations varies from a few percent to 100% depending on the polarization of the incident wave. Such large modulation opens new possibilities for efficient SPP modulation with considered magneto-optical intensity effect.

Photo-thermal modulation of surface plasmon polariton propagation at telecommunication wavelengths

We report on photo-thermal modulation of thin film surface plasmon polaritons (SPP) excited at telecom wavelengths and traveling at a gold/air interface. By operating a modulated continuous-wave or a Q-switched nanosecond pump laser, we investigate the photo-thermally induced modulation of SPP propagation mediated by the temperature-dependent ohmic losses in the gold film. We use a fiber-to-fiber characterization set-up to measure accurately the modulation depth of the SPP signal under photo-thermal excitation. On the basis of these measurements, we extract the thermo-plasmonic coefficient of the SPP mode defined as the temperature derivative of the SPP damping constant. Next, we introduce a figure of merit which is relevant to characterize the impact of temperature onto the properties of bounded or weakly leaky SPP modes supported by a given metal at a given wavelength. By combining our measurements with tabulated values of the temperature-dependent imaginary part of gold dielectric function, we compute the thermo-optical coefficients (TOC) of gold at telecom wavelengths. Finally, we investigate a pulsed photo-thermal excitation of the SPP in the nanosecond regime. The experimental SPP depth of modulation obtained in this situation are found to be in fair agreement with the modulation depths computed by using our values of gold TOC.

Unraveling the relationship between electromagnetic field intensity and the magnetic modulation of the wave vector of coupled surface plasmon polaritons

The magnetic-field-induced wave-vector modulation of surface plasmon polaritons (SPPs) is analytically derived for dielectric/metal and dielectric/metal/dielectric systems when a very thin magneto-optical metallic film is placed at different positions in the metal. In the simplest case of a single dielectric/metal interface, the SPP wave-vector modulation is found to be proportional to the intensity of the electromagnetic field at the location of the magneto-optically active layer. For the more complex dielectric/metal/dielectric systems, the SPPs existing at each dielectric/metal interface interact to give rise to modes with a symmetric- and anti-symmetric-like character. We show that in this case the relationship between wave-vector modulation and field distribution is more complex and does not follow a proportionality law for coupled eigenmodes.

Magnetic modulation of surface plasmon modes in magnetoplasmonic metal-insulator-metal cavities

The magnetic modulation of the surface plasmon-polariton (SPP) wavevector is experimentally and theoretically studied for the plasmonic modes excited in metal-insulator-metal (MIM) magnetoplasmonic cavities. For this purpose, Ag/SiO₂/Ag multilayers with different SiO₂ layer thickness in which a thin Co layer is positioned near the top Ag/SiO₂ interface, near the bottom SiO₂/Ag one, or near both of them, are studied. The magnetoplasmonic MIM cavities present symmetric (SM) and antisymmetric (AM) plasmonic modes, of different wavevector and electromagnetic field profiles inside the MIM cavity. We show that the magnetic SPP wavevector modulation strongly depends on which mode is considered, the cavity thickness, and the number and specific location of Co layers within the structure. With only one ferromagnetic layer, a net modulation is obtained, of higher magnitude as we reduce the SiO₂ layer thickness. The introduction of a second Co layer in the structure reduces the modulation due to the non-reciprocal character of SPP modes under an applied magnetic field. Moreover, we demonstrate that the non-reciprocal nature of the SPP modulation can be experimentally visualized in the magnetic hysteresis loops under plasmon excitation conditions by using two Co layers with different magnetization switching fields.

Strong Intensity Modulation of Surface Plasmon Polaritons by a Dielectric Layer

We find strong intensity modulations in surface plasmon polaritons (SPPs) by changing the refractive index and thickness (i.e., the optical length) of the dielectric layer in metal-dielectric nanodevices. We interpret the phenomena by considering the dielectric layer as a Fabry–Pérot cavity. Analysis from finite-difference time-domain numerical studies reveals that the strong periodic variation of the enhancement in SPP fields is due to alternating constructive and destructive interference of optical waves in the dielectric layer. The present results can be applied in designing planar metal-dielectric nanodevices that require optical intensity modulation or high-field enhancement, such as optical switches or thin-film solar cells.

Short-range surface plasmon propagation supported by stimulated amplification using electrical injection

We have investigated the propagation of the long-range mode (LRSP) and the short-range mode (SRSP) surface plasmon polaritons (SPPs) along the waveguide made from Au film and quantum wells (QWs) gain medium. Influenced by the gain spectral nonuniformity, the SRSP showed narrower spectrum than the LRSP in output, denoting that the SRSP propagation was supported by stimulated amplification (SA) in electrically-pumped QWs. An SRSP output power as large as 1.6 times of that of the LRSP was obtained over a travelling distance of 80 μm. The mechanism of SA-supported SRSP propagation can be adopted for electrical modulation of SPPs.

An Integrated Electrochromic Nanoplasmonic Optical Switch

We demonstrate an electrochemically driven optical switch based on absorption modulation of surface plasmon polaritons (SPPs) propagating in a metallic nanoslit waveguide containing nanocrystals of electrochromic Prussian Blue dye. Optical transmission modulation of ∼96% is achieved by electrochemically switching the dye between its oxidized and reduced states using voltages below 1 V. High spatial overlap and long interaction length between the SPP and the active material are achieved by preferential growth of PB nanocrystals on the nanoslit sidewalls. The resulting orthogonalization between the directions of light propagation and that of charge transport from the electrolyte to ultrathin active material inside the nanoslit waveguide offers significant promise for the realization of electrochromic devices with record switching speeds.