Subwavelength Mode Research Articles

Improving the room-temperature confinement of light by miniaturizing mode sizes into a deep subwavelength scale using dielectric spheres in metal cavities

The confinement of light within nanometer-scale regions may result in the significant enhancement of light-matter interactions. However, light confinement to nanometers is hindered by the diffraction limit of a dielectric material. For a dielectric cavity, if the material loss is negligible, reducing the cavity size usually causes a significantly increase in radiation loss. Surface plasmons show great promise for potential subwavelength light confinement. However, in most circumstances, light confinement by dissipative metallic materials can cause ohmic losses at optical frequencies. In such cases, the realization of light confinement with deep subwavelength mode sizes results in great losses and thus has low quality factors. In the present study, a three-dimensional light confinement with deep subwavelength mode sizes is achieved using dielectric spheres in metal cavities. Contrary to other mechanisms for subwavelength light confinement that are based on the use of dielectric or metal cavities, the nanometer-scale regions ensure that most of the light energy is confined away from the metal-dielectric interfaces, thereby decreasing light absorption in the metal cavity. In turn, the metal cavity decreases the radiation loss of light. Thus, high quality factors ranging from 2×10(2) to 6×10(2) can be obtained at room temperature. An effective electrical mode volume ranging from 7×10(-5)λ(0)(3) to 2×10(-4)λ(0)(3) (where λ(0) is the resonant wavelength in a vacuum) can be achieved. Therefore, this method of three-dimensional light confinement with deep subwavelength mode sizes using dielectric spheres in metal cavities may have potential applications in the design of nanolasers, nanophoton detectors, nonlinear optical switches, and so on.

Far field subwavelength imaging of magnetic patterns

Far field imaging of subwavelength magnetic objects in real time is a very challenging issue. We propose an original solution based on a planar array of closely spaced split ring resonators. Hybridization between the resonators of such metalens induces subwavelength modes with different frequencies. Thanks to these high Q resonating modes, Purcell like effect allows an evanescent source, close to the metalens, to emit waves that can be collected efficiently in the far field. We present the first microwave experimental demonstration of such metalens to image of a subwavelength magnetic pattern. Numerical simulation shows that this approach is still valid at THz frequencies.

Guiding of Long-Range Hybrid Plasmon Polariton in a Coupled Nanowire Array at Deep-Subwavelength Scale

A novel type of hybrid plasmonic waveguiding structure that integrates semiconductor and metallic nanowires has been proposed and investigated at telecommunication wavelengths. Semiconductor nanowires symmetrically placed on both sides of a metallic nanowire provide an additional degree of freedom for tuning the characteristics of the plasmonic nanowire mode. Theoretical analysis reveals that at appropriate geometrical parameters, the symmetric hybrid plasmonic mode of the waveguide could achieve subwavelength mode confinement with ultra-long propagation distance (even exceeding the millimeter range). Such a hybrid plasmonic nanowire structure could facilitate ultra-strong light-matter interaction between semiconductor and metal materials, and enable important applications in nanolasers and nonlinear photonics.

Creating electromagnetic cavities using transformation optics

We investigate the potential of transformation optics for the design of novel electromagnetic cavities. First, we determine the dispersion relation of bound modes in a device performing an arbitrary radial coordinate transformation and we discuss a number of such cavity structures. Subsequently, we generalize our study to media that implement azimuthal transformations, and show that such transformations can manipulate the azimuthal mode number. Finally, we discuss how the combination of radial and azimuthal coordinate transformations allows for perfect confinement of subwavelength modes inside a cavity consisting of right-handed materials only.

Infrared surface plasmons on heavily doped silicon

Conductors with infrared plasma frequencies are potentially useful hosts of surface plasmon polaritons (SPP) with sub-wavelength mode confinement for sensing applications. A challenge is to identify such a conductor that also has sharp SPP excitation resonances and the capability to be functionalized for biosensor applications. In this paper we present experimental and theoretical investigations of IR SPPs on doped silicon and their excitation resonances on doped-silicon gratings. The measured complex permittivity spectra for p-type silicon with carrier concentration 6×1019 and 1×1020 cm−3 show that these materials should support SPPs beyond 11 and 6 μm wavelengths, respectively. The permittivity spectra were used to calculate SPP mode heights above the silicon surface and SPP propagation lengths. Reasonable merit criteria applied to these quantities suggest that only the heaviest doped material has sensor potential, and then mainly within the wavelength range 6 to 10 μm. Photon-to-plasmon coupling resonances, a necessary condition for sensing, were demonstrated near 10 μm wavelength for this material. The shape and position of these resonances agree well with simple analytic calculations based on the theory of Hessel and Oliner (1965).

Subwavelength slow-light waveguides based on a plasmonic analogue of electromagnetically induced transparency

We introduce a plasmonic waveguide system, based on a plasmonic analogue of electromagnetically induced transparency, which supports a subwavelength slow-light mode, and exhibits a small group velocity dispersion. The system consists of a periodic array of two metal-dielectric-metal (MDM) stub resonators side-coupled to a MDM waveguide. Decreasing the frequency spacing between the two resonances increases the slowdown factor and decreases the bandwidth of the slow-light band. We also show that there is a trade-off between the slowdown factor and the propagation length of the slow-light mode.

Surface Plasmonic Nanodisk/Nanopan Lasers

We report the demonstration of subwavelength plasmonic lasers from a semiconductor nanodisk with a silver nanopan cavity. Full 3-D surface-plasmon-polariton (SPP) lasing was achieved because the nanodisk/nanopan structure enables excitation of high-quality SPP modes with subwavelength mode volumes. The optical properties of all possible resonant modes including SPP and optical modes excited in the nanodisk/nanopan were calculated and analyzed systematically using the finite-difference time-domain method. To fabricate the nanodisk/nanopan structure with an ultra-smooth silver surface, conformal deposition of silver was performed on the nanodisk. Rich SPP lasing actions were demonstrated through optical pumping of the fabricated structures. The observed SPP lasing modes were indentified unambiguously from measurements of the spectrum, mode image, and polarization state. These measurements compared well with the simulation results. In particular, the significant temperature-dependent threshold of the SPP lasers, which distinguishes SPP modes from conventional optical modes, was measured. This subwavelength SPP laser is a significant step toward the further miniaturization of a coherent light source in ultra-compact photonic integrated circuits.

Ultraconfined Interlaced Plasmons

We describe a mesoscopic excitation in strongly coupled grids of metallic nanorods, resulting from the hybridization of weakly bounded plasmons. It is shown both theoretically and experimentally that the characteristic spatial scale of the interlaced plasmons is determined by geometrical features, rather than from the electrical length of the nanorods, and that due to their wide band nature, weak sensitivity to metallic absorption, and subwavelength mode sizes, such plasmons may have exciting applications in waveguiding in the nanoscale.

Purcell effect in the inductor-capacitor laser

The recently demonstrated inductor-capacitor (LC) laser has a strongly subwavelength mode volume. In this Letter we investigate the Purcell effect in the LC laser resonator. An average Purcell factor of 17 is computed for the LC laser. The laser rate equations are formulated and solved for the LC laser. Evidence of a large Purcell factor in agreement with the theoretical one is obtained through comparison of the threshold and the emission characteristics of the LC laser with the rate equations.

Ultrasmall subwavelength nanorod plasmonic cavity

We propose an ultrasmall plasmonic cavity consisting of a high-index/low-index dielectric nanorod covered with silver. Full three-dimensional subwavelength confinement of the surface-plasmon polaritons was achieved at the high-index dielectric-silver interface without propagating to the low-index dielectric-silver interface. The numerical simulations showed that the plasmonic mode excited in this cavity has a deep subwavelength mode volume of 0.0038(λ/2n)(3) and a quality factor of 1500 at 40 K, and consequently a large Purcell factor of ∼2×10(5). Therefore, this plasmonic cavity is expected to be useful for the demonstration of high-efficiency single photon sources or low-threshold lasers in an ultracompact nanophotonic circuit.

Hybrid Photonic−Plasmonic Crystal Nanocavities

We propose a hybrid optical nanocavity consisting of photonic crystals coupled to a metal surface with a nanoscale air gap between. The hybridization of photonic crystal modes and surface plasmons across the gap forms hybrid cavity modes, which are highly confined in the low-loss air gap region. Deep subwavelength mode volume and high quality factor are demonstrated at telecommunication wavelength, resulting in an extremely large Q/V(m) ratio of 60,000 λ(-3). This new type of high-Q/V(m) broad-band hybrid nanocavity opens up opportunities for various applications in enhanced light-matter interactions.

Dielectric waveguide with deep subwavelength mode confinement based on coupled nanowires

Dielectric waveguide with deep subwavelength mode confinement based on coupled semiconductor nanowires is proposed.Through the coupling between two adjacent nanowires with high refractive indexes, light can be efficiently confined in the nano-gap between the nanowires with a low refractive index. Numerical simulations indicate that the effective mode area of such a waveguide can be as small as Λ20/200, which is one order of magnitude smaller than that of a single nanowire, and such a mode confinement is comparable to that of hybrid plasmonic waveguide. It is also shown that from the view of real applications, possible existing low refractive index oxidization layers of nanowires, low refractive index substrate and small deviation of nanowire dimensions do not have significant influence on the property of the waveguide. As the propagation length is theoretically infinite for dielectric waveguides, such a coupled nanowire waveguide with deep subwavelength mode confinement may have important applications in future integrated photonic circuits.

Low-loss silicon-based hybrid plasmonic waveguide with an air nanotrench for sub-wavelength mode confinement

A hybrid plasmonic waveguide based on a silicon-on-insulator substrate with an air trench is proposed and investigated. Theoretical analysis demonstrates that the nano-scale air trench could result in strong local field enhancement and tight mode confinement. Besides, the transmission loss of the fundamental hybrid plasmonic mode could also be reduced despite the existence of the high-index silicon rib. The proposed structure is compatible with standard nanofabrication process based on the silicon wafers and could enable various nanophotonic integrated components.

Guided subwavelength slow-light mode supported by a plasmonic waveguide system

We introduce a plasmonic waveguide system, which supports a subwavelength broadband slow-light guided mode with a tunable slowdown factor at a given wavelength. The system consists of a metal-dielectric-metal (MDM) waveguide side-coupled to a periodic array of MDM stub resonators. The slowdown factor at a given wavelength can be tuned by adjusting the geometrical parameters of the system. In addition, there is a trade-off between the slowdown factor and the propagation length of the supported optical mode. Finally, we show that light can be coupled efficiently from a conventional MDM waveguide to the plasmonic waveguide system.

Nanowaveguides and couplers based on hybrid plasmonic modes

Experimental demonstration of silicon nanowires based hybrid plasmonic waveguides and couplers with subwavelength mode confinement at the near infrared wavelength λ=980 nm are presented. By measuring the radiating light from the discontinuities in a nanowire, the estimated propagation length of the hybrid plasmonic waveguide is about 30 μm (corresponding to a propagation loss of ∼0.14 dB/μm). For the coupler, the experimental results show that the hybrid plasmonic modes can be efficiently coupled between two overlapping nanowires only with a 1.9 μm long coupling length.

Dielectric-loaded surface plasmon polariton waveguide with a holey ridge for propagation-loss reduction and subwavelength mode confinement

A novel dielectric-loaded surface plasmon polariton (DLSPP) waveguide with an air nanohole within a high-index dielectric ridge is proposed and analyzed. It is demonstrated by simulations that the introduced air nanohole could strongly modify the modal behavior, and it could alleviate the transmission loss caused by the high-index ridge with rather small sacrifice in the mode area. Under certain geometric parameter ranges, a shallow and wide air nanohole at the metal surface could result in strong local field enhancement while improves the figure of merit (FOM). The proposed structure could enable the realization of the DLSPP waveguide with a high-index ridge to achieve subwavelength mode confinement with relatively low transmission loss.

Deep subwavelength waveguiding and focusing based on designer surface plasmons

We experimentally demonstrate focusing and guiding electromagnetic (EM) waves in a designer surface plasmonic waveguide with deep subwavelength mode cross section. Our experiments show that a metal grating with suitable parameters, functioning as a designer surface plasmonic waveguide, can support deep subwavelength surface modes and the width of the modes can be squeezed also into deep subwavelength by tapering the width of the waveguide. The results provide a new insight into deep subwavelength waveguiding and focusing.

Confining light in deep subwavelength electromagnetic cavities

We demonstrate that it is possible to confine electromagnetic radiation in cavities that are significantly smaller than the wavelength of the radiation it encapsulates. To this aim, we use the techniques of transformation optics. First, we present a "perfect cavity" of arbitrarily small size in which such confined modes can exist. Furthermore, we show that these eigenmodes have a continuous spectrum and that bending losses are absent, in contrast to what is observed in traditional microcavities. Finally, we introduce an alternative cavity configuration that is less sensitive to material imperfections and still exhibits deep subwavelength modes combined with high quality factor, even if considerable material losses are included. Such a cavity may be interesting for the storage of information in optical data processing and for applications in quantum optics.

Reducing crosstalk between nanowire-based hybrid plasmonic waveguides

Hybrid plasmonic waveguides based on a surface oxidized dielectric nanowire placed on a metal surface can facilitate simultaneously deep subwavelength mode confinement and large propagation length. Directional coupling based on such waveguides are theoretically investigated. Much lower crosstalk is noticed for such hybrid plasmonic waveguides compared to conventional waveguides based on bare dielectric nanowires. Some modifications, such as vertically placing the metal surfaces or using a metallic block between the nanowires, are studied which can further reduce the crosstalk between two waveguides. The proposed low crosstalk structures based on hybrid plasmonic waveguides can provide a simple platform for plasmonic integration which can at the same time easily interface with traditional photonic circuits.

Design of a surface-emitting, subwavelength metal-clad disk laser in the visible spectrum

We analyze metal-clad disk cavities designed for nanolasers in the visible red spectrum with subwavelength device size and mode volume. Metal cladding suppresses radiation loss and supports low order modes with room temperature Q of 200 to 300. Non-degenerate single-mode operation with enhanced spontaneous emission coupling factor β is expected with the TE(011) mode that has a 0.46(λ(0)/n)(3) mode volume and Q = 210 in a device of size 0.12λ(0)(3). Threshold gain calculations show that room temperature lasing is possible using multiple GaInP/AlGaInP quantum wells as the gain medium. Placing a planar metal reflector under the cavity can enhance radiation and extraction efficiencies or increase the Q, without incurring additional metallic absorption loss. We show that the far-field radiation characteristics are strongly affected by the devices' immediate surroundings, such as changes in metal cladding thickness, even as the resonant mode profile, frequency, and Q remain the same. When the metal cladding is 1 mm thick, light radiates upward with a distinct intensity maximum at 45° when the cladding is 100 nm thick, the emitted light spreads in a near-horizontal direction.