In addition to surface plasmon polaritons (SPPs), quasi-cylindrical waves (CWs) are also important components of electromagnetic fields on metal surfaces. In this study, we present a closed-form expression for CW fields on a dielectric-film-coated metal surface. It is found that the effective refractive index of CWs roughly keeps unchanged when the coated dielectric film becomes thicker, while the effective refractive index of SPPs increases rapidly. These different responses are explained by referring to a waveguide perspective, in which the SPP and CW waves are, respectively, identified as a waveguide mode and superposition of radiation modes. The results and related analyses with the waveguide perspective explicitly show the different physical natures of SPPs and CWs. (C) 2015 Optical Society of America
Surface plasmon polaritons (SPPs) have sparked enormous interest on nanophotonics beyond the diffraction limit for their remarkable capabilities of subwavelength confinements and strong enhancements. Due to the inherent polarization sensitivity of the SPPs [transverse-magnetic (TM) polarization], it is a great challenge to couple the s-polarized free-space light to the SPPs. Here, an ultrasmall defect aperture (<(2)/2) is designed to directionally couple both the p- and s-polarized incident beams to the single SPP mode in a broad bandwidth, which is guided by a subwavelength plasmonic waveguide. Simulations show that hot spots emerge at the sharp corners of the defect aperture when the incident beams illuminate it from the back side. The strong radiative fields from the hot spots are directionally coupled to the SPP mode because of the symmetry breaking of the defect aperture. By adjusting the structural parameters, both the unidirectional and bidirectional SPP coupling from the two orthogonal linear-polarization incident beams are experimentally demonstrated. The polarization-free coupling of the SPPs is of importance in circuits for fully optical processing of information with a deep-subwavelength footprint.
A sharp trapped resonance is numerically predicted in a single metal-insulator-metal (MIM) resonator by exciting the anti-symmetric waveguide mode. This MIM resonator consists of a wide-gap MIM structure which is connected with a narrow-gap MIM waveguide. By introducing a small structural break in the plasmonic resonator, both of the symmetric and anti-symmetric waveguide modes are excited in the wide-gap MIM structure. By reducing the propagation loss as well as taking advantage of the different field distribution of the anti-symmetric waveguide mode, a strongly trapped resonance with a quality factor as high as about Q a parts per thousand 570 emerges in the MIM resonator. This quality factor is significantly greater than that of the MIM resonators based on the widely used symmetric waveguide mode, which has much longer propagation lengths. The utilization of the anti-symmetric mode in the MIM waveguide provides a new possibility for designing high-performance plasmonic devices.
Using a double-slit structure fabricated on a gold film or a subwavelength (300 nm) plasmonic waveguide, high-contrast and broadband plasmonic sensors based on the interference of surface plasmon polaritons (SPPs) are experimentally demonstrated on chips. By adjusting the focused spot position of the p-polarized incident light on the double-slit structure to compensate for the propagation loss of the SPPs, the interfering SPPs from the two slits have nearly equal intensities. As a result, nearly completely destructive interference can be experimentally achieved in a broad bandwidth (>200 nm), revealing the robust design and fabrication of the double-slit structure. More importantly, a high sensing figure of merit (FOM*) of >1 x 10(4) RIU-1 (refractive index unit), which is much greater than the previous experimental results, is obtained at the destructive wavelength because of a high contrast ratio (C = 0.96). The high-contrast and broadband on-chip sensor fabricated on the subwavelength plasmonic waveguide may find important applications in the real-time sensing of particles and molecules.
Surface-plasmon-polariton (SPP) launchers, which can couple the free space light to the SPPs on the metal surface, are among the key elements for the plasmonic devices and nano-photonic systems. Downscaling the SPP launchers below the diffraction limit and directly delivering the SPPs to the desired subwavelength plasmonic waveguides are of importance for high-integration plasmonic circuits. By designing a submicron double-slit structure with different slit widths, an ultra-broadband (>330 nm) unidirectional SPP launcher is realized theoretically and experimentally based on the different phase delays of SPPs propagating along the metal surface and the near-field interfering effect. More importantly, the broadband and unidirectional properties of the SPP launcher are still maintained when the slit length is reduced to a subwavelength scale. This can make the launcher occupy only a very small area of
A submicron asymmetric dielectric-coated metal slit with a Fabry-Perot (FP) nano-resonator is experimentally fabricated to realize an ultra-small on-chip polarization splitter. In the hybrid plasmonic structure, both of the transverse-electric (TE) and transverse-magnetic (TM) modes can be efficiently generated on the front metal surface. Based on the quite different resonant conditions and the different field confinements of the two orthogonal polarization modes in the FP resonator, the TM and TE modes are generated to propagate in the opposite directions along the metal surface. In this device, there are no coupling waveguide regions, and the excitation and the splitting of the TE and TM modes are integrated into the same asymmetric nano-slit. This considerably shrinks the device dimension to only about 850 nm (about one wavelength). In such a submicron asymmetric slit, the measured extinction ratios for the two opposite directions can reach up to (eta(L)/eta(R))(TM) approximate to 1:14 and (eta(L)/eta(R))(TE) approximate to 11:1 at lambda = 820 nm. This on-chip submicron polarization splitter is of importance in highly integrated photonic circuits. (C) 2014 AIP Publishing LLC.
Two Fano resonances are theoretically predicted in a single defect nanocavity, consisting of a rectangular cavity with a small stub defect, side-coupled with a plasmonic waveguide. These two Fano resonances are found to originate from two different mechanisms. One is caused by the excitation of a high-order resonant mode in the rectangular cavity owing to the structural breaking, and the other is attributed to the inherent resonant mode in the small stub defect. The narrow high-order mode and inherent mode couple with the broad low-order resonant mode in the rectangular cavity, giving rise to two Fano resonances. Because of the different origins, these two Fano resonances exhibit quite different responses to the variations of the structural dimensions. This has important applications in highly sensitive and multiparameter sensing in the complicated environments. (C) 2013 Optical Society of America
By manipulating the surface-plasmon-polariton (SPP) excitation properties of two nanogrooves, we demonstrate unidirectional launching of SPPs using a groove-doublet structure both numerically and experimentally, with the groove separation being downscaled to 1/4 and even 1/8 of the wavelength. Thus, the total lateral dimension of the SPP launcher is only about 1/3 and 1/6 of the wavelength, which is truly subwavelength. The measured extinction ratio at incident wavelength of 800 nm reaches as high as 130 and 18. Such subwavelength SPP unidirectional launchers may have important applications in highly integrated plasmonic circuits. (C) 2014 AIP Publishing LLC.
Surface plasmon polariton, a kind of surface electromagnetic wave propagating along the interface between metals and dielectrics, provides an excellent platform for the realization of integrated photonic devices due to its unique properties of confining light into subwavelength scales. Our recent research progresses of nanoscale integrated photonic devices based on surface plasmon polaritons, including all-optical switches, all-optical logic discriminator, and all-optical routers, are introduced in detail.
Highly efficient plasmonic nanofocusing is numerically predicted in a single step-like microslit, which is placed on a high-index dielectric layer. Because of the high throughput of the impinging light on the wide microslit, highly efficient nanofocusing is achieved in the proposed structure based on the multimode interferences in the microslits, the constructive interference between the transmitted light and the scattered surface plasmon polaritons, and the Fabry-Perot resonator effect in the high-index dielectric layer. Compared with previous nanofocusing structures containing plenty of substructures arranged laterally, the proposed structure has a much smaller lateral dimension because of the vertical arrangement of the microslits. This is of importance for realizing densely integrated plasmonic circuits. (C) 2013 Optical Society of America
By integrating a vertical cavity into an asymmetric nanoslit, we demonstrate numerically and experimentally that such a composite cavity structure is capable of generating and splitting surface plasmon polaritons (SPPs) of two different wavelengths to opposite directions. The reason is that the horizontal cavity in the upper part of the asymmetric nanoslit and the added vertical cavity can manipulate SPPs nearly independently. High splitting ratios of 1:24 and 23:1 at splitting wavelengths of 767 nm and 847 nm are numerically presented with a device lateral dimension of only 790 nm. Moreover, the splitting wavelengths can easily be tuned. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794803]
Ultra-small all-optical switches are of importance in highly integrated optical communication and computing networks. However, the weak nonlinear light-matter interactions in natural materials present an enormous challenge to realize efficiently switching for the ultra-short interaction lengths. Here, we experimentally demonstrate a submicron bidirectional all-optical plasmonic switch with an asymmetric T-shape single slit. Sharp asymmetric spectra as well as significant field enhancements (about 18 times that in the conventional slit case) occur in the symmetry-breaking structure. Consequently, both of the surface plasmon polaritons propagating in the opposite directions on the metal surface are all-optically controlled inversely at the same time with the on/off switching ratios of >6 dB for the device lateral dimension of <1 mu m. Moreover, in such a submicron structure, the coupling of free-space light and the on-chip bidirectional switching are integrated together. This submicron bidirectional all-optical switch may find important applications in the highly integrated plasmonic circuits.
The manipulation of light propagation is a basic subject in optics and has many important applications. With the development of nano-optics, this area has been downscaled to wavelength or even subwavelength scales. One of the most efficient ways to control light propagation is to exploit interference effects. Here, by manipulating the interference between two nanogrooves on a metal surface, we realize a submicron broadband surface-plasmon-polariton (SPP) unidirectional coupler. More importantly, we find an anomalous bandwidth shrinking behavior in the proposed SPP unidirectional coupler as the groove separation is down to a subwavelength scale of one-quarter of the SPP wavelength. This abnormal behavior is well explained by considering the contribution of the near-field quasi-cylindrical waves in addition to the interference of propagating SPPs and the dispersion effects of individual grooves. Such near-field effects provide new opportunities for the design of ultracompact optical devices.
An asymmetric T-shape nanoslit in a metal film is proposed to act as an efficient dichroic surface-plasmon-polariton (SPP) splitter, which is composed of a single nanoslit in immediate contacting with two nanogrooves with different widths. Simulations show that, due to the interferences of SPPs in the upper part of the asymmetric T-shape nanoslit, the generated SPPs propagating to the left and right directions on the front metal surface can be manipulated nearly independently by altering the right and left groove widths, respectively. Based on such effects, a dichroic SPP splitter is demonstrated and the splitting wavelengths can easily be adjusted. High splitting ratios of 31: 1 and 1: 12 at splitting wavelengths of 680 nm and 884 nm are numerically presented with a device's lateral dimension of only 1200 nm. Further experimental results match the simulations well. (C)2013 Optical Society of America
Efficient all-optical molecule-plasmon modulation is experimentally demonstrated by employing a compact T-shape single slit on a metal film coated with an azopolymer film, in which the azobenzene molecules can be reoriented by a pump beam. In the T-shape single slit, the transmission spectra exhibit periodic behaviors and are quite sensitive to variations of the refractive index of the azopolymer in the groove. Under a pump beam, the azobenzene molecules are reoriented, so the SPPs in the groove feel a refractive index quite different from that of the originally isotropic azopolymer with randomly orientations. This leads to a high modulation depth of about 53 % (3.3 dB) and a phase variation of >pi experimentally.
A compact plasmonic coupled-resonator system, consisting of a stub resonator and baffles in the metal-insulator-metal waveguide, is numerically investigated with the finite element method. Simulations show that sharp and asymmetric response line-shapes can occur in the system. The asymmetric line-shapes in the transmission spectra depend on the relative positions of the resonant wavelengths between the single-stub resonator and the inner resonator constructed by the baffle and the stub resonator, while the other part of the transmission spectra (except the asymmetric part) maintains the spectral features of the structure constructed by the baffles. An analytic model and a relative phase analysis based on the scattering matrix theory are used to describe and explain this phenomenon. These sharp and asymmetric response line-shapes are important for improving the nano-plasmonic devices' performances.
Fano resonances are numerically predicted in an ultracompact plasmonic structure, comprising a metal-isolator-metal (MIM) waveguide side-coupled with two identical stub resonators. This phenomenon can be well explained by the analytic model and the relative phase analysis based on the scattering matrix theory. In sensing applications, the sensitivity of the proposed structure is about 1.1 x 10(3) nm/RIU and its figure of merit is as high as 2 x 10(5) at lambda=980 nm, which is due to the sharp asymmetric Fano line-shape with an ultra-low transmittance at this wavelength. This plasmonic structure with such high figure of merits and footprints of only about 0.2 mu m(2) may find important applications in the on-chip nano-sensors.
Based on the strongly coupled-resonator effects, a high wavelength-resolution plasmonic Y-splitter, consisting of a Y-branch metal-insulator-metal waveguide with a baffle in each channel, is numerically investigated using the finite element method. Due to the coupling of different resonators (with nearly equal bandwidths) in the Y-splitter, sharp and asymmetric transmission spectra occur. This greatly increases the wavelength resolution of the Y-splitter to be Delta lambda a parts per thousand aEuro parts per thousand 15 nm, which is significantly narrower than the bandwidth of the single resonator (Delta lambda (FWHM) a parts per thousand aEuro parts per thousand 110 nm). An analytic model based on the scattering matrix theory is provided to describe and explain this phenomenon.