Beaming light from a subwavelength metal slit surrounded by dielectric surface gratings

Advances in Waveguide Bragg Grating Structures, Platforms, and Applications: An Up-to-Date Appraisal

A Bragg grating (BG) is a one-dimensional optical device that may reflect a specific wavelength of light while transmitting all others. It is created by the periodic fluctuation of the refractive index in the waveguide (WG). The reflectivity of a BG is specified by the index modulation profile. A Bragg grating is a flexible optical filter that has found broad use in several scientific and industrial domains due to its straightforward construction and distinctive filtering capacity. WG BGs are also widely utilized in sensing applications due to their easy integration and high sensitivity. Sensors that utilize optical signals for sensing have several benefits over conventional sensors that use electric signals to achieve detection, including being lighter, having a strong ability to resist electromagnetic interference, consuming less power, operating over a wider frequency range, performing consistently, operating at a high speed, and experiencing less loss and crosstalk. WG BGs are simple to include in chips and are compatible with complementary metal-oxide-semiconductor (CMOS) manufacturing processes. In this review, WG BG structures based on three major optical platforms including semiconductors, polymers, and plasmonics are discussed for filtering and sensing applications. Based on the desired application and available fabrication facilities, the optical platform is selected, which mainly regulates the device performance and footprint.

Large-angle beaming from asymmetric nanoslit-corrugation structures

The beaming effect in single apertures surrounded by periodic corrugations and the manipulation of beaming directions from such structures has gained considerable attention since discovery. Different materials and structural profiles have been studied in this context but directional beaming at angles larger than 45° has not been achieved. We design and demonstrate nanoslits in a gold film flanked by corrugations, which give rise to beaming angles ranging from 45° to 60°. While the previous designs are based on achieving constructive interference at the aimed beaming angle, our approach complements such constructive interference with destructive interference at 0° and, as a result, enhances the directional beaming effect at angles larger than 45°. The structures are fabricated by electron beam lithography with two consecutive lift-off processes. The experimental far-field intensity distributions agree well with the designs.

Beaming light through a bow-tie nanoaperture at the tip of a single-mode optical fiber

We demonstrate coupling and directivity enhancement of electromagnetic fields emerging from a single metallic nanoaperture at the tip of a single-mode optical fiber. We achieve this by using circular grooves flanking the nanoaperture perforated in a 100 nm thick gold film. The film with nanostructure is transferred to the fiber tip by aligned stripping with optical epoxy. When incident from both sides of the nanoaperture, enhancement factors of 2.2 and 2.4 in power coupling into the fiber and in beaming into free-space were obtained. Numerical simulations show that the optimum grating period is nearly identical to the surface plasmon polariton wavelength that can be supported at the gold-epoxy interface. This integrated platform couples light between the single mode fiber and the nanoapeture without the need for cumbersome optics, with applications for optical trapping and single-photon detection.

Plasmonic Implanted Nanogrooves for Optical Beaming

Surface plasmon polaritons are electromagnetic surface waves, which, due to their nanoscale nature, are efficiently used for modifying an output of optical field through a metallic nanoslit, e.g., extraordinary optical transmission and beaming of light. Herein, the phenomenon of optical beaming by employing a regular array of semicylinder-shaped grooves around a nanoslit has been investigated based on numerical simulations. By analyzing the behavior of Poynting vectors in near surroundings of the slit, we have successfully demonstrated that grooves which are embedded on the layer at the exit side of the slit produce enhanced directionality of the output light than the unembedded ones. In case of semicylinder-shaped grooves, the calculated intensity of the output beam was 1.5-times, at near and far distances, higher than that of the grating grooves. Our analysis shows that positioning of the groove right at the exit of the slit is crucial for the enhancement of the beaming effect. This is due to the conversion of surface plasmon polaritons into a freely propagating field and the possible excitation of localized surface plasmons because of the presence of nanogroove. Furthermore, the proposed geometries are made of Aluminum, which is a plasmonic material and commonly applied for the fabrication of optical nanostructures. Manipulating of light (beaming, focusing/guiding, and splitting) by nanoslit can be beneficial to several applications such as nano-resolution optical imaging, sensors, and plasmonic circuits.

A systematic approach to enhance off-axis directional electromagnetic wave by two-dimensional structure design

In this study, we propose a two-dimensional (2D) dielectric structure tailored by a systematic design approach on the exit side of a metallic aperture to enhance the off-axis electromagnetic (EM) wave. We adopted a phase field method based topology optimization scheme and designed an arbitrary 2D dielectric structure in order to steer outward beaming through an aperture to a specific direction. Beyond previous one-dimensional structure, we proposed an arbitrary 2D dielectric structure through the introduced design process defining not only x- but also y-directional dielectric structural boundaries simultaneously and experimentally confirmed enhanced EM wave transmission to a desired direction.

Effect of Nanohole Spacing on the Self-Imaging Phenomenon Created by the Three-Dimensional Propagation of Light through Periodic Nanohole Arrays

We present a detailed study of the inter-nanohole distance that governs the self-imaging phenomenon created by the three-dimensional propagation of light through periodic nanohole arrays on plasmonic substrates. We used scanning near-field optical microscopy (SNOM) to map the light intensity distributions at various heights above 10×10 nanohole arrays of varying pitch sizes on silver films. Our results suggest the inter-hole spacing has to be greater than the wavelength of the incident light to create the self-imaging phenomenon. We also present Finite-Difference Time-Domain (FDTD) calculations which show qualitative corroboration of our experimental results. Both our experimental and FDTD results show that the self-imaging phenomenon is more pronounced for structures with larger pitch sizes. We believe this self-imaging phenomenon is related to the Talbot imaging effect that has also been modified by a plasmonic component and can potentially be used to provide the basis for a new class of optical microscopes.

All-optical ultrafast control of beaming through a single sub-wavelength aperture in a metal film

We propose an ultrafast all-optical technique to control and beam the light emerging from a sub-wavelength slit in a planar gold film by exciting a transient grating in the area around the slit. A FDTD model is used to show how excitation of surface plasmon polaritons by the grating governs the beaming process. Both the grating and the beaming effect are shown to decay on a picosecond time-scale. An on-off contrast of 5 dB is obtained for the beaming, with a divergence angle of only 2.4 degrees.

Beam splitting at the output of photonic crystal waveguides with discrete surface point defects

With the method of adding two point defects on modulated surface, novel photonic crystal (PC) waveguide-based beam splitters were presented. The modulated surface layer supports surface states, and introduced discrete point defects can serve as discrete light emitters. The finite-difference time-domain (FDTD) simulations show that the number of beams is sensitive to the distance of two point defects. By adjusting the positions of the point defects, 1-to-N beam splitters can be realized. These simple, easy-to-fabricate and controllable structures have important potential applications in integrated optical circuits.

Modeling and design of nano-plasmonic structures using transmission line modeling

For the first time, we demonstrate the application of the time domain transmission line method (TLM) to accurate modeling of surface plasmon polariton (SPP) structures. The constructed TLM node allows for modeling of dispersive materials through simple time-difference equations. Using such node, an ultra-wide band excitation can be applied to obtain the response over the band of interest. Bérenger's perfectly matched layer (PML) boundary condition can readily be implemented using the same node. We illustrate our TLM approach through the modeling of different challenging structures including SPPs filters and focusing structures.

Design and characterization of a micron-focusing plasmonic device

We design and experimentally demonstrate a focusing plasmonic device consisting of a central slit surrounded by the grooves with the fixed width and depth in Au/Cr metal layers. By modulating the phases of the radiation lights decoupled by the grooves from surface plasmon polaritons waves, the focal length can be controlled in the range of several wavelengths. A micron-focusing device is fabricated by the focused ion beam and the focusing performance is characterized using the scanning near-field optical microscope. The experimental results show a significant focusing effect and have agreement with the theoretical analysis and simulation results. Such a focusing plasmonic device with the predicted focal length has potential practical applications in integrated photonics due to the simplicity of the design and fabrication.

On the Feasibility of Using the Intrinsic Fluorescence of Nucleotides for DNA Sequencing

There is presently a worldwide effort to increase the speed and decrease the cost of DNA sequencing as exemplified by the goal of the National Human Genome Research Institute (NHGRI) to sequence a human genome for under $1000. Several high throughput technologies are under development. Among these, single strand sequencing using exonuclease appear very promising. However, this approach requires complete labeling of at least two bases at a time, with extrinsic high quantum yield probes. This is necessary because nucleotides absorb in the deep ultra-violet (UV) and emit with extremely low quantum yields. Hence intrinsic emission from DNA and nucleotides is not being exploited for DNA sequencing. In the present paper we consider the possibility of identifying single nucleotides using their intrinsic emission. We used the finite-difference time-domain (FDTD) method to calculate the effects of aluminum nanoparticles on nearby fluorophores that emit in the UV. We find that the radiated power of UV fluorophores is significantly increased when they are in close proximity to aluminum nanostructures. We show that there will be increased localized excitation near aluminum particles at wavelengths used to excite intrinsic nucleotide emission. Using FDTD simulation we show that a typical DNA base when coupled to appropriate aluminum nanostructures leads to highly directional emission. Additionally we present experimental results showing that a thin film of nucleotides show enhanced emission when in close proximity to aluminum nanostructures. Finally we provide Monte Carlo simulations that predict high levels of base calling accuracy for an assumed number of photons that is derived from the emission spectra of the intrinsic fluorescence of the bases. Our results suggest that single nucleotides can be detected and identified using aluminum nanostructures that enhance their intrinsic emission. This capability would be valuable for the ongoing efforts towards the $1000 genome.

Experimental investigation of superfocusing of plasmonic lens with chirped circular nanoslits

A plasmonic lens with metallic chirped circular nanoslits corrugated on Au film supported on quartz substrate for the purpose of superfocusing was put forth and fabricated by means of focused ion beam direct milling technique. Topography of the lens was imaged using an atomic force microscope. After that a near-field scanning optical microscope was employed for optical characterization of focusing performance of the lens. Our experimental results verify the focusing performance and further demonstrate that they are in agreement with the theoretical calculation results. Focusing performance is significantly improved in comparison to that of the non-chirped lens. The lenses are possible to be used for the applications of bioimaging, detection, and inspection in submicron scale resolution.

Recording Bessel-like beam shapes generated by plasmonics lens

In our study, we spun a negative photoresist layer on top of a plasmonic lens which was formed by adopting a metallic ring structure with a nano-scale width opening. We recorded the beam shape of the Bessel-like beam emitting from the plasmonic lens which formed a high aspect ratio structure. We found that the high aspect ratio structure was higher after exposure as the inner and outer diameter had increased. In addition, we used an oblique incidence on the negative resist metallic ring structure to produce an inclined micro-structure. Different exposure results were obtained with the two different metal thicknesses. Therefore, in our study, we not only proved that it is possible to record the shape of a Bessel-like beam, but we also demonstrated that it is possible to create a plasmonic lens which is capable of creating a high aspect ratio structure through exposure.

Simultaneous realization of transmission enhancement and directional beaming of dual-wavelength light by a metal nanoslit

Transmission enhanced dual-wavelength light beaming is realized by a dielectric-metal-dielectric structure. Guided-mode resonance theory is used on the dielectric grating flanked single nanoslit in an optically thick metal to predict the original structure parameters for transmission enhanced dual-wavelength (442 nm and 633 nm) light beaming. Finite-difference time-domain numerical simulations confirm the theoretical prediction and demonstrate that the normalized-to-area transmittance of 10.8 and 14.7 and beaming angle of 2.40 degrees and 2.65 degrees for 442 nm and 633 nm, respectively, are achieved with a single structure, suggesting the potential applications of such structures in optical data storage, nanoscale wavelength multiplexing, directional light sources and emitters.

Propagation characteristics of silver and tungsten subwavelength annular aperture generated sub-micron non-diffraction beams

We examined the optical properties such as propagation modes, focal length, side lobes, etc. of metallic subwavelength annular apertures (SAA) and used finite-difference time-domain (FDTD) simulation to compare our experimental findings. Using two different metals, silver and tungsten, we examined the different optical transmission properties of the two metallic SAA structures. The far-field propagation of the silver SAA structure was found to be a type of quasi-Bessel beam when compared with a quasi-Bessel beam generated by a perfect axicon. The propagation characteristics of these two beams were found to match qualitatively. The far-field transmitted light generated by the silver SAA structure was found to possess a 390 nm sub-micron focal spot with a 24 microm depth of focus, which was much smaller than the focal spot generated by a perfect axicon. We also found that a silver SAA structure can generate a sub-micron quasi- Bessel beam that has a much lower far-field side-lobe when compared to that of non-diffraction beams generated by a tungsten SAA structure.

Plasmon-controlled fluorescence: a new paradigm in fluorescence spectroscopy

Fluorescence spectroscopy is widely used in biological research. Until recently, essentially all fluorescence experiments were performed using optical energy which has radiated to the far-field. By far-field we mean at least several wavelengths from the fluorophore, but propagating far-field radiation is usually detected at larger macroscopic distances from the sample. In recent years there has been a growing interest in the interactions of fluorophores with metallic surfaces or particles. Near-field interactions are those occurring within a wavelength distance of an excited fluorophore. The spectral properties of fluorophores can be dramatically altered by near-field interactions with the electron clouds present in metals. These interactions modify the emission in ways not seen in classical fluorescence experiments. In this review we provide an intuitive description of the complex physics of plasmons and near-field interactions. Additionally, we summarize the recent work on metal-fluorophore interactions and suggest how these effects will result in new classes of experimental procedures, novel probes, bioassays and devices.

Directional light beaming control by a subwavelength asymmetric surface structure

We propose a direct experimental set-up to observe the directional beaming effect of surface plasmon. A single diffracted beam from an asymmetric-sided surface corrugation is demonstrated. A single subwavelength slit with an asymmetric structure was fabricated using a focused ion beam (FIB) onto a metal surface with a glass substrate. By means of surface plasmon (SP) diffraction, the directionality of the light can be changed by the period of the metallic gratings. We show corresponding numerical simulations achieved by a Rigorous Coupled-Wave Analysis (RCWA) method and a Finite-Difference Time-Domain (FDTD) method. The simulation results were in agreement with the experimental data.

Imaging three-dimensional light propagation through periodic nanohole arrays using scanning aperture microscopy

The authors introduce a technique for three-dimensional (3D) imaging of the light transmitted through periodic nanoapertures using a scanning probe to perform optical sectioning microscopy. For a 4×4 nanohole array, the transmitted light displays intensity modulations along the propagation axis, with the maximum intensity occurring at 450 μm above the surface. The propagating fields show low divergence, suggesting a beaming effect induced by the array. At distances within 25 μm from the surface, they observe subwavelength confinement of light propagating from the individual nanoholes. Hence, this technique can potentially be used to map the 3D distribution of propagating light, with high spatial resolution.