Terahertz near-field imaging using subwavelength plasmonic apertures and a quantum cascade laser source

The first demonstration, to our knowledge, of near-field imaging using subwavelength plasmonic apertures with a terahertz quantum cascade laser source is presented. "Bull's-eye" apertures, featuring subwavelength circular apertures flanked by periodic annular corrugations were created using a novel fabrication method. A fivefold increase in intensity was observed for plasmonic apertures over plain apertures of the same diameter. Detailed studies of the transmitted beam profiles were undertaken for apertures with both planarized and corrugated exit facets, with the former producing spatially uniform intensity profiles and subwavelength spatial resolution. Finally, a proof-of-concept imaging experiment is presented, where an inhomogeneous pharmaceutical drug coating is investigated.     

Near-field light localization using subwavelength apertures incorporated with metamaterials

We report strong near-field electromagnetic localization by using subwavelength apertures and metamaterials that operate at microwave frequencies. We designed split ring resonators with distinct configurations in order to obtain extraordinary transmission results. Furthermore, we analyzed the field localization and focusing characteristics of the transmitted evanescent waves. The employed metamaterial configurations yielded an improvement on the transmission efficiency on the order of 27 dB and 50 dB for the deep subwavelength apertures. The metamaterial loaded apertures are considered as a total system that offered spot size conversion ratios as high as 7.12 and 9.11 for the corresponding metamaterial configurations. The proposed system is shown to intensify the electric fields of the source located in the near-field. It also narrows down the electromagnetic waves such that a full width at half maximum value of λ/29 is obtained.     

Extraordinary THz transmission through subwavelength semiconductor slits under antiparallel external magnetic fields

We theoretically study the resonant transmission of electromagnetic waves at the THz frequencies through subwavelength semiconductor slits under external static magnetic fields. The dispersion relations of the surface plasmon polaritons (SPPs) inside a subwavelength slit are analytically derived. It is found that the SPPs propagating along one direction and its reverse are symmetric when parallel external magnetic fields are applied, but are asymmetric when antiparallel external magnetic fields are applied. The transmission properties of periodic subwavelength semiconductor slit arrays with the antiparallel magnetic fields in each unit cell are investigated by the mode expansion technique. The two significant transmission characteristics are observed: (i) The resonant peaks are redshifted with increasing external magnetic fields; (ii) The transmissions in the two opposite directions through the slit arrays are asymmetric. The origin of the transmission asymmetry is reasonably explained by the magnetic-field induced asymmetric SPP propagation losses.     

Theory of resonant acoustic transmission through subwavelength apertures

A complete landscape is presented of the acoustic transmission properties of subwavelength apertures (slits and holes). First, we study the emergence of Fabry-Perot resonances in single apertures. When these apertures are placed in a periodic fashion, a new type of transmission resonance appears in the spectrum. We demonstrate that this resonance stems from the excitation of an acoustic guided wave that runs along the plate, which hybridizes strongly with the Fabry-Perot resonances associated with waveguide modes in single apertures. A detailed discussion of the similarities and differences with the electromagnetic case is also given.     

微型腔单一亚波长金属狭缝的光波透射

 利用时域有限差分法模拟了厚金属层上单一亚波长狭缝内壁刻有一微型腔结构光波传输,结果发现在特定波长发生异常的增强透射。结合透射峰值对应波长近场分布,利用麦克斯韦方程组和波导理论对增强透射进行解释,认为光波在微型腔和狭缝中形成共振腔模起了重要作用:在狭缝内形成纵模,可以形成透射峰;在腔内形成不同的腔模也可以形成透射模。     

Detection of subwavelength slit-width variation with irradiance measurements in the far field

We demonstrate that, under suitable conditions, subwavelength feature variations of an object can affect the corresponding far-field diffraction pattern in a measurable way. We present an experiment in which width variations of less than 1/100 of the wavelength are measured with a slit whose width is 100 times the wavelength. Integral and differential intensity measurements in the far field are fully consistent with standard diffraction theory even in the subwavelength variation regime. In particular, slit modulations of 6 nm with a wavelength of 670 nm are shown to follow theoretical calculations within the experimental sensitivity of ~10(-5) .     

Interference-induced enhancement of intensity and energy of a quantum optical field by a subwavelength array of coherent light sources

Recently, we have shown a mechanism that could provide great resonant and nonresonant transmission enhancements of the classical (nonquantum) light waves passed through subwavelength aperture arrays in thin metal films not by the plasmon–polariton waves, but by the constructive interference of diffracted waves (beams generated by the apertures) at the detector placed in the far-field zone. We now present a quantum reformulation of the model. The Hamiltonian describing the phenomenon of interference-induced enhancement and suppression of both the intensity and energy of a quantum optical field is derived. The basic properties of the field energy determining by the Hamiltonian are analyzed. Normally, the interference (addition) of two or more waves causes enhancement or suppression of the light intensity, but not the light energy. The model shows that the phenomenon could be observed experimentally, for instance, by using a subwavelength array of the coherent quantum light-sources (one- and two-dimensional subwavelength apertures, fibers, dipoles, and atoms).     

Remarkable zeroth-order resonant transmission of microwaves through a single subwavelength metal slit

A slit in a thick metal plate that is extremely subwavelength will not transmit microwaves polarized parallel to it. It is shown here that cuts perpendicular to the slit allow parallel polarized radiation to resonantly transmit. Furthermore, a zero-order mode may be excited within the slit, the frequency of which, to first order, is independent of the plate depth. Remarkably, for this novel type of resonance, the field in the slit oscillates with a constant phase and little amplitude variation throughout the plate depth, while the resonant wavelength tends to infinity as the slit width approaches zero.     

Plasmonic splitter based on the metal-insulator-metal waveguide with periodic grooves

In this paper, we have proposed a plasmonic splitter which is composed of a subwavelength slit and two different metal-insulator-metal (MIM) waveguides with periodic grooves. The slit is used to excite surface plasmon polaritons (SPPs) at certain wavelengths. By setting the SPPs resonance wavelengths of the slit as the Bragg wavelengths of MIM waveguides, the SPPs of different wavelengths are able to be confined and guided in the two different MIM waveguides. The numerical results of two-dimensional finite difference time domain (2D-FDTD) demonstrate that our proposed structure is capable of splitting light into two MIM waveguides.     

Anomalous terahertz transmission in bow-tie plasmonic antenna apertures

Arrays of subwavelength dipole bow-tie apertures are designed and characterized at terahertz frequencies. For an incident terahertz field perpendicular to the longer axis of the bow tie, a strong resonance enhancement, line narrowing, and a nonmonotonic frequency shift were observed with increasing length of the tapered bow-tie arms. Such characteristic behaviors primarily originate from localized surface plasmon resonances. In addition, with a decreasing aperture size, the contribution of localized plasmons becomes prominent due to an increase in plasmonic lifetime as the terahertz pulses strongly couple with the metallic surface surrounding the bow-tie apertures.     

Resonant transmission of light through finite chains of subwavelength holes in a metallic film

In this Letter we show that the extraordinary optical transmission phenomenon found before in 2D hole arrays is already present in a linear chain of subwavelength holes, which can be considered as the basic geometrical unit showing this property. In order to study this problem, we have developed a new theoretical framework, able to analyze the optical properties of finite collections of subwavelength apertures and/or dimples (of any shape and placed in arbitrary positions) drilled in a metallic film.     

Localization of Surface Plasmon Waves in Hybrid Photodetectors with Subwavelength Metallic Arrays

The spectral characteristics of the hole photocurrent in plasmon photodetectors based on Ge/Si heterostructures with Ge quantum dots combined with regular arrays of subwavelength apertures of various shapes in a gold film on the semiconductor surface are investigated. Dispersion relations characterizing the propagation of surface plasmon waves along the metal–semiconductor interface are determined from the dependences of the photocurrent on the angle of incidence of light. It is established that the plasmonic enhancement of the photocurrent in rectangular aperture array is suppressed as compared to that in circular and square aperture arrays. It is found that, in hybrid structures with rectangular apertures, there exists a range of wave vectors where the energy of surface plasmons is independent of the wave vector of incident radiation. The results are explained by the excitation of dipole modes localized at rectangular apertures with a large aspect ratio by light waves.     

Limits for superfocusing with finite evanescent wave amplification

Perfect lensing using negative refractive index materials and radiationless electromagnetic interference both provides extreme subwavelength focusing by "amplifying" evanescent wave components that are usually lost. This Letter provides a relation between the achievable focus spot size, the amplification available, and the focal length. This may be considered as a revised version of Abbe's diffraction limit for focusing systems that have evanescent wave amplification. It is useful in comparing the amplification achieved in various subwavelength focusing implementations as well as determining when it is better to use existing near-field techniques, such as simple diffraction from an aperture or slit, than to attempt complicated superfocusing.     

Interaction of light with subwavelength apertures: a comparison of approximate and rigorous approaches

The interaction of an electromagnetic wave with various classes of subwavelength one dimensional apertures is studied in detail, using several approximate and rigorous approaches. The attention is given to a comparison of the revealing basic physical features of the interaction. Several semi-analytical models which are able to predict the enhanced transmission phenomenon are reviewed, applied, and extended: the one-mode periodic model, the model based on the composite diffracted evanescent wave approach, and the so called combined model. Then, a detailed rigorous modeling is performed, using the finite difference time domain method, to study the parametrical dependencies of all critical parameters (i.e. the parameters of the aperture—diameter and thickness, and also the parameters of the surrounding corrugations). The transmission properties of the subwavelength single apertures and apertures with the supporting corrugations on the input side and on both sides of the structure are compared in detail, and the options for their optimizations are discussed. Finally, the results of approximate models are compared with that from the rigorous finite difference time domain modeling, for the case of two important cases, a periodic array of slits and the slit-groove diffraction problem, i.e. a combination of the aperture and one corrugation.     

Plasmonic quarter-wave plate

Here we present a strategy for designing wave plates utilizing resonances of subwavelength apertures in metallic films. Specifically, we show that it is possible to tune the geometry in a periodic array of cross-shaped apertures in a silver film to produce a quarter-wave plate at a particular wavelength in the near-infrared. This is achieved by introducing an asymmetry into the lengths of the arms of the crosses.     

Inward electromagnetic wave coupling in hybrid subwavelength structures illuminated with secondary imaging

Subwavelength structures hybridized with slits and grooves in metallic films exhibit extraordinary optical properties for manipulation of light. A Green method is developed to investigate the wave coupling physics in the hybrid subwavelength structures, while source-image arrangement in bounded spaces is employed to obtain the Green's functions for the hybrid units. We find that secondary imaging is induced at a horizontal boundary to provide an inward wave propagation mechanism to couple the wave scattered by groove back into slit. Sophisticated self-consistent framework derived from our method that involves the wave couplings is then developed to study the physics origin of the extraordinary optical properties. The case of a slit in a metal film with patterned grooves on the output side for beaming and that of a two-layer metal film with indented double slit for beyond-limit focusing are analyzed. Our Green method, which automatically includes the Fourier high-order modes, is concise in formulation and thus yields accurate solutions. Explicit analytical models are rigorously developed to address wave coupling physics problems in subwavelength structure.     

Transmission through array of subwavelength metallic slits curved with a single step or mutli-step

The transmission of normally incident plane wave through an array of subwavelength metallic slits curved with a sin- gle step or mutli-step has been explored theoretically. The transmission spectrum is simulated by using the finite-difference time-domain method. The influences of surface plasmon polaritons make the end of finite long sub-wavelength metallic slit behaves as magnetic-reflecting barrier. The electromagnetic fields in the subwavelength metallic slits are the superpo- sition of standing wave and traveling wave. The standing electromagnetic oscillation behaves like LC oscillating circuit to decide the resonance wavelength. Therefore, the parameters of adding step may change the LC circuit and influence the transmission wavelength. A new explanation model is proposed in which the resonant wavelength is decided by four factors： the changed length for electric field, the changed length for magnetic field, the effective coefficient of capacitance, and the effective coefficient of inductance. The effect of adding step is presented to analyze the interaction of two steps in slit with mutli-step. This explanation model has been proved by the transmission through arrayed subwavelength metallic slits curved with two steps and fractal steps. All calculated results are well explained by our proposed model.     

Characterization of the signature of subwavelength variation from far-field irradiance

The dynamic signature of the subwavelength variation of a slit is shown to be determinable from far-field irradiance with a precision of better than 1 nm. One can increase the efficiency of measurement of the subwavelength's signature by adjusting the detection width over which the subwavelength variation is detected. The subwavelength variation of a rectangular aperture was also examined to show the general feasibility.     

Enhanced transmission and reflection of few-cycle pulses by a single slit

We show that a physical mechanism responsible for the enhanced transmission and reflection of ultrashort (few-cycle) pulses by a single subwavelength slit in a thick metallic film is the Fabry-Perot-like resonant excitation of stationary, quasistationary, and nonstationary waves inside the slit, which leads to the field enhancement inside and around the slit. The mechanism is universal for any pulse-scatter system, which supports the stationary resonances. We point out that there is a pulse duration limit below which the slit does not support the intraslit resonance.