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Chondroitin sulfate (CS), derived from cartilage tissues, is an important type of biomacromolecule. In this paper, the terahertz time-domain spectroscopy (THz-TDS) was investigated as a potential method for content detection of CS. With the increase of the CS content, the THz absorption coefficients of the CS/polyethylene mixed samples linearly increase. The refractive indices of the mixed samples also increase when the CS content increases. The extinction coefficient of CS demonstrates the THz frequency dependence to be approximately the power of 1.4, which can be explained by the effects of CS granular solids on THz scattering., Competing Interests: The authors declare that there are no conflicts of interest related to this article.

Protein analysis is the foundation to understanding the mechanisms of complex biological processes. As one of the most widely used techniques to determine protein species and contents, protein dot blot aids biology research but needs corresponding antibodies for marking. A label-free detection method based on terahertz time-domain spectroscopy (THz-TDS) is proposed and demonstrated to improve this traditional technology. A membrane loaded with protein samples is directly scanned using a transmission THz-TDS system for spectral imaging. Different kinds of proteins can be distinguished by the refractive index extracted from the THz transmission spectrum. The intensity or shade imaged with the THz transmission spectrum can help detect the protein quantitatively. The feasibility of this new protein assay is demonstrated by the results of systematic testing with actual samples prepared with the dot-blot protocol., Competing Interests: The authors declare that there were no conflicts of interest related to this article.

The mechanism of propagating graphene plasmons excitation using a nano-grating and a Fabry-Pérot cavity as the optical coupling components is studied. It is demonstrated that the system could be well described within the temporal coupled mode theory using two phenomenological parameters, namely, the intrinsic loss rate and the coupling rate of a graphene plasmonic mode, and their analytical expressions are derived. It is found that the intrinsic loss rate is solely determined by the electron relaxation time of graphene, while independent of the field distributions of the modes. Such result originates from the negligible magnetic field energy of the graphene plasmonic mode. The coupling rate is governed by the optical coupling components parameters, and varies periodically with the Fabry-Pérot cavity length. By modulating the two rates, quality factors and absorption rates can be adjusted. Furthermore, it is revealed that low refractive index of the Fabry-Pérot cavity material is vital to the enlargement of tunable band, and the underlying physics is discussed. Such plasmon excitation configuration is insensitive to light incident angle and could serve as a platform for many tunable infrared photonic device, such as surface-enhanced infrared absorption spectroscopies, infrared detectors and modulators.

We investigate the excitations of graphene surface plasmon waves in grating-spacer-graphene hybrid systems. It is demonstrated that the resonant absorption rate is scaling invariant as the geometric parameters of the hybrid system are scaled, and this phenomenon is nearly unaffected by the dispersions of the optical parameters of graphene and the grating material. We present an analytical model to calculate the absorption rate and elucidate that the scaling invariant phenomenon originates from the scalabilities of the graphene surface plasmon modes. This study could benefit the development of graphene plasmonic devices at infrared and terahertz frequencies.

Multi-focus plasmonic lens with metallic nanoslits of variant widths have great potential applications in optical interconnection, integrated optics and nanophotonics. But the design method with simulated annealing algorithm or Yang-Gu algorithm requires complex calculation and multi focuses are limited to be set on the same output plane. In this paper, we propose a design method based on holography. The desired light field distribution and the incident plane wave can be treated as object wave and reference wave, respectively. So the calculation is relative simple and multi focuses can be located in different output plane. Numerical simulation of multi-focus lens design is performed through finite-difference time-domain (FDTD) method and the result confirms the feasibility of our method.

A sensitive surface enhanced Raman scattering (SERS) substrate with metallic nanogap array (MNGA) is fabricated by etching of an assembled polystyrene (PS) spheres array, followed by the coating of a metal film. The substrate is reproducible in fabrication and sensitive due to the nanogap coupling resonance (NGCR) enhancement. The NGCR is analyzed with the finite difference time domain (FDTD) method, and the relationship between the gap parameter and the field enhancement is obtained. Experimental measurements of R6G on demonstrate that the enhancement factor (EF) of the MNGA SERS substrate is increased by more than two fold compared with the control sample.

A new thin-film solar cell structure with a broadband absorption enhancement is proposed. The active a-Si:H film is sandwiched by two periodic pyramidal structured layers. The upper dielectric pyramidal layer acts as matching impedance by gradual change of the effective refractive index to enhance the absorption of the active layer in the short wavelength range. The lower metallic pyramidal layer traps light by the excitation of Fabry-Perot (FP) resonance, waveguide (WG) resonance and surface plasmon (SP) mode to enhance the absorption in the long wavelength range. With the cooperation of the two functional layers, a broadband absorption enhancement is realized. The structure parameters are designed by the cavity resonance theory, which shows that the results are accordant with the finite-difference time-domain (FDTD) simulation. By optimizing, the absorption of the sandwich structure is enhanced up to 48% under AM1.5G illumination in the 350-900 nm wavelength range compared to that of bare thin-film solar cells.

A polarization bifocal lens based on the polarization effect caused by asymmetrical hole arrays had been designed, fabricated, and characterized experimentally. By considering the fact that the skin depth of an infrared electromagnetic field inside metal is much shorter than the incident wavelength, a polarization bifocal lens composed of high deep-width ratio metallic holes was realized by using a gold-coated silicon structure to replace the one directly formed on a thick metal film. An infrared optical experiment setup is built based on the secondary imagery method for characterizing the focal length of the designed bifocal lens. The measured focal lengths of the fabricated bifocal lens coincide well with the designed values, which proves the validity for realizing the polarization elements with the proposed structure and the feasibility of the fabrication process., (© 2011 Optical Society of America)

A general conformal transformation method (CTM) is proposed to construct the conformal mapping between two irregular geometries. In order to find the material parameters corresponding to the conformal transformation between two irregular geometries, two polygons are utilized to approximate the two irregular geometries, and an intermediate geometry is used to connect the mapping relations between the two polygons. Based on these manipulations, the approximate material parameters for TE and TM waves are finally obtained by calculating the Schwarz-Christoffel (SC) mappings. To demonstrate the validity of the method, a phase modulator and a plane focal surface Luneburg lens are designed and simulated by the finite element method. The results show that the conformal transformation can be expanded to the cases that the transformed objects are with irregular geometries.

We have recently shown that patterns with 30 nm line width and micrometer scale periodicity could be steadily fabricated by employing localized surface plasmons lithography based on a soft mold [Opt. Lett. 35, 13 (2009)]. In this paper, the dependence of the resolution (pattern periodicity), critical dimension, and electric field intensity on the geometrical parameters of the soft mold, such as ridge width, mold periodicity, ridge depth, and slope, have been systematically studied and analyzed. The relevant simulation results by finite-difference time-domain demonstrate that the critical dimension exhibits a perfect stabilization and the value of electric field intensity would be especially large, when the ridge depth is in the range from 100 to 270 nm and the slope angle is below 35°. Importantly, the optimal resolution and critical dimension can reach 100 and 17 nm, respectively, by reasonably designing the corresponding mold periodicity and ridge width, which indicates that the method is particularly suitable for obtaining patterns with high density and is extremely promising for bio-sensing and photonic crystals application.

The effective plasma frequency fp of periodic metallic wires whose characteristic dimensions are comparable to their skin depth has been analyzed. And a relevant analytic model is constructed by considering the skin effect and making a reasonable shape approximation, which is suitable for the case that the cross section of the wire is noncircular. To verify this model, a wires array with rectangle cross section is designed and the corresponding stacked Au-SiO(2) nanostrips are fabricated. The experimental and simulational transmittances of the metamaterial have been evaluated with a good agreement, although the presence of quartz substrate and structural imperfections in experiment will have an impact, which validates that the multilayer Au-SiO(2) nanostrips could function similarly to a natural bulk metal with discrepancies of fp values less than 8%. It could be confirmed that the theoretic formula is trustworthy in predicting fp for designing and realizing a controllable artificial metal in optical region.

Three-dimensional (3D) nanoscale focusing of radially polarized light in far field by a simple plasmonic lens composed of an annular slit and a single concentric groove is reported. The numerical calculations reveal that the incident light is coupled to surface plasmon polaritons (SPP) by the annular slit and a focal spot with a size less than a half of the illumination wavelength is formed in the far field due to the constructive interference of the scattered light by the groove. More importantly, the focal length can be modulated by changing the groove diameter. This structure provides an admirable choice for the nano-optical devices.

We propose a photolithographic method to fabricate nanostructures by employing a localized surface plasmon (LSP) mask generated by a soft mold on a thin metal film. The soft mold can be formed by transparent materials, such as polydimethylsiloxane, contacting firmly to the metal film. The pattern edges of the mold, serving as the fine tapers, can be used to excite LSPs and accumulate a large amount of localized energy from the incident light field, providing a modulated optical field in the resist with nanometer feature size. Nanolithographic results with a minimum feature size of 30 nm are demonstrated.

A design method was proposed for transforming the scattering cross section's shape of PEC objects to other arbitrary shapes based on transformation optics. The parameters of the transformer which is a kind of metamaterial being tightly covered to the original PEC object can be determined by the transformation expressions derived in the paper. With the method, the virtual PEC objects with their definite shape of the scattering cross section can be produced visually through the original PEC objects with a designed transformer. The validation was made by comparing the distribution of electromagnetic field of the PEC objects with and without transformer by means of finite-element method. Some examples are also given for demonstrating the effectiveness of the method.

We have proposed and demonstrated an artificial medium consisting of arrays of circular metal rods embedded in a dielectric host, which holds a real metal behavior but the extracted effective plasma frequency is in near-infrared region. The electromagnetic responses of such medium and the retrieved effective material parameters have been particularly shown. In addition, an analytic model about effective plasma frequency is constructed by uniquely considering the skin effect and introducing the parameter-skin depth, whose predicting results are in well accordance with the FDTD simulation. This artificial material may open possibilities for many metal-based applications in near-infrared regime.

In this paper, a design method for a multiwavelength photon sieve is described based on a random-area-divided approach, where the whole aperture of a multiwavelength imaging photon sieve is divided into multiple discrete spaces corresponding to the number of the selected working wavelengths. The micropinhole distribution in each discrete space can be calculated for the defined wavelength with one fixed focal length in terms of the normal design for photon sieve. A three-wavelength photon sieve was designed and fabricated in the lab, and its imaging properties are analyzed in the experimental optical system with satisfactory results.

A method is brought forward for realizing polarization dependent devices by employing sub-wavelength asymmetrical hole array on a metallic film. Based on the fundamental mode approximation, the phase retardations of rectangular hole for two orthogonal polarization incident waves are analyzed and calculated. Using rectangular hole array, a bifocal-polarization lens for the infrared radiation with 10.6 microm wavelength is designed. Its focal lengths for x- and y- polarized incident wave are examined by the finite difference time domain (FDTD) method and the Rayleigh-Sommerfeld diffraction integrals and the obtained results agree well with the designed values.

A nanolithography technique based on the interference of surface plasmons (SPs) is proposed and demonstrated to modulate the localized exposure energy. The SP waves participating in interference are excited by two distinct structures, namely, the grating and the nanotaper. Constructive or destructive interference, which ultimately causes an enhanced or reduced modulation to the localized energy, can be obtained merely by adjusting the distance of the grating and the taper. Detailedly speaking, the localized energy can be modulated consecutively with a constant periodicity, and the modulation range of energy is extremely wide, for instance, the maximum energy is nearly 3 orders of magnitude larger than the minimum by our FDTD simulation results. Moreover, since the localized electric field at the taper tip, which leads to the exposure of the photoresist, is extremely sensitive to interference, it suggests a potential way to produce patterns with different depths and critical widths in one chip via beforehand programming and reasonably controlling the corresponding interference of SPs.

Based on surface plasmon resonant enhancement, a method to realize photolithography beyond diffraction limit by using polystyrene spheres (PSs) self-assembled on silver slab was proposed in this paper. The optimum parameters for PS with different diameters were presented. In order to verify this method, numerical simulation on a typical configuration with 1.5 microm diameter of PS was carried out by using the finite-difference time-domain (FDTD) method, and the minimum feature size of 88 nm beyond diffraction limit at 365 nm working wavelength was obtained.

We present that an interference lithography technique beyond the diffraction limit can be theoretically achieved by positing an anisotropic metamaterial under the conventional lithographic mask. Based on the special dispersion characteristics of the metamaterial, only the enhanced evanescent waves with high spatial frequencies can transmit through the metamaterial and contribute to the lithography process. Rigorous coupled wave analysis shows that with 442nm exposure light, one-dimensional periodical structures with 40nm features can be patterned. This technique provides an alternative method to fabricate large-area nanostructures.

Omni-directional electromagnetic field concentrators have been recently reported by Marco Rahm et al. [Photon. Nanostruct.: Fundam. Appl. 6, 87 (2008)] based on form-invariant coordinate transformations related to its Jacobi transformation matrix. Using transverse-electric wave illumination, we reduced the complex material parameters of the concentrator for future practical implementation. Concentrators with different set of permittivity and permeability tensors are proposed. The electromagnetic concentrating performance and the scattering properties at the inner and outer boundary of these concentrators are theoretically and numerically analyzed. Finally we obtain a set of material tensors for a concentrator that simultaneously has perfect matched interior and exterior interfaces.

A circular cylindrical and an oblate cylindrical perfect lens are designed by using coordinate transformation theory. Theoretical analyses are performed to give an insight into the variant angular magnification in the oblate cylindrical perfect lens. We further take advantage of the oblate cylindrical coordinate system to make the object surface flat for future practical imaging and lithography applications. We also for the first time make systematical simulations of various kinds of perfect lens, including numerical confirmation of Mankei Tsang's statement about the magnification of the planar perfect lens and the imaging and magnifying performance beyond the diffraction limit of our designed perfect lens. All the calculated results agree well with our mathematical derivations, thus verifying the coordinate transformation method in designing perfect lenses.

A three-dimensional cone-shaped concentrator was designed and analyzed through an approach of coordinate transformation theory. The device can provide varying performances for concentrating along the symmetric axis. The physical picture regarding concentrating ability of this structure was revealed and quantitative analyses were performed for the purpose of investigating the dependence of the concentrating properties on the structural parameters. Moreover, reduced material parameters were theoretically derived and the corresponding mismatched impedance at boundaries was analyzed. Finite element method-based numerical simulations results of the device were further presented to verify our theoretical design.

In this letter we propose a compensating oblique lens which can realize super resolution imaging. The imaging structure consists of two parts constructed by metallodielectric films but positioned in different orientation. Super resolution optical imaging can be obtained with uniform light intensity and tunable magnification by changing parameters of the structure. Design principles and examples are given and illustrated with numerical simulation.

An imaging mechanism for demagnifing features above wavelength into desired images beyond the diffraction limit is proposed in this letter. The super resolution ability (about two times and even more that of diffraction limit) arises from the surface plasmon wave excitation and amplification associated with metallic grating structure. Two specifically designed masks are projected to the grating surface from both sides, at one of which the superimposed field forms the desired images. Conceptually formalism of the imaging process is presented using spatial Fourier analysis and illustrated with numerical simulations.

A method is proposed to modulate phase using variant square holes in a metallic film based on a fundamental mode approximation model. Phase retardation through square holes in a subwavelength scale in a thin metal film has been analyzed and calculated. Based on the model, a structured lens with a numerical aperture of 0.583 and a focal length of 240 microm formed by a 2D square hole array in a metallic film is designed. Numerical simulation using the finite-difference time-domain method is carried out, and the results agree with the theoretical analysis. A focal spot close to the diffraction limit can be obtained.

It is demonstrated that multiple directional beaming effect can be realized by a metallic subwavelength slit surrounded by finite number of grooves based on mode expansion method. Each of the directional beaming is formed by superimposing two diffraction orders of spoof surface plasmon excited on the two corrugated sides of the slit. This delivers high contrast and considerably uniform energy distribution for the beaming directions.

The authors theoretically demonstrate a plasmonic beam deflector based on the particular properties of surface plasmon polaritons in metallic nanoslits. Beam deflection ranging from 0 degrees to 90 degrees can be achieved by designing the deflector with appropriate structural parameters. Numerical illustrations of deflectors for variant deflection angles are presented through finite-difference time-domain simulation, showing good agreement with theoretical analysis. The efficiency and some factors influencing the deflection behavior are also discussed.

Subwavelength imaging can be obtained with alternately layered metallodielectric films structure, even when the permittivity of metal and dielectric are not matched. This occurs as the effective transversal permittivity tends to be zero or the vertical one approaches infinity, depending on the permittivity value of the utilized dielectric and metal material. Evanescent waves can be amplified through the structure, but not in a manner of fully compensating the exponentially decaying property in dielectric. Numerical illustration of subwavelength imaging is presented for variant configuration of anisotropic permittivity with finite layer number of metallodielectric films.

An aperiodic mask design method for fabricating a microlens array with an aspherical profile is proposed. The nonlinear relationship between exposure doses and lens profile is considered, and the select criteria of quantization interval and fabrication range of the method are given. The mask function of a quadrangle microlens array with a hyperboloid profile used in the infrared was constructed by using this method. The microlens array can be effectively fabricated during a one time exposure process using the mask. Reactive ion etching was carried out to transfer the structure into the substrate of germanium. The measurement results indicate that the roughness is less than 10 nm (pv), and the profile error is less than 40 nm (rms).

A type of metallic double nanoslit with different widths is proposed to investigate Young's interference mediated by surface plasmon polaritons (SPPs). Numerical calculations show that the Young's interference order could be shifted readily by adjusting the width difference between two slits. Further calculations indicate that the interference order shift related to the additional phase retardation is caused by the distinct surface plasmon mode in two slits. Since the surface plasmon mode in a nanoslit is extremely sensitive to the incident wavelength, it suggests a potential way of ultrahigh resolution spectral analysis via measuring the shift of Young's interference.

A method is proposed for designing an arbitrary continuous refracting surface that can realize conversion between two specified distributions of wavefront. Optimization technique is avoided by solving a partial differential equation governing the surface-profile function. This equation is obtained using the property of congruence and the law of refraction, and its solution in this special case is found to be the same as that obtained in wavefront conjugation. A method based on finding the local coefficients of the surface is developed to solve the equation numerically, and an example is given to illustrate its feasibility.

Based on the theoretical formalism derived from the diffraction theory and electromagnetic boundary condition [Phys. Rev. Lett. 167401 (2003)], we investigate numerically the diffraction behavior of light passing through a subwavelength metallic slit-grooves structure, slit surrounded symmetrically by a finite array of grooves at the output surface. The diffraction dependence with the geometrical parameters, groove depth, width and number is analyzed in detail. It is found that variant profile of angle spectra and beaming intensity can be obtained in the far field by tuning the geometrical parameters. Numerical analysis shows that these diffraction behaviors are associated with different excitation states of surface electromagnetic wave.

The propagation and combination of surface plasmon polaritons (SPPs) in Y-shaped metallic nanochannels are investigated numerically via finite difference time domain (FDTD). It is shown that the behavior of SPPs in nano-size channels resembles that of light guiding in conventional waveguides, and SPPs can also be combined effectively with appropriately designed structures. The loss associated with metal absorption and scattering with the multiple reflections between slit openings on the bend angle are analyzed numerically. The Fabry-Perot cavity effect displayed by SPPs traveling in channels with finite length is discussed as well.

A novel method is proposed to manipulate beam by modulating light phase through a metallic film with arrayed nano-slits, which have constant depth but variant widths. The slits transport electro-magnetic energy in the form of surface plasmon polaritons (SPPs) in nanometric waveguides and provide desired phase retardations of beam manipulating with variant phase propagation constant. Numerical simulation of an illustrative lens design example is performed through finite-difference time-domain (FDTD) method and shows agreement with theory analysis result. In addition, extraordinary optical transmission of SPPs through sub-wavelength metallic slits is observed in the simulation and helps to improve elements' energy using factor.

A beam-shaping technique is presented for asymmetrical laser beams with different beam waists and divergences in both vertical and horizontal directions. We utilize a pair of two-dimensional micrograting arrays to equalize the beam parameter products of an asymmetrical beam in orthogonal directions by deflecting the appointed parts of the beam on the longer side of the beam and by recombining the parts on the shorter side. When combined with divergent transformation by means of collimating optical components, the beam-shaping system can produce a symmetrical beam in orthogonal directions with optimized beam waists and divergences. A beam-equalization system is designed and demonstrated for a typical asymmetrical beam of a laser diode bar. In the experiment an emission beam with dimensions of 1 microm x 10 mm and half-divergences of 148 mrad x 576 mrad in the far field is transformed into an almost-square distribution with dimensions of -12 mm x 12 mm and half-divergences of -2 mrad x 2 mrad, which confirm the effectiveness of the proposed technique.

We describe a novel control approach for form accuracy of microlenses with continuous relief by control of exposure threshold. The approach can be achieved by analysis of the exposure distribution on photoresist and by deriving a relationship between the exposure distributions and the internal photoactive compound concentration (PAC) of the photoresist. The feature of parallel PAC curves for a specific value is determined in this way. We then analyze the development process and discover its critical effect on the approach. Finally we establish the relationship between the PAC distribution and the form accuracy of the relief. Not only is the fabricated relief height significantly increased to as high as 100 microm by use of this method, but we also realized effective fine control of the form accuracy of the continuous relief. With this approach we obtained micro-optic elements with 100-microm relief height and a form error (rms value) of less than 1 microm.

Optical second harmonic generation (OSHG) in a two-dimension photonic crystal consisting of centro-symmetric dielectric is investigated. The calculation model and analyzing method for OSHG are discussed. Based on Finite Difference-Time Domain algorithm, the electromagnetic field distribution in the structure and the intensity of second harmonic generation along the waveguide are analyzed. The results show that the acute spatial variation of electro-magnetic field results in the radiation of OSHG, and the intensity of OSHG is proportional to the square of the waveguide length. When the beam intensity of the pumping wave with a wavelength of 10.6 microm is 1.3MW/mm2, the power conversion efficiency is 0.268% for a silicon photonic crystal with a length of 40 microm.