L-shaped Gold Research Articles

Multi-Mode Long-Wavelength GaAs/AlGaAs Quantum Well Infrared Photodetectors for Circular Polarization Detection

We present an integrated device combining a double L-shaped chiral metasurface with long-wavelength GaAs/AlGaAs quantum well infrared photodetectors (QWIPs), achieving a circular polarized extinction ratio (CPER) as high as 45 in the long-wavelength infrared range of 7–9 μm. The unit of the chiral metasurface array consists of two structurally identical L-shaped gold structures with central symmetry. The CPER of the double L-shaped QWIPs is 14 times higher than that of a single L-shaped QWIP. The device operates in three modes within the detection band: the microcavity mode, the surface plasmon polariton (SPP) mode, and the hybrid mode. The double L-shaped chiral structure selects and reflects a small portion of left-handed circularly polarized light (LCP), while the majority enters the device and excites SPP modes with the bottom gold grating layer, leading to an absorption enhancement. In contrast, right-handed circularly polarized light (RCP) is mostly reflected with limited excitation of SPP waves. QWIPs exhibit a peak absorption of 0.8 and a coupling efficiency of 2700% in the active region of the quantum well due to the combined effects of the microcavity and SPP modes, in which the SPP mode plays a dominant role. The proposed device maintains high circular polarization discrimination capability under large incident angles and can be applied in spectral imaging.

Highly sensitive tunable terahertz absorber for biosensing applications

A multicontrollable terahertz perfect absorber has been proposed and demonstrated numerically through an L-shaped metasurface tri-layer structure. The proposed tri-layer structure comprises a top L-shaped InAs layer and gold ground plane separated by a dielectric polyimide layer which shows a perfect absorption of more than 99.8% with the application of magnetic field from 0.5–0.8 T along with a higher tunability rate of 0.76 THz/T in the frequency spectrum. The multicontrollability and tunability in the frequency response are achieved by the application of a magnetic field as well as the geometrical rotation of meta-atom. Thus, the proposed structure act as a bicontrollable metasurface providing an efficient approach for control over the maximum absorptance frequency. Furthermore, the performance of the proposed absorber shows greater applicability for the bio-chemical sensors with a high value of sensitivity of around 1109 GHz/RIU for the detection of different chemicals, 1012 GHz/RIU for different pathogens, and 1066 GHz/RIU for different hemoglobin concentrations. Thus, the proposed metasurface provides a bicontrollable approach for efficient absorption and prevents transmission as well as acts as multifunctional metasurface which provides potential applications in designing bio-chemical sensors at terahertz frequency regimes.

Thermal tracing of a highly reconfigurable and wideband infrared heat sensor based on vanadium dioxide

In this paper, we investigate the thermodynamic properties of a highly reconfigurable and broadband perfect metamaterial absorber in the near-infrared region. Our model is a periodic nanostructure composed of an array of L-shaped hybrid gold and vanadium dioxide nanostructures on gold and glass substrates. The reconfigurability of the nanostructure is based on the metal–insulator transition of vanadium dioxide. The degree of reconfigurability depends on the temperature of the vanadium dioxide nanostructure. In order to study the photo-thermal effect on the reconfigurability of the nanostructure, we irradiate it with a time-dependent Gaussian pulse laser with a 1.5 ns pulse width. Our main purpose is twofold. On the one hand, we aim to investigate how the laser increases the temperature of the nanostructure, and on the other hand, we intend to explore whether the temperature modifies the absorption and reflection of the array. Our results reveal that the VO2 fully transitions to its metallic phase when the fluence of the incident laser is adjusted to 17.8 mJ/cm2.

Efficient method for evaluation of second-harmonic generation by surface integral equation

A frequency-domain method based surface integral equation of the tangential Poggio–Miller–Chang–Harrington–Wu–Tsai formulation is presented as a full wave analysis to evaluate surface second-harmonic generation from noble metal nanoparticles of virtually arbitrary shape. According to the similar solution of fields and boundary conditions at fundamental and second-harmonic frequency, we get the derivation of surface integral equation formulations with only half unknowns compared with the conventional surface integral equation method. Simultaneously, the condition number of impedance matrix has been sharply declined. Numerical examples of gold nanospheres of different radius are presented to demonstrate the accuracy and efficiency of the proposed method. To further research the distribution of surface nonlinear polarization and properties of the second-harmonic radiation, we apply our method to a noncentrosymmetric L-shaped gold nanoparticle studied experimentally. This method provides an efficient and promising approach for evaluation of nonlinear optical radiation generated from metal nanoparticles array and optimization design of nonlinear nanoantennas.

Tailoring Second-Harmonic Generation in Single L-Shaped Plasmonic Nanoantennas from the Capacitive to Conductive Coupling Regime

We investigate the efficiency of second-harmonic generation (SHG) over the transition from capacitive to conductive coupling in orthogonal L-shaped dimer gold antennas. By tuning both the gap and antenna length, the bonding and antibonding resonances are individually addressed. Results on the intensity and polarization of SHG are compared quantitatively with microscopic numerical simulations taking into account the nanoscale nonlinear surface dipole distribution, elucidating the interplay between symmetry at macroscopic and microscopic levels and optical resonance effects. Microscopic modeling reveals strong cancellations of nonlinear dipoles by capacitive coupling in plasmonic nanogaps, resulting in only small changes in SHG efficiency despite large local field enhancement in the gap. Experimentally, irreproducible polarization properties are obtained in a range of parameters associated with strong optical near fields in the gap of the antennas, which is interpreted as a consequence of nanoscopic asymmet...

Ultra-thin optical vortex phase plate based on the L-shaped nanoantenna for both linear and circular polarized incidences

Based on the L-shaped gold nanoantennas, the ultra-thin optical vortex phase plates (VPPs) have been designed to generate the optical vortex beams with different topological charges, which are independent of the incident polarization states and suitable for both X/Y linear and circular polarization incidences simultaneously. The phase and amplitude of transmitted cross-polarization light can be simultaneously manipulated by changing two degrees of freedom (the length and the width) in the L-shaped nanoantenna unit. Evolution properties of the generated vortex beam are demonstrated and analyzed. The different interactions of angular momentums between light and the VPP in the different incident polarization states have also been investigated fully. The designed VPP shows a superior broadband characteristics in near-infrared wavelength ranging from 750nm to 1200nm, which enable a potential implication for integrated optics and vortex optics.

High Performance Broadband Asymmetric Polarization Conversion Due to Polarization-dependent Reflection

We present the underlying theory, the design specifications, and the simulated demonstration of a high performance broadband asymmetric polarization conversion composed of an L-shaped gold particle and a gold nanoan- tenna array for the near-infrared regime. It can transform linearly polarized light to its cross polarization in the trans- mission mode for one propagation direction and efficiently reflect the light for the opposite propagation direction. The broadband asymmetric polarization conversion can be attributed to the polarization-dependent reflection of the nanoantenna array, which enhances the polarization con- version efficiency of the L-shaped particle and makes it asymmetric and devisable. This work offers a further step in the development of a high efficiency broadband optical activity device.

L-shaped metasurface for both the linear and circular polarization conversions

A new type of optical polarization converter based on the L-shaped gold nanoantenna array supercell has been presented, which is suitable for both circular and X/Y linear polarization conversion simultaneously. Both the amplitude and phase of transmitted cross-polarization light can be modulated precisely by changing the lengths and widths of the L-shaped nanoantenna units. For circular or X/Y linear polarization incident lights, the corresponding cross-polarized lights can be obtained in the transmitted lights, and the corresponding bending angles of the cross-polarized lights can be modulated by the structures and the incident angles according to the generalized Snell’s law. The multi-spectral characteristics have also been investigated, in which the operating bandwidth of the designed optical polarization converter is 450 nm (in the range of 750 ∼ 1200 nm).

Wide-angle, polarization-independent and dual-band infrared perfect absorber based on L-shaped metamaterial.

We propose a wide-angle, polarization-independent and dual-band infrared perfect metamaterial absorber made of double L-shaped gold patches on a dielectric spacer and opaque gold ground layer. Numerical and experimental results demonstrate that the absorber has two near-unity absorption peaks, which are result from magnetic polariton modes generated at two different resonant wavelengths. In addition, the proposed structure also shows good absorption stability in a wide range of incident anglesθfor both TE and TM incidences at azimuthal angle φ = 0°. Moreover, we demonstrate that such structure has good absorption stability for a wide range of azimuthal angles due to the excitation of perpendicular magnetic polariton modes within the asymmetric double L-shaped structure. Such structure will assist in designing magnetic polaritons absorbing element for infrared spectroscopy and imaging.

Active control of chirality in nonlinear metamaterials

An all-optical tunabe chirality is realized in a photonic metamaterial, the metamolecule of which consists of a nonlinear nano-Au:polycrystalline indium-tin oxide layer sandwiched between two L-shaped gold nano-antennas twisted 90° with each other. The maximum circular dichroism reached 30%. Under excitation of a 40 kW/cm2 weak pump light, the peak in the circular dichroism shifts 45 nm in the short-wavelength direction. An ultrafast response time of 35 ps is maintained. This work not only opens up the possibility for the realization of ultralow-power and ultrafast all-optical tunable chirality but also offers a way to construct ultrahigh-speed on-chip biochemical sensors.

Boundary element method for surface nonlinear optics of nanoparticles

We present the frequency-domain boundary element formulation for solving surface second-harmonic generation from nanoparticles of virtually arbitrary shape and material. We use the Rao-Wilton-Glisson basis functions and Galerkin's testing, which leads to very accurate solutions for both near and far fields. This is verified by a comparison to a solution obtained via multipole expansion for the case of a spherical particle. The frequency-domain formulation allows the use of experimentally measured linear and nonlinear material parameters or the use of parameters obtained using ab-initio principles. As an example, the method is applied to a non-centrosymmetric L-shaped gold nanoparticle to illustrate the formation of surface nonlinear polarization and the second-harmonic radiation properties of the particle. This method provides a theoretically well-founded approach for modelling nonlinear optical phenomena in nanoparticles.

Effective medium multipolar tensor analysis of second-harmonic generation from metal nanoparticles

We present a detailed multipolar tensor analysis of second-harmonic (SH) generation from arrays of L-shaped gold nanoparticles. We define three effective nonlinear tensors, which include electric dipoles only (Aeee) and lowest-order magnetic (and quadrupole) effects at the fundamental (Aeem) and the SH (Amee) frequency. The components of the various tensors are distinguished through their different transformations as the experimental geometry is varied. The response is dominated by electric-dipole effects. However, the higher multipoles also play a significant role and are more important at the fundamental frequency than at the SH frequency. The results correlate well with the particles’ plasmonic resonances and symmetry rules.

Particle plasmon resonances in L-shaped gold nanoparticles

We present an extensive experimental and theoretical study of the particle plasmon resonances of L-shaped gold nanoparticles. For the small characteristic size of the particles, we observe more higher-order resonances than previously from related shapes, and show that a short-wavelength resonance arises from the particle arm width and is not the suggested volume plasmon. We interpret the resonances through the local vector electric field in the structure and by fully taking into account the particle symmetry.

Resonant Modes of L-Shaped Gold Nanoparticles

We analyze the electric field modes excited in resonant L-shaped gold nanoparticles using a finite-difference time domain method. Compared to a single gold nanorod, both the odd and even modes of the L-shaped nanoparticles can be excited due to the symmetry breaking. The nanoparticles with equal and unequal arms have different dependence of field enhancement and mode on the incident polarization.

Multipolar analysis of second-harmonic radiation from gold nanoparticles

We present a multipolar tensor analysis of second-harmonic radiation from arrays of noncentrosymmetric L-shaped gold nanoparticles. Our approach is based on the fundamental differences in the radiative properties of electric dipoles and higher multipoles, which give rise to differences in the nonlinear response tensors for the reflected and transmitted second-harmonic signals. The results are analyzed by dividing the tensors into symmetric (dipolar) and antisymmetric (higher multipolar) parts between the two directions. The nonlinear response is found to be dominated by a tensor component, not resolved earlier [Phys. Rev. Lett. 98, 167403 (2007)], which is associated with chiral symmetry breaking of the sample and which also contains a strong multipolar contribution. The results are explained by a phenomenological model where asymmetrically-distributed defects on opposite sides of the particles give rise to dipolar and quadrupolar second-harmonic emission.

Fourier modal method for the analysis of second-harmonic generation in two-dimensionally periodic structures containing anisotropic materials

The rigorous Fourier modal method for crossed anisotropic gratings is extended to the analysis of second-harmonic generation in two-dimensionally periodic structures. The method takes the undepleted-pump approximation that uncouples the calculations of the fundamental and the second-harmonic fields. The effectiveness of the method is verified by solving a one-dimensional problem, which has been analyzed by two previously developed methods, and by comparing the simulation results of the L-shaped gold nanoparticle arrays with the previous experimental measurements.

A macroscopic formalism to describe the second-order nonlinear optical response of nanostructures

We present a macroscopic formalism to describe the second-order nonlinear optical response of nanostructures. Rapid variations in local nonlinearity and electric field distributions on scales smaller than a wavelength preclude a simple, direct relationship between the macroscopic and nanoscopic nonlinear response functions in arrays of metal nanoparticles. We develop an approach that bypasses these difficulties by focusing on the macroscopic nonlinear optical response of the sample in terms of the input and output fields. The main advantage of this macroscopic formalism is that it naturally includes contributions from higher multipoles, although symmetry properties can be addressed using electric-dipole-type selection rules. It is limited by being specific to the experimental geometry, although experimental variations are expected to provide additional insight into the underlying physical processes. The formalism is applied to the second-harmonic response of an array of L-shaped gold nanoparticles.

Remarkable polarization sensitivity of gold nanoparticle arrays

In an array of low-symmetry, L-shaped gold nanoparticles, slight distortions of particle shape and arm lengths eliminate the array’s mirror plane. Such asymmetries induce large angular shifts (∼10°) of the resonant extinction axes directions from those of mirror-symmetric particles. The axes directions exhibit dispersion, as allowed by the lack of any structural features dictating them. The nanostructures are chiral, and evidence of optical activity is observed. Rigorous diffraction theory calculations qualitatively reproduce the data.

Polarization effects in the linear and nonlinear optical responses of gold nanoparticle arrays

We summarize our work on polarization effects in arrays of low-symmetry L-shaped gold nanoparticles. A collection of experiments used to characterize both the linear and second-order optical responses of the arrays is discussed. The responses of the arrays are found to be exceptionally sensitive to polarization. This sensitivity is determined to arise from structural properties including particle size, shape, spacing, and orientation. Nonlinear polarization measurements are shown to yield unexpected and very interesting information concerning the symmetry of the nanoparticle arrays. The combined linear and nonlinear results confirm that the smallest details of arrays influence optical responses.

Linear and nonlinear optical responses influenced by broken symmetry in an array of gold nanoparticles

An array of low-symmetry, L-shaped gold nanoparticles is shown to exhibit high sensitivity to the state of incident polarization. Small imperfections in the shape of the actual particles, including asymmetric arm lengths and edge distortions, break the symmetry attributed to an ideal particle. This broken symmetry leads to a large angular displacement of the extinction axes from their expected locations. More significantly, second-harmonic generation experiments reveal significant second-order susceptibility tensor components forbidden to the ideal symmetry.