Liwei Fu

Periodic Large‐Area Metallic Split‐Ring Resonator Metamaterial Fabrication Based on Shadow Nanosphere Lithography

Metamaterials have gained substantial attention because of their potential for negative permeability as well as negative refractive index in the optical frequency range[1,2]. Due to their unique electromagnetic properties, these nanostructures show promise for numerous applications such as perfect lenses and optical cloaking devices.

Optical resonances of bowtie slot antennas and their geometry and material dependence

In order to provide a guide for the design and optimization of bowtie slot antennas in the visible and near infrared spectral regime, their optical properties have been investigated with emphasis on geometry and materials. Although primarily theoretical, experimental investigations for reduced thickness cases are also included. As characterized by their field patterns, two types of resonances are discussed: plasmonic and Fabry-Pérot-like resonances. These resonance types show a linear dependence on aperture perimeter and film thickness, respectively, while showing a complementary behavior with near independence of the other respective parameter. Metal properties, as in the Drude model, are also considered. Various metals with respectively different skin depths are studied, showing a nearly linear dependence of the resonance wavelength on skin depth.

Resonance hybridization in double split-ring resonator metamaterials

We introduce a plasmon hybridization picture to understand the optical properties of double split-ring resonator metamaterials. The analysis is based on the calculated reflectance spectra from a finite-integration time-domain algorithm. Field distributions of the double split-ring resonators at the resonant frequencies confirm the results from the plasmon hybridization analysis. We demonstrate that the plasmon hybridization is a simple and powerful tool for understanding and designing metamaterials in the near infrared and visible regime.

Optical properties of metallic meanders

A vertical metallic meander structure with a rectangular corrugated surface profile represents a frequency-selective surface in which the excitation and interaction of localized surface plasmon modes are controlled in a flexible fashion by its geometrical parameters over a large spectral range. In this report we investigate the optical properties of metallic meanders numerically. Although the structure is simple from both the structural geometry and the nanofabrication point of view, its plasmonic band structure manifests rich features that would be very attractive for plasmonic functional devices. In particular, the short-range surface plasmon mode can be tuned by changing the meander depth without altering the long-range surface plasmon mode. To obtain deeper physical insight into the relationship between the structural geometry and its optical response, a transmission line equivalent circuit model is used. It is revealed that circuit parameters that were fitted from numerical scattering parameters have physical relationships with the structural parameters, which can be described by quasi-static or radiative descriptions. In certain frequency ranges, enhanced transmission occurs due to the interaction of magnetic and electric dipole resonances. The calculated effective material parameters reveal that enhanced transmission occurs around the near-zero index frequencies. The application potential of these structures as frequency filters is discussed.

Design and realization of high-power ripple-free superluminescent diodes at 1300 nm

To realize high-power superluminescent diodes (SLDs) emitting at the 1300-nm wavelength and to see how different structure parameters influence the device performances, three different epitaxial layers have been studied. It was found that the structure employing a graded-refractive-index separate-confinement heterostructure, with linearly graded p-doped cap layer and with eight quantum wells, is most suitable for high-power SLDs. With a proper design of a geometrical structure for SLDs, the obtained output power at 20/spl deg/C is about 25 mW under CW operation and 100 mW at 1.1 A under pulsed operation with no observable ripple.

Design of high-transmission metallic meander stacks with different grating periodicities for subwavelength-imaging applications

When replacing a bulk negative index material (NIM) with two resonant surfaces that allow for surface plasmon polariton (SPP) propagation it is possible to recreate the same near-field imaging effects as with Pendrys perfect lens. We show that a metallic meander structure is perfectly suited as such a resonant surface due to the tunability of the short (SRSPP) and long range surface plasmon (LRSPP) frequencies by means of geometrical variation. Furthermore, the Fano-type pass band between the SRSPP and LRSPP frequencies of a single meander sheet retains its dominant role when being stacked. Hence, the pass band frequency position, which is determined by the meander geometry, controls also the pass band of a meander stack. When building up stacks with different periodicities the pass band shifts in frequency for each sheet in a different way. We rigorously calculate the spectra of various meander designs and show that this shift can be compensated by changing the remaining geometrical parameters of each single sheet. We also present a basic idea how high- transmission stacks with different periodicities can be created to enable energy transfer at low loss over practically arbitrary distances inside such a stack. The possibility to stack meander sheets of varying periodicity might be the key to far field superlenses since a controlled transformation of evanescent modes to traveling wave modes of higher diffraction order could be enabled.

Polarization scramblers with plasmonic meander-type metamaterials

Due to plasmonic excitations, metallic meander structures exhibit an extraordinarily high transmission within a well-defined pass band. Within this frequency range, they behave like almost ideal linear polarizers, can induce large phase retardation between s- and p-polarized light and show a high polarization conversion efficiency. Due to these properties, meander structures can interact very effectively with polarized light. In this report, we suggest a novel polarization scrambler design using spatially distributed metallic meander structures with random angular orientations. The whole device has an optical response averaged over all pixel orientations within the incident beam diameter. We characterize the depolarizing properties of the suggested polarization scrambler with the Mueller matrix and investigate both single layer and stacked meander structures at different frequencies. The presented polarization scrambler can be flexibly designed to work at any wavelength in the visible range with a bandwidth of up to 100 THz. With our preliminary design, we achieve depolarization rates larger than 50% for arbitrarily polarized monochromatic and narrow-band light. Circularly polarized light could be depolarized by up to 95% at 600 THz.

The extension of gain spectra and accurate determination of the quasi-Fermi-level separation from measured amplified spontaneous emission spectra

A method for obtaining the gain spectra of semiconductor lasers in an extended energy range from amplified spontaneous emission (ASE) spectra is presented. Hakki–Paoli gain measurement is first used to determine the quasi-Fermi-level separation. By using the fitting process proposed, a self-consistent correction in determining quasi-Fermi separation leads to a reduced error (<1 meV) and a recalibration of the intrinsic absorption coefficient is also self-consistently possible. Subsequently, with measured gain in a restricted energy range, we can obtain gain spectra in a much wider energy range by our proposed algorithm in conjunction with ASE data. The application of this method in obtaining the extended gain spectra of a double-quantum-well GaInP ridge waveguide laser is demonstrated.

Short-range surface plasmonics: Localized electron emission dynamics from a 60-nm spot on an atomically flat single-crystalline gold surface

We demonstrate nanofocusing down to 60 nm with 800-nm light in atomically flat single-crystalline 22-nm-thick gold flakes. We experimentally and theoretically visualize the propagation of short-range surface plasmon polaritons using atomically flat single-crystalline gold platelets on silicon substrates. We study their excitation and subfemtosecond dynamics via normal-incidence two-photon photoemission electron microscopy. By milling a plasmonic disk and grating structure into a single-crystalline gold platelet, we observe nanofocusing of the short-range surface plasmon polariton. Localized two-photon ultrafast electron emission from a spot with a smallest dimension of 60 nm is observed. Our novel approach opens the door toward reproducible plasmonic nanofocusing devices, which do not degrade upon high light intensity or heating due to the atomically flat surface without any tips, protrusions, or holes. Our nanofoci could also be used as local emitters for ultrafast electron bunches in time-resolved electron microscopes.

Coupling between surface plasmons and Fabry-Pérot modes in metallic double meander structures

The excitation and transfer of evanescent electromagnetic waves appears as key challenge for the realization of optical imaging devices with super resolution. In this process surface plasmon polaritons (SPP) overtake the role as indispensable mediators between source fields and propagating fields. Therefore, the interaction between SPPs and the vacuum field in a double meander structure (DMS) is investigated. The occurrence of Fabry-Pérot (FP) modes within such a cavity and the SPP modes of the meander structure is analyzed to understand the interaction of both mode systems in the combined double meander structure. We show that the known Fano-type passband of single meander structures keeps its dominant role in the DMS and demonstrate the frequency selective role of meander mirrors within this meander cavity. The meander geometry determined passband frequency position also controls nearly solely the passband of the DMS. For far field superlenses (FSL) the energy transfer at low loss over practically arbitrary distances inside the structure is a key property. A resonant amplitude transfer can be obtained between resonantly coupled meander surfaces for unlimited distances in practical cases. This property enables a controlled transformation of evanescent modes to traveling wave modes of higher diffraction order useful for superlens operation.