Lina Shi

Guided-mode resonance photonic crystal slab sensors based on bead monolayer geometry

Using finite-difference time-domain method, we investigate photonic crystal slabs consisting of spherical voids or silica beads embedded into a dielectric slab as bio-chemical sensors. We study the dependence of the spectral position of guided-mode resonances on the refractive index of a slab material. The most sensitive design is based on voids filled with analyte. We also study the effects of the slab and analyte thicknesses on guided-mode resonance properties. We eventually demonstrate an aqueous analyte sensor with high sensitivity at visible wavelength as electro-magnetic energy distribution in some guided-mode resonances can be strongly localized in the analyte region.

Spiral photon sieves apodized by digital prolate spheroidal window for the generation of hard-x-ray vortex

We extend the work of photon sieves to spiral photon sieves (SPSs) for the generation of a hard-x-ray vortex. A robust digital prolate spheroidal window, which has an optimal energy concentration at low frequencies, was used to adjust the number of pinholes on each ring of the SPS. It was demonstrated that an SPS has better spatial resolution and lower background than a spiral zone plate using the specified smallest structures condition. The intensity at the center of the dark core is difficult to damp to true zero, primarily owing to current limitations of high-aspect-ratio metal nanostructures. The diameter of the dark core increases as the charge value increases. However, the FWHM of the doughnut-shaped ring and background will also increase. Our results pave the way toward the design of high-performance SPSs for the generation of a hard-x-ray vortex.

Feasibility study of hard-x-ray nanofocusing above 20 keV using compound photon sieves.

Combining the advantages of photon sieves (PSs) and compound Fresnel zone plates (CZPs), we designed compound photon sieves (CPSs) for hard-x-ray nanofocusing. A CPS consists of an inner PS using the first-order diffraction surrounded by an outer zone plate using the third-order diffraction. A robust digital prolate spheroidal window was used as an apodization window for the inner PS, making CPSs more flexible than CZPs. CPSs can provide not only slightly better resolution compared to CZPs, but also it can significantly suppress the sidelobes, leading to a high signal-to-noise ratio. Further improvement of the high-aspect-ratio metal nanostructure process will allow CPSs to be a promising candidate for hard-x-ray nanofocusing in the high-energy region above 20 keV.

Tunable structures comprising two photonic crystal slabs--optical study in view of multi-analyte enhanced detection.

Using finite-difference time-domain method, we characterize the normal-incidence transmission properties of a two slab photonic crystal device in a view of its applications in fluorescence enhancement and multi-analyte detection. Individual slabs consist of a square or a triangular lattice of air holes embedded into a silicon nitride slab. The geometrical parameters are chosen so that the individual slabs operate in a guided resonance regime where strong reflectivity under the normal incidence angle is observed in a broad spectral range. When placed in the close proximity of each other, the two photonic crystal slab system exhibits a narrow Fabry-Perot type transmission peak corresponding to the excitation of a resonant mode in the cavity formed by the two slabs. We then study the effects of the size of the air gap between the two photonic crystal slabs on the spectral position and bandwidth of a resonance transmission peak. Finally, we investigate the electromagnetic energy distributions at the wavelength of a transmission resonance in the double slab photonic crystals. As a final result we demonstrate that this structure can provide electric field enhancement at the slab surface, which can be used for fluorescence enhancement.

Toward two-dimensional nanometer resolution hard X-ray differential-interference-contrast imaging using modified photon sieves

In this Letter, we report a significant step forward in the design of single-optical-element optics for two-dimensional (2D) hard X-ray differential-interference-contrast (DIC) imaging based on modified photon sieves (MPSs). MPSs were obtained by a modified optic, i.e., combining two overlaid binary gratings and a photon sieve through two logical XOR operations. The superior performance of MPSs was demonstrated. Compared to Fresnel zone plates-based DIC diffractive optical elements (DOEs), which help to improve contrast only in one direction, MPSs can provide better resolution and 2D DIC imaging. Compared to normal photon sieves, MPSs are capable of imaging at a significantly higher image contrast. We anticipate that MPSs can provide a complementary and versatile high-resolution nondestructive imaging tool for ultra-large-scale integrated circuits at 45 nm node and below.

Fabrication of x-ray diffractive optical elements for laser fusion applications

Abstract. We review our recent progress on the fabrication of x-ray diffractive optical elements (DOEs) by combining complementary advantages of electron beam, x-ray, and proximity optical lithography. First, an electron beam lithography tool with an accelerating voltage of 100 kV is used to expose initial x-ray mask based on SiC membrane with a low aspect ratio. Second, x-ray lithography is used to replicate x-ray DOEs and amplify the aspect ratio up to 14:1. Third, proximity optical lithography is used to fabricate a large-scale gold mesh as the supporting structures. We demonstrate that this method can achieve high aspect ratio metal nanometer structures without the need of a complicated multilayer resist process. A large number of x-ray DOEs have been fabricated with feature sizes down to 100 nm for the purpose of laser plasma fusion applications. Among them, the ninth-order diffraction peak on the positive side of the zeroth order can be observed for both 3333 and 5000  lines/mm x-ray gold transmission gratings.

NEW COLLECTOR OF PLANAR INSULATED GATE BIPOLAR TRANSISTOR FOR BROAD APPLICATIONS

A new concept of Insulated Gate Bipolar Transisitor (IGBT) with a Si/Ge layer collector is proposed to meet different requirements for turn-on voltage and turn-off time. The operation principles of IGBT are discussed and the energy band diagram of Si/Ge heterojunction is employed to explain the inner dynamic mechanism of the proposed IGBT. Two-dimensional (2D) device-circuit mixed-mode simulations indicate that the tail-current, which is a major cause of the power loss and limits the operation speed of the device, is suppressed effectively by using the Si/Ge layer collector. On the other hand, turn-on voltage is increased by the use of the Si/Ge collector. Furthermore, the turn-on voltage is increasing with the increase of the areal rate of the Ge region in the whole collector, while the turn-off time is reversed. This valuable information leads to the freely tunable planar IGBT by adapting the different areal rates of the Ge region to cast to different actual situations. Detailed physical explanations ar...

ORIGIN AND MANIFESTATION OF THE ANHARMONIC POTENTIAL FELT BY AN ION-CLOUD IN AN ACTUAL PAUL TRAP

The anharmonic potential felt by a single-species ions confined in a rf quadrupole trap which results from a non-ideal trap configuration and the charge distribution of the ion cloud is studied. The rf resonance-absorption spectra are explained by a Duffing oscillator and a representation of the line-shape parameter η is derived. Forη > 0.77, the electric signals will exhibit hysteresis. The relation with the anharmonic potential is discussed.

A high-power solid-state p+–n–n+ diode for picosecond-range closing switching

The solid-state delayed breakdown diode (DBD) is investigated in this paper. Special attention is given to the internal dynamics and physical nature that underlie the delayed breakdown process. We investigate the source of initial carriers which trigger the front, explain the origin of the time delay in triggering the front, and detail the mechanism of front propagation. Then, a Si-based DBD with an improved three-step gradually changing doping structure is reviewed in this paper. The output voltage of an identical-sized improved DBD has been shown to increase from 2.12 kV to 2.25 kV and the dV/dt peak of the output voltage has been increased from 30 kV ns−1 to 43.87 kV ns−1 with the same driving voltage through two-dimensional device simulations. Finally, a Si/SiGe heterojunction DBD with high-speed capability for picosecond switching is presented. Switch speed of the identical-sized Si/SiGe DBD (144.9 kV ns−1) appears to be more than four times higher than that of the conventional Si device (30 kV ns−1).

Determination of guided-mode resonances in photonic crystal slabs

We present a design and analysis study of guided-mode resonances in photonic crystal slabs. Three-dimensional finite-difference time-domain (FDTD) simulations are used in parallel with a simplified model of guided-mode resonances to produce a representation of their evolution with structural parameters. From the analysis of the effective medium behavior of the system, we propose a simplified method able to predict the first guided-mode resonances at normal incidence with a good accuracy (∼1%) for holes with radius-to-period ratio smaller than 0.3 for the transverse magnetic polarization created internally. A substantial gain of time is, therefore, provided compared to FDTD (from the hours level to the seconds level). We also focus on two other important parameters, the quality factor and asymmetry of peaks, and present a way to design symmetric peaks with low sidebands.