Hailiang Li

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.

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.

Quasi-periodic gratings: diffraction orders accelerate along curves

Light diffracting to different diffraction orders of a periodic grating generally propagates along a set of straight trajectories. Here we show that certain quasi-periodic gratings can produce curved diffraction orders. These curved lobes are created by the caustic interference of the originally straight diffraction orders and manifest themselves as accelerating beams. Both numerical simulations and experimental results demonstrate the validity of multiple accelerating beam generation with a single binary grating. Our work makes a quantitative link between the quasi-periodicity of a grating and the resulting caustic diffraction orders. Furthermore, the use of binary devices has important applications in acoustics, x-ray optics, and electron beam engineering and is also useful when high optical power is needed.

Enhanced optical transmission through asymmetric nanostructured gold films

We investigate optical properties of a gold structure comprising a two-dimensional array of gold nanoblocks placed on top of a thin gold film. We observe enhanced transmission through asymmetric nanostructured gold films, which can be attributed to the surface plasmon (SP) mode at the air-gold interface leaking to the substrate when the substrate index is larger than the superstrate index. When the substrate and superstrate are the same dielectric, the SPs at both superstrate-gold and gold-substrate interfaces strongly interact with each other and even and odd SPs are then excited. In addition, we investigate effects of oblique incidence and electronic interband transition on SP resonances. Our results provide a guideline for engineering novel devices with enhanced transmission based on nanostructures.

Higher-order diffraction suppression of free-standing quasiperiodic nanohole arrays in the x-ray region

Nanohole array is particularly advantageous for light field manipulation. Here, we report a strategy to mimic the function of x-ray transmission gratings with free-standing quasiperiodic nanohole array. An analytical description is developed to reveal the physical mechanism of the free-standing quasiperiodic nanohole array that reduces higher-order contamination, and is verified by rigorous numerical simulations. An x-ray free-standing quasiperiodic nanohole array consisting of 1.6 × 109 nanoholes over an active area of 10 mm × 10 mm was fabricated. Experimental results of near-complete suppression of higher-order diffractions were obtained in the x-ray region. The capability to separate multiple overlapping orders makes this kind of nanohole array attractive for future development and application of high-resolution spectroscopy.

High Q Plasmonic Lasing of Band Edge Modes in an Asymmetry Environment

We theoretically propose a novel plasmonic laser based on the band edge modes of one-dimensional plasmonic crystal in an asymmetric dielectric environment. We find that the quality factors of the band edge modes are high and comparable to that of the collective resonances of surface plasmon in symmetric background. Moreover, surface plasmon lasing of single mode is numerically demonstrated with a full-wave Maxwell-Bloch approach. And it is also shown simultaneously lasing of both radiative and non-radiative modes due to spatial hole burning, enabling the realization of coherent source of both light and surface plasmon in a single platform. Finally, we discuss the dependence of the sensitivity of the plasmonic laser mode on the geometrical and material parameters. Our model provides an easy-to-use approach for future silicon-based on-chip applications such as low-threshold plasmonic laser and solid-state lighting emission.

Two-dimensional gratings of hexagonal holes for high order diffraction suppression

We propose two-dimensional gratings comprised of a large number of identical and similarly oriented hexagonal holes for the high order diffraction suppression. An analytical study of the diffraction property for such gratings, based on both square and triangle arrays, is described. The dependence of the high order diffraction property on the hole shape and size is investigated. Notably, theoretical calculation reveals that the 2nd, 3rd and 4th order diffractions adjacent to the 1st order diffraction can be completely suppressed, and the 5th order diffraction efficiency is as low as 0.01%, which will be submerged in the background noise for most practical applications. The 1st order diffraction intensity efficiency 6.93% can be achieved as the hexagonal holes along y-axis connect with each other. For the case of b=Py/3, the 1st order diffraction intensity efficiency is 3.08%. The experimental results are also presented, confirming the theoretical predictions. Especially, our two-dimensional gratings have the ability to form free-standing structures which are highly desired for the x-ray region. Comparing with the grating of the square array, the grating of the triangle array is easy to be fabricated by silicon planar process due to the large spacing between any two adjacent holes. Our results should be of great interest in a wide spectrum unscrambling from the infrared to the x-ray region.

Omnidirectional diffraction control with rotational topological defects

We present a new scheme for the directional diffraction management of light with incompatible transformation optics. By introducing the concept of disclinations into transformation optics, we demonstrate that the rotational incompatible mapping violates the integrability condition of the coordinate transformation and gives rise to the non-vanishing Frank vector. It is revealed that such special coordinate transformations can produce rotational topological defects in physical space, which magnifies or compresses the diffraction of light beams propagating in arbitrary directions. We verify our theoretical analysis by numerical simulations of the light scattering from a cylinder of a generic disclination medium.

Fabrication of ultralarge single order diffraction grating with high surface flatness

In this work, we present the fabrication process of a large (100mmx40mm) and thick (30mm) single order diffraction grating with a very flat reflecting surface. This grating is designed for a soft X-ray monochromator. Three single order diffraction gratings with different line densities were integrated on one silicon plate. The gratings are patterned on a 6.35mm thick substrate using direct e-beam writing and etched using high density plasma. Then the grating is glued on to a 23.7mm thick bulk silicon. The flatness of the surface of the gratings was well controlled and tested with an interferometer, the test results show that the peak-to-valley value of the surface is less than 60nm, which meet the requirement for a soft X-ray monochromator.