Yilei Hua

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.

Quasi suppression of higher-order diffractions with inclined rectangular apertures gratings

Advances in the fundamentals and applications of diffraction gratings have received much attention. However, conventional diffraction gratings often suffer from higher-order diffraction contamination. Here, we introduce a simple and compact single optical element, named inclined rectangular aperture gratings (IRAG), for quasi suppression of higher-order diffractions. We show, both in the visible light and soft x-ray regions, that IRAG can significantly suppress higher-order diffractions with moderate diffraction efficiency. Especially, as no support strut is needed to maintain the free-standing patterns, the IRAG is highly advantageous to the extreme-ultraviolet and soft x-ray regions. The diffraction efficiency of the IRAG and the influences of fabrication constraints are also discussed. The unique quasi-single order diffraction properties of IRAG may open the door to a wide range of photonic applications.

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.

Single-focus spiral zone plates

We extend the concept of spiral zone plates along the optical axis and define a specific single optical element, termed as single-focus spiral zone plates (SFSZPs), for the generation of a single-focus vortex beam. The key idea is to make the transmittance of the spiral zone plates sinusoidal in the azimuthal direction. Furthermore, a two-parameter modified sinusoidal apodization window is introduced to modulate the transmittance function. Theoretical analysis reveals that the third-order diffraction light intensity of the SFSZPs could be reduced by more than three orders of magnitude compared to a conventional spiral zone plate. The experimental results are also presented, confirming the desired single-focus characteristics. The unique single-focus phase singularity properties imply that SFSZPs may find a wide range of imaging and microscopy applications, as well as fundamental studies of vortex beams.

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.

Fabrication and test of quasiperiodic x-ray reflection gratings for high-order diffraction suppression (Conference Presentation)

X-ray diffraction gratings with periodic structures have been widely used in various x-ray instruments and systems, such as synchrotron radiation, x-ray interferometer, x-ray astronomy and plasma diagnostics in the field of laser fusion. However, conventional diffraction gratings suffer from so-called high order diffraction contamination. Here we present a large-area quasiperiodic x-ray reflection grating fabricated by high-speed electron beam direct writing technique. The grating consists of a large number of circular holes for the high order diffraction suppression. The 3rd and even order diffractions can be completely eliminated, and the 5th order diffraction is as low as 0.02% of the 1st order diffraction. Shipley SAL-601 with high-resolution, high sensitivity and good resistance is used for electron beam lithography, followed by dry silicon etching and Au thin film deposition using magnetron sputtering. Since the surface roughness and flatness of the x-ray reflection gratings have a great impact on the dispersion performance, we optimized the fabrication the inductively coupled plasma (ICP) silicon etching process, and tested the surface roughness and flatness of the x-ray reflection gratings by an atomic force microscope and a Zygo interferometer, respectively. The optical characterization of the fabricated quasiperiodic x-ray reflection gratings was performed at the spectral radiation standard and metrology beamline BL08B, national synchrotron radiation laboratory of China. The test results demonstrated the effectiveness of high order diffraction suppression. The capability of high order diffraction suppression and fabrication constraints and the limitation of the diffraction efficiency of the quasiperiodic x-ray reflection gratings are also discussed. The unique high order diffraction suppression properties of the quasiperiodic x-ray reflection gratings may provide a platform for x-ray spectroscopic instruments in laboratory sciences and synchrotron light sources.

Fabrication of ultralarge single order diffraction grating for soft X-ray monochromator

In this work, we present the fabrication process of a large area (100mm×40mm) and thick (30mm) single order diffraction grating for a 10-1000eV soft X-ray monochromator. Three types of diffraction gratings were integrated on one single piece of bulk silicon. The gratings were patterned on a 4 inch silicon wafer using direct e-beam writing with SAL-601. The wafer was etched and diced into a 100mm×40mm×0.5mm slice, and this slice is bonded to a bulk silicon using low temperature Au-Au bonding technology. The performance of this grating is tested in soft X-ray region.

Hybrid lithography for x-ray diffractive optical elements

X-ray diffractive optical elements (DOEs) are used in x-ray astronomy, interferometry, extreme UV lithography, and plasma diagnostics for laser-fusion targets. To fabricate DOEs, microlithographic techniques determine the minimum feature size and functional capabilities critical to the elements’ function. However, DOE requirements for membrane-based metal nanostructures with high aspect ratio and fidelity present challenges for lithography. Over the past 50 years, microlithography has been developed for the advancement of very small semiconductor and metal device structures, but only in the last two decades has it started to meet the requirements of x-ray DOEs.1 Existing lithographic techniques for nanopatterning include electron-beam lithography,2, 3 interference lithography,4 and focused-ion-beam technology.5 Using interference lithography, researchers from the Massachusetts Institute of Technology’s Space Nanotechnology Laboratory have fabricated a large number of x-ray transmission gratings for laser plasma fusion and space flight missions.6 However, this approach is limited to periodic geometries and (as with all the aforementioned techniques) often requires multilayer resist processes to obtain nanometer structures with high aspect ratio. Alternatively, we report a strategy for low-volume x-ray DOE nanofabrication that combines electron-beam, x-ray, and proximity optical lithographies on a silicon carbide (SiC) membrane. The idea is to use the strengths of each appropriate lithographic technique for the various processing steps. We prepared the membrane using a home-made plasma-enhanced chemical vapor deposition system with a temperature of up to 1000C, making the membrane impervious to radiation attack during laser fusion experiments. There are three main steps in our approach (see Figure 1). The first is to pattern fine complex structures with low aspect ratio on a master x-ray mask using 100kV electron-beam lithography, Figure 1. The fabrication process for x-ray diffractive optical elements (DOEs) based on a silicon carbide (SiC) membrane. First, electron beam lithography patterns x-ray masks (a, b). Next, x-ray lithography replicates the DOEs (c–d), and finally, proximity optical lithography patterns a large-scale gold (Au) mesh (e, f). PMMA: poly(methyl methacrylate). Cr: Chromium.