Binary Diffractive Optics Research Articles

Improving the Performances of Optical Tweezers by Using Simple Binary Diffractive Optics

Improving the Performances of Optical Tweezers by Using Simple Binary Diffractive Optics

Improving the beam quality factor (M2) by phase-only reshaping of structured light.

Laser brightness is crucial in many optical processes, and is optimized by high power, high beam quality (low M2) beams. Here we show how to improve the laser beam quality factor (reducing the M2) of arbitrary structured light fields in a lossless manner using continuous phase-only elements, thus allowing for the increase in brightness by a simple linear optical transformation. We demonstrate the principle with four high M2 initial beams, converting each to a Gaussian (M2≈1) with a dramatic increase in brightness of >10×. This work puts a new (to the best of our knowledge) perspective on the old debate of improving laser beam quality with binary diffractive optics, while providing a practical approach to enhancing laser brightness for arbitrary input beams.

Ultrathin transmissive metasurfaces for multi-wavelength optics in the visible

Metasurface-based optical elements have the ability to shape wavefronts by locally changing the properties of incident illumination. They hold great potential to promote a new generation of wearable devices and thin optical systems for imaging and sensing. However, due to the diffractive nature of these metasurfaces, chromatic aberration represents a critical challenge toward practical implementations, especially for the visible spectrum. Existing solutions for multi-wavelength meta-optics inevitably increase the device thickness and system complexity, similar to the conventional refractive and diffractive counterparts. They are also limited to polarization sensitive lenses with a small size and numerical aperture (NA). In this manuscript, we present the design of single-layer multi-wavelength metasurfaces using ultrathin dielectric resonators (thickness ≪ wavelength) as structural elements. The electromagnetic response of these resonators can be tailored by the in-plane geometrical parameters and used to manipulate the transmitted light. We introduce a digital transmission modulation scheme which enables opportunities to design multi-wavelength optics beyond conventional binary diffractive optics. The design versatility of our approach is demonstrated by polarization-independent achromatic metalenses with a larger lens size and larger NA than existing approaches. The simplicity of the design and fabrication process makes these ultrathin metasurfaces good candidates for building flat optical elements that can be easily integrated onto CMOS electronics and MEMS devices.

Shaping coherent x-rays with binary optics.

Diffractive lenses fabricated by lithographic methods are one of the most popular image forming optics in the x-ray regime. Most commonly, binary diffractive optics, such as Fresnel zone plates, are used due to their ability to focus at high resolution and to manipulate the x-ray wavefront. We report here a binary zone plate design strategy to form arbitrary illuminations for coherent multiplexing, structured illumination, and wavefront shaping experiments. Given a desired illumination, we adjust the duty cycle, harmonic order, and zone placement to vary both the amplitude and phase of the wavefront at the lens. This enables the binary lithographic pattern to generate arbitrary structured illumination optimized for a variety of applications such as holography, interferometry, ptychography, imaging, and others.

Binary diffractive optics for 3D-demultiplexing of OAM beams

We propose to use a combination of spiral Fresnel zone plates (SFZPs) and binary diffractive gratings in order to realize 3D-demultiplexing of vortex beams – laser beams having an orbital angular momentum (OAM). It is known that binary and blazed fork-shaped diffractive gratings are widely used for demultiplexing of OAM beams and their analysis. In contrast to the fork-shaped gratings, in the case of SFZPs the diffraction orders differ in their focus positions along the propagation axis. A combination of these two approaches allows us to perform the socalled 3D-demultiplexing of OAM beams.

Focusing anomalies with binary diffractive optical elements.

Binary diffractive optics have been extensively studied to date as tools for arbitrary laser beam shaping and experimentally implemented with etched transparent optics and spatial light modulators. Here we demonstrate that a simple one-step binary optic is able to enhance the intensity of a focused beam, displaying some counterintuitive focusing anomalies. We explain these effects by considering the optical aberrations in binary diffractive optics and outline how this may be exploited for further improvements in refractive/diffractive combinations for super-resolution microscopy.

Versatile bilayer resist for laser lithography at 405 nm on glass substrates

We describe a simple bilayer photoresist that is particularly well suited for laser lithography at an exposure wavelength of 405 nm on glass substrates, which are often used for the fabrication of binary diffractive optics and computer-generated holograms. The resist consists of a poly-dimethyl glutarimide (PMGI) bottom layer that is used as an antireflection coating between a glass substrate and a positive or negative photoresist. The optical properties of the PMGI layer at 405 nm result in excellent suppression of reflections into the photoresist and good process latitude.

Design and optimization of broadband wide-angle antireflection structures for binary diffractive optics

We propose a class of antireflecting structures that can effectively suppress reflections for binary diffractive optics. In this structure, multiple periodic thin films with gradually varying refractive indices are used to shift all reflected diffraction to the transmitted orders. The structure is optimized to operate over broad bands and wide angles using rigorous coupled-wave analysis and genetic algorithms. We validated the structure numerically using finite-difference time-domain methods. The proposed structure may lead to more efficient diffractive devices for applications in thin-film photovoltaic, waveguide coupler, and holographic optical elements.

Z-scan sensitivity enhancement using a binary diffractive optics

We report a theoretical study of the sensitivity enhancement of Z-scan technique, using a simple binary diffractive optical element. The latter is a phase aperture made from a transparent plate that has a circular relief introducing a π phase shift in the central region of the incident beam. We demonstrate that the phase aperture acts like a divergence multiplier allowing the Z-scan sensitivity to be increased by a factor of several hundred.

Improvement of the self-Q-switching behavior of a Cr:LiSrAlF6 laser by use of binary diffractive optics.

It has been shown experimentally and theoretically that Q-switching behavior is possible in a flashlamp-pumped Cr-doped LiSrAlF6 (Cr3+:LiSAF) laser that consists only of two mirrors, a laser crystal, and a diaphragm. We demonstrate that insertion into a laser of a binary diffractive optical element can speed up the dynamics of the self-Q-switched laser such that the output pulse is shortened (from 60 to 33 ns) and its energy is increased (from 36 to 54 mJ). The self-Q-switching behavior of the laser has the ability to produce a laser pulse with a duration that one can adjust continuously from 60 to 700 ns just by opening the diaphragm.

Preparation of microfocusing optics using synchrotron radiation sources (invited, abstract)

X-rays are used for microfabrication of high-aspect-ratio structures because of their high penetrating power for microfabrication of integrated circuits because of their very high spatial resolution. An uncommon application is the development of phase-shifting optics for hard x-rays, where both requirements (high aspect ratio and high resolution) must be met at the same time. In the hard x-ray region (8 keV and up), conventional absorbing binary diffractive optics are very inefficient because of the low contrast of most elements. It is instead possible to achieve very high efficiency using phase-shifting optics. To achieve a phase shift of Pi, several microns of material are needed. At the same time, the spatial resolution is determined by the finest line in the pattern, of the order of 0.2–0.1 μm. This yields aspect ratios of 10-50, posing great challenges to microfabrication because the submicron dimensions require a much more delicate process than that found, for instance, in LIGA. We have overcome these hurdles, and succeeded in manufacturing Fresnel phase plates with 0.1 μm smallest features for operation at 8 keV. We use a combination of x-ray and electron-beam lithography to process the masks. In particular, the use of an x-ray aligner, capable of multiple exposures, has allowed us to fabricate multilevel FPPs, with ‘‘blazed’’ zone profile of efficiency approaching 80% in the first order focus. We will review the techniques that are at the basis of our fabrication process, describe our approach, and present some of the most recent results.