Focusing Grating Research Articles

Efficient low-density grating setup for monochromatization of XUV ultrafast light sources.

Ultrafast light sources have become an indispensable tool to access and understand transient phenomenon in material science. However, a simple and easy-to-implement method for harmonic selection, with high transmission efficiency and pulse duration conservation, is still a challenge. Here we showcase and compare two approaches for selecting the desired harmonic from a high harmonic generation source while achieving the above goals. The first approach is the combination of extreme ultraviolet spherical mirrors with transmission filters and the second approach uses a normal-incidence spherical grating. Both solutions target time- and angle-resolved photoemission spectroscopy with photon energies in the 10-20 eV range but are relevant for other experimental techniques as well. The two approaches for harmonic selection are characterized in terms of focusing quality, photon flux, and temporal broadening. It is demonstrated that a focusing grating is able to provide much higher transmission as compared to the mirror+filter approach (3.3 times higher for 10.8 eV and 12.9 times higher for 18.1 eV), with only a slight temporal broadening (6.8% increase) and a somewhat larger spot size (∼30% increase). Overall, our study establishes an experimental perspective on the trade-off between a single grating normal incidence monochromator design and the use of filters. As such, it provides a basis for selecting the most appropriate approach in various fields where an easy-to-implement harmonic selection from high harmonic generation is needed.

Design Principles of Apodized Grating Couplers

To optimize the coupling efficiency of grating couplers, we extend an analytical model for the design of apodized grating couplers, taking into account the constraints on the upper and lower bounds of the scattering strength as determined by fabrication technology. We prove that our model gives the global optimum distribution of the scattering strength with the presence of such constraints. We apply our model to the standard problem of coupling from a silicon chip to a single-mode fiber, as well as more complex problems including the coupling to a vortex beam, and the design of a focusing grating. In the problem of coupling to a vortex beam, we validate our model with full-wave numerical simulations. For this problem, our design obtains efficiency that is significantly higher compared with existing designs. Our theoretical study provides a solid foundation for apodized grating coupler design in different material platforms, and our deterministic algorithm can function as a near-optimum starting point for advanced optimizations.

Alignment-tolerant broadband compact taper for low-loss coupling to a silicon-on-insulator photonic wire waveguide.

We experimentally demonstrate a broadband, fabrication-tolerant compact silicon waveguide taper (34.2 μm) in a silicon-on-insulator wire waveguide. The taper works on multimode interference along the length of the taper. A single taper design has broadband operation with coupling efficiency >70% over 700 nm that can be used in O-, C-, and L-bands. The compact taper is highly tolerant to fabrication variations; ±100 nm change in the taper and end waveguide width varies the taper transmission by <5%. The footprint of the device, i.e., the taper along with linear gratings, is ≈442 μm2, 11.5× smaller than the adiabatic taper. The taper with linear gratings provides coupling efficiency comparable to standard focusing gratings. We have also experimentally compared the translational and rotational alignment tolerance of the focusing grating with linear grating couplers.

High-density electrical and optical probes for neural readout and light focusing in deep brain tissue

To advance neuroscience in vivo experiments, it is necessary to probe a high density of neurons in neural networks with single-cell resolution and be able to simultaneously use different techniques, such as electrophysiological recordings and optogenetic intervention, while minimizing brain tissue damage. We first fabricate electrical neural probes with a high density of electrodes and small tip profile (cross section of shank: 47-μm width × 16-μm thickness). Then, with similar substrate and fabrication techniques, we separately fabricate optical neural probes. We finally indicate a fabrication method that may allow integrating the two functionalities into the same device. High-density electrical probes have been fabricated with 64 pads. Interconnections to deliver the signal are 120-nm wide, and the pads are 5 × 25 μm. Separate optical probes with similar shank dimensions with silicon dioxide and silicon nitride ridge single-mode waveguides have also been fabricated. The waveguide core cross section is 250 nm × 160 nm. Light is focused above the waveguide plane in 2.35-μm diameter spots. The actual probes present two output focusing gratings on the shank.

Polarization-independent high-speed photodetector based on a two-dimensional focusing grating

We demonstrate a reflection-enhanced high-speed photodetector, which integrated a mushroom-mesa p–i–n structure on a two-dimensional (2D) nonperiodic focusing grating. Mushroom-mesa p–i–n photodetectors exhibit a high frequency response owing to their low resistance capacity (RC) time constant. 2D nonperiodic focusing gratings not only can increase the external quantum efficiency of the device owing to their reflecting and focusing abilities, but also are not sensitive to the polarization of the incident light. The external quantum efficiency of this device is 44.71% and the measured 3 dB bandwidth is up to 32 GHz.

Precise and diffraction-limited waveguide-to-free-space focusing gratings

We present the design and characterization of waveguide grating devices that couple visible-wavelength light at λ = 674 nm from single-mode, high index-contrast dielectric waveguides to free-space beams forming micron-scale diffraction-limited spots a designed distance and angle from the grating. With a view to application in spatially-selective optical addressing, and in contrast to previous work on similar devices, deviations from the main Gaussian lobe up to 25 microns from the focus and down to the 5 × 10−6 level in relative intensity are characterized as well; we show that along one dimension the intensity of these weak sidelobes approaches the limit imposed by diffraction from the finite field extent in the grating region. Additionally, we characterize the polarization purity in the focal region, observing at the center of the focus a low impurity <3 × 10−4 in relative intensity. Our approach allows quick, intuitive design of devices with such performance, which may be applied in trapped-ion quantum information processing and generally in any systems requiring optical routing to or from objects 10 s–100 s of microns from a chip surface, but benefitting from the parallelism and density of planar-fabricated dielectric integrated optics.

3D Printed Terahertz Focusing Grating Couplers

We have designed, constructed and characterized a grating that focuses electromagnetic radiation at specific frequencies out of a dielectric waveguide. A simple theoretical model predicts the focusing behaviour of these chirped gratings, along with numerical results that support our assumptions and improved the grating geometry. The leaky waveguide was 3D printed and characterized at 120 GHz demonstrating its potential for manipulating terahertz waves.

Combined Focusing and Flat-Field Spectrometer System for Extreme Ultraviolet Pulse

For reducing both extreme ultraviolet attosecond pulses energy loss in the focusing reflection process and measurement error caused by pulse focusing aberration, as well as improving the operability of pulse spectroscopy monitoring, a combined focusing and flat-field spectrometer analysis system for attosecond pulse is proposed and designed through step-by-step performance optimization. The focusing and spectrum-analyzing components are gold- coated grazing incidence to roidal mirror and grazing incidence concave focusing grating, respectively. The characteristic parameters of the system are given in details. The system proposed can find application in research platform of attosecond spectroscopy using high energy short attosecond pulse as basic probe tool.

Optical zooming based on focusing grating in direct drive ICF.

In direct drive ICF, optical zooming is an effective way to mitigate cross-beam energy transfer and increase the hydrodynamic efficiency, by reducing the spot size of the laser beams while target compressing. In this paper, a novel optical zooming scheme is proposed, which employs a focusing grating to focus the broadband laser pulse, changing the spot size on the target within single beamlet. Experimentally, a focusing grating with clean aperture of 40-mm × 40-mm placed after the collimated light successfully realized the peak-valley of defocusing wavefront distribution of 0.73 µm as the wavelength ranging from 1052.43 nm to 1053.23 nm. Extended to the full-sized focusing grating with laser beam of 360-mm × 360-mm, it is derived that the focal spot reduction reaches to 21.8% with the 3rd harmonic light ranging from 350.81 nm to 351.08 nm, decreasing from 375 µm to 294 µm with 300 µm shaping continuous phase plate.

Total internal reflection orders in transmission grating

In a current high-power laser system, transmission grating has been used to sample, separate and focus laser beam. Under high power laser irradiation, besides the 0-order shooting beam and target sampling beam, the detrimental influence of other diffraction orders should be taken into consideration seriously. These useless orders may damage other nearby optical elements and mechanical devices, disturb and confuse the measurement of time pulse and near/far-field intensity. Especially the total internal diffraction order will lead to some series diffraction patterns, causing the above problems. First, relevant theoretical calculation and analysis are carried out for transmission grating (including beam sample grating and focusing grating), which can predict and indicate these inconspicuous diffraction orders. These orders appear on four receiving screens regularly and periodically, and the periodic distance between them is determined by ray-tracing draft. Second, the phenomenon of total internal reflection order is observed and measured by combining with anti-reflection film. The measured periodic spacings on three screens are 24 mm, 26 mm and 35 mm, respectively. Moreover, energy intensities of these redundant orders are measured finely, which shows that their contrasts or SNRs to 0-order main laser is in a range of 10-8-10-4). Finally, some appropriate and effective approaches to eliminating or avoiding total internal reflection and other useless orders are proposed and discussed, which include 1) coating by anti-reflection film with pre-deep etching; 2) optimizing the grating design to make redundant orders far from target spot; 3) placing laser scattering or absorbing devices at corresponding position to avoid being damaged by the side-leakage energy and ghost image of total internal reflection and other redundant orders.

Visible wavelength silicon nitride focusing grating coupler with AlCu/TiN reflector

High-performance silicon nitride focusing grating couplers with AlCu/TiN reflectors for a visible wavelength (660 nm) have been designed and fabricated in a standard complementary metal-oxide-semiconductor pilot line. The influence of the bottom oxide cladding thickness on the grating decay length and efficiency is theoretically and experimentally investigated. It is shown how the metal reflector not only increases the efficiency but also allows reduction of the radiated beam size. Coupling efficiencies above 59% have been measured for compact focusing gratings.

Broadband focusing grating couplers for suspended-membrane waveguides

We propose and demonstrate broadband focusing grating couplers for suspended-membrane waveguides on silicon-on-insulator both in near-infrared (near-IR) and in mid-IR wavelength range. Finite-difference time-domain simulation predicts ∼100 nm 3dB bandwidth with -1.7 dB coupling efficiency for an apodized grating in near-IR. -3.5 dB maximum coupling efficiency and ∼90 nm 3dB bandwidth are realized experimentally. In mid-IR, -5.5 dB maximum coupling efficiency from a uniform focusing grating is measured at 2.75μm, and ∼500 nm 3dB bandwidth is predicted theoretically.

Numerical simulations of the focal spot generated by a set of laser beams : LMJ

In order to get the fusion of small capsules containing a deuterium-tritium nuclear fuel, the MegaJoule laser (LMJ) will focus a large number of laser beams inside a cylinder (Hohlraum) which contains the fusion capsule. In order to control this process we have to know as well as possible the electromagnetic field created by the laser beams on both Hohlraum’s apertures. This article describes a numerical tool which computes this electromagnetic field from the definition of the laser beams in front of the focusing gratings. Despite its apparent simplicity, this computation necessitates a two step plane wave decomposition and the size of the data requires the use of a parallel strategy with a large number of processors.

Amplitude modulation filtering of FM-to-AM conversion due to the focusing grating of LMJ

FM-to-AM conversion is an important issue for high power lasers such as the Laser MegaJoule. Indeed temporal intensity modulation may prevent a good ignition of the target. FM is necessary to achieve optical smoothing: phase modulation broadens the spectrum and the different generated wavelengths will create different speckle patterns thanks to dispersion systems (gratings) that spatially average the focal spot on the target. We show in this paper that dispersion and focusing systems linearly filter intensity modulations by averaging due to time delay introduced by the gratings. For the LMJ configuration this filter is equivalent to a Gaussian low-pass filter with a 7GHz 3dB-bandwidth.

The LIL facility: An experimental tool for laser-matter interaction

The MegaJoule (LMJ) under construction at CESTA, a CEA laboratory located near Bordeaux, is designed to produce 240 beams, 1.8 MJ, 500 TW of ultraviolet light for high energy density physics experiments and thermonuclear burn of DT targets. The first step of LMJ program consisted in the construction of a full scale prototype the Ligne d'Integration Laser (LIL) which was developed to demonstrate LMJ performances and qualify all its components and systems as well as maintenance procedures. Commissioned in 2002, LIL facility mainly includes a laser bay which houses a bundle of 8 beamlines similar to the LMJ ones and a target bay where are located the 1ω-3ω conversion system, the focusing gratings and the 4.4-meter diameter target chamber equipped with plasma diagnostics. This paper describes the LIL facility as a tool for laser-matter interaction pointing out the laser system.

Double-Corrugated C-Shaped Aperture for Near-Field Recording

We applied surface corrugations surrounding the C-aperture on both incident and exit interfaces of the metal film, which can generate surface plasmon (SP) waves coupling to the propagation mode and function as a focusing grating, respectively. Compared with a 60 nm square aperture, the double-corrugated C-aperture can enhance power throughput by five orders of magnitude and reduce spot area by 30%.

Diffraction-limited performance of flat-substrate reflective imaging gratings patterned by DUV photolithography

We report on the first demonstration of flat substrate imaging gratings fabricated by deep ultraviolet (DUV) photoreduction lithography, which uniquely offers sub-100-nm resolution and spatial coherence over centimeter scales. Reflective focusing gratings, designed according to holographic principle, were fabricated on 300-mm silicon wafers by immersion DUV lithography. Spatial coherence of the fabrication process is evident in measured diffraction-limited imaging function. Flat-substrate gratings, with lines of arbitrary spacing and curvature, offer both dispersion and general spatial wavefront transformation combining the function of multiple optical elements. Fabrication at the sub-100-nm resolution level allows high-line-count, low-order efficient gratings even into the deep ultraviolet region. Nanoreplication of gratings at the wafer level provides a pathway to devices of ultimate low cost.

Holographic focusing gratings on the surfaces of optical waveguides

An account is given of a simple holographic system for the formation of diffraction gratings designed to couple out radiation from a waveguide and to focus it in air. Relationships that help to avoid astigmatic aberrations in the course of grating formation are given. Experimental results are reported for a structure that focuses the radiation coupled out of a waveguide, where this radiation has a plane front, to a symmetric spot 6 μm in diameter.

Holographic antenna gratings on optical waveguide surfaces

A holographic method is proposed for the formation of diffraction-grating couplers that focus coupled-in radiation inside a waveguide. Relationships ensuring the formation of aberration-free focusing gratings are given. A prototype structure, capable of coupling He—Ne laser radiation with λ = 1.15 μm into a waveguide and focusing this radiation to form a spot of 10 μm size, is described.

Some focusing properties of chirped gratings

In the past the focusing properties of chirped grating structures have been investigated in various disciplines including optics, acousto-optics and acoustics. In this paper we present the results of a detailed theoretical and experimental study of one such amplitude grating structure, namely the chirped optical diffraction grating, although many of the conclusions are of more general validity. We derive some useful fundamental properties of such focusing gratings, and show that to first order their behaviour is similar to that of various other classical components such as focusing lenses, grating spectrometers and matched filters for chirped waveforms. However, we also describe a range of second-order differences in behaviour, for example concerning the physical location of the multiple foci, and the sidelobe structures within and perpendicular to the focal plane. For devices of large numerical aperture we demonstrate the advantages of using a non-linear grating structure — for example, for sidelobe suppression in the focal plane - and point out that surface acoustic wave (SAW) techniques can be used to generate the non-linear chirp waveforms necessary to implement such gratings in acousto-optic devices.