99% efficiency measured in the -1st order of a resonant grating

Internal mechanism of perfect-reflector-backed dielectric gratings to achieve 100% diffraction efficiency

The work started 20 years ago [Appl. Opt.42, 6255 (2003)APOPAI0003-693510.1364/AO.42.006255] investigating the physical mechanism of multilayer dielectric reflection gratings to achieve 100% diffraction efficiency is completed to offer much deeper insight than before. How different scattering matrix elements of the top periodic surface corrugation contribute to the -1st-order efficiency of such a compound grating is unveiled analytically using a minimum set of real parameters. The two diffraction amplitudes transmitted through the top corrugation play a dominant role in enabling 100% diffraction efficiency. Simple necessary and sufficient conditions for 100% efficiency are derived. Moreover, the role of the reflection phase of the perfect reflector, including the contribution of the optical path between the top corrugation and the reflector, is emphasized.

The Development Progress of Surface Structure Diffraction Gratings: From Manufacturing Technology to Spectroscopic Applications

The high-precision diffraction grating is an important chromatic dispersion component that has been widely used in many fields, including laser beam combining, chirped pulse compression, spectroscopy, among others. In this paper, we review the development status of reflection and transmission gratings with high diffraction efficiency and high laser-induced damage thresholds, such as metal-film and multilayer-dielectric-film gratings. Then, we review the basic principles and most recent stages in the development of manufacturing techniques, such as mechanical scribing, holographic exposure, electron-beam lithography, and nanoimprinting.

Polarization-independent multi-channel retroreflective metasurfaces based on extraordinary optical diffraction

Retroreflection can be achieved by phase gradient imparted by super-cells of metasurfaces. Nevertheless, in most cases, retroreflection can only be achieved for one specific polarization. In this paper, we propose an alternative design strategy and reveal that a polarization-independent multi-channel metasurface based on extraordinary optical diffraction (EOD) can achieve high-efficient retroreflection. A unary unit cell, instead of binary unit cells, is employed to canalize impinging EM waves along targeted diffraction channels. Under oblique incidence, only the -1st diffraction order is maintained and the 0th order and others are suppressed through structural design while the reflection is unaffected under normal incidence. In this way, we can achieve retroreflection in three channels. A proof-of-principle prototype was designed, fabricated and measured to verify this design strategy. The prototype can operate at 20.0 GHz under the incident angle of ±48.6° and 0° with the efficiency of retroreflection about 90%. Both the simulated and measured results show an excellent performance of retroreflection along the three channels, regardless of the polarization state of incident waves. This method offers a fast implementation for retrodirective characteristics with facile planar fabrication and can also be easily extended to THz or optical regimes.

Near-infrared broadband Si:H/SiO2 multilayer gratings with high tolerance to fabrication errors

Si:H and TiO₂ multilayer dielectric gratings (MDGs) were studied comparatively to highlight the influence of refractive indices on fabrication tolerances, including tolerances for grating width errors and cross-sectional shape errors. The fabrication tolerance of Si:H MDGs is two to four times greater than that of TiO₂ MDGs, because the higher refractive index of Si:H has a stronger ability to restrain electric fields. It was further revealed in these studies that a Si:H MDG with positive trapezoidal errors has minor influence on high diffraction efficiency bandwidth. Finally, a Si:H MDG was prepared without iterative corrections. Although large fabrication errors of grating width and cross-sectional shape existed, the MDG still had a 146 nm bandwidth with diffraction efficiency over 97%, which verifies the robustness of the proposed Si:H MDG.

Broadly tunable laser based on novel metallic resonant leaky-mode diffraction grating

We present a fibre laser broadly tunable in a wavelength range from 1058 nm to 1640 nm based on a new type of metallic resonant leaky-mode diffraction grating and three fibre-pigtailed semiconductor optical amplifiers. For TM polarization in Littrow configuration, the grating has experimentally measured diffraction efficiency into the -1^st reflected order of more than 90 % over a spectral range of 1417 nm to 1700 nm. The laser covered a spectral range of 331 nm within a tuning band of 558 nm without any adjustment of optics and its tuning range was limited by amplification bands of available semiconductor optical amplifiers.

Resonant Grating without a Planar Waveguide Layer as a Refractive Index Sensor

Dielectric grating-based sensors are usually based on the guided mode resonance (GMR) obtained using a thin planar waveguide layer (PWL) adjacent to a thin subwavelength grating layer. In this work, we present a detailed investigation of thick subwavelength dielectric grating structures that exhibit reflection resonances above a certain thickness without the need for the waveguide layer, showing great potential for applications in biosensing and tunable filtering. Analytic and numerical results are thoroughly discussed, as well as an experimental demonstration of the structure as a chemical sensor in the SWIR (short wave infrared) spectral range (1200-1800 nm). In comparison to the GMR structure with PWL, the thick grating structure has several unique properties: (i) It gives higher sensitivity when the spaces are filled, with the analyte peaking at certain space values due to an increase in the interaction volume between the analyte and the evanescent optical field between the grating lines; (ii) the TM (transverse magnetic) resonance, in certain cases, provides a better figure of merit; (iii) the sensitivity increases as the grating height increases; (iii) the prediction of the resonance locations based on the effective medium approximation does not give satisfactory results when the grating height is larger than a certain value, and the invalidity becomes more severe as the period increases; (iv) a sudden increase in the Q-factor of the resonance occurs at a specific height value accompanied by the high local field enhancement (~10³) characteristic of a nano-antenna type pattern. Rigorous numerical simulations of the field distribution are presented to explain the different observed phenomena.

Extreme Asymmetry in Metasurfaces via Evanescent Fields Engineering: Angular-Asymmetric Absorption

On the quest towards full control over wave propagation, the development of compact devices that allow asymmetric response is a challenge. In this Letter, we introduce a new paradigm for the engineering of asymmetry in planar structures, revealing and exploiting unilateral excitation of evanescent waves. We test the idea with the design and experimental characterization of a metasurface for angular-asymmetric absorption. The results show that the contrast ratio of absorption (the asymmetry level) can be arbitrarily engineered from zero to infinity for waves coming from two oppositely tilted angles. We demonstrate that the revealed asymmetry effects cannot be realized using conventional diffraction gratings, reflectarrays, and phase-gradient metasurfaces. This Letter opens up promising possibilities for wave manipulation via evanescent waves engineering with applications in one-side detection and sensing, angle-encoded steganography, flat nonlinear devices, and shaping the scattering patterns of various objects.

Design of dense transmission diffraction gratings for high efficiency

We propose a design method for dense surface-relief diffraction gratings with high efficiency in transmission mode. Closed-form analytical relations between diffraction efficiency, polarization, and grating parameters are derived and verified in the resonance domain of diffraction under general three-dimensional angles of incidence traditionally termed conical mounting. A powerful tool for rigorous design of computer-generated holograms and diffractive optical elements with spectroscopic scale periods is now enabled.

Design of high-efficiency ultrabroadband dielectric gratings

We present a design concept of dielectric gratings containing resonant high (TiO₂) and resonant low (SiO₂) index dielectric thin-film layers between the grating and the underlying multilayer reflector. We use numerical simulations and the genetic algorithm optimization method to achieve high diffraction efficiency (>97%) in the first diffracted order over a wide wavelength range (∼160  nm) at around 800 nm. The basic concept of the structural optimization contains a high refractive index binary grating with alternating low- and high-index reflector layers, the thicknesses of which are also among the optimization parameters. We introduce two resonant dielectric layers directly below the corrugated TiO₂ grating structure and we choose a small (<10%) filling fraction for the grating. This concept allows us to broaden the spectral range where high DE can be achieved. We investigate here gratings for both Littrow-mount and off-Littrow applications.

Subwavelength diffractive color beam combiner

A high-efficiency subwavelength diffractive beam combiner operating in a visible spectral range is designed, fabricated, and demonstrated. Such a device combines red, green, and blue color beams into one output light beam. Diffraction efficiencies of different types of gratings are calculated for various materials, incidence angles, and polarizations of light. It is shown that the plasmon resonance via a grating coupling occurs at the determined conditions. Subwavelength gratings with a period of 400 nm are fabricated and tested using laser and laser diode sources.

Broadband pulse compression gratings with measured 99.7% diffraction efficiency

We present experimental investigations of grating mirrors with high diffraction efficiencies exceeding 99.7% in the -1st order for TE polarization at a wavelength of 1060 nm, and exceeding a diffraction efficiency of 99% in the wavelength range from 1025 nm to at least 1070 nm. The total efficiency of a four-pass compressor for chirped pulse amplification was >96%. The design, fabrication, and characterization of the fully dielectric grating mirrors are presented.

Thermal behavior of resonant waveguide-grating mirrors in Yb:YAG thin-disk lasers

We present the experimental investigations of different designs of resonant waveguide-grating (RWG) mirrors, used as intracavity folding mirrors in an Yb:YAG thin-disk laser (TDL). The investigation was focused on the rise of the surface temperature due to the coupling of the incident radiation to a waveguide mode as well as on laser efficiency, polarization, and wavelength selectivity. It was found that the damage threshold and efficiency can be increased significantly with a proper design of the structure in comparison to the simplest design with a single waveguide layer. So far, the presented RWG allow the generation of linear polarization with a narrow spectral linewidth down to 25 pm FWHM in a fundamental mode Yb:YAG TDL. Damage thresholds of 60 kW/cm(2) have been reached where only 63 K of surface temperature increase was observed. This showed that the improved mirrors are suitable for the generation of kW-class narrow linewidth, linearly polarized Yb:YAG TDL.

Resonance domain surface relief diffractive lens for the visible spectral region

Early expectations for a role of diffractive lenses were dramatically lessened by their high order overlapping foci, low optical powers, and competing advances in refractive micro-optics. By bringing the Bragg properties of volume holograms to diffractive lenses we got rid of ghost diffractive orders and the critical trade-off between diffraction efficiency, number of phase levels, and spatial feature-size. Binary off-axis resonance domain diffractive lens with high numerical aperture of 0.16 was designed with analytical effective grating theory, fabricated by direct e-beam writing, etched in fused silica and experimentally investigated. More than 81% measured diffraction efficiency exceeds twice the limits of thin binary optics.

Linearly polarized, narrow-linewidth, and tunable Yb:YAG thin-disk laser

We report on the design, fabrication, and implementation of grating-waveguide structures (GWS) for intracavity polarization and wavelength selection as well as wavelength tuning. The GWS discussed in this Letter is a combination of a low-index leaky-mode waveguide, a subwavelength diffraction grating, and a highly reflective mirror that was designed to operate in Littrow configuration. Using our device as the end mirror of an Yb:YAG thin-disk laser allowed the extraction of beams that exhibit an extremely narrow laser emission bandwidth of ≈25 pm FWHM and a high degree of linear polarization of 99±0.5%. Moreover, the GWS allows a wavelength tuning range from 1007 to 1053 nm. The high-power suitability and the low loss of the GWS was demonstrated by the intracavity use in an Yb:YAG thin-disk laser with an output power of 325 W in multimode operation (M(2)=6) and with 110 W in fundamental-mode operation (M(2)≈1.2), exhibiting optical efficiencies of 53.2% and 36.2%, respectively. An output power of 1.8 kW, corresponding to a power density of 125 kW/cm(2) on the grating, was achieved in further higher-power experiments.

Diffractive properties of imaginary-part photonic crystal slab

The diffraction spectra of Imaginary-Part Photonic Crystal (IPPC) slabs are analyzed by using Scattering-Matrix Method. By investigating the thickness dependence of the diffraction, we find the remarkable red shift of central wavelength of diffraction spectrum, which obviously distinguishes from the phenomenon of spectral hole. We observe that diffraction efficiency can be enhanced more than twentyfold by optimizing the geometry parameters. These imply that the diffraction spectra of the IPPC slab can be controlled at will and used to guide the design to achieve useful nanoscale devices.

Whispering gallery modes and other cavity modes for perfect backscattering and blazing

We demonstrate the possibility to obtain perfect blazing both in Littrow and off-Littrow mountings using diffractive systems consisting of a plane metallic substrate and dielectric structures that can support cavity modes. The resonances are located at a relatively large distance between the metal and the dielectric structure, a condition that prevents the resonance increase of absorption. The high efficiency can be obtained in transverse electric or transverse magnetic polarization and at high incident angles. When cylindrical rods with circular cross-sections are used, the so-called whispering gallery modes can be used to provide the resonances, necessary for the blazing.

Polarization insensitive resonance-domain blazed binary gratings

Three variants of binary blazed gratings with subwavelength features are considered, which have high first-order efficiencies in the non-paraxial domain for arbitrarily polarized light. A combination of effective medium theory and further parametric optimization with the Fourier modal method are used in design. Experimental demonstration is provided by electron beam lithography on a structure etched in a Si3N4 layer on top of a SiO2 substrate, with period approximately 3.5lambda at lambda = 633 nm. The measured efficiency (81% for TE and 85% for TM polarization) agrees well with the calculated value, 84%.

Design and analysis of broadband high-efficiency pulse compression gratings

We report on two effective methods, multiparameter optimization and local optimization combined with the diffraction bandwidth merit function, to design a broadband pulse compression grating (PCG), and we present broadband, high-efficiency PCGs based on both the multilayer dielectric grating (MDG) and metal-multilayer dielectric grating (MMDG) models. For MDG, the average diffraction efficiency is higher than 97.5% for TE polarization light over the 100 nm bandwidth centered at 800 nm. Moreover, a novel multilayer structure, which comprises higher index material in the high-reflectivity mirror and relatively lower index material on top, is first proposed to yield higher average efficiency, broader bandwidth, and excellent fabrication tolerance. For MMDG, it exhibits an ultrabroadband top-hat diffraction spectrum with average efficiency exceeding 97% over the 200 nm wavelength wide centered at 1053 nm. In addition, the MMDG structure, which has the best tolerance for grating fabrication, is determined by investigating characteristics of MMDGs with different thin-film structures.

Optimization design of an ultrabroadband, high-efficiency, all-dielectric grating

More than 97% flat-top diffraction efficiency in the -1st-order TE polarization over a 110 nm wavelength range around 800 nm in an all-dielectric grating is designed by a simulated annealing algorithm and the Fourier mode method. Its band is near to the maximum bandwidth provided by a dielectric high-reflectivity mirror under the match layer. This result will provide a way for high-efficiency chirped-pulse amplification to be used in an ultrashort high-power pulse laser system based on all-dielectric gratings. Furthermore, an effective method for broadband high-efficiency all-dielectric grating design is presented in this Letter.

High-repetition-rate picosecond pump laser based on a Yb:YAG disk amplifier for optical parametric amplification

We report an optically synchronized picosecond pump laser for optical parametric amplifiers based on an Yb:YAG thin-disk amplifier. At 3 kHz repetition rate, pulse energies of 25 mJ with 1.6 ps pulse duration were achieved with an rms fluctuation in pulse energy of <0.7% by utilizing a broadly intermittent single-energy regime in the deterministic chaos of a continuously pumped regenerative amplifier.

Optical characterization of ultrahigh diffraction efficiency gratings

We report on the optical characterization of an ultrahigh diffraction efficiency grating in a first-order Littrow configuration. The apparatus used was an optical cavity built from the grating under investigation and an additional high-reflection mirror. The measurement of the cavity finesse provided precise information about the grating's diffraction efficiency and its optical loss. We measured a finesse of 1580 from which we deduced a diffraction efficiency of (99.635+/-0.016)% and an overall optical loss due to scattering and absorption of just 0.185%. Such high-quality gratings, including the tool used for their characterization, might apply for future gravitational wave detectors. For example, the demonstrated cavity itself presents an all-reflective, low-loss Fabry-Perot resonator that might replace conventional arm cavities in advanced high-power Michelson interferometers.

Light coupling into an optical microcantilever by an embedded diffraction grating

By measuring the excitation efficiency of an optical waveguide on a diffraction grating one can accurately register the changes in the incidence angle of the exciting light beam. This phenomenon was applied to detect ultrasmall deflections of silicon dioxide cantilevers of submicrometer thickness that were fabricated with corrugation on top to act as diffraction grating couplers. The power of light coupled into the cantilevers was monitored with a conventional photodetector and modulated using mechanical vibration of the cantilever, thus changing the spatial orientation of the coupler with respect to the incident light beam. The technique can be considered as an alternative to the methods known for detection of cantilever deflection.