Abstract
Diffraction gratings are core optical components featuring wavelength splitting, phase modulation and wavefront manipulation capabilities. They have been widely developed and applied across spin-wave photonics, extreme ultraviolet (EUV)/soft X-ray spectroscopy, neutron optics, silicon photonic communication devices, and large-aperture high-precision optical systems. This review summarizes five recent research studies on functional diffraction gratings, focusing on their structural designs, fabrication techniques, working mechanisms, key performance indicators and application scenarios. The current technical achievements, existing limitations and future development directions of gratings in different fields are systematically analyzed, aiming to provide a comprehensive reference for follow-up research on grating-based optical devices.
1. Introduction
As fundamental diffractive optical elements, diffraction gratings realize spectral separation, beam focusing, polarization management and wavefront control via periodic micro-nano structures. With the advancement of microfabrication, photonic integration and cross-disciplinary optical technologies, traditional gratings have evolved into diversified functional devices, breaking the application boundaries of classical optics. The collected five studies cover spin-wave concave gratings, EUV/soft X-ray off-plane spectrometric gratings, holographic polymer composite neutron gratings, silicon photonic polarization-independent demultiplexing gratings, and meter-level large-aperture holographic gratings. These works target different technical demands including on-chip magnonics, high-energy ray spectroscopy, neutron interferometry, optical communication and high-power laser systems. This review sorts out the core innovations, technical parameters and practical values of each research, and discusses the common challenges and development trends of grating technology.
2. Overview of Researches by Field
2.1 Concave Gratings for Spin-Wave Optics (Document 1)
This work experimentally demonstrated a Rowland-type concave grating fabricated on yttrium iron garnet (YIG) thin films for spin-wave manipulation, which is designed to serve as a microwave spectrometer for spin-wave signal analysis.
Core Design & Fabrication
The grating pattern was patterned by focused ion beam (FIB) irradiation instead of material removal. The high-dose Ga⁺ ion bombardment locally eliminated the ferrimagnetic properties of YIG, forming periodic modulation for spin-wave diffraction. The device adopted the classic Rowland arrangement, which integrates diffraction and focusing functions without additional lens components, adapting to the short propagation length of spin waves.
Working Mechanism
An unexpected nonlinear indirect spin-wave excitation mechanism was discovered: instead of direct excitation by coplanar waveguide (CPW) magnetic fields, spin waves were mainly generated by the dipole fields of high-amplitude long-wavelength standing waves formed behind the grating. This indirect excitation greatly improves the excitation efficiency of short-wavelength spin waves at a distance from the waveguide.
Performance & Limitations
· The diffraction patterns measured by time-resolved Magneto Optical Kerr Effect (trMOKE) imaging are consistent with theoretical calculations and micromagnetic simulations (MuMax3). Diffraction focal points basically follow the Rowland circle rule.
· Key limitation: The nonlinear resonance mechanism only works for single-frequency signals and causes spectral pollution for multi-frequency inputs, so the current device cannot realize multi-frequency spectral decomposition.
Application Prospect
This research paves the way for on-chip spin-wave optical devices and post-von Neumann wave-based computing. By adopting linear excitation modes, the Rowland concave grating is expected to be developed into a practical on-chip microwave spectrometer.
2.2 High-Efficiency Aberration-Free Gratings for EUV & Soft X-Ray Spectroscopy (Document 2)
Targeting the low throughput and pulse distortion problems of traditional EUV/soft X-ray spectrometers, this study proposed a dual-optic off-plane grating spectrometer/monochromator based on conical diffraction geometry.
Core Innovation
Combining a toroidal mirror and a single uniform blazed grating, the system realizes spatial aberration compensation by simply adjusting the azimuthal angle of the toroidal mirror. The aberration generated by grating diffraction is counteracted by the mirror, eliminating the need for complex variable-line-spacing gratings or multi-stage optical structures.
Key Performance
· Throughput efficiency: Theoretical efficiency exceeds 60%, and the experimental average efficiency is over 40%, far higher than traditional EUV spectral devices.
· Spectral resolution: λ/Δλ>200 across the working band.
· Temporal dispersion: The pulse broadening is limited to 80 fs (tail-to-tail) and 29 fs (FWHM), which meets the requirements of femtosecond ultrafast spectroscopy and pump-probe experiments.
· Polarization retention: The device maintains circular and elliptical polarization states for 1st, 2nd and 3rd diffraction orders.
Application Scenarios
It is applicable to high-order harmonic generation (HHG) light source detection, nanoimaging, material ultrafast dynamic analysis, and can be flexibly switched between spectrometer and monochromator modes for broad EUV-soft X-ray band use.
2.3 Holographic Hyperbranched Polymer Composite Gratings for Neutron Optics (Document 3)
This research developed a holographic nanoparticle-polymer composite (NPC) grating using hyperbranched polymer (HBP) as the functional medium, and verified its excellent neutron diffraction performance for cold and very cold neutron (VCN) optical systems.
Material & Fabrication
The grating was fabricated via two-beam holographic recording. HBP nanoparticles with ultra-high refractive index (nNP=1.82) were dispersed in the polymer host, forming periodic structures with large refractive index modulation. The grating period is 500 nm, and the physical thickness is about 8.4μm.
Neutron Optical Performance
· Under a neutron wavelength of 2 nm, the grating achieves an extremely high neutron scattering length density (SLD) modulation amplitude: the first-order modulation amplitude reaches 14.9 μm−2, which is about 8 times higher than existing nanodiamond-based neutron gratings.
· Clear 0th, ±1st, ±2nd diffraction spots were observed in SANS-I neutron diffraction tests, and the multi-wave coupling analysis confirms excellent diffraction characteristics.
Challenges & Outlook
Secondary scattering caused by high modulation will degrade the interference fringe during holographic recording when the grating thickness increases. Optimizing material composition and recording conditions to increase grating thickness while maintaining structural integrity is the main follow-up direction. The grating is a promising candidate for next-generation neutron interferometers and slow neutron optical components.
2.4 Polarization-Independent Wavelength Demultiplexing Gratings for Silicon Photonics (Document 4)
A single etched diffraction grating (EDG) on silicon nitride (Si3N4) platform was proposed for wavelength division multiplexing (WDM) optical communication, solving the polarization dependence problem of traditional silicon photonic demultiplexers.
Working Principle
The design ingeniously utilizes the waveguide birefringence effect: TE and TM polarized lights have different effective refractive indices in the slab waveguide, leading to different diffraction angles on the Rowland circle. Thus, wavelength demultiplexing and polarization splitting are realized simultaneously on a single grating device.
Device Parameters & Performance
· The device footprint is only 320×230 μm2, supporting four-channel CWDM signal demultiplexing (output 8 single-polarization channels).
· Insertion loss: 0.5–2.4 dB; Polarization-dependent loss (PDL): 0.5–1.8 dB; Crosstalk: lower than−30 dB.
· Tests with arbitrary polarization (linear, elliptical, circular) incident light prove stable output, fully realizing polarization-independent operation.
Technical Value
Different from traditional polarization diversity schemes requiring cascaded multiple devices, this single EDG simplifies the system structure. It provides a universal solution for polarization-insensitive devices on birefringent silicon photonic platforms and can be monolithically integrated with photodiodes for high-density optical transceivers.
2.5 Large-Aperture High-Precision Holographic Gratings (Document 5)
A set of scanning interference field exposure systems was developed to fabricate meter-level large-aperture holographic gratings, focusing on the control of grating groove errors and wavefront aberration.
Key Technical Breakthroughs
Three core technical problems were solved:
1. Integrated displacement measurement: Combining grating sensing and laser interferometry, the stage positioning repeatability reaches ±6 nm under long-range movement (over 1 m), suppressing environmental interference.
2. Interference fringe precision control: The fringe direction error is optimized to 0.677 μrad, and the period error RMS is 0.42616 ppm.
3. Real-time phase compensation: A dynamic phase-locking model based on heterodyne interferometry compensates stage-induced phase errors, with static/dynamic fringe phase 3σ values below 0.355 rad.
Fabrication Results
A large-aperture grating of 1500 mm × 420 mm was successfully fabricated:
· Wavefront aberration: 0.327λ @ 632.8 nm;
· Wavefront gradient: 16.444 nm/cm;
· The grating groove consistency is excellent, avoiding splicing seams of traditional segmented gratings.
Application Fields
This large-aperture high-precision grating is a key component for chirped pulse amplification (CPA) systems, high-energy lasers, astronomical spectrometers and ultra-precision displacement measurement equipment.
3. Comprehensive Analysis & Discussion
3.1 Common Technical Characteristics
1. Universal adoption of Rowland/concave grating configuration: Multiple studies (spin-wave grating, communication EDG grating) adopt the Rowland circle structure, which integrates diffraction and focusing, omitting auxiliary lenses and adapting to miniaturization and on-chip integration demands.
2. Diversified microfabrication technologies: FIB patterning, holographic exposure, dry etching, scanning interference lithography are applied for different material systems (YIG, polymer composite, silicon nitride, photoresist), forming targeted processing schemes.
3. Performance optimization based on optical aberration/modulation control: Whether for EUV gratings (aberration compensation), neutron gratings (SLD modulation enhancement) or large-aperture gratings (groove error suppression), the core optimization direction is to improve diffraction quality and energy utilization.
3.2 Existing Common Challenges
1. Working condition constraints: Some functional gratings (e.g., spin-wave concave gratings) are limited by excitation mechanisms and can only work under single-frequency input, restricting practical application scope.
2. Material and processing trade-offs: For polymer neutron gratings, high modulation is accompanied by secondary scattering; for silicon nitride communication gratings, balancing waveguide birefringence and reflection bandwidth requires strict thickness optimization.
3. Large-aperture device difficulties: Meter-level gratings face long-term exposure, environmental interference and splicing errors, which raise the threshold of fabrication equipment and technology.
4. Bandwidth and polarization balance: In EUV, optical communication and other fields, realizing high efficiency, high resolution and full polarization adaptation simultaneously remains a difficulty.
3.3 Development Trends
1. Integration and miniaturization: Gratings will be further combined with on-chip photonic, magnonic and microwave devices to develop fully integrated miniature systems for computing and communication.
2. Broadband and multi-functional integration: Realizing multi-frequency, multi-band and multi-polarization compatible grating devices to break the limitation of single working state.
3. Ultra-large aperture & ultra-high precision: With the demand of high-energy lasers and large scientific instruments, seamless meter-level gratings with nanoscale precision will become an important development direction.
4. Cross-medium expansion: Extending grating technology from traditional optics to spin waves, neutrons and other wave systems to expand the application boundary of diffractive optics.
4. Conclusion
Diffraction gratings, as versatile diffractive elements, have achieved remarkable progress in spin-wave photonics, EUV/soft X-ray spectroscopy, neutron optics, silicon-based optical communication and large laser systems. The five representative studies cover different material systems, fabrication methods and working mechanisms, and respectively solve the pain points of low efficiency, polarization dependence, limited aperture and poor spectral resolution in corresponding fields.
At present, grating technology still faces challenges such as working condition limitations, processing trade-offs and environmental interference resistance. In the future, with the continuous innovation of micro-nano fabrication, optical field control and material design, functional diffraction gratings will move towards higher precision, larger aperture, multi-function and on-chip integration. It will continue to play a core role in next-generation computing, ultrafast spectroscopy, high-energy laser, quantum measurement and optical communication technologies.
