Diamond diffractive optics—recent progress and perspectives
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-12-03 |
| Journal | Advanced Optical Technologies |
| Authors | Marcell Kiss, Sichen Mi, Gergely Huszka, Niels Quack |
| Institutions | École Polytechnique Fédérale de Lausanne |
| Citations | 13 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This review details the rapid progress in manufacturing high-performance Diamond Diffractive Optical Elements (DOEs) using micro- and nanofabrication techniques.
- Core Value Proposition: Diamond’s exceptional material properties—including the highest thermal conductivity (2200 W/mK), extreme hardness, broad transparency (UV to Far-IR), and high refractive index (2.4)—enable DOEs for applications inaccessible to traditional materials.
- Technological Shift: Fabrication has moved from traditional machining (ruling, turning) to high-precision bulk micromachining, primarily involving lithography (E-beam or Photo) and Reactive Ion Etching (RIE) using O2 plasma.
- Precision and Scale: Microfabrication allows unprecedented dimensional control, achieving feature sizes down to 30 nm pitch and aspect ratios up to 1:13.5, enabling high-quality DOEs across the entire optical spectrum.
- Spectral Coverage: Demonstrated devices span from Far-IR (10.6 µm anti-reflection structures and half-wave plates) to Visible/UV (beam shapers, grating couplers) and Hard X-ray (gratings capable of withstanding 59,000 mJ/cm2 fluence).
- High Power Capability: Diamond DOEs exhibit superior Laser-Induced Damage Threshold (LIDT), making them critical components for high-power laser systems and free-electron lasers.
- Emerging Applications: The ability to precisely structure single-crystal diamond is accelerating developments in quantum photonics, where DOEs are used to enhance the collection efficiency of single photons emitted by color centers (e.g., Nitrogen-Vacancy centers).
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Refractive Index (1550 nm) | 2.3878 | - | Single-Crystal Diamond (SCD) |
| Thermal Conductivity | 2200 | W/mK | SCD (Highest known bulk material) |
| Transparency Window | 0.22-20 | µm | SCD (Ultraviolet to Far-Infrared) |
| Hardness | 50-110 | GPa | SCD |
| Laser Damage Threshold | 20x better | - | Compared to fused silica (in certain conditions) |
| Minimum Grating Pitch | 30 | nm | X-ray gratings (E-beam lithography) |
| Maximum Aspect Ratio | 1:13.5 | - | Mid-IR gratings (using Al resputtering) |
| High Contrast Grating Reflectance | 95.85 | % | NIR (1550 nm) HCG mirror |
| Grating Coupler Loss (1550 nm) | -6.3 | dB | Diamond-on-Insulator (DOI) substrate |
| Etch Sidewall Angle (Crystallographic) | 57° or 87° | - | Depending on crystal orientation (<100> vs <110>) |
| Etch Rate (O2 Plasma) | 90-100 | nm/min | Typical rate for NIR HCG fabrication |
| Etched Surface Roughness (Ra) | < 2 | nm | After 3D laser lithography pattern transfer |
| X-ray Fluence Withstand (8.2 keV) | 59000 | mJ/cm2 | Diamond gratings (118x better than tungsten) |
| AR Structure Transmission Increase | 71% to 97% | % | Far-IR (10.6 µm) application |
Key Methodologies
Section titled “Key Methodologies”The fabrication of diamond DOEs relies on generalized bulk micromachining, adapting semiconductor processes to diamond’s extreme hardness and chemical resistance.
- Substrate Preparation:
- High-quality SCD or Polycrystalline Diamond (PCD) substrates are used.
- Mechanical polishing is often problematic due to anisotropy; non-contact methods like Ion Beam Etching (IBE) are preferred to achieve sub-nanometer smooth surfaces (peak-to-valley < λ/100).
- Patterning (Lithography):
- E-beam Lithography (EBL): Essential for nanometer-scale features (pitch < 1 µm), using resists like Hydrogen Silsesquioxane (HSQ).
- Photolithography: Used for larger features (pitch > 1 µm), including contact, DUV, and 2-photon absorption lithography (for 3D structures).
- Template Growth: For large-scale PCD components, pulsed laser machining structures a silicon template, diamond is grown over it, and the template is etched away.
- Hardmask Application:
- Hardmasks (e.g., SiO2, Al, Ti, amorphous Si) are required because photoresists have low selectivity against diamond during aggressive etching.
- A dual-purpose metal mask can serve as both a hardmask and a conductive layer for EBL.
- Diamond Etching (Dry Process):
- Reactive Ion Etching (RIE) / Inductively Coupled Plasma (ICP): The primary method, typically using Oxygen (O2) plasma to remove carbon atoms as COx byproducts.
- Crystallographic Etching: Utilizing the anisotropic etch rate of diamond along crystal planes to naturally form V-groove (57°) or rectangular (87°) gratings when aligned to specific crystal directions (<100> or <110>).
- High Aspect Ratio Control: Techniques like periodic resputtering of the Al hardmask are used to maintain near-vertical sidewalls and achieve high aspect ratios (e.g., 1:13.5).
- Alternative Structuring:
- Ion Implantation: Boron ion implantation (40 keV) can graphitize selected areas, causing local expansion and refractive index change (na-c = 2.1-2.223), creating an optical path difference for X-ray optics.
Commercial Applications
Section titled “Commercial Applications”Diamond DOEs are positioned to disrupt several high-value markets where material limitations currently restrict performance.
- High-Power Laser Optics:
- Products: Laser windows, beam splitters, and anti-reflection (AR) structured surfaces for high-power CO2 lasers (10.6 µm) and Raman lasers.
- Advantage: Exploits diamond’s high LIDT and thermal conductivity, preventing thermal lensing and damage.
- Quantum Technology & Sensing:
- Products: Integrated circular gratings and photonic circuits designed to enhance the collection efficiency of single photons from Nitrogen-Vacancy (NV) or Silicon-Vacancy (SiV) color centers.
- Advantage: Enables scalable, high-performance quantum sensors and single-photon emitters.
- Aerospace and Astronomy:
- Products: Mid-IR achromatic half-wave plates and subwavelength gratings for coronagraphy imaging (1-13 µm).
- Advantage: High durability and broad spectral coverage.
- Short Wavelength & X-ray Systems:
- Products: Hard X-ray diffraction gratings and Fresnel zone plates for synchrotrons and free-electron lasers.
- Advantage: Extreme radiation hardness (59,000 mJ/cm2) and high precision (30 nm features).
- Compact and UV/VIS Optics:
- Products: Compact spectrometers, UV/VIS grating couplers, and Top-Hat beam shapers (e.g., for copper welding applications).
- Advantage: High refractive index allows for more compact designs; broad transparency extends into the deep UV where silicon is opaque.
- Harsh Environment Components:
- Products: Optics requiring chemical resistance (e.g., exit windows submerged in corrosive fluids) or abrasion resistance (e.g., gratings in dusty environments).
- Advantage: Diamond’s chemical inertness and extreme hardness ensure longevity and cleanability.
View Original Abstract
Abstract Diamond is an exceptional material that has recently seen a remarkable increase in interest in academic research and engineering since high-quality substrates became commercially available and affordable. Exploiting the high refractive index, hardness, laser-induced damage threshold, thermal conductivity and chemical resistance, an abundance of applications incorporating ever higher-performance diamond devices has seen steady growth. Among these, diffractive optical elements stand out—with progress in fabrication technologies, micro- and nanofabrication techniques have enabled the creation of gratings and diffractive optical elements with outstanding properties. Research activities in this field have further been spurred by the unique property of diamond to be able to host optically active atom scale defects in the crystal lattice. Such color centers allow generation and manipulation of individual photons, which has contributed to accelerated developments in engineering of novel quantum applications in diamond, with diffractive optical elements amidst critical components for larger-scale systems. This review collects recent examples of diffractive optical devices in diamond, and highlights the advances in manufacturing of such devices using micro- and nanofabrication techniques, in contrast to more traditional methods, and avenues to explore diamond diffractive optical elements for emerging and future applications are put in perspective.