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6CCVD Research

A Metalens with a Near-Unity Numerical Aperture

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2018-02-27
JournalNano Letters
AuthorsRamón Paniagua‐Domínguez, Yefeng Yu, Egor Khaidarov, Sumin Choi, Victor Leong
InstitutionsNanyang Technological University, Data Storage Institute
Citations486
AnalysisFull AI Review Included

Technical Documentation & Analysis: Near-Unity Numerical Aperture Metalens

Section titled “Technical Documentation & Analysis: Near-Unity Numerical Aperture Metalens”

This document analyzes the research demonstrating a near-unity numerical aperture (NA > 0.99) flat metalens, focusing on its implications for quantum optics and high-resolution imaging, and connecting the material requirements to 6CCVD’s advanced MPCVD diamond capabilities.

Executive Summary

Section titled “Executive Summary”
  • Record-Breaking NA: The research successfully demonstrated a diffraction-limited flat metalens achieving a near-unity Numerical Aperture (NA > 0.99) at 715 nm, corresponding to a maximum collection angle of 82°.
  • Sub-Wavelength Thickness: The metalens utilizes an amorphous silicon (a-Si) film with a sub-wavelength thickness of approximately 250 nm (~λ/3), enabling ultra-compact optical systems.
  • Novel Design Methodology: The high NA and efficiency are achieved using asymmetric dielectric nanoantennas (dimers, trimers, quadrumers) to engineer diffracted energy redistribution, bypassing the efficiency limitations of traditional phase mapping approaches.
  • Quantum Application Validation: The metalens was successfully employed in a confocal configuration to map photoluminescence (PL) from Nitrogen-Vacancy (NV) color centers embedded in sub-diffractive nanodiamonds (NDs).
  • Enhanced Collection Efficiency: The near-unity NA translates to a truly significant increase in the collection solid angle, which is critical for boosting the efficiency of low-light quantum optics experiments involving isotropic photon emission.
  • Fabrication Relevance: The structures were fabricated using high-resolution techniques (EBL, RIE) on quartz, demonstrating a pathway for integrating high-index dielectric metasurfaces onto high-quality substrates.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Numerical Aperture (NA)>0.99DimensionlessExperimental maximum achieved by the flat lens.
Maximum Bending Angle (Ξ)82°Corresponds to the NA > 0.99 design.
Operating Wavelength (λ)715nmUsed for focusing and NV center PL collection.
Metalens MaterialAmorphous Silicon (a-Si)N/A250 nm thick film deposited on fused silica.
Lens Diameter (d)600”mTotal size of the fabricated metalens.
Focal Distance (f)42 - 47”mMeasured focal spot distance.
Measured FWHM (Focal Spot)385nmMeasured resolution (limited by NAobj=0.95).
Diffraction Efficiency (T+1)35 (p-pol), 32 (s-pol)%Measured efficiency into the desired order at 715 nm.
Focused Power Efficiency~10%Percentage of incident power focused by the lens.
Nanoantenna Height (H)250nmHeight of the Si nanodisks.
Dimer Diameters (D₁, D₂)150, 190nmUsed for large-angle bending (82° to 55°).
Diffractive Period (Pd)721nmUsed for 82° bending array.

Key Methodologies

Section titled “Key Methodologies”

The high-NA metalens was realized through precise nanofabrication of high-contrast dielectric structures.

  1. Substrate and Film Preparation:
    • Fused silica (quartz) substrates were used for optical transparency.
    • Amorphous silicon (a-Si) films of 250 nm thickness were deposited via Plasma Enhanced Chemical Vapor Deposition (PECVD).
  2. Patterning (Electron Beam Lithography):
    • Single-step Electron Beam Lithography (EBL) was performed using Hydrogen silsesquioxane (HSQ) resist to define the complex nanoantenna patterns.
  3. Etching (Reactive Ion Etching):
    • The patterns were transferred into the a-Si film using Reactive Ion Etching (RIE) in an Inductively Coupled Plasma system with chlorine gas chemistry.
  4. Nanoantenna Design:
    • The lens was discretized into Fresnel zones, requiring varying bending angles (82° down to 31°).
    • Dimer, Trimer, and Quadrumer asymmetric nanoantenna unit cells were designed, with cylinder diameters ranging from 110 nm to 190 nm and gaps (g) of 50 nm or 60 nm, to achieve the required angle-dependent light bending.
  5. Characterization:
    • Diffraction efficiencies were measured using both inverted microscopy (for low angles) and a custom free-space microscopy setup (for large angles).
    • Imaging capability was validated using a scanning confocal microscope setup to map photoluminescence (PL) from NV centers in nanodiamonds (NDs).

6CCVD Solutions & Capabilities

Section titled “6CCVD Solutions & Capabilities”

This research highlights the critical need for high-quality, high-index dielectric materials and precision fabrication for next-generation flat optics, particularly in quantum applications. 6CCVD is uniquely positioned to supply the foundational materials and integration services required to replicate and advance this work using superior diamond platforms.

Research Requirement/Challenge6CCVD Solution & CapabilityCore Value Proposition
Quantum Emitter Host MaterialOptical Grade Single Crystal Diamond (SCD): Provides the lowest background fluorescence and highest purity host for stable, high-coherence NV centers, essential for quantum sensing and computing applications.Superior Quantum Platform: Enables integrated NV creation directly within the substrate, eliminating the need for nanodiamond drop-casting and improving emitter stability and collection efficiency.
High Refractive Index MetasurfacesOptical Grade Polycrystalline Diamond (PCD): Diamond (n≈2.4) offers a wide bandgap and high refractive index, providing low-loss operation across the visible and NIR spectrum (715 nm operating wavelength).Low-Loss, High-Contrast Optics: Diamond metasurfaces are inherently superior to silicon for visible light applications due to minimal dissipative losses, leading to higher achievable bending efficiencies than the 32-35% reported here.
Large-Scale Integration & ScalingCustom PCD Wafers up to 125 mm: While the demonstrated lens was 600 ”m, scaling up for industrial applications (e.g., photolithography or large-array quantum sensors) requires large substrates. 6CCVD supplies PCD plates up to 125 mm diameter.Industrial Scalability: Provides the necessary large-area, high-quality substrates for mass production of flat optics arrays.
Precision Nanofabrication SubstratesUltra-Smooth Polishing (Ra < 1 nm for SCD, < 5 nm for PCD): The fabrication relies on EBL and RIE on a smooth surface. Our precision polishing minimizes surface roughness, crucial for high-fidelity patterning of sub-wavelength nanoantennas.Optimized Fabrication Yield: Ensures minimal scattering losses and high-resolution pattern transfer for features down to the 50 nm gap size used in the asymmetric dimers.
Integrated Device ArchitectureCustom Metalization Services (Au, Pt, Ti, W, Cu): For advanced integration, such as creating on-chip electrical contacts or thermal control layers adjacent to the metalens, 6CCVD offers internal metal deposition capabilities.Seamless Device Integration: Supports complex device architectures required for active or tunable metasurfaces.

Engineering Support

Section titled “Engineering Support”

6CCVD’s in-house team of PhD material scientists and engineers specializes in optimizing MPCVD diamond properties for advanced photonics and quantum applications. We can assist researchers in selecting the optimal diamond grade (SCD, PCD, or Boron-Doped Diamond (BDD)) and customizing dimensions (thickness 0.1 ”m to 500 ”m) and surface preparation to maximize the efficiency and performance of similar high-NA flat optics and integrated quantum emitter projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

The numerical aperture (NA) of a lens determines its ability to focus light and its resolving capability. Having a large NA is a very desirable quality for applications requiring small light-matter interaction volumes or large angular collections. Traditionally, a large NA lens based on light refraction requires precision bulk optics that ends up being expensive and is thus also a specialty item. In contrast, metasurfaces allow the lens designer to circumvent those issues producing high-NA lenses in an ultraflat fashion. However, so far, these have been limited to numerical apertures on the same order of magnitude as traditional optical components, with experimentally reported NA values of <0.9. Here we demonstrate, both numerically and experimentally, a new approach that results in a diffraction-limited flat lens with a near-unity numerical aperture (NA > 0.99) and subwavelength thickness (∌λ/3), operating with unpolarized light at 715 nm. To demonstrate its imaging capability, the designed lens is applied in a confocal configuration to map color centers in subdiffractive diamond nanocrystals. This work, based on diffractive elements that can efficiently bend light at angles as large as 82°, represents a step beyond traditional optical elements and existing flat optics, circumventing the efficiency drop associated with the standard, phase mapping approach.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 1999 - Principles of Optics [Crossref]

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