Diamond step-index nanowaveguide to structure light efficiently in near and deep ultraviolet regimes
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-10-28 |
| Journal | Scientific Reports |
| Authors | Nasir Mahmood, Muhammad Qasim Mehmood, Farooq A. Tahir |
| Institutions | Information Technology University, National University of Sciences and Technology |
| Citations | 19 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Core Innovation: Introduction of Diamond Step-Index Nanowaveguides (DSINs) as ultra-low loss meta-atoms for highly efficient light structuring in the Near and Deep Ultraviolet (UV) regimes (250 nm to 400 nm).
- Material Advantage: Diamond is selected for its ultra-low loss (extinction coefficient k approx 0) and large bandgap (5.5 eV), providing transparency where conventional dielectrics (e.g., TiO2, Si3N4) absorb UV light.
- Wavefront Control: The DSINs achieve complete (0-2pi) phase coverage and high transmission amplitude by spatially varying the nanowaveguide diameter (D) based on indexed waveguide theory.
- Efficiency: The designed DSIN metasurfaces exhibit high average transmission efficiency, approximately 84%, across the entire spectrum of interest (250 nm to 400 nm), with peak efficiency reaching 96.5% (at 400 nm).
- Validation: The absolute control over phase and amplitude is verified by designing polarization-insensitive meta-holograms (NUST and ITU images) at the operational wavelength of lambda = 250 nm.
- Miniaturization: The high refractive index of diamond (n approx 2.4) allows for a low aspect ratio (AR = 5.7) of the meta-atoms, facilitating easier fabrication and miniaturized on-chip integration.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Core Material | Diamond | N/A | Ultra-low loss dielectric. |
| Substrate Material | SiO2 (Glass) | N/A | Used for patterning the DSINs. |
| Bandgap (Eg) | 5.5 | eV | Determines UV transparency window. |
| Cutoff Wavelength (lambdac) | 226 | nm | Wavelength threshold for transparency (k approx 0). |
| Refractive Index (n) | 2.406 | N/A | High index at 633 nm (facilitates low aspect ratio). |
| Extinction Coefficient (k) | 0.00 | N/A | Absolute transparency in the UV regime. |
| DSIN Height (H) | 400 | nm | Fixed thickness of the nanowaveguide. |
| Aspect Ratio (AR) | 5.7 | N/A | Height-to-minimum diameter ratio (within fabricate-able limits). |
| Operational Wavelength (lambdad) | 250, 300, 350, 400 | nm | Spectrum of interest (Near and Deep UV). |
| Diameter Range (lambdad=250 nm) | 70 to 98 | nm | Range used for achieving 0-2pi phase coverage. |
| Optimized Periodicity (lambdad=250 nm) | 140 | nm | Lattice constant (U) of the unit cell. |
| Average Transmission Efficiency | approx 84 | % | Efficiency across the entire spectrum of interest. |
| Peak Transmission Efficiency | 96.5 | % | Achieved at lambdad = 400 nm. |
| Metasurface Size | 12.2 x 12.2 | ”m2 | Total area of the simulated meta-hologram. |
| Pixel Array Size | 175 x 175 | Pixels | Number of DSIN elements in the simulation. |
| Focal Length (f) | 14 | ”m | Distance for holographic image reconstruction. |
Key Methodologies
Section titled âKey Methodologiesâ- Material Selection and Characterization: Diamond was chosen based on energy bandgap analysis (Eg = 5.5 eV) to confirm its transparency window (lambda > 226 nm) and high refractive index, comparing favorably against TiO2, Si3N4, and a-Si:H.
- Theoretical Waveguide Modeling: The underlying physics was established using indexed waveguide theory, deriving transcendental equations (Eqs. 6 and 7) from Maxwellâs equations to determine the propagation constant (beta) and effective refractive index (neff) for the hybrid (HE and EH) modes within the DSIN.
- Numerical Optimization (FDTD): Full-wave Finite-Difference Time-Domain (FDTD) simulations were performed on the circular cylindrical DSIN unit cell (H = 400 nm) using Perfect Matching Layers (PML) along the z-axis and periodic boundaries along the x- and y-axes.
- Phase and Amplitude Control: The DSIN diameter (D) was spatially swept (e.g., 70 nm to 98 nm for lambdad = 250 nm) while optimizing the unit cell periodicity (U) to achieve complete (0-2pi) phase coverage and maximize transmission amplitude simultaneously.
- Phase Retrieval Algorithm: The Gerchberg-Saxton (GS) iterative Fourier transform algorithm was employed to calculate the required discrete phase distribution (175 x 175 array) necessary to generate the target holographic images.
- Meta-Hologram Implementation: The calculated phase distribution was translated into a corresponding spatial variation of the optimized DSIN diameters (D) to construct the final polarization-insensitive meta-hologram for validation at lambda = 250 nm.
Commercial Applications
Section titled âCommercial Applicationsâ- Deep UV Photolithography: The high efficiency and short operational wavelengths (down to 250 nm) make this technology suitable for next-generation, high-resolution patterning in semiconductor fabrication.
- Miniaturized CMOS Integration: The compact 2D metasurface design and low aspect ratio (AR = 5.7) ensure compatibility with existing Complementary Metal-Oxide-Semiconductor (CMOS) fabrication processes, enabling on-chip UV optical systems.
- High-Resolution UV Imaging: Applicable in advanced microscopy and high-resolution imaging systems that require Deep UV light for enhanced spatial accuracy, such as biological imaging and non-invasive diagnostics.
- UV Sensing and Spectroscopy: Diamondâs ultra-low loss nature provides an ideal platform for highly sensitive UV sensors and compact spectroscopic devices, particularly in harsh or high-temperature environments.
- Flat Optics and Lenses: Used to create compact, high-numerical-aperture flat lenses and other optical elements (like wave-plates and beam generators) operating efficiently in the UV spectrum.
- Information Security and Displays: Implementation of high-fidelity, computer-generated meta-holograms for applications in data storage, information encryption, and 3D displays.