Photonic waveguides evanescently coupled with single NV-centers

At a Glance

Metadata	Details
Publication Date	2015-01-01
Journal	Elektronische Hochschulschriften der LMU München (Ludwig-Maximilians-Universität München)
Authors	Lars Liebermeister
Citations	1
Analysis	Full AI Review Included

Photonic Waveguides Evanescently Coupled with Single NV-centers

Analysis by 6CCVD Technical Sales Engineering

Executive Summary

This research demonstrates fundamental steps toward building scalable, integrated quantum photonic devices by efficiently coupling single Nitrogen-Vacancy (NV) centers in diamond to dielectric waveguides.

Integrated Quantum Platform: A platform combining single NV-centers with single-mode dielectric strip waveguides (Ta${2}$O${5}$ on SiO$_{2}$) was designed for robust, compact quantum information technology (QIT) applications.
High Coupling Efficiency: Simulations predict a broadband coupling efficiency ($\beta$) of approximately 35% between the single emitter and the optimized on-chip strip waveguide geometry (100 nm x 700 nm).
Deterministic Emitter Placement: Single NV-centers hosted in nanodiamonds were deterministically positioned onto the waveguide surface (tapered optical fiber in the initial experiment) using AFM-based nanomanipulation.
Proof of Concept Achieved: Experimental results confirmed evanescent coupling in a tapered optical fiber system, yielding a measured coupling efficiency of $(10 \pm 5)%$ for a single NV-center emission at 666 nm.
Low-Loss Waveguide Fabrication: On-chip Ta${2}$O${5}$ waveguides were fabricated using Electron Beam Lithography (EBL) and Reactive Ion Etching (RIE), achieving low propagation losses (below 1.8 dB/mm).
Efficient Fiber Coupling: An inverted adiabatic taper design achieved high off-chip coupling efficiencies up to 57% to standard single-mode fibers, crucial for connecting the integrated platform to external measurement apparatus.
6CCVD Value Proposition: The core challenge—high-purity diamond material control and precise fabrication—is directly addressed by 6CCVD’s specialized MPCVD Single Crystal Diamond (SCD) platforms for next-generation integrated NV-photonics.

Technical Specifications

Parameter	Value	Unit	Context
Core Emitter	NV-center	Defect	Hosted in $\approx$ 16 nm nanodiamond
Excitation Wavelength ($\lambda_{exc}$)	532	nm	Continuous-wave laser
NV$^{-}$ Zero Phonon Line (ZPL)	637	nm	Emission peak wavelength
Measured Coupling Efficiency ($\beta_{meas}$)	10 ± 5	%	TOF system at 666 nm
Predicted Coupling Efficiency (Strip WG)	$\approx$ 35	%	Ta${2}$O${5}$ (100 nm x 700 nm)
Strip Waveguide Core Material	Ta${2}$O${5}$ (n₂=2.1)	Dielectric	High refractive index core
Substrate Material	Fused Silica (n₁=1.46)	Dielectric	Lower cladding/substrate
Strip Waveguide Dimensions	100 x 700	nm	Height x Width
Propagation Loss ($l_{prop}$)	< 1.8	dB/mm	Ta${2}$O${5}$ waveguide at 658 nm
Off-Chip Insertion Loss (Coupling)	$\le$ 2.4 (57%)	dB (%)	Via inverted taper to SM fiber
EBL Acceleration Voltage	10	kV	Patterning waveguides/tapers
EBL Write Dose	95	µC/cm²	Used for optimal pattern transfer

Key Methodologies

The experimental approach relies on nanoscale material precision for both the quantum emitter host (diamond) and the photonic infrastructure (waveguides).

Diamond Nanocrystal Preparation: Commercial nanodiamonds (mean diameter $\approx$ 16 nm) are pre-characterized using a confocal microscope to ensure they host a single, fluorescing NV-center.
Hybrid AFM-Confocal Setup: A combined Atomic Force Microscope (AFM) and confocal microscope is utilized for in-situ monitoring and precise nanomanipulation.
Deterministic Placement: The preselected single NV-nanodiamond is picked up by the AFM tip and deterministically placed onto the surface of the photonic structure (Tapered Optical Fiber, or designed On-Chip Waveguide).
On-Chip Waveguide Fabrication (Ta${2}$O${5}$ on SiO$_{2}$):
- Layer Deposition: 100 nm thick Ta${2}$O${5}$ layer is sputtered onto a synthetic fused silica substrate.
- Patterning: Electron Beam Lithography (EBL) is used with PMMA resist and a conductive polymer discharge layer to define features down to 100 nm.
- Etching: The pattern is transferred into the Ta${2}$O${5}$ layer using Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) with an SF₆/Ar gas mixture and a Chromium (Cr) hard mask.
Mode Conversion Component: Linear inverted tapers (700 nm width down to 110 nm over 80 µm length) are fabricated at the waveguide ends for efficient adiabatic mode conversion and coupling to standard single-mode optical fibers.
Performance Characterization: Coupling efficiency ($\beta$), excited state lifetime ($\tau_{tot}$), and propagation losses ($l_{prop}$) are determined using power-dependent fluorescence and second-order correlation functions (Hanbury-Brown-Twiss configuration).

6CCVD Solutions & Capabilities

This research highlights the necessity of highly controlled diamond material coupled with precise, nanoscale fabrication for integrated quantum optics. 6CCVD provides the enabling MPCVD diamond platforms essential for replicating and scaling this technology.

Research Requirement / Challenge	6CCVD Solution & Capability	Sales Advantage
High-Quality NV Emitter Platform	Electronic Grade SCD Wafers: Required for creating NV-centers directly in the bulk material via ion implantation or during growth, offering greater spin coherence control than nanocrystals.	Provides the ultra-pure, low-defect foundation critical for next-generation quantum sensing and Qubit coherence.
Precise Wafer Dimensions/Orientation	Custom Dimensions & Orientation: We supply SCD/PCD plates up to 125 mm and substrates up to 10 mm thickness. We routinely manufacture specific crystal orientations (e.g., [111], [100]) crucial for alignment with the evanescent field of on-chip waveguides.	Ensures geometric compatibility and maximizes NV-dipole coupling efficiency for integrated devices, moving beyond nanodiamonds.
Integrated Photonic Structures (RIE)	Ultra-Smooth SCD/PCD Polishing: Research notes sidewall roughness limits propagation loss. Our single crystal diamond offers Ra < 1 nm polishing, minimizing scattering loss when etching photonic circuits directly into the diamond.	Enables the fabrication of low-loss, high-confinement diamond slot waveguides (as referenced in the paper’s outlook) with ultra-precise geometries.
Complex Integration & Metal Masking	Custom Metalization Services: We offer in-house deposition of metals (Au, Pt, Pd, Ti, W, Cu). We can supply diamond wafers pre-coated with hard masks (e.g., Cr or Ta${2}$O${5}$ via collaboration) ready for EBL patterning and RIE.	Streamlines the fabrication process by providing customized diamond platforms prepared for advanced lithographic techniques outlined in this research.
Scalable On-Chip Architecture	Large-Area MPCVD PCD: For high throughput or non-SCD requirements, 6CCVD provides polycrystalline diamond (PCD) plates up to 125mm with thicknesses up to 500 µm, enabling large-scale R&D and pilot manufacturing.	Supports industrial scaling of integrated quantum platforms requiring large wafer sizes not achievable with traditional SCD methods.

Engineering Support

The design and optimization of evanescent coupling ($\beta$ predicted at 35%) requires careful control over material refractive indices, waveguide geometry, and surface quality. 6CCVD’s in-house PhD team can assist with material selection, SCD/PCD substrate orientation, and surface preparation to maximize coupling efficiency for similar Integrated Quantum Photonics and Single Photon Source projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. Global shipping (DDU/DDP) is available.

View Original Abstract

The interaction of photons with individual quantum systems is a very fundamental process in physics. Thereby, the emission rate as well as the angular emission pattern of a quantum emitter are not only a function of intrinsic properties of the emitter itself, but are also strongly modified by its surrounding. For instance, by restricting the optical modes which are allowed at the position of the dipole, the emission rate can be strongly modified and the emitted photons can be directed into specific optical modes. This effect can be demonstrated by the interaction of a single optically active quantum emitter with the strongly confined optical mode of a single-mode dielectric waveguide. Efficient coupling of the emitter to the dielectric structure can be achieved by placing the quantum emitter inside the evanescent field of the guided mode. This evanescent field coupling mechanism is discussed and demonstrated experimentally. A single nitrogen-vacancy center (NV-center), hosted in a nanodiamond is deterministically coupled to a tapered optical fiber (TOF) via the evanescent field of its guided mode (coupling efficiencies exceeding 30% are predicted). By employing an AFM-based nanomanipulation technique, the diamond nanocrystal is placed on the nanofiber waist of the TOF. Beforehand, the diamond nanocrystal has been characterized to guarantee that it hosts only one fluorescing NV-center. While the diamond nanocrystal is optically exited, single photon fluorescence of the NV-center is detected at both outputs of the tapered optical fiber. This verifies the evanescent coupling of the emitter to the guided mode. In order to quantify the coupling, the comparison of the emission rate into free space with the rate into the fiber yields that 10.0(5) of the emitted photons are coupled into the tapered optical fiber. In the determination of this value, the orientation of the emitting dipoles and the emission pattern, which are modified by the TOF, have been considered. The NV-center features a broad emission spectrum which can be used to investigate the wavelength-dependence of the coupling. Comparing the spectra of the emission into the fiber mode with the emission into free space modes roughly resembles the expected wavelength dependency of the coupling efficiency. The evanescent coupling and the deterministic positioning of preselected fluorescing diamond nanocrystals, which has been demonstrated with the TOF, can be applied to other waveguide structures as well. Dielectric single-mode waveguides made of Ta2O5 on a SiO2 substrate promise similar coupling efficiencies to tapered optical fibers (above 30%). With the design of the on-chip wave-guiding structure being flexible, the combination with other optical on-chip elements is feasible, rendering it a promising platform for on-chip photonic experiments. Test structures of this waveguide design are realized using lithographic processes and are characterized. These waveguides are equipped with inverted taper structures to allow efficient off-chip coupling with butt-coupling to standard single-mode fibers. The evanescent coupling of a single quantum emitter to a singe optical mode can be used to efficiently collect emission of the quantum emitter. This can help building a compact single photon source and is beneficial for the optical read-out of the quantum emitter’s internal degree of freedom, which can be either used as probe (sensing) or as information-storage. Utilizing the high coupling efficiency, for instance, the non-linearities of the quantum system can be exploited to build a single photon transistor. The evanescent coupling is very broadband (about hundred nanometers), allowing to efficiently collect emission from broadband emitters like the NV-center, but it can also be used for multi-wavelength manipulation schemes.

Tech Support

Original Source

DOI: https://doi.org/10.5282/edoc.19101