Visible to Infrared Diamond Photonics Enabled by Focused Femtosecond Laser Pulses

At a Glance

Metadata	Details
Publication Date	2017-02-17
Journal	Micromachines
Authors	Belén Sotillo, Vibhav Bharadwaj, John P. Hadden, Stefano Rampini, Andrea Chiappini
Institutions	Istituto di Fotonica e Nanotecnologie, Koç University
Citations	38
Analysis	Full AI Review Included

Technical Documentation and Collateral: Integrated Diamond Photonics via Femtosecond Laser Inscription

Executive Summary

The analyzed research demonstrates a significant advancement in diamond integrated optics, fabricating both single-mode optical waveguides and deterministically placed single Nitrogen-Vacancy (NV) centers within the bulk of quantum-grade diamond using focused femtosecond laser pulses. This integrated approach leverages diamond’s superior properties for quantum technologies and sensing.

Integrated Photonics Platform: Achieved Type II optical waveguides in bulk diamond, tunable to operate across a broad spectral range from the visible (532 nm) to the infrared (1550 nm).
Wavelength Flexibility: Single-mode guiding was optimized for long-wavelength applications (1550 nm) by increasing track separation to 19 µm, critical for coupling to silicon photonics or microspheres.
Quantum Defect Control: Demonstrated deterministic creation of single NV centers in ultrapure quantum-grade diamond using static, ultra-low energy (24 nJ) laser pulses, achieving up to an 80% success probability without complex beam shaping.
Material Quality Preservation: High repetition rate inscription (500 kHz) minimizes graphite formation and reduces compressive stress in the guiding region (stress shift of only 1.5 cm^-1), preserving the high-quality optical and spin properties of the NV centers.
Fundamental Building Blocks: The ability to co-locate high-quality NV centers with optical circuits provides the essential components for next-generation 3D quantum information systems and ultrasensitive optical magnetometers.
6CCVD Advantage: Replication and advancement of this work require pristine, high-purity Single Crystal Diamond (SCD) material and specialized post-growth annealing, core capabilities provided by 6CCVD.

Technical Specifications

The following hard data points define the parameters and outcomes of the femtosecond laser microfabrication process used to create integrated diamond photonic circuits.

Parameter	Value	Unit	Context
Waveguide Operating Wavelength Range	532 to 1550	nm	Visible (Quantum) to Near-Infrared (Telecom) operation.
Lowest Measured Insertion Loss (WL)	11	dB	Includes 1.4 dB/facet coupling loss (0.5 cm sample length).
Optimal Laser Repetition Rate (RR)	500	kHz	Used for waveguide inscription to minimize stress and graphite.
Optimal Pulse Energy (Waveguide)	60 to 100	nJ	Used for stable Type II modification tracks.
Optimized Track Separation (Visible)	13 to 15	µm	For single mode guiding at shorter wavelengths.
Optimized Track Separation (1550 nm)	19	µm	For single mode guiding at long wavelengths (IR).
Minimum Waveguide Depth	12	µm	Shallowest depth achieved without surface ablation.
Compressive Stress in Guiding Region (RR 500 kHz)	1.5	cm^-1	Raman shift, indicating minimal crystal disorder.
Optimal Pulse Energy (Single NV Creation)	24	nJ	Used static exposure; below amorphization threshold (~50 nJ).
NV Creation Success Probability (Optimal)	80	%	Probability of forming a single NV center per exposure.
Post-Fabrication Annealing Temperature	1000	°C	Held for 3 h in N₂ atmosphere to mobilize vacancies.
Diamond Material Purity (Quantum Grade)	<5	ppb	Nitrogen impurities (Type II, required for high-quality NVs).

Key Methodologies

The study relied on a multi-step femtosecond laser processing and post-processing regimen to fabricate and characterize the integrated diamond devices.

Laser Setup and Inscription:
- A regeneratively amplified Yb:KGW system was used, operating at 515 nm (frequency doubled) with a 230-fs pulse duration.
- Focusing was achieved using a high numerical aperture (1.25-NA) oil immersion objective.
- Type II Waveguide Writing: Two parallel modification lines were written using translation stages, achieving optimized single-mode performance at a 500 kHz repetition rate and 60-100 nJ pulse energy.
- Geometry Control: Waveguide separation (13 µm to 19 µm) and depth (12 µm to 40 µm) were precisely controlled to tune operation from visible to 1550 nm.
NV Center Deterministic Placement:
- Single NV centers were created using static exposures of single femtosecond laser pulses (24 nJ optimal energy) focused approximately 25 µm below the surface of quantum-grade diamond (<5 ppb N).
- This pulse energy was specifically chosen to be below the bulk amorphization threshold (~50 nJ) to create isolated vacancies.
Post-Processing Annealing:
- All laser-modified samples were subjected to high-temperature annealing (1000 °C for 3 h in N₂ atmosphere) to mobilize the laser-created vacancies, allowing them to bind with native nitrogen impurities to form the desired NV centers.
Characterization:
- Structural Analysis: µ-Raman spectroscopy (532 nm laser) was employed to map the stress distribution (diamond peak shift) and crystal quality (peak width) within the guiding regions.
- Optical Performance: Near-field mode profiles and insertion losses were measured using single-mode fibers coupled to 532 nm, 635 nm, 808 nm, and 1550 nm sources.
- Quantum Quality Confirmation: Confocal photoluminescence (PL) and intensity autocorrelation measurements (g⁽²⁾(0)) confirmed the existence of high-quality single NV centers (ZPL at 637 nm) demonstrating single photon emission (antibunching dip well below 0.5).

6CCVD Solutions & Capabilities

This breakthrough research, demonstrating an integrated platform for quantum information and sensing, relies fundamentally on access to ultra-high-quality diamond materials and precise fabrication support. 6CCVD is uniquely positioned to supply the foundational SCD substrates required to replicate and scale this technology.

Applicable Materials for Quantum Diamond Photonics

The reported work utilized “quantum grade” diamond (<5 ppb nitrogen impurities) to ensure the highest quality NV centers and long spin coherence times. 6CCVD offers the necessary material platform:

Optical Grade Single Crystal Diamond (SCD): Essential for minimizing light scattering and absorption losses across the visible-to-infrared spectrum (532 nm to 1550 nm).
- 6CCVD Recommendation: High-Purity Electronic/Optical Grade SCD Wafers. Available in customized thicknesses, essential for controlling the spherical aberration effects seen at various waveguide depths (12 µm to 40 µm).
Nitrogen Doping Control: While the paper used existing N impurities, future work requires precise control over the initial N concentration or subsequent ion implantation sites.
- 6CCVD Capability: We offer SCD with tightly controlled nitrogen content to optimize the density and location of NV precursors for deterministic creation methods.
Boron-Doped Diamond (BDD): Although not the focus of the paper, BDD thin films are vital for related applications requiring electrochemistry or surface-charge management on integrated diamond circuits.

Precision Substrate Engineering for Laser Processing

Success in femtosecond laser inscription, especially with high-NA oil immersion objectives, depends critically on the flatness and surface quality of the diamond substrate.

Research Requirement	6CCVD Capability	Direct Benefit to Client
Pristine Surface Quality	Ultra-Smooth SCD Polishing: Achieves surface roughness Ra < 1 nm.	Maximizes coupling efficiency, minimizes scattering loss, and ensures stable focal plane for high-NA objectives.
Custom Wafer Dimensions	Custom Dimensions: Plates/wafers up to 125 mm (PCD). SCD wafers in common research sizes (e.g., 5x5 mm, 10x10 mm) and custom thickness (0.1 µm - 500 µm).	Supports rapid prototyping and eventual scale-up of 3D photonic circuits and integrated quantum platforms.
Post-Processing Annealing	Internal Thermal Treatment: Capabilities for high-temperature (>1000 °C) annealing in controlled atmospheres (N₂, Ar, Vacuum).	Ensures efficient conversion of laser-created vacancies into high-quality NV centers, critical for achieving 80% success probability.
Integrated Electrode Design	Custom Metalization Services: Offers Au, Pt, Pd, Ti, W, Cu layers.	Enables post-fabrication integration of microwave electrodes (required for ODMR/spin manipulation) directly onto the same substrate used for optical guiding.

Engineering Support

6CCVD’s in-house PhD team provides authoritative support in selecting the optimal MPCVD diamond grade and post-processing recipe required for complex integration tasks. We can assist with material selection for similar Integrated Diamond Quantum Sensing and Information Processing projects, ensuring material properties align perfectly with femtosecond laser processing requirements (e.g., matching N content to pulse energy for efficient vacancy conversion).

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

View Original Abstract

Diamond’s nitrogen-vacancy (NV) centers show great promise in sensing applications and quantum computing due to their long electron spin coherence time and because they can be found, manipulated, and read out optically. An important step forward for diamond photonics would be connecting multiple diamond NVs together using optical waveguides. However, the inertness of diamond is a significant hurdle for the fabrication of integrated optics similar to those that revolutionized silicon photonics. In this work, we show the fabrication of optical waveguides in diamond, enabled by focused femtosecond high repetition rate laser pulses. By optimizing the geometry of the waveguide, we obtain single mode waveguides from the visible to the infrared. Additionally, we show the laser writing of individual NV centers within the bulk of diamond. We use µ-Raman spectroscopy to gain better insight on the stress and the refractive index profile of the optical waveguides. Using optically detected magnetic resonance and confocal photoluminescence characterization, high quality NV properties are observed in waveguides formed in various grades of diamond, making them promising for applications such as magnetometry, quantum information systems, and evanescent field sensors.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi8020060

References

2009 - Ultralong spin coherence time in isotopically engineered diamond [Crossref]
2008 - Diamond waveguides fabricated by reactive ion etching [Crossref]
2010 - Evidence of light guiding in ion-implanted diamond [Crossref]
2016 - Diamond photonics platform enabled by femtosecond laser writing [Crossref]
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2006 - Efficient frequency doubling in femtosecond laser-written waveguides in lithium niobate [Crossref]
2011 - Engineering of nitrogen-vacancy color centers in high purity diamond by ion implantation and annealing [Crossref]
2017 - Laser writing of coherent colour centres in diamond [Crossref]
1977 - Temperature and pressure variation of the refractive index of diamond [Crossref]
2010 - Controlled variation of the refractive index in ion-damaged diamond [Crossref]