Diamond photonics platform enabled by femtosecond laser writing

At a Glance

Metadata	Details
Publication Date	2016-10-17
Journal	Scientific Reports
Authors	Belén Sotillo, Vibhav Bharadwaj, J P Hadden, Masaaki Sakakura, Andrea Chiappini
Institutions	Italian Institute of Technology, Istituto di Fotonica e Nanotecnologie
Citations	112
Analysis	Full AI Review Included

6CCVD Technical Analysis: Diamond Photonics Platform Enabled by Femtosecond Laser Writing

Paper Reference: Scientific Reports 6, Article number: 35566 (2016).

Executive Summary

This paper successfully demonstrates the first realization of three-dimensional (3D) buried optical waveguides in single-crystal diamond (SCD) using focused high repetition rate femtosecond (fs) laser writing. This breakthrough provides a scalable fabrication method for integrated quantum photonic devices.

Novel Fabrication: Development of a disruptive Type II fs-laser writing technique to create bulk optical waveguides in SCD, analogous to established methods in silicon photonics.
Graphitization Suppression: Optimal use of a high laser repetition rate (500 kHz) to reduce the formation of highly absorptive graphitic clusters, resulting in barriers composed of amorphous carbon.
Waveguide Performance: Demonstrated single-mode waveguiding behavior at visible and near-infrared wavelengths (635 nm, 808 nm, 1550 nm), supporting only the Transverse Magnetic (TM) mode.
NV Center Preservation: Crucially, the process preserves the high crystalline quality of the diamond core region, maintaining the electronic structure and quantum properties of embedded Nitrogen-Vacancy (NV) centers.
Quantum Metrics Maintained: NV centers within the guiding region exhibited an excited state lifetime (11.0 ± 1.5 ns) and ground state hyperfine structure comparable to pristine diamond.
Application Potential: This 3D photonics toolkit enables efficient collection and routing of NV fluorescence, paving the way for integrated quantum magnetometers and scalable quantum information processing (QIP) systems.

Technical Specifications

The following hard data points define the materials, processing, and performance metrics achieved in the research.

Parameter	Value	Unit	Context
Diamond Material	Type II SCD	N impurities	Optical Grade (100 ppb) and Electronic Grade (5 ppb)
Material Dimensions	3 x 3 x 0.2, 5 x 5 x 0.5	mm	Fabricated SCD plate sizes
Laser Type	Yb:KGW (Amplified)	N/A	Femtosecond writing system
Laser Wavelength (Writing)	515	nm	Frequency-doubled output
Pulse Duration	230	fs	Ultrashort pulse regime
Optimal Repetition Rate	500	kHz	Used to minimize graphitization and absorption
Average Power (Writing)	50	mW	Used with 500 kHz repetition rate
Waveguide Type	Type II	N/A	Undamaged diamond core confined by modified barriers
Optimal Line Separation	13	µm	For lowest loss, two-line modification
Mode Field Diameter (MFD)	10 x 11	µm	Central mode at 635 nm (two-line guide)
Insertion Loss (Measured)	14	dB	At 635 nm (butt-coupled SM fibers)
Propagation Loss (Estimated)	16	dB/cm	Main loss attributed to scattering overlap with modification tracks
NV Center Excitation	532	nm	Wavelength used for incoherent excitation
NV Center Lifetime (τ)	11.0 ± 1.5	ns	Within waveguide region, comparable to pristine diamond
NV Center Dephasing Time (T2)*	0.2	µs	Lower bound, derived from ODMR linewidth (1.7 ± 0.4 MHz)
Waveguide Stress (Guiding Region)	+1.5	cm^-1 shift	Corresponds to compressive stress (via µRaman)

Key Methodologies

The core of this research relied on controlling the highly localized material modification induced by femtosecond laser pulses to create 3D photonic barriers without compromising the quantum functionality of the diamond matrix.

Material Acquisition: High-purity, low-birefringence synthetic Single-Crystal Diamond (SCD) of both optical grade (100 ppb N impurities) and electronic grade (5 ppb N impurities) were used as substrates.
Femtosecond Laser Inscription: A regeneratively amplified Yb:KGW system producing 230-fs, 515-nm pulses was focused using a high Numerical Aperture (NA) 1.25 oil immersion objective to ensure highly localized energy absorption.
Optimization of Repetition Rate: The laser repetition rate was systematically optimized, confirming that 500 kHz suppressed the formation of highly absorptive, nanocrystalline graphite clusters, instead favoring less absorptive amorphous carbon.
Type II Waveguide Fabrication: Pairs or four-line arrays of parallel modification lines were written 50 µm below the diamond surface along the <110> crystallographic direction. The modification lines act as regions of reduced refractive index due to stress and damage, confining the guided mode in the pristine diamond core (Type II waveguiding).
Optical Characterization: Waveguide transmission measurements utilized single-mode fibers and free-space coupling at 532 nm, 635 nm, 808 nm, and 1550 nm to determine insertion loss, mode field diameter (MFD), and polarization dependence (only TM mode was supported).
Structural and Stress Analysis: µRaman spectroscopy was performed to analyze the change in the diamond lattice (1332 cm^-1 peak shift/broadening), confirming the presence of compressive stress (+1.5 cm^-1 shift) in the guiding region and verifying the preservation of crystalline structure.
Quantum Performance Verification: Photoluminescence (PL) spectroscopy and Optically Detected Magnetic Resonance (ODMR) were used to confirm that NV centers within the waveguide core retained their characteristic ZPL (Zero Phonon Line) structure, stable excited state lifetime, and preserved hyperfine spin structure.

6CCVD Solutions & Capabilities

This research validates the critical need for high-quality, ultra-pure diamond substrates—a requirement perfectly matched by 6CCVD’s Material Science portfolio. Researchers and engineers seeking to replicate or advance this 3D quantum photonics platform can rely on 6CCVD for customized MPCVD diamond materials and engineering services.

Applicable Materials for Quantum Photonics

The success of this work is entirely dependent on the quality of the SCD substrate. 6CCVD offers two primary grades directly applicable to this research:

Material Specification	Application Requirement (Based on Paper)	6CCVD Material Solution
Ultra-Low Nitrogen/Defect Density	Required for high-coherence NV- centers (Electronic Grade, 5 ppb N).	Electronic Grade SCD (High Coherence): Guaranteed ultra-low nitrogen content necessary for maximizing spin coherence time (T2).
NV Ensemble Integration	Required for high-sensitivity magnetometers (Optical Grade, 100 ppb N).	Optical Grade SCD (Controlled N): Customizable nitrogen doping levels (e.g., [N] < 100 ppb) to ensure a high, uniform density of NV centers suitable for ensemble sensing applications.
Material Dimensions	Plates down to 0.2 mm thick; need for large-scale integration.	Custom Substrates: We supply plates/wafers up to 125 mm (PCD) and SCD up to 500 µm thickness, ideal for large-scale integrated chip fabrication.

Customization Potential & Engineering Support

Replicating the results of this cutting-edge research requires bespoke material specifications and advanced processing capabilities, all of which are offered in-house by 6CCVD.

Precision Thickness and Dimensions: The paper used thin diamond films (0.2 mm and 0.5 mm). 6CCVD specializes in growing and supplying SCD plates with precise thickness control, ranging from 0.1 µm up to 500 µm, critical for controlling dispersion and mode matching in integrated circuits.
Ultra-Low Surface Roughness: Minimizing scattering loss (cited as 16 dB/cm) is paramount. Our SCD polishing capability achieves surface roughness Ra < 1 nm, significantly reducing interface scattering loss prior to laser inscription.
Custom Metalization and Interfacing: The original work required a 20 µm copper wire for ODMR measurements. 6CCVD offers internal metalization services (including Ti, Pt, Au, Cu, Pd) necessary for integrating microwave strips, electrodes, or passive optics required for advanced NV control and integrated cavity structures (like the Bragg gratings discussed in the paper).
Cryogenic and High-Pressure Applications: For researchers pursuing ground-state NV applications at low temperatures or high strain, our SCD growth and post-processing capabilities ensure materials maintain crystalline integrity under extreme conditions.

Engineering Support

6CCVD’s in-house team of PhD material scientists and engineers is available to consult on material selection and substrate preparation for projects involving femtosecond laser writing, integrated quantum photonics, and spin-based sensing. We can assist in tuning nitrogen concentration, crystal orientation, and surface termination to optimize results for specific laser inscription parameters.

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/srep35566