Laser-Inscribed Diamond Waveguide Resonantly Coupled to Diamond Microsphere

At a Glance

Metadata	Details
Publication Date	2020-06-10
Journal	Molecules
Authors	Nurperi Yavuz, Mustafa Mert Bayer, Hüseyin Ozan Ҫirkinoğlu, Ali Serpengüzel, Thien Le Phu
Institutions	Koç University, Center for Biomolecular Nanotechnologies
Citations	4
Analysis	Full AI Review Included

Technical Documentation & Analysis: All-Diamond Integrated Photonic Circuits

This document analyzes the research paper “Laser-Inscribed Diamond Waveguide Resonantly Coupled to Diamond Microsphere” to provide technical specifications and demonstrate how 6CCVD’s advanced MPCVD diamond materials and processing capabilities can support and extend this critical research area in integrated diamond photonics.

Executive Summary

All-Diamond Photonic Circuit Demonstrated: The research successfully implemented an integrated photonic circuit by coupling a femtosecond-laser-written bulk diamond waveguide (WG) to a 1 mm CVD diamond microsphere.
High Q-Factor Performance: The system achieved high-quality factor (Q-factor) Whispering Gallery Modes (WGMs) up to 1.6 x 10⁵ in the near-infrared (1427 nm) region, validating diamond as a superior platform for high-performance optical resonators.
Material Purity Criticality: The study identified that the achievable Q-factor limits are determined primarily by material loss (Q_mat) and external coupling loss (Q_ext), emphasizing the need for ultra-high purity CVD diamond.
Waveguide Fabrication: Stress-induced waveguiding was achieved using the Type II femtosecond laser fabrication method, creating shallow WGs (20 µm depth) suitable for evanescent coupling.
Integrated Applications: The configuration shows immediate promise for high-resolution optical filtering, advanced sensing, and nonlinear optical applications, particularly when leveraging existing Nitrogen-Vacancy (NV) centers within the diamond structure.
6CCVD Value Proposition: 6CCVD specializes in providing the ultra-high purity Single Crystal Diamond (SCD) and highly polished Polycrystalline Diamond (PCD) required to minimize Q_mat and Q_ss losses, enabling Q-factors in the order of 10⁶ or higher, as targeted by the authors.

Technical Specifications

The following hard data points were extracted from the experimental results detailing the performance of the all-diamond photonic circuit:

Parameter	Value	Unit	Context
Maximum Measured WGM Q-Factor	1.6 x 10⁵	Dimensionless	TM-polarized light at 1427.96 nm
Waveguide FP Resonance Q-Factor	10⁴	Dimensionless	Highest measured value
WGM Mode Spacing (∆λ)	0.33	nm	Measured in elastic scattering spectra
FP Resonance FSR (∆λ_WG)	87	pm	Measured in transmission spectra
Operating Wavelength (Central)	1427.7	nm	Near-Infrared (Near-IR)
Waveguide Insertion Loss	12.4	dB	At 1550 nm wavelength
Waveguide Depth (Center to Surface)	20	µm	Minimum depth for Type II WG
Waveguide MFD (Vertical x Horizontal)	20 x 16	µm	Elongated in the vertical axis
Diamond Microsphere Diameter	1	mm	Type-Ib CVD Diamond
Substrate Nitrogen Impurity	~100	ppb	Optical Grade Diamond (WG substrate)
Required Surface Roughness (Ra)	<2	nm	Essential for minimizing scattering losses (Q_ss)

Key Methodologies

The all-diamond photonic circuit was fabricated and characterized using the following key parameters and techniques:

Waveguide Fabrication (Femtosecond Laser Inscription):
- Laser Type: Yb:KGW femtosecond (fs) pulsed laser.
- Central Wavelength: 515 nm.
- Repetition Rate: 500 kHz.
- Pulse Duration: 230 fs.
- Fabrication Method: Type II stress-induced waveguiding.
- Laser Power Range: 30 mW to 40 mW.
- Scan Speed: 0.5 mm/s.
- Track Spacing (Type II): 19 µm (between tracks forming the WG).
- WG Spacing (Successive WGs): 50 µm.
- Substrate Material: Optical grade diamond (5 mm x 5 mm x 0.5 mm) with ~100 ppb Nitrogen impurity.
Microsphere Preparation:
- Material: Type-Ib CVD diamond (Nitrogen impurity >5 ppm).
- Diameter: 1 mm.
- Finishing: Lapped to achieve a form accuracy (roundness) of <250 nm.
- Cleaning: Isopropanol and acetone mixture in an ultrasonic bath to achieve surface roughness Ra < 2 nm.
Optical Measurement Setup:
- Light Source: Narrow linewidth Distributed Feedback (DFB) semiconductor laser (CW mode).
- Central Wavelength: 1427.7 nm (Near-IR).
- Spectral Resolution: 1 pm (via Thermo-Electric Control).
- Coupling: Single Mode Fiber (SMF) butt-coupling to the diamond WG facet.
- Detection: Simultaneous measurement of 90° elastic scattering (PD1) and 0° transmission spectra (PD2).

6CCVD Solutions & Capabilities

This research highlights the critical role of high-quality CVD diamond material in achieving high-performance integrated photonic devices. 6CCVD is uniquely positioned to supply the advanced materials and processing required to replicate this work and push Q-factors into the next order of magnitude (10⁶+).

Applicable Materials for Replication and Extension

The paper notes that material loss (Q_mat) is a limiting factor, calculated to be in the order of 10⁵. To achieve higher Q-factors, lower absorption diamond is essential.

Research Requirement	6CCVD Solution & Specification	Technical Advantage
Ultra-Low Loss Substrates (Minimizing Q_mat)	Optical Grade Single Crystal Diamond (SCD)	Ultra-low nitrogen content (<1 ppb available) minimizes absorption losses in the near-IR, enabling Q-factors >10⁶.
Large-Scale Integration (Future Devices)	High Purity Polycrystalline Diamond (PCD)	Available in plates/wafers up to 125 mm diameter, ideal for scaling integrated photonic circuits beyond the 5 mm x 5 mm substrate used in the study.
Quantum Emitter Integration (NV Centers)	Tailored Nitrogen-Doped SCD/PCD	We offer controlled, intentional nitrogen doping during MPCVD growth to optimize the density and coherence of NV centers for quantum sensing and computing applications.
Microsphere Material (Type-Ib equivalent)	High-Quality SCD or PCD Blanks	We supply thick, high-pquality substrates (up to 10 mm) suitable for precision machining into high-form-accuracy microresonators.

Customization Potential & Processing Services

The success of this experiment relies heavily on precise dimensions and surface quality. 6CCVD’s in-house processing capabilities directly address these needs:

Superior Polishing for Low Scattering Loss (Q_ss):
- The paper required Ra < 2 nm. 6CCVD guarantees Ra < 1 nm for Single Crystal Diamond (SCD) and Ra < 5 nm for inch-size Polycrystalline Diamond (PCD). This ultra-smooth finish minimizes surface inhomogeneity and scattering losses, crucial for maximizing WGM Q-factors.
Custom Dimensions and Thickness:
- We provide custom plates and wafers in the exact dimensions required for femtosecond laser inscription, including substrates up to 10 mm thick, offering flexibility for deeper or multi-layer waveguide structures.
Integrated Metalization Services:
- While not used in this specific coupling experiment, future integrated diamond photonics often require electrodes or thermal management layers. 6CCVD offers internal metalization capabilities, including Au, Pt, Pd, Ti, W, and Cu, allowing researchers to integrate electrical components directly onto the diamond platform.

Engineering Support

6CCVD maintains an in-house team of PhD-level material scientists and engineers specializing in MPCVD diamond growth and characterization. We offer comprehensive support for projects involving:

Material selection to optimize Q-factor performance in integrated diamond photonics.
Consultation on achieving specific nitrogen concentrations for NV center quantum applications.
Guidance on substrate preparation and polishing requirements for femtosecond laser writing and high-efficiency evanescent coupling.

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

View Original Abstract

An all-diamond photonic circuit was implemented by integrating a diamond microsphere with a femtosecond-laser-written bulk diamond waveguide. The near surface waveguide was fabricated by exploiting the Type II fabrication method to achieve stress-induced waveguiding. Transverse electrically and transverse magnetically polarized light from a tunable laser operating in the near-infrared region was injected into the diamond waveguide, which when coupled to the diamond microsphere showed whispering-gallery modes with a spacing of 0.33 nm and high-quality factors of 105. By carefully engineering these high-quality factor resonances, and further exploiting the properties of existing nitrogen-vacancy centers in diamond microspheres and diamond waveguides in such configurations, it should be possible to realize filtering, sensing and nonlinear optical applications in integrated diamond photonics.

Tech Support

Original Source

DOI: https://doi.org/10.3390/molecules25112698

References

2006 - Silicon photonics [Crossref]
2011 - Diamond photonics [Crossref]
1979 - Quantum photoyield of diamond(111)—A stable negative-affinity emitter [Crossref]
2013 - Whispering gallery mode diamond resonator [Crossref]
1962 - Optical Properties of Diamond in the Vacuum Ultraviolet [Crossref]
2010 - Spherical whispering-gallery-mode microresonators [Crossref]
2016 - Optical Fiber Excitation of Fano Resonances in a Silicon Microsphere [Crossref]
2000 - Observation of Critical Coupling in a Fiber Taper to a Silica-Microsphere Whispering-Gallery Mode System [Crossref]
2009 - Ultracompact SOI Microring Add-Drop Filter With Wide Bandwidth and Wide FSR [Crossref]
2004 - Microsphere-Based Channel Dropping Filter with an Integrated Photodetector [Crossref]