On-chip diamond Raman laser

At a Glance

Metadata	Details
Publication Date	2015-10-21
Journal	Optica
Authors	Pawel Latawiec, Vivek Venkataraman, Michael J. Burek, Birgit J. M. Hausmann, İrfan Bulu
Institutions	Harvard University, University of California, Berkeley
Citations	140
Analysis	Full AI Review Included

Technical Analysis & Documentation: On-Chip Diamond Raman Laser

6CCVD Product Focus: Optical Grade Single Crystal Diamond (SCD) for Integrated Photonics and Nonlinear Optics.

Executive Summary

This research demonstrates a significant advancement in integrated optics by successfully developing a high-performance, on-chip continuous-wave (CW) Raman laser utilizing synthetic Single Crystal Diamond (SCD). The results validate diamond as the superior material for high-power, broadband integrated nonlinear applications compared to silicon or silica platforms.

Platform Validation: Confirmed SCD’s suitability for integrated photonics, overcoming the limitations of Si (two-photon absorption) and Silica (low Raman gain, thermal issues).
Superior Raman Shift: Leverages diamond’s giant ~40 THz Raman shift, enabling access to exotic wavelengths (near 2 µm) unattainable with conventional materials (~12-15 THz shift).
Low-Threshold CW Lasing: Achieved stable continuous-wave operation with a record-low pump threshold of only ~85 mW in the feeding waveguide, vastly improving efficiency over bulk systems (~W-kW).
High-Q Performance: Fabricated diamond racetrack micro-resonators demonstrated high loaded quality factors (Q) up to 440,000 at the telecom pump wavelength (~1.6 µm).
Broad Tunability: Demonstrated discrete wavelength tuning over >100 nm (7.5 THz) around the 2 µm region, alongside mode-hop-free continuous tuning over 7.5 GHz.
Thermal Management: Diamond’s superior thermal conductivity (~1800 W/m/K @ 300K) and low thermo-optic coefficient minimize thermal lensing, crucial for stable, high-power CW operation.
Future Trajectory: The study highlights the need for custom-oriented, thick SCD plates ([111] orientation) to further enhance efficiency and support all-diamond structures for mid-infrared cascaded lasing.

Technical Specifications

The following key operational and material parameters were achieved or leveraged in the diamond Raman laser demonstration:

Parameter	Value	Unit	Context
Material Used	Type IIa Single Crystal Diamond (SCD)	N/A	CVD Grown
Diamond Thermal Conductivity	~1800	W/m/K	At 300K
Diamond Raman Shift (Ω_R)	~40	THz	Corresponds to ~1993 nm Stokes output from 1575 nm pump
Pump Wavelength Range (λ_p)	1535 - 1610	nm	Telecom C-band or L-band
Stokes Wavelength Range (λ_s) - Discrete	1950 to >2050	nm	Total discrete tuning range of >100 nm
CW Pump Threshold Power	~85	mW	Measured in the coupling waveguide
Maximum Output Stokes Power	>250	µW	Coupled into the output waveguide
Loaded Q (Pump Mode)	~440,000	N/A	Measured at 1574.8 nm (under-coupled)
Loaded Q (Stokes Mode)	~30,000	N/A	Measured near 1966 nm (under-coupled)
External Slope Efficiency	~0.43	%	Above threshold
Waveguide Cross-Section	800 x 700	nm	Diamond core embedded in silica cladding
Racetrack Path Length	~600	µm	Defining the resonator size
Free Spectral Range (FSR)	~180	GHz	Corresponds to ~1.5 nm
Continuous Tuning Range	~7.5	GHz	Mode-hop-free tuning via thermal redshift

Key Methodologies

The complex device fabrication requires highly specialized handling and processing of ultra-thin, high-quality CVD SCD plates, emphasizing the need for 6CCVD’s expertise in material preparation.

Initial Substrate Preparation: Started with a ~20 µm thick, Type IIa CVD single crystal diamond plate (Delaware Diamond Knives).
Chemical Cleaning: Intensive refluxing acid mixture (nitric, sulphuric, and perchloric) used to ensure pristine surface quality.
High-Precision Thinning: The diamond plate was bonded to a sapphire carrier wafer via Van der Waals forces and thinned via cycling Ar/Cl₂ and O₂ ICP-RIE etching down to <1 µm thickness specification.
Stress Management: Diamond was etched on both sides to eliminate residual internal stress/strain resulting from initial bulk polishing.
Substrate Transfer: The ultra-thin diamond film was transferred and bonded onto a SiO₂/Si substrate (featuring a 2 µm thermal oxide layer).
Nanophotonic Patterning: Fox 16 electron-beam resist and multipass exposure were used to define racetrack micro-resonators (Path Length: ~600 µm) and bus waveguides.
Final Etching: Final high-aspect-ratio structures (800 nm wide, 700 nm high) were defined using a deep oxygen etch.
Cladding and Integration: The completed device was capped with a ~3 µm layer of silica via Plasma-Enhanced Chemical Vapor Deposition (PECVD) to provide mechanical stability and index confinement.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the foundational materials and advanced processing required to replicate and significantly advance this integrated diamond Raman laser technology. Our specialized CVD growth and post-processing capabilities address the precise material specifications identified as limitations in this research.

Applicable Materials

To replicate and enhance the on-chip Raman laser performance, the following 6CCVD materials are essential:

Optical Grade Single Crystal Diamond (SCD, Type IIa): Guaranteed high purity and low residual stress (Ra < 1nm polished) to ensure the highest intrinsic quality factors (Q) and minimize propagation losses necessary for high-efficiency resonators.
Custom Orientation Plates (Crucial for Efficiency): While the researchers used [100]-oriented diamond (where Raman gain is sub-optimal), the conclusion strongly recommends using thick [111] diamond plates to ensure the light polarization aligns with the optimal <111> direction, thereby significantly enhancing the Raman efficiency (up to 20x higher gain is mentioned for visible wavelengths).
SCD Thin Films: 6CCVD specializes in supplying SCD films pre-thinned to specifications between 0.1 µm and 500 µm, reducing customer processing time and improving thickness uniformity.

Customization Potential

The success of this research relies heavily on precise thickness control, custom geometry, and integration with silicon platforms.

Research Requirement	6CCVD Capability & Advantage
Sub-Micron Thickness Control	We supply SCD plates pre-thinned to <1 µm, matching the exact film thickness required for nanophotonic mode confinement (700 nm height). This drastically reduces customer fabrication complexity (e.g., ICP-RIE cycling).
Custom Wafer Dimensions	While the paper focused on small chips, 6CCVD can supply SCD plates up to 125mm (PCD) or custom sizes for larger integrated runs, facilitating scalable manufacturing efforts.
Precision Polishing	Our standard SCD polishing achieves Ra < 1nm. This ultra-smooth surface finish is critical for minimizing scattering losses in high-Q micro-resonators and increasing intrinsic Q factors (Q_int), leading to lower lasing thresholds.
All-Diamond Structure Support	For the next generation of devices (eliminating silica absorption limits for cascaded Mid-IR lasers), 6CCVD provides SCD substrates up to 10mm thickness, ideal for developing robust, all-diamond integrated platforms.
Metalization Services	While not used in this iteration, future devices requiring thermal heaters or integrated electronics will benefit from 6CCVD’s internal capabilities for custom metalization stacks (e.g., Au, Pt, Ti, Cu) applied directly to the SCD surface.

Engineering Support

6CCVD’s in-house team of PhD material scientists and technical engineers is available to consult on optimizing diamond properties for integrated nonlinear optics projects. We provide expert advice on:

Orientation Selection: Guiding researchers toward high-gain orientations (e.g., [111] SCD) for maximal Raman efficiency.
Thinning Procedures: Ensuring precise wafer-level thickness uniformity across large substrates for reproducible high-Q fabrication.
Packaging and Integration: Supporting the mechanical requirements for bonding and heterogenous integration with standard silicon platforms (like the SiO₂-on-Si used here).

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

View Original Abstract

Synthetic single-crystal diamond has recently emerged as a promising platform for Raman lasers at exotic wavelengths due to its giant Raman shift, large transparency window and excellent thermal properties yielding a greatly enhanced figure-of-merit compared to conventional materials. To date, diamond Raman lasers have been realized using bulk plates placed inside macroscopic cavities, requiring careful alignment and resulting in high threshold powers (~W-kW). Here we demonstrate an on-chip Raman laser based on fully-integrated, high quality-factor, diamond racetrack micro-resonators embedded in silica. Pumping at telecom wavelengths, we show Stokes output discretely tunable over a ~100nm bandwidth around 2-{\mu}m with output powers >250 {\mu}W, extending the functionality of diamond Raman lasers to an interesting wavelength range at the edge of the mid-infrared spectrum. Continuous-wave operation with only ~85 mW pump threshold power in the feeding waveguide is demonstrated along with continuous, mode-hop-free tuning over ~7.5 GHz in a compact, integrated-optics platform.

Tech Support

Original Source

DOI: https://doi.org/10.1364/optica.2.000924