Diamond as a material for monolithically integrated optical and optomechanical devices

At a Glance

Metadata	Details
Publication Date	2015-11-01
Journal	physica status solidi (a)
Authors	Patrik Rath, S. Ummethala, Christoph E. Nebel, Wolfram H. P. Pernice
Institutions	Karlsruhe Institute of Technology, University of Münster
Citations	51
Analysis	Full AI Review Included

Diamond for Monolithically Integrated Optomechanical Circuits: A 6CCVD Technical Analysis

This document analyzes the research paper “Diamond as a material for monolithically integrated optical and optomechanical devices” (Rath et al., 2015) to highlight the critical role of high-quality CVD diamond and connect the material requirements directly to 6CCVD’s advanced manufacturing capabilities.

Executive Summary

Diamond is confirmed as the premier material platform for integrated optomechanical circuits due to its unique combination of properties, enabling next-generation sensing and quantum technologies.

Superior Material Properties: Diamond exhibits the highest Young’s modulus (1100 GPa), thermal conductivity (2200 W/m K), and sound velocity (17700 m/s), translating directly into ultra-high mechanical resonance frequencies and high power handling capacity.
Record Performance: The research demonstrates SCD optical resonators achieving Quality (Q) factors up to 1,000,000 (at 1545 nm) and mechanical resonators achieving Q factors up to 1,510,000 (at 3 K).
Integrated Functionality: The platform supports complex integrated components, including waveguides, Mach-Zehnder Interferometers (MZI), and H-resonators, essential for phase-sensitive readout and on-chip actuation.
Quantum Potential: Single Crystal Diamond (SCD) is critical for integrating color centers (e.g., NV centers) for strain-mediated quantum sensing and single-photon emission.
Scalability Challenge Addressed: Polycrystalline Diamond (PCD) thin films, available in wafer-scale formats, are shown to be highly effective for integrated circuits in the Near-Infrared (NIR) and telecom bands, overcoming the size limitations of bulk SCD templates.
6CCVD Solution: 6CCVD provides both high-purity SCD and scalable, large-area PCD wafers (up to 125 mm) with precise thickness control (0.1 µm to 500 µm) and custom metalization, directly enabling the replication and advancement of this research.

Technical Specifications

The following table summarizes the key material and performance metrics extracted from the research, emphasizing diamond’s advantages over competing materials (Si, Si₃N₄, SiC).

Parameter	Value	Unit	Context
Band Gap (E_g)	5.47	eV	Wide optical transparency
Young’s Modulus (E)	1100	GPa	High mechanical stiffness (3x Si)
Thermal Conductivity (k)	2200	W/m K	Essential for high-power handling
Sound Velocity (c)	17700	m/s	Highest known material value
Refractive Index (n)	2.4	Dimensionless	High contrast for waveguiding
Highest Optical Q Factor (SCD)	1,000,000	Dimensionless	Ring Resonator, 1545 nm (NIR/Telecom)
Highest Mechanical Q Factor (SCD)	1,510,000	Dimensionless	Cantilever, Cryogenic (3 K)
Record Mechanical Q*f Product	> 10¹³	Hz	PCD Nanomechanical Resonator
SCD Thickness Range Demonstrated	0.1 - 10	µm	Thin films for freestanding membranes
PCD Wafer Size Availability	Up to 6	Inch	Wafer-scale fabrication platform
Nonlinear Refractive Index (n₂)	1.3 x 10^-19	m²/W	Attractive for on-chip nonlinear optics

Key Methodologies

The fabrication of freestanding diamond photonic and optomechanical components relies on specialized techniques, particularly for Single Crystal Diamond (SCD) where thin films are not readily available on low-index buffer layers.

Thin-Down Strategy: Starting with thick bulk SCD (tens of µm) acquired from commercial suppliers, the material is thinned down to hundreds of nanometers via dry etching. Photonic structures are then defined using Electron Beam Lithography (EBL) and dry etching.
Lift-Off Technique: Circumvents thickness inhomogeneity by irradiating the diamond with ions, graphitizing the damaged layer, and selectively removing it during annealing. A pristine diamond layer is regrown, patterned, and transferred to a different substrate (e.g., oxidized silicon).
Angle-Etching Procedure: EBL patterning followed by angled anisotropic plasma etching to directly structure freestanding devices into bulk diamond. This method restricts design freedom as height is fixed by the etch angle.
Undercut Etching Method: EBL patterning and transfer into bulk diamond using a series of steps involving Si₃N₄ coating and quasi-isotropic Reactive Ion Etching (RIE) to create vertical sidewalls and undercut the structures, resulting in freestanding devices.
Focused Ion Beam (FIB) Milling: Uses Ga⁺-ions for direct cutting without a mask. While effective for small-scale research, it presents drawbacks concerning damage, ion implantation, and scalability.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials necessary to replicate and extend the integrated optomechanics research described in this paper, addressing key limitations in material availability and scalability.

Applicable Materials

To successfully implement integrated optomechanical circuits, researchers require materials optimized for specific spectral and mechanical performance regimes:

Optical Grade Single Crystal Diamond (SCD): Required for visible wavelength applications, particularly for integrating Nitrogen-Vacancy (NV) centers for quantum sensing and single-photon sources. 6CCVD provides high-purity SCD plates with Ra < 1 nm polishing essential for achieving the ultra-high optical Q factors (up to 1,000,000) demonstrated in the paper.
Polycrystalline Diamond (PCD) Wafers: Ideal for scalable, wafer-level fabrication of complex photonic networks (waveguides, MZI) operating in the Near-Infrared (NIR) and telecom C-band (1550 nm). 6CCVD offers PCD wafers up to 125 mm in diameter, directly supporting the wafer-scale fabrication process flow outlined in Figure 7 of the paper.
Custom Thin Films and Membranes: The research relies heavily on thin diamond films (hundreds of nanometers) for creating freestanding mechanical resonators. 6CCVD specializes in providing both SCD and PCD materials with precise thickness control, ranging from 0.1 µm up to 500 µm, suitable for all “thin-down” and “lift-off” fabrication strategies.

Customization Potential

The complexity of integrated optomechanical devices necessitates highly customized material preparation, which 6CCVD provides as a core service:

Research Requirement	6CCVD Customization Capability	Benefit to Researcher
Large-Area Substrates	PCD plates/wafers up to 125 mm (5 inches).	Enables scalable, commercializable fabrication of complex circuits.
Precise Thickness Control	SCD/PCD thickness from 0.1 µm to 500 µm.	Critical for defining mechanical resonance frequency and optical mode confinement.
Custom Metalization	In-house deposition of Au, Pt, Pd, Ti, W, Cu.	Essential for creating on-chip electrodes (e.g., Cr/Au/Cr) for electrostatic actuation and for integrating superconducting nanowire single photon detectors (SNSPDs).
Surface Quality	SCD polishing to Ra < 1 nm; PCD polishing to Ra < 5 nm.	Minimizes scattering losses, crucial for achieving high optical Q factors (Q > 10⁶).
Substrate Thickness	Substrates available up to 10 mm thick.	Provides robust templates for bulk-etching methods (angle-etching, undercut) used to create high-Q SCD resonators.

Engineering Support

6CCVD’s in-house PhD team possesses deep expertise in MPCVD growth parameters and post-processing techniques. We can assist researchers and engineers with material selection for similar integrated optomechanics, quantum sensing, and high-frequency MEMS projects, ensuring optimal material purity and crystal orientation for desired performance metrics (e.g., maximizing NV center coherence time or mechanical Q*f product).

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

View Original Abstract

Diamond provides superior optical and mechanical material properties, making it a prime candidate for the realization of integrated optomechanical circuits. Because diamond substrates have matured in size, efficient nanostructuring methods can be used to realize full‐scale integrated devices. Here we review optical and mechanical resonators fabricated from polycrystalline as well as single crystalline diamond. We present relevant material properties with respect to implementing optomechanical devices and compare them with other material systems. We give an overview of diamond integrated optomechanical circuits and present the optical readout mechanism and the actuation via optical or electrostatic forces that have been implemented to date. By combining diamond nanophotonic circuits with superconducting nanowires single photons can be efficiently detected on such chips and we outline how future single photon optomechanical circuits can be realized on this platform.