Cavity-Enhanced Raman Scattering for In Situ Alignment and Characterization of Solid-State Microcavities

At a Glance

Metadata	Details
Publication Date	2020-01-22
Journal	Physical Review Applied
Authors	Daniel Riedel, Sigurd Flågan, Patrick Maletinsky, Richard J. Warburton
Institutions	University of Basel
Citations	25
Analysis	Full AI Review Included

Technical Documentation & Analysis: Cavity-Enhanced Raman Scattering in SCD Microcavities

Reference: Riedel et al., Cavity-enhanced Raman scattering for in situ alignment and characterization of solid-state microcavities, arXiv:1909.12333v1 [quant-ph] (2019).

Executive Summary

This research demonstrates a powerful, generic method for optimizing solid-state microcavities, specifically utilizing the intrinsic Raman scattering of the diamond host material to achieve rapid in situ alignment and characterization.

Core Value Proposition: Cavity-enhanced Raman scattering provides a high-intensity, narrowband internal light source, facilitating the precise mode-matching required for efficient outcoupling of weak single-photon emitters (e.g., NV centers).
Performance Achievement: A measured Raman intensity enhancement factor (F) of 58.8-fold was achieved compared to confocal measurements, resulting from the Purcell effect.
Material Requirement: The experiment relies on high-purity, low-strain Single Crystal Diamond (SCD) membranes (1 µm thick) to ensure narrow Raman linewidths (47.8 GHz / 77 pm).
Cavity Metrics: The tunable Fabry-Pérot microcavity achieved a high Finesse (F) of ~1,000 and a Q-factor (Qc,res) of 8,200 at the Stokes resonance wavelength (572.67 nm).
Alignment Utility: The strong, ubiquitous Raman signal is independent of lateral emitter position, enabling fast, single-shot imaging of cavity modes and determination of geometric parameters (e.g., beam waist w₀ = 0.88 µm).
Applicability: The technique is immediately applicable to optimizing the spin-photon interface efficiency for a wide range of solid-state qubits, including NV centers in diamond and rare-earth ions.

Technical Specifications

The following hard data points were extracted from the experimental results concerning the diamond material and cavity performance:

Parameter	Value	Unit	Context
Raman Intensity Enhancement (F)	58.8	Fold	Total measured enhancement (F = Fp * Fc)
Calculated Purcell Factor (Fp)	4.7	Dimensionless	Calculated for the cavity structure
Diamond Raman Shift	1,332	cm^-1	Optical phonon energy in diamond
Raman Gain Coefficient	~75	GW·cm^-1	At 532 nm (cited reference)
Diamond Membrane Thickness	1	µm	Typical thickness used for the SCD membrane
Diamond Membrane Side Length	10 to 50	µm	Square-shaped membranes
Pump Excitation Wavelength	532	nm	Green laser (outside DBR stopband)
Stokes Emission Wavelength (Resonance)	572.67	nm	Cavity tuned to this resonance
Cavity Finesse (F)	~1,000	Dimensionless	At Stokes wavelength (~573 nm)
Cavity Q-factor (Qc,res)	8,200	Dimensionless	Measured at resonance (70 pm linewidth)
Measured Beam Waist (w₀)	0.88	µm	Gaussian fit of the fundamental mode (q,0,0)
Raman Linewidth (δν_s)	47.8	GHz	Corresponds to 77 pm at 636 nm excitation
Operating Temperature	4	K	Demonstrated cryogenic operation

Key Methodologies

The experiment involved precise fabrication and integration of high-purity diamond membranes into a tunable Fabry-Pérot microcavity structure.

Material Sourcing: High-purity, single-crystal diamond (SCD) was sourced (Element 6) and subsequently implanted with nitrogen ions and annealed to create NV centers.
Membrane Fabrication: Electron-beam lithography and inductively-coupled plasma-etching were used to define square-shaped SCD membranes (1 µm thickness, 10-50 µm side length).
Bottom Mirror Preparation: A planar SiO₂ substrate was coated with a highly reflective Distributed Bragg Reflector (DBR) consisting of 15 layers of SiO₂/Ta₂O₅.
Top Mirror Preparation: Curved microtemplates (Radius of Curvature R ~ 10 µm) were fabricated on a SiO₂ chip via CO₂ laser ablation, followed by coating with a 14-layer Ta₂O₅/SiO₂ DBR.
Cavity Assembly: The SCD membrane was bonded to the planar DBR mirror using a micromanipulator, relying on strong van der Waals forces due to the extremely smooth surfaces (Ra < 0.3 nm).
In Situ Tuning: The mirror separation (air-gap width) and lateral position were controlled using xyz nanopositioners to tune the cavity resonance frequency and align the cavity antinode.
Raman Enhancement: The cavity length was tuned to achieve resonance with the Stokes line (572.67 nm), while the pump laser (532 nm) was coupled outside the DBR stopband.
Mode Characterization: The enhanced Raman signal was used to measure cavity mode dispersion and to perform single-shot imaging of the transverse electromagnetic (TEM) cavity mode profiles (q,n,m).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-specification MPCVD diamond materials required to replicate and advance this research in quantum photonics and solid-state qubit development.

Applicable Materials for Quantum Cavity Integration

The success of this experiment hinges on the use of high-purity, low-strain SCD. 6CCVD provides materials engineered specifically for quantum applications:

Research Requirement	6CCVD Material Recommendation	Technical Specification Match
High Coherence / Low Strain	Optical Grade Single Crystal Diamond (SCD)	Ultra-low nitrogen and defect density, ensuring minimal inhomogeneous strain fields and maximizing NV center spin coherence and phonon lifetime.
Thin Film Structure	Custom Thickness SCD Wafers	We supply SCD plates in thicknesses ranging from 0.1 µm up to 500 µm, ideal for subsequent thinning and membrane fabrication (e.g., the 1 µm membranes used here).
High-Q Interface	SCD Polishing Service (Ra < 1 nm)	Our proprietary polishing techniques achieve surface roughness Ra < 1 nm, critical for promoting strong van der Waals bonding and minimizing scattering losses at the diamond-air interface, thereby maintaining high cavity Q-factors (Qc,res = 8,200).
Alternative Qubit Hosts	Boron-Doped Diamond (BDD)	For research requiring electrochemical or sensing applications, 6CCVD offers BDD films, which can also serve as a host material for other color centers or quantum defects.

Customization Potential & Engineering Support

6CCVD’s in-house capabilities directly address the complex fabrication and integration challenges inherent in microcavity quantum systems:

Custom Dimensions and Geometry: The paper utilized small, square membranes (10-50 µm). 6CCVD offers precision laser cutting and dicing services to deliver SCD or PCD wafers in custom shapes and dimensions, ready for lithography and etching processes. We support plates/wafers up to 125 mm (PCD).
Advanced Metalization Services: While the paper used dielectric DBRs, many quantum devices require electrical contacts or integrated heaters. 6CCVD provides in-house metalization using materials such as Au, Pt, Pd, Ti, W, and Cu, enabling the integration of electrodes for Stark tuning or thermal control.
Substrate Supply: We can provide SCD substrates up to 10 mm thick for use as robust, high-thermal-conductivity carriers or as bulk material for high-power Raman laser applications (as cited in the paper).
Engineering Support: 6CCVD’s in-house PhD team specializes in material selection and optimization for quantum applications, including NV center coupling, quantum sensing, and high-power optics. We can assist researchers in selecting the optimal diamond grade and processing parameters to replicate or extend the high-performance cavity results demonstrated here.

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

View Original Abstract

We report cavity-enhanced Raman scattering from a single-crystal diamond membrane embedded in a highly miniaturized fully-tunable Fabry-P’{e}rot cavity. The Raman intensity is enhanced 58.8-fold compared to the corresponding confocal measurement. The strong signal amplification results from the Purcell effect. We show that the cavity-enhanced Raman scattering can be harnessed as a narrowband, high-intensity, internal light-source. The Raman process can be triggered in a simple way by using an optical excitation frequency outside the cavity stopband and is independent of the lateral positioning of the cavity mode with respect to the diamond membrane. The strong Raman signal emerging from the cavity output facilitates in situ mode-matching of the cavity mode to single-mode collection optics; it also represents a simple way of measuring the dispersion and spatial intensity-profile of the cavity modes. The optimization of the cavity performance via the strong Raman process is extremely helpful in achieving efficient cavity-outcoupling of the relatively weak emission of single color-centers such as nitrogen-vacancy centers in diamond or rare-earth ions in crystalline hosts with low emitter density.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.13.014036