Cavity-Enhanced Raman Scattering for In Situ Alignment and Characterization of Solid-State Microcavities
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-01-22 |
| Journal | Physical Review Applied |
| Authors | Daniel Riedel, Sigurd FlÄgan, Patrick Maletinsky, Richard J. Warburton |
| Institutions | University of Basel |
| Citations | 25 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research demonstrates the use of cavity-enhanced Raman scattering as a powerful, generic tool for the in situ alignment and characterization of tunable solid-state microcavities, specifically using diamond membranes.
- Signal Enhancement: The Stokes Raman signal from the diamond host material is enhanced 58.8-fold compared to confocal measurement, resulting from the Purcell effect (calculated Fp = 4.7).
- Alignment Tool: The strong, narrowband Raman emission (Stokes wavelength 572.67 nm) acts as a ubiquitous internal light source, enabling fast and precise alignment of the cavity mode to external single-mode collection optics.
- Lateral Independence: Unlike single emitters, the Raman signal depends only weakly on the lateral position, facilitating alignment regardless of the exact location of a specific qubit.
- Mode Characterization: The strong signal allows for rapid, single-shot imaging of the transverse electromagnetic (TEM) cavity mode profiles (e.g., beam waist w0 = 0.88 ”m) and fast determination of mode dispersion and geometric parameters (e.g., mirror radius Rcav = 10 ”m).
- Cavity Performance: The Fabry-Pérot cavity, incorporating a 0.77 ”m thick diamond membrane, achieved high performance with a finesse of ~1,000 and Q-factors up to 9,600.
- Generic Utility: This technique is applicable to optimizing the performance and collection efficiency for various low-density solid-state emitters, such as nitrogen-vacancy (NV) centers in diamond or rare-earth ions in crystalline hosts, at both room and cryogenic temperatures.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Raman Enhancement Factor (F) | 58.8 | N/A | Ratio of cavity-enhanced to confocal signal. |
| Calculated Purcell Factor (Fp) | 4.7 | N/A | Enhancement of Stokes photon generation rate. |
| Cavity Finesse (F) | ~1,000 | N/A | At Stokes wavelength (~573 nm). |
| Cavity Q-Factor (Qc,res) | 8,200 to 9,600 | N/A | Measured on resonance (572 nm to 573.4 nm). |
| Diamond Raman Shift | 1,332 | cm-1 | Optical phonon energy. |
| Pump Wavelength | 532 | nm | Green laser excitation (outside DBR stopband). |
| Stokes Emission Wavelength | 572.67 | nm | First-order Stokes scattering. |
| Diamond Membrane Thickness (td) | 0.77 | ”m | Used in coupled-cavity model simulation. |
| Top Mirror Radius of Curvature (Rcav) | 10 | ”m | Fabricated via CO2 laser ablation. |
| Fundamental Mode Beam Waist (w0) | 0.88 | ”m | Measured via Gaussian fit of CCD image. |
| Effective Mode Volume (Veff) | 84.9(1/n)3 | N/A | Calculated for cubic resonator in diamond. |
| Cavity Linewidth (Ύλc) | 70 | pm | Corresponds to 55 GHz (on resonance). |
| Stokes Linewidth (ÎŽÎœS) | 47.8 | GHz | Measured independently (corresponds to 3.9 ps phonon lifetime). |
| DBR Layer Structure (Bottom) | 15 layers SiO2/Ta2O5 | N/A | Planar mirror. |
| DBR Stopband Center (λcenter) | 625 | nm | Calculated for the planar mirror. |
Key Methodologies
Section titled âKey MethodologiesâThe experiment relies on precise nanofabrication, bonding, and a highly tunable optical setup:
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Diamond Membrane Fabrication:
- High-purity, single-crystal diamond (Element 6) was patterned using electron-beam lithography and inductively-coupled plasma (ICP) etching.
- Membranes were fabricated with typical dimensions of 1 ”m thickness and 10 to 50 ”m side lengths.
- NV centers were introduced prior to nanofabrication via nitrogen-ion implantation and subsequent annealing.
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Curved Mirror Template Creation:
- A SiO2 substrate was patterned using focused CO2 laser ablation to create microindentations, yielding atomically-smooth curved templates with a radius of curvature (Rcav) of approximately 10 ”m.
- The templates were coated with a highly reflective 14-layer Ta2O5/SiO2 DBR.
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Cavity Assembly and Bonding:
- The diamond membrane was transferred and bonded to the planar DBR substrate using a micromanipulator, relying on strong van der Waals forces due to the smooth surfaces.
- The assembled structure was placed on a z-nanopositioner (for tuning cavity length) and x,y-nanopositioners (for lateral alignment).
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Optical Excitation and Tuning:
- The system was pumped with a continuous-wave green laser (532 nm), which lies outside the DBR stopband, allowing excitation independent of mirror separation.
- The cavity resonance frequency (λc) was tuned in situ by adjusting the air-gap width (ta) via voltage applied to the z-nanopositioner.
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Raman Signal Detection and Analysis:
- The Stokes signal (572.67 nm) was collected via the excitation objective, filtered heavily to remove pump light and fiber fluorescence, and coupled into a single-mode fiber leading to a spectrometer.
- The strong Raman signal was used to monitor the cavity output while tuning the cavity length (dispersion measurement) and to directly image the spatial intensity distribution of the TEM modes onto a CCD camera (single-shot imaging).
Commercial Applications
Section titled âCommercial ApplicationsâThe methodologies and results presented are highly relevant for the development of next-generation quantum and photonic devices:
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Quantum Networking and Communication:
- High-Rate Entanglement: The ability to efficiently couple NV center ZPL emission into a cavity mode is critical for achieving high entanglement rates necessary for long-distance quantum links.
- Solid-State Qubits: Optimization of light collection from low-density solid-state emitters (NV centers, SiV centers, rare-earth ions like Erbium) in crystalline hosts.
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Advanced Photonic Integration:
- Microcavity Manufacturing: The technique provides a robust, non-destructive method for quality control and performance verification of tunable Fabry-PĂ©rot microcavities, ensuring optimal mode-matching and high Q-factors.
- Internal Light Sources: Utilizing cavity-enhanced Raman scattering to create stable, narrowband internal light sources for integrated photonic circuits and sensing applications.
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Cryogenic Quantum Systems:
- The demonstration that the Raman alignment technique works effectively at cryogenic temperatures (liquid helium bath) ensures its utility for quantum experiments where phonon-related broadening must be eliminated.
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High-Precision Sensing:
- The diamond host material, known for its high Raman gain and stability, combined with cavity enhancement, could be leveraged in highly sensitive Raman spectroscopy systems or phonon-based quantum memories.
View Original Abstract
We report cavity-enhanced Raman scattering from a single-crystal diamond membrane embedded in a highly miniaturized fully-tunable Fabry-Pâ{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.