Microcavity platform for widely tunable optical double resonance
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2022-08-29 |
| Journal | Optica |
| Authors | Sigurd FlƄgan, Patrick Maletinsky, Richard J. Warburton, Daniel Riedel |
| Institutions | University of Calgary, University of Basel |
| Citations | 18 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled āExecutive Summaryā- Platform Innovation: Developed a widely-tunable, open-access Fabry-Perot (FP) microcavity utilizing a high-quality single-crystalline diamond membrane for enhanced nonlinear optics.
- Doubly-Resonant Enhancement: Achieved simultaneous resonance for both the pump and Stokes wavelengths, maximizing stimulated Raman scattering efficiency in the visible regime.
- Exceptional Quality Factors (Q): Experimentally measured pump Q-factors (Qp) exceeding 300,000 (297,000 Ā± 500) in the visible spectrum, promoting strong light-matter interaction.
- Widely Continuous Tuning: Demonstrated a continuous tuning range greater than 1 THz by exploiting a slight thickness gradient (0.17 nm/Āµm) in the diamond membrane via lateral displacement.
- Low Threshold Prediction: Current setup predicts a lasing threshold (Pth) of 187.5 mW, representing a significant reduction (more than an order of magnitude) compared to bulk diamond lasers.
- Sub-mW Potential: Theoretical modeling suggests that with optimized geometry (shorter air-gap, thicker diamond) and realistic surface scattering losses, the threshold can be reduced to 1.87 mW (sub-mW regime).
- Universal Frequency Shifter: The generic, open-access design paves the way for a universal, low-power frequency shifter and Raman laser, potentially integrating other wide-bandgap materials (e.g., AlN, SiC).
Technical Specifications
Section titled āTechnical Specificationsā| Parameter | Value | Unit | Context |
|---|---|---|---|
| Pump Q-Factor (Qp) | 297,000 Ā± 500 | Dimensionless | Measured at 473.233 THz |
| Stokes Q-Factor (Qs,dres) | 6,650 Ā± 50 | Dimensionless | At double resonance condition |
| Raman Shift (ĪVR) | 39.914 (or 1331.4) | THz (or cm-1) | Fixed phonon energy in diamond |
| Bulk Raman Gain (gRB) | ~40 | cm/GW | At pump wavelength range |
| Pump Wavelength (Ī»pcav) | 634.57 | nm | Double resonance condition |
| Stokes Wavelength (Ī»Rcav) | 693.13 | nm | Double resonance condition |
| Effective Mode Volume (VR) | 109.85 | Āµm3 | Current experimental geometry |
| Predicted Lasing Threshold (Pth) | 187.5 | mW | Based on current experimental parameters |
| Optimized Pth (w/ scattering) | 1.87 | mW | Predicted for optimized geometry (sub-mW regime) |
| Continuous Tuning Range | >1 | THz | Achieved via lateral diamond displacement |
| Diamond Thickness Gradient | 0.17 Ā± 0.2 | nm/Āµm | Used for in situ tuning |
| Diamond Membrane Size | ~20 x 20 x 0.8 | Āµm3 | Typical dimensions |
| Cavity Beam Waist (w0,I) | ~1 | Āµm | Gaussian fundamental mode |
| Mirror Curvature (Rcav) | ~10 | Āµm | Spherical microindentations |
| Pump Laser Wavelength Range | 630-640 | nm | CW narrowband tunable red diode laser |
Key Methodologies
Section titled āKey Methodologiesā- Microcavity Construction: A plano-concave Fabry-Perot cavity was assembled using two fused silica (SiO2) substrates coated with Distributed Bragg Reflectors (DBR). Spherical microindentations (Rcav ~10 Āµm) were fabricated on one substrate via CO2 laser ablation to define the Gaussian mode.
- Diamond Membrane Preparation: High-purity, (100)-cut single-crystal diamond was thinned to a membrane (~0.8 Āµm thick) using inductively-coupled reactive-ion etching and electron-beam lithography. A slight thickness gradient was intentionally introduced during the thinning process.
- Integration and Alignment: The diamond membrane (~20 x 20 Āµm2) was transferred and embedded onto the planar mirror using a micromanipulator, placing it within the cavity mode.
- In Situ Spatial and Spectral Tuning: Piezoelectric nanopositioners (attocube ANPx51, ANPz51) were used to control the mirror separation (air-gap ta) and the lateral position of the cavity mode relative to the diamond. This allowed precise control over both the absolute frequency and the effective diamond thickness (td).
- Double Resonance Verification: A tunable red diode laser (630-640 nm) was used as the pump. The double resonance condition (pump and Stokes resonant simultaneously, separated by ~40 THz) was confirmed by observing a strong signal enhancement (>3 orders of magnitude) in the cavity emission spectrum.
- Q-Factor Measurement: The cavity linewidth was measured by scanning the cavity length while monitoring reflected light. An Electro-Optic Modulator (EOM) was used to generate laser sidebands (Ā±3.9 GHz) which served as a precise frequency ruler for linewidth extraction.
- Tuning Range Demonstration: The lateral displacement of the cavity mode across the diamondās thickness gradient was used to continuously shift the double resonance condition, demonstrating a >THz continuous tuning range.
Commercial Applications
Section titled āCommercial ApplicationsāThe demonstrated platform and technology are highly relevant for advanced photonics and nonlinear optics, particularly in areas requiring low-power, tunable frequency conversion:
- Universal Frequency Shifters: Creation of highly versatile, low-power devices capable of shifting coherent light across large frequency gaps (tens of THz) using the fixed Raman shift of diamond.
- Exotic Wavelength Lasers: Generating coherent light in wavelength regimes (e.g., yellow, mid-infrared) that are inaccessible or inefficiently served by standard semiconductor laser diodes.
- Nonlinear Optics: The open-access, high-Q/V design is ideal for integrating materials exhibiting strong second-order (Ļ(2)) nonlinearity, enabling low-threshold processes such as:
- Second-Harmonic Generation (SHG).
- Sum- and Difference-Frequency Mixing (SFM/DFM).
- Material Integration: The generic platform design allows for the incorporation of other wide-bandgap Raman materials (e.g., Aluminum Nitride) or materials like Silicon Carbide (SiC), Lithium Niobate (LiNbO3), or Gallium Phosphide (GaP) for enhanced nonlinear performance.
- Quantum Photonics: The use of high-quality single-crystal diamond membranes in open microcavities is a core technology for coupling to solid-state quantum emitters (like NV or SiV centers), enabling enhanced photon generation and quantum network components.
View Original Abstract
Tunable open-access Fabry-Perot microcavities are versatile and widely applied in different areas of photonics research. The open geometry of such cavities enables the flexible integration of thin dielectric membranes. Efficient coupling of solid-state emitters in various material systems has been demonstrated based on the combination of high quality factors and small mode volumes with a large-range in situ tunability of the optical resonance frequency. Here, we demonstrate that by incorporating a diamond micromembrane with a small thickness gradient, both the absolute frequency and the frequency difference between two resonator modes can be controlled precisely. Our platform allows both the mirror separation and, by lateral displacement, the diamond thickness to be tuned. These two independent tuning parameters enable the double-resonance enhancement of nonlinear optical processes with the capability of tuning the pump laser over a wide frequency range. As a proof of concept, we demonstrate a <mml:math xmlns:mml=āhttp://www.w3.org/1998/Math/MathMLā display=āinlineā> <mml:mrow class=āMJX-TeXAtom-ORDā> <mml:mo>></mml:mo> </mml:mrow> <mml:mrow class=āMJX-TeXAtom-ORDā> <mml:mi mathvariant=ānormalā>T</mml:mi> <mml:mi mathvariant=ānormalā>H</mml:mi> <mml:mi mathvariant=ānormalā>z</mml:mi> </mml:mrow> </mml:math> continuous tuning range of doubly resonant Raman scattering in diamond, a range limited only by the reflective stopband of the mirrors. Based on the experimentally determined quality factors exceeding 300,000, our theoretical analysis suggests that, with realistic improvements, a <mml:math xmlns:mml=āhttp://www.w3.org/1998/Math/MathMLā display=āinlineā> <mml:mrow class=āMJX-TeXAtom-ORDā> <mml:mo>ā¼</mml:mo> </mml:mrow> <mml:mrow class=āMJX-TeXAtom-ORDā> <mml:mi mathvariant=ānormalā>m</mml:mi> <mml:mi mathvariant=ānormalā>W</mml:mi> </mml:mrow> </mml:math> threshold for establishing Raman lasing is within reach. Our findings pave the way to the creation of a universal, low-power frequency shifter. The concept can be applied to enhance other nonlinear processes such as second harmonic generation or optical parametric oscillation across different material platforms.