Dephasing by optical phonons in GaN defect single-photon emitters
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-05-29 |
| Journal | Scientific Reports |
| Authors | Yifei Geng, Jialun Luo, Len van Deurzen, Huili Grace Xing, C. Jena |
| Institutions | Cornell University |
| Citations | 10 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research investigates the temperature-dependent dephasing mechanisms in GaN defect single-photon emitters (SPEs), providing critical insight into linewidth broadening for quantum applications.
- Novel Dephasing Mechanism: The study proposes and confirms that the ZPL linewidth broadening is caused by the absorption/emission of low-energy optical phonons (specifically the E2(low) mode) via an elastic Raman process, rather than the typical acoustic phonon interactions (T3, T5, T7 power laws) observed in diamond or SiC SPEs.
- Phonon Energy Match: The optical phonon energy extracted from fitting the temperature-dependent linewidth data is 19 meV (± 0.5 meV), which matches the zone center energy of the lowest Raman-active optical phonon band (E2(low) at ~18 meV) in bulk wurtzite GaN.
- Temperature Regimes: Below 50 K, the ZPL lineshape is Gaussian, dominated by spectral diffusion (FWHM 0.7-1 meV). Above 50 K, the lineshape evolves into a Lorentzian, where the optical phonon interaction dominates the broadening.
- High Purity and Speed: The GaN SPEs exhibit strong single-photon emission (g(2)(0) < 0.2) and fast lifetimes (2.2-3.2 ns), making them promising for high-repetition-rate quantum communication.
- Engineering Implication: The broad ZPL linewidths (up to 7.12 meV at 270 K) caused by this strong optical phonon coupling pose a significant challenge for generating highly indistinguishable photons required for scalable quantum systems.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Material System | GaN (HVPE grown) | N/A | Semi-insulating epitaxial layers on sapphire. |
| SPE Emission Range | 600-700 | nm | Visible wavelength range at room temperature. |
| Fitted Optical Phonon Energy (ħÏop) | 19 ± 0.5 | meV | Energy responsible for dephasing via elastic Raman process. |
| Bulk GaN Phonon Match | ~18 | meV | Zone center energy of the E2(low) optical phonon mode. |
| Temperature Range Studied | 10-270 | K | Range over which ZPL spectra were measured. |
| Low-T Linewidth (FWHM, < 50 K) | 0.7-1.0 | meV | Dominated by spectral diffusion (Gaussian lineshape). |
| High-T Linewidth (FWHM, 270 K) | 6.82-7.12 | meV | Dominated by optical phonon dephasing (Lorentzian lineshape). |
| Single-Photon Purity (g(2)(0)) | 0.17 (E3), 0.19 (E4) | N/A | Confirms single-photon emission at room temperature. |
| Excited State Lifetime (Ï1) | 2.2 ± 0.17 to 3.18 ± 0.24 | ns | Fast decay component of the second-order correlation function. |
| Photon Collection Enhancement | ~4.5 | Factor | Achieved using Solid Immersion Lenses (SILs). |
| SIL Radius | 2.5 | ”m | Hemisphere fabricated via Focused Ion Beam (FIB) milling. |
| Spectral Resolution (Setup) | ~0.18 | meV | Measured at 650 nm wavelength. |
Key Methodologies
Section titled âKey MethodologiesâThe study combined advanced material growth, nanofabrication, and cryogenic spectroscopy to characterize the GaN SPEs.
-
Material Growth:
- SPEs were hosted in ~4 ”m thick semi-insulating GaN epitaxial layers.
- Growth method: Hydride Vapor Phase Epitaxy (HVPE).
- Substrate: 430 ”m thick sapphire (Ga-polar growth).
-
Nanofabrication for Collection Enhancement:
- Solid Immersion Lenses (SILs) were fabricated as hemispheres (radius 2.5 ”m) directly on top of the SPEs.
- Fabrication method: Focused Ion Beam (FIB) milling of GaN.
- Surface preparation: A 30 nm Al layer was sputtered prior to milling to prevent ion beam deflection due to charge accumulation, and subsequently removed via wet etch.
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Optical Characterization Setup:
- Excitation: Custom built confocal scanning microscope using a 532 nm pump laser.
- Detection: Collected Photoluminescence (PL) was split 50:50 into a spectrometer (for ZPL analysis) and a Hanbury-Brown and Twiss setup (for g(2)(Ï) measurement).
- Photon Counting: Time-tagged time-resolved (TTTR) mode using a MultiHarp150 correlator.
- Objectives: 0.9 NA objective for room temperature; 0.7 NA objective with a correction collar for cryogenic measurements.
-
Temperature-Dependent Spectroscopy:
- Samples were mounted inside a cryostat for measurements in the 10 K to 270 K range.
- ZPL spectra were analyzed using Voigt functions (convolution of Gaussian and Lorentzian components) to extract the temperature dependence of the linewidth (FWHM).
-
Theoretical Modeling:
- A theoretical model based on the elastic scattering of thermally excited optical phonons from the defect was developed.
- The model fit the Lorentzian component linewidth (fL = 2Îł) using a dephasing rate Îł proportional to $n(\omega_{op})[n(\omega_{op}) + 1]$, where $n(\omega_{op})$ is the thermal boson occupation factor.
Commercial Applications
Section titled âCommercial ApplicationsâThe unique properties of GaN SPEs, particularly their brightness, stability, and integration potential within a mature semiconductor platform, target several high-value engineering sectors.
| Application Area | Relevance to GaN SPE Technology | Engineering Significance |
|---|---|---|
| Quantum Communication | Bright, stable, and fast single-photon sources (lifetimes ~2-3 ns) operating in the visible range. | Enables high-repetition-rate quantum key distribution (QKD) and quantum networking protocols. |
| Integrated Photonics | GaN is a technologically mature, wide bandgap material compatible with high-quality epitaxy and integration with existing photonic devices (waveguides, resonators). | Facilitates the creation of scalable, on-chip quantum light sources and quantum circuits. |
| Quantum Computing | SPEs with addressable spin states (though not explicitly confirmed for these GaN defects) are crucial for solid-state quantum memory and qubit operations. | GaNâs wurtzite structure, shared with AlN and hBN, suggests potential for robust defect engineering in Group III-V nitrides. |
| Quantum Sensing/Metrology | Defect states are sensitive to local electric and magnetic fields. | Potential for high-resolution sensing platforms, leveraging the stability and brightness of the GaN host material. |
| Material Science & Defect Engineering | The identification of the E2(low) optical phonon as the dominant dephasing mechanism provides a roadmap for mitigating linewidth broadening in GaN and related nitrides (AlN, hBN). | Directs future material growth efforts toward minimizing coupling to low-energy optical modes to improve photon indistinguishability. |
View Original Abstract
Abstract Single-photon defect emitters (SPEs), especially those with magnetically and optically addressable spin states, in technologically mature wide bandgap semiconductors are attractive for realizing integrated platforms for quantum applications. Broadening of the zero phonon line (ZPL) caused by dephasing in solid state SPEs limits the indistinguishability of the emitted photons. Dephasing also limits the use of defect states in quantum information processing, sensing, and metrology. In most defect emitters, such as those in SiC and diamond, interaction with low-energy acoustic phonons determines the temperature dependence of the dephasing rate and the resulting broadening of the ZPL with the temperature obeys a power law. GaN hosts bright and stable single-photon emitters in the 600-700 nm wavelength range with strong ZPLs even at room temperature. In this work, we study the temperature dependence of the ZPL spectra of GaN SPEs integrated with solid immersion lenses with the goal of understanding the relevant dephasing mechanisms. At temperatures below ~ 50 K, the ZPL lineshape is found to be Gaussian and the ZPL linewidth is temperature independent and dominated by spectral diffusion. Above ~ 50 K, the linewidth increases monotonically with the temperature and the lineshape evolves into a Lorentzian. Quite remarkably, the temperature dependence of the linewidth does not follow a power law. We propose a model in which dephasing caused by absorption/emission of optical phonons in an elastic Raman process determines the temperature dependence of the lineshape and the linewidth. Our model explains the temperature dependence of the ZPL linewidth and lineshape in the entire 10-270 K temperature range explored in this work. The ~ 19 meV optical phonon energy extracted by fitting the model to the data matches remarkably well the ~ 18 meV zone center energy of the lowest optical phonon band ( $$E_{2}(low)$$ <mml:math xmlns:mml=âhttp://www.w3.org/1998/Math/MathMLâ><mml:mrow><mml:msub><mml:mi>E</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>l</mml:mi><mml:mi>o</mml:mi><mml:mi>w</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> ) in GaN. Our work sheds light on the mechanisms responsible for linewidth broadening in GaN SPEs. Since a low energy optical phonon band ( $$E_{2}(low)$$ <mml:math xmlns:mml=âhttp://www.w3.org/1998/Math/MathMLâ><mml:mrow><mml:msub><mml:mi>E</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>l</mml:mi><mml:mi>o</mml:mi><mml:mi>w</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math> ) is a feature of most group III-V nitrides with a wurtzite crystal structure, including hBN and AlN, we expect our proposed mechanism to play an important role in defect emitters in these materials as well.