Time-resolved cathodoluminescence in an ultrafast transmission electron microscope
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-08-09 |
| Journal | Applied Physics Letters |
| Authors | Sophie Meuret, Luiz H. G. Tizei, Florent Houdellier, SĂ©bastien Weber, Yves Auad |
| Institutions | Centre dâĂlaboration de MatĂ©riaux et dâĂtudes Structurales, Laboratoire dâOptique AppliquĂ©e |
| Citations | 21 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis paper reports the successful implementation and demonstration of Time-Resolved Cathodoluminescence (TR-CL) spectroscopy within an Ultrafast Transmission Electron Microscope (UTEM), opening a new domain for nanoscale dynamics studies.
- First UTEM TR-CL: This is the first reported time-resolved cathodoluminescence study performed in an Ultrafast Transmission Electron Microscope (UTEM).
- High Resolution Achieved: The technique simultaneously achieves high spatial resolution (12 nm) and sub-nanosecond temporal resolution (Instrumental Response Function ~873 ps).
- Instrumentation Core: The system utilizes a laser-driven cold-field emission source generating 400 fs electron pulses and a high numerical aperture parabolic mirror for efficient light collection, crucial for low-signal UTEM operation.
- Target System: The method was validated by mapping the excited state lifetime of Nitrogen-Vacancy (NV) centers in nano-diamonds.
- Key Finding: Spatially-resolved lifetime maps revealed significant local variations in the NV center lifetime, decreasing from 23.4 ns ± 0.7 ns to 15.8 ns ± 0.8 ns over distances less than 50 nm within the diamond cluster.
- Correlative Potential: This development enables correlative studies, combining atomic-scale structural and chemical analysis (TEM/EELS) with the dynamics of light emission (TR-CL) in optically active nanostructures.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Electron Acceleration Voltage | 150 | keV | Operating voltage of the Hitachi HF2000 UTEM |
| Electron Pulse Width | 400 | fs | Duration of the pulsed electron beam |
| Repetition Rate | 2 | MHz | Frequency of the femtosecond laser trigger |
| Spatial Resolution (Effective) | 12 | nm | Resolution achieved in the lifetime maps (after 4x binning) |
| Spatial Resolution (Probe Size) | 5 | nm | Electron probe size used for decay trace acquisition |
| Temporal Resolution (IRF) | 873 | ps | Instrumental Response Function derived from the fit |
| Pulsed Electron Beam Current | ~100 | fA | Current on the sample (low signal environment) |
| NV0 Zero-Phonon Line (ZPL) | 575 | nm | Characteristic emission line of the neutral NV center |
| Measured Lifetime (Maximum) | 23.4 ± 0.7 | ns | Maximum lifetime observed in the nano-diamond cluster |
| Measured Lifetime (Minimum) | 15.8 ± 0.8 | ns | Minimum lifetime observed in the nano-diamond cluster |
| Nano-diamond Mean Diameter | ~100 | nm | Size of the studied diamond particles |
| Photon Detection Rate | 40 | Cts/s | Detected photons above dark noise (200 s integration) |
Key Methodologies
Section titled âKey MethodologiesâThe experiment relies on a highly customized Ultrafast Transmission Electron Microscope (UTEM) setup integrated with advanced photon detection and correlation hardware.
- Ultrafast Electron Source: A femtosecond laser (515 nm) is used to trigger the emission of 400 fs electron pulses from a sharp tungsten tip (cold-field emission source) at a 2 MHz repetition rate.
- Electron Beam Excitation: The 150 keV pulsed electron beam is focused into a nanometric probe (down to 5 nm spot size) and scanned over the sample in STEM mode.
- High-Efficiency CL Collection: Cathodoluminescence (CL) generated by the electron interaction is collected using a high numerical aperture parabolic mirror positioned close to the sample, maximizing photon flux.
- Signal Routing and Correlation: The collected light is coupled via an optical fiber to a Single Photon Counting Module (SPCM). The SPCM output is fed into a correlator, which measures the delay histogram between the laser trigger signal and the detected CL photons.
- Time-Resolved Mapping: The electron beam is raster-scanned over the sample area (200 nm region). A decay trace is recorded at each pixel (6 nm width, 5s integration time).
- Data Binning and Fitting: To achieve sufficient Signal-to-Noise Ratio (SNR) for accurate fitting, four adjacent pixels were binned, yielding an effective spatial resolution of 12 nm and an integration time of 20s per effective pixel.
- Lifetime Extraction: The decay traces were fitted using the convolution of an exponential decay (representing the lifetime, Ï) and a Gaussian function (representing the Instrumental Response Function, IRF) to extract the spatially-resolved lifetime maps.
Commercial Applications
Section titled âCommercial ApplicationsâThe ability to map excited state dynamics with nanometer precision in an electron microscope environment has direct implications for several high-tech sectors, particularly those relying on quantum emitters and nanoscale optoelectronics.
- Quantum Technology:
- NV Center Characterization: Essential for quality control and optimization of solid-state quantum emitters (like NV centers) used in quantum sensing (magnetometry, thermometry) and quantum computing.
- Qubit Engineering: Characterizing how local strain, defects, and surface effects influence the coherence and lifetime of engineered qubits.
- Optoelectronics and Photonics:
- Device Optimization: Studying carrier relaxation dynamics in semiconductor devices (LEDs, quantum wells, nanowires) to correlate structural defects (e.g., threading dislocations) with non-radiative recombination pathways.
- Plasmonics: Investigating the Purcell effect and coupling between emitters and localized surface plasmons at the nanometer scale, crucial for designing efficient nanoscale light sources.
- Advanced Materials Science:
- Defect Analysis: Spatially resolving the optical properties associated with atomic defects and color centers in wide bandgap materials (e.g., diamond, GaN, ZnO).
- Correlative Microscopy Platforms: Integration of TR-CL with existing TEM/STEM capabilities (EELS, diffraction imaging) to provide a comprehensive, multi-modal analysis of functional nanomaterials.
View Original Abstract
Ultrafast transmission electron microscopy (UTEM) combines sub-picosecond time-resolution with the versatility of TEM spectroscopies. It allows us to study the ultrafast materialsâ response using complementary techniques. However, until now, time-resolved cathodoluminescence was unavailable in UTEM. In this paper, we report time-resolved cathodoluminescence measurements in an ultrafast transmission electron microscope. We mapped the spatial variations of the emission dynamics from nano-diamonds with a high density of NV centers with a 12 nm spatial resolution and sub-nanosecond temporal resolution. This development will allow us to study the emission dynamics from quantum emitters with a unique spatiotemporal resolution and benefit from the wealth of complementary signals provided by transmission electron microscopes. It will further expand the possibilities of ultrafast transmission electron microscopes, paving the way to the investigation of the quantum aspects of an electron/sample interaction.
Tech Support
Section titled âTech SupportâOriginal Source
Section titled âOriginal SourceâReferences
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