Ultrafast room-temperature valley manipulation in silicon and diamond
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-04-14 |
| Journal | Nature Physics |
| Authors | Adam Gindl, Martin Ämel, F. TrojĂĄnek, P. MalĂ˝, Martin KozĂĄk |
| Institutions | Charles University |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research demonstrates a novel, ultrafast method for generating and reading out valley-polarized electron populations in bulk silicon and diamond at room temperature, paving the way for next-generation valleytronic devices.
- Core Achievement: Successful generation and detection of valley-polarized electron populations in bulk Silicon and Diamond using non-resonant optical methods.
- Speed and Frequency: Valley manipulation occurs on subpicosecond timescales, supporting the development of valleytronic devices operating at terahertz (THz) frequencies.
- Mechanism: Unidirectional intervalley scattering of electrons, accelerated by the oscillating electric field of linearly polarized infrared femtosecond pulses (strong-field interaction).
- Relaxation Dynamics: Valley polarization relaxation times (Trel) at room temperature were measured at 730 fs in Silicon and 9.7 ps in Diamond.
- Switching Capability: Subpicosecond switching of the valley polarization direction was demonstrated using a pair of orthogonally polarized pump pulses separated by 1.4 ps.
- Compatibility: The technique is compatible with contemporary silicon-based technology and is universal, requiring only anisotropic effective mass and energy-dependent intervalley scattering, applicable to many bulk semiconductors.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Operating Temperature | 295 | K | Room temperature experiments. |
| Valley Polarization (V) | â 0.10 | (rel.u.) | Simulated maximum degree in Silicon. |
| Valley Polarization (V) | â 0.33 | (rel.u.) | Simulated maximum degree in Diamond. |
| Relaxation Time (Si) | 730 | fs | Room temperature, N = 1.8 x 1017 cm-3. |
| Relaxation Time (Diamond) | 9.7 | ps | Room temperature, N = 6.4 x 1016 cm-3. |
| Pump Photon Energy | 0.62 | eV | Central wavelength 2000 nm (Infrared). |
| Pump Pulse Duration (Tp) | 40 | fs | Full-width at half-maximum. |
| Peak Electric Field (F0) Si | 0.7 | V nm-1 | Applied in Silicon experiments. |
| Peak Electric Field (F0) Diamond | 1.3 | V nm-1 | Applied in Diamond experiments. |
| Longitudinal Effective Mass (Si) | 0.92 | m0 | Relative to free electron mass. |
| Transverse Effective Mass (Si) | 0.19 | m0 | Relative to free electron mass. |
| Switching Delay | 1.4 | ps | Time delay between orthogonal pump pulses. |
| Low Temperature Trel (Si) | â 80 | ns | Measured at 7 K. |
| Low Temperature Trel (Diamond) | â 10 | ns | Measured at 7 K. |
Key Methodologies
Section titled âKey MethodologiesâThe experimental setup utilized a pump-probe spectroscopy scheme combined with numerical Monte Carlo simulations to model electron dynamics.
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Carrier Excitation (Pre-excitation):
- Electrons and holes were generated using indirect single-photon excitation (1.2 eV) in Silicon or two-photon excitation (3.6 eV) in Diamond.
- A waiting time of 100 ps was enforced after excitation to ensure the electronic system relaxed to the lattice temperature and achieved an isotropic distribution across all six degenerate conduction band valleys.
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Valley Polarization Generation (Pump):
- A linearly polarized infrared femtosecond pump pulse (0.62 eV, 40 fs) was applied along the [100] or [010] crystallographic direction.
- The strong oscillating electric field accelerated electrons anisotropically, leading to higher intervalley scattering rates for electrons in valleys with a low effective mass in the direction of the field, thus generating valley polarization.
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Valley Polarization Detection (Probe):
- The valley polarization was detected by measuring the polarization anisotropy (ÎÎą) of free-carrier absorption using a linearly polarized probe pulse (0.62 eV, 40 fs).
- The probe polarization was rotated 45° relative to the pump polarization to maximize sensitivity to the anisotropic electron distribution.
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Ultrafast Switching Demonstration:
- A pair of infrared pump pulses, separated by 1.4 ps, were used. The first pulse generated polarization along [100]. The second pulse, polarized orthogonally along [010], switched the direction of the valley polarization on a subpicosecond timescale.
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Numerical Modeling:
- The Boltzmann transport equation was solved using a Monte Carlo approach to simulate electron dynamics, incorporating the anisotropic effective mass tensor and energy-dependent intravalley and intervalley (f-scattering) electron-phonon scattering mechanisms.
Commercial Applications
Section titled âCommercial ApplicationsâThis technology, based on ultrafast valley manipulation in standard semiconductor materials, has significant implications for future electronics and information technology.
- Terahertz (THz) Electronics: Enables the creation of logic and memory devices operating at THz clock speeds, far exceeding the limits of conventional charge-based electronics.
- Valleytronic Memory and Logic: Provides a pathway for storing and processing information using the electronâs valley quantum number rather than its charge or spin, potentially leading to higher density and lower power consumption.
- Compatibility with Silicon Technology: The use of bulk silicon allows for seamless integration into existing semiconductor fabrication infrastructure, accelerating commercialization.
- Advanced Semiconductor Materials: The non-resonant nature of the technique makes it applicable to other technologically important bulk semiconductors and dielectric crystals (e.g., GaN, SiC) that possess multiple degenerate conduction band minima.
- Ultrafast Sensing: Potential use in ultrafast optical or electrical sensors that rely on manipulating and reading anisotropic carrier distributions in momentum space.
View Original Abstract
Abstract Some semiconductors have more than one degenerate minimum of the conduction band in their band structure. These minimaâknown as valleysâcan be used for storing and processing information, if it is possible to generate a difference in their electron populations. However, to compete with conventional electronics, it is necessary to develop universal and fast methods for controlling and reading the valley quantum number of the electrons. Even though selective optical manipulation of electron populations in inequivalent valleys has been demonstrated in two-dimensional crystals with broken time-reversal symmetry, such control is highly desired in many technologically important semiconductor materials, including silicon and diamond. We demonstrate an ultrafast technique for the generation and read-out of a valley-polarized population of electrons in bulk semiconductors on subpicosecond timescales. The principle is based on the unidirectional intervalley scattering of electrons accelerated by an oscillating electric field of linearly polarized infrared femtosecond pulses. Our results are an advance in the development of potential room-temperature valleytronic devices operating at terahertz frequencies and compatible with contemporary silicon-based technology.