A novel scheme for ultrashort terahertz pulse generation over a gapless wide spectral range - Raman-resonance-enhanced four-wave mixing
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-02-02 |
| Journal | Light Science & Applications |
| Authors | Jiaming Le, Yudan Su, Chuanshan Tian, A. H. Kung, Y. R. Shen |
| Institutions | State Key Laboratory of Surface Physics, University of California, Berkeley |
| Citations | 29 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThe research introduces a novel, highly effective scheme for generating ultrashort, energetic, and widely tunable Terahertz (THz) pulses using Raman-Resonance-Enhanced Four-Wave Mixing (R-FWM) in centrosymmetric diamond.
- Gapless Wide Tuning: The system successfully generates stable, few-cycle THz pulses continuously tunable from 5 THz to >20 THz, effectively filling the critical 5-12 THz âTHz gapâ important for materials science.
- High Field Strength: The output pulses carry up to 41 nJ of energy (at 17 THz) and achieve a peak electric field strength of approximately 1 MV cm-1 when focused (at 10 THz).
- Material Advantage: Diamond is utilized due to its exceptional properties: wide transparency (0 to 5.5 eV), low dispersion, and a high optical damage threshold (~7 TW cm-2), allowing for high input pump intensities necessary for third-order nonlinear effects.
- Mechanism: The process uses picosecond (ps) infrared (IR) pulses (E1, E2) to coherently excite the 40 THz Raman resonance in diamond, which then beats with a femtosecond (fs) IR pulse (E3) to generate the fs THz output.
- Pulse Quality: The generated THz pulses exhibit near-Gaussian spatial and temporal profiles, are transform-limited, and possess a stable, controllable Carrier-Envelope Phase (CEP).
- Scalability: The scheme is highly scalable. Future optimization using thicker, birefringent diamond plates could increase output energy by at least one order of magnitude, potentially reaching >1 ”J per pulse.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| THz Frequency Tuning Range | 5 to >20 | THz | Gapless, continuously tunable output. |
| Peak THz Pulse Energy (Max) | 41 | nJ | Achieved at 17 THz output frequency. |
| Peak THz Field Strength | ~1 | MV cm-1 | At 10 THz output, focused to ~100 ”m spot. |
| Energy Conversion Efficiency (Max) | 4 | % | R-FWM in diamond (at 17 THz). |
| Nonlinear Material | CVD Diamond | 0.5 mm | (001)-cut plate used in the experiment. |
| Diamond Raman Resonance | 40 | THz | Corresponds to 1332 cm-1 vibrational mode. |
| Diamond Optical Damage Threshold | ~7 | TW cm-2 | For ~60 fs pulses at 50 THz IR frequency. |
| Diamond Nonresonant Ï(3) | 4.6 x 10-14 | esu | Non-enhanced third-order susceptibility. |
| Input Pulse Width (E1, E2) | 1.3 | ps | Stretched pulses used to coherently excite the Raman resonance. |
| Input Pulse Width (E3) | 63 | fs | Used for frequency down-conversion/beating. |
| Input Energy (W1, W2, W3) | 85, 35, 10 | ”J | Energies incident on the overlapping area. |
| Phonon Dephasing Time (T2) | 4.8 | ps | Measured in diamond. |
| Potential Scaled Output Energy | >1 | ”J | Projected using >6 mm thick, birefringent diamond. |
Key Methodologies
Section titled âKey MethodologiesâThe R-FWM process relies on precisely timed and tuned input pulses interacting within a high-quality diamond crystal:
- Laser System: A commercial 33-fs, 5-W, 1-kHz Ti:sapphire laser system was used to pump two Optical Parametric Amplifiers (OPA-1 and OPA-2).
- Raman Excitation Pulses (E1, E2):
- Signal (E1) and idler (E2) from OPA-1 (centered at 206 THz and 166 THz, respectively) were generated such that their difference frequency was 40 THz (matching the Raman resonance).
- These pulses were positively chirped and stretched to 1.3 ps to maximize the coherent excitation of the narrow Raman resonance.
- Converting Pulse (E3):
- A 63-fs pulse was derived from OPA-2, tunable between 40 and 60 THz. This pulse beats with the excited vibrational wave Q(t) to generate the THz output (Es).
- Interaction Medium: A 0.5-mm thick, (001)-cut CVD diamond plate was used.
- Geometry and Phase Matching (PM):
- All three input beams were p-polarized and incident on the diamond at approximately 45°.
- Noncollinear phase matching was employed, where the angle (Ξ) between the input wave vectors (k1 and k2||K3) was adjusted to tune the output THz frequency (e.g., Ξ = 0.42° for 5 THz PM).
- Characterization:
- The THz output was analyzed using a Fourier Transform Infrared Interferometer (FTIR) for spectral measurements.
- Electro-Optic Sampling (EOS) in GaP was used for time-domain measurements, confirming few-cycle pulse structure and controllable Carrier-Envelope Phase (CEP).
Commercial Applications
Section titled âCommercial ApplicationsâThe development of a stable, high-quality, gapless THz source based on R-FWM in diamond opens new avenues for research and industrial applications:
- Ultrafast Dynamics and Spectroscopy:
- THz Gap Research: Provides a powerful tool for studying low-frequency excitations (phonons, magnons, intermolecular vibrations) in the previously difficult 5-12 THz range.
- Coherent Control: Enables coherent manipulation of material states (e.g., driving phase transitions, controlling superconductivity) using high-field, CEP-locked pulses.
- Advanced Materials Science:
- Nonlinear Optics: Facilitates the study of third-order nonlinear optical effects in wide band gap materials, leveraging diamondâs high damage threshold.
- Ferroelectricity/Superconductivity: Used as a strong pump source for optical-field-induced phenomena, such as transient ferroelectricity in quantum paraelectrics.
- High-Resolution Imaging and Sensing:
- THz Time-Domain Spectroscopy (TDS): The near-Gaussian spatial and temporal profiles of the pulses are ideal for high-fidelity THz imaging and spectroscopy systems.
- Diamond Technology Integration:
- High-Power THz Devices: The use of CVD diamond, known for its superior thermal and optical properties, suggests a path toward robust, high-repetition-rate THz sources suitable for industrial environments.
View Original Abstract
Abstract Ultrashort energetic terahertz (THz) pulses have created an exciting new area of research on light interactions with matter. For material studies in small laboratories, widely tunable femtosecond THz pulses with peak field strength close to MV cm â1 are desired. Currently, they can be largely acquired by optical rectification and difference frequency generation in crystals without inversion symmetry. We describe in this paper a novel scheme of THz pulse generation with no frequency tuning gap based on Raman-resonance-enhanced four-wave mixing in centrosymmetric media, particularly diamond. We show that we could generate highly stable, few-cycle pulses with near-Gaussian spatial and temporal profiles and carrier frequency tunable from 5 to >20 THz. They had a stable and controllable carrier-envelop phase and carried ~15 nJ energy per pulse at 10 THz (with a peak field strength of ~1 MV cm â1 at focus) from a 0.5-mm-thick diamond. The measured THz pulse characteristics agreed well with theoretical predictions. Other merits of the scheme are discussed, including the possibility of improving the THz output energy to a much higher level.