A novel scheme for ultrashort terahertz pulse generation over a gapless wide spectral range - Raman-resonance-enhanced four-wave mixing

At a Glance

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	University of California, Berkeley, State Key Laboratory of Surface Physics
Citations	29
Analysis	Full AI Review Included

Technical Documentation & Analysis: MPCVD Diamond for Ultrashort Terahertz Generation

Executive Summary

This research successfully demonstrates a novel, gapless scheme for generating high-quality, ultrashort Terahertz (THz) pulses using Raman-resonance-enhanced four-wave mixing (R-FWM) in diamond. This application leverages the unique material properties of MPCVD Single Crystal Diamond (SCD).

Core Achievement: Generation of stable, few-cycle THz pulses tunable from 5 to >20 THz, effectively covering the critical “THz gap” (5-15 THz).
Material Advantage: CVD diamond was selected for its exceptional transparency (0 to 5.5 eV) and extremely high optical damage threshold (~7 TW cm^-2), enabling the high input intensities necessary for resonant $\chi$⁽³⁾ enhancement.
Performance Metrics: Achieved peak THz field strengths up to 1.7 MV cm^-1 (41 nJ per pulse at 17 THz) from a 0.5-mm-thick SCD plate.
Scaling Potential: Theoretical projections indicate that using thicker SCD plates (up to 6 mm) and higher input energy could increase THz output energy by at least one order of magnitude, potentially reaching microjoule (µJ) levels.
6CCVD Value Proposition: 6CCVD is the ideal supplier for the required high-purity, custom-thickness SCD substrates (up to 10 mm) and precision polishing (Ra < 1 nm) needed to scale this R-FWM technology.

Technical Specifications

The following data points summarize the key material and performance parameters achieved using the CVD diamond plate in the R-FWM scheme, compared against critical material limits.

Parameter	Value	Unit	Context
THz Tuning Range	5 to >20	THz	Gapless spectral coverage achieved by R-FWM
Peak THz Field Strength	1.7	MV cm^-1	Achieved at 17 THz (41 nJ pulse energy)
THz Pulse Energy (Max)	41	nJ	Measured output energy at 17 THz
Conversion Efficiency (Max)	4	%	Ratio of THz output energy to fs input pulse energy (W_out/W_E3)
Optical Damage Threshold (Diamond)	~7	TW cm^-2	Critical limit allowing high input intensity
Raman Resonance Frequency	40	THz	Used for resonant enhancement
Resonantly Enhanced $\chi$⁽³⁾	2.8 x 10^-12	esu	Deduced effective third-order susceptibility
Diamond Thickness (Used)	0.5	mm	(001)-cut CVD plate used in experiment
Diamond Thickness (Required for PM)	>6	mm	Proposed thickness for collinear phase matching (PM)
SCD Polishing Requirement	Ra < 1	nm	Essential for minimizing scattering at high intensity

Key Methodologies

The THz generation relies on a four-wave mixing process enhanced by the diamond’s intrinsic Raman resonance.

Material Selection: A 0.5-mm-thick, (001)-cut CVD diamond plate was used as the centrosymmetric nonlinear medium, chosen for its high damage threshold and wide transparency.
Raman Excitation: Two picosecond (ps) input pulses (E₁ and E₂, centered at 206 THz and 166 THz, respectively) were used to coherently excite the transient vibrational wave, Q(t), associated with the 40 THz Raman resonance.
Frequency Conversion: A third femtosecond (fs) input pulse (E₃, tunable 40-60 THz) was introduced. The beating of E₃ with the excited vibrational wave Q(t) generated the fs THz pulse ($\omega_S = \omega_3 - \omega_1 + \omega_2$).
Phase Matching: Noncollinear phase matching geometry was employed, with all three p-polarized input beams incident on the diamond at approximately 45°.
Characterization: The resulting THz output was characterized using knife-edge beam profiling, a Fourier transform infrared interferometer (FTIR), and electro-optic sampling (EOS) to confirm near-Gaussian spatial/temporal profiles and controllable carrier-envelope phase (CEP).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to support the scaling and commercialization of R-FWM THz generation by providing the necessary high-specification diamond materials.

Applicable Materials

To replicate and extend this high-power THz generation research, the following 6CCVD material is required:

Optical Grade Single Crystal Diamond (SCD): High-purity, low-nitrogen SCD is mandatory to ensure minimal absorption in the IR and THz ranges and to maintain the high optical damage threshold (~7 TW cm^-2) required for the intense pump pulses.
Custom Orientation: The experiment utilized a (001)-cut plate. 6CCVD provides SCD wafers with precise crystallographic orientations, including (001), (110), and (111), to meet specific phase-matching requirements.

Customization Potential

The research explicitly identifies material thickness as the primary bottleneck for scaling output energy and achieving collinear phase matching (PM).

Research Requirement	6CCVD Capability	Benefit to Customer
Thick Substrates	SCD Substrates up to 10 mm thick	Exceeds the 6 mm thickness required for collinear PM, enabling potential µJ output levels.
Precision Polishing	SCD Polishing to Ra < 1 nm	Ensures ultra-low surface scattering losses, critical for maintaining beam quality and preventing damage at TW cm^-2 intensities.
Custom Dimensions	Plates/wafers up to 125 mm (PCD)	While SCD is used here, 6CCVD can provide custom dimensions for large-area scaling of the beam overlap area, which directly increases THz output energy.
Birefringence/Stress	Custom Material Engineering	The paper notes that uniaxial stress (~1 GPa) could enable collinear PM. 6CCVD can assist in sourcing or engineering materials suitable for post-processing stress application.
Metalization (Future)	Au, Pt, Pd, Ti, W, Cu (Internal)	Although not used in this R-FWM scheme, 6CCVD offers custom metalization for integrated diamond devices (e.g., BDD electrodes or heat sinks).

Engineering Support

6CCVD’s commitment extends beyond material supply. Our in-house team of PhD material scientists and engineers provides critical support for advanced applications:

Material Optimization: We assist researchers in selecting the optimal SCD grade, thickness, and orientation to maximize conversion efficiency and achieve the highest possible peak field strength for Ultrafast Terahertz Generation projects.
Scaling Consultation: We provide technical guidance on material specifications required to transition from noncollinear to collinear phase matching, facilitating the path toward high-energy (µJ) THz pulse generation.
Global Logistics: We ensure reliable, global shipping (DDU default, DDP available) of high-value diamond materials, minimizing lead times for critical research projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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 −1 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 −1 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.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41377-023-01071-z