Gapless tunable intense terahertz pulse generation in strained diamond

Ultrashort intense terahertz (THz) pulses have created an exciting new area of research on light-matter interactions. These intense coherent sources allow researchers to not only observe novel nonlinear optical effects but to actively steer material properties, e.g. effectively "switching on" transient ferroelectricity, magnetism, and even superconductivity. Yet, a persistent "new THz gap" between 5 and 15 THz has long hindered the field. In this spectral window, conventional nonlinear crystals suffer from intense phonon absorption, leaving a significant void in our spectroscopic toolkit.

In 2023, a team of scientists led by Professor Y. Ron Shen from the University of California Berkeley and Professor Chuanshan Tian from Fudan University introduced a sophisticated solution for generation of intense broadband THz pulse covering over a gapless spectral range (Light Sci Appl 12, 34 (2023)). This scheme is based on a third order nonlinear process: Resonant enhanced four wave mixing(R-FWM) or Coherent Stokes Raman Scattering (CSRS) process in diamond. The process starts with coherent Raman excitation of phonons in the medium by a pair of input light pulses followed by beating of the coherent phonons with another input Mid infrared pulse to frequency down-convert to the THz frequency range. Owing to the strong Raman resonance enhancement and the high optical damage threshold of diamond, the induced THz polarization can approach that achieved via difference-frequency generation in second-order nonlinear crystals, enabling peak fields exceeding ~1 MV/cm at focus. More importantly, diamond is THz-transparent and by adjusting the input MIR frequency, the THz center frequency can be continuously tuned from 5 THz to MIR without any gap. While groundbreaking, that initial scheme was constrained by a non-collinear geometry required for phase matching, which limited the effective interaction length and, consequently, the total energy output.

Now, reporting in Light: Science & Applications, the same team has elegantly overcome this bottleneck. By introducing precise mechanical strain to the diamond crystal, they have successfully unlocked collinear phase matching across the challenging 5 to >15 THz range. The brilliance of this approach lies in its simplicity and efficiency. In the collinear configuration, the interacting optical and vibrational waves remain spatially overlapped over several millimeters. Compared with the earlier non-collinear implementation, the collinearly phase-matched scheme boosts the THz output energy by approximately a factor of three. The corresponding peak electric field strength at the focal point reaches about ~2.3 MV/cm at 10 THz. Notably, the energy conversion efficiency of collinearly phase matched CSRS can surpass that of THz pulse generation by the difference frequency generation in usual nonlinear crystals, thus being high enough to bridge the 5~15 THz“new THz gap”.

Beyond the experimental advance, the authors provide a rigorous theoretical framework. Their study elucidates how phonon-photon coupling dictates efficiency and offers a strategic roadmap for optimizing output in the strong-pump depletion regime.

This work is more than a technical refinement; it is a natural and powerful evolution that establishes strain-engineered diamond as a scalable, robust platform for intense THz generation. By finally bridging the 5–15 THz gap with a tunable, high-field source, the team has provided the community with a vital tool for driving and probing the next generation of ultrafast dynamics in quantum materials and complex molecular systems.