[Paper Review] 1.4-mJ High Energy Terahertz Radiation from Lithium Niobates

Baolong Zhang, Zhenzhe Ma+9 more|arXiv (Cornell University)|Jul 18, 2020

Terahertz technology and applications60 references4 citations

TL;DR

This paper demonstrates the generation of 1.4-mJ terahertz (THz) pulses in lithium niobate crystals using cryogenically cooled, chirped, 214-mJ femtosecond laser pulses via the tilted pulse front technique. The method achieves 0.7% energy conversion efficiency and produces free-space THz fields of 6.3 MV/cm and 2.1 T, enabling extreme nonlinear THz applications and field-induced intervalley scattering in silicon.

ABSTRACT

Free-space super-strong terahertz (THz) electromagnetic fields offer multifaceted capabilities for reaching extreme nonlinear THz optics, accelerating and manipulating charged particles, and realizing other fascinating applications. However, the lack of powerful solid-state THz sources with single pulse energy >1 mJ is impeding the proliferation of extreme THz applications. The fundamental challenge lies in hard to achieve high efficiency due to high intensity pumping caused crystal damage, linear absorption and nonlinear distortion induced short effective interaction length, and so on. Here, through cryogenically cooling the crystals, delicately tailoring the pump laser spectra, chirping the pump pulses, and magnifying the laser energies, we first successfully realized the generation of 1.4-mJ THz pulses lithium niobates under the excitation of 214-mJ femtosecond laser pulses via tilted pulse front technique. The 800 nm-to-THz energy conversion efficiency reached 0.7%, and a free-space THz peak electric and magnetic fields reached 6.3 MV/cm and 2.1 Tesla. Our numerical simulations based on a frequencydomain second-order nonlinear wave equation under slowly varying envelope approximation reproduced the experimental optimization processes. To show the capability of this super-strong THz source, nonlinear absorption due to field-induced intervalley scattering effect in high conductive silicon induced by strong THz electric field was demonstrated. Such a high energy THz source with a relatively low peak frequency is very appropriate not only for electron acceleration towards table-top X-ray sources but also for extreme THz science and nonlinear applications.

Motivation & Objective

• To overcome the lack of high-energy solid-state THz sources with single-pulse energy >1 mJ.
• To address crystal damage, linear absorption, and nonlinear distortion limiting efficient THz generation in lithium niobate.
• To develop a scalable, high-efficiency method for generating super-strong THz fields suitable for extreme nonlinear optics.
• To demonstrate the capability of the source in inducing nonlinear effects such as field-induced intervalley scattering in high-conductivity silicon.
• To establish a platform for table-top X-ray sources and advanced THz science through high-field THz generation.

Proposed method

• Cryogenic cooling of lithium niobate crystals to reduce thermal damage and improve damage threshold.
• Tailoring the pump laser spectrum and applying temporal chirping to the pump pulses to extend effective interaction length.
• Utilizing the tilted pulse front technique to achieve spatially overlapped, high-intensity pump beams with large beam area.
• Employing a frequency-domain second-order nonlinear wave equation under the slowly varying envelope approximation for numerical simulation and optimization.
• Scaling laser pump energy to 214 mJ to maximize THz output while maintaining high conversion efficiency.
• Measuring and characterizing the generated THz pulses for peak electric and magnetic field strength.

Experimental results

Research questions

• RQ1 Can cryogenic cooling significantly enhance the damage threshold of lithium niobate for high-intensity THz generation?
• RQ2 To what extent can pulse chirping and spectral tailoring extend the effective interaction length in nonlinear THz generation?
• RQ3 What is the maximum single-pulse energy achievable in lithium niobate using the tilted pulse front technique with high-energy femtosecond lasers?
• RQ4 How strong are the free-space THz electric and magnetic fields produced, and can they induce measurable nonlinear effects?
• RQ5 Can this source enable new applications such as electron acceleration or nonlinear manipulation of charge carriers in semiconductors?

Key findings

• The experiment achieved 1.4-mJ single-pulse THz output from lithium niobate using 214-mJ femtosecond laser pulses.
• The energy conversion efficiency from 800 nm to THz radiation reached 0.7%, a significant improvement over previous methods.
• The generated free-space THz peak electric field reached 6.3 MV/cm, and the magnetic field reached 2.1 T, enabling extreme field studies.
• Numerical simulations based on the frequency-domain second-order nonlinear wave equation accurately reproduced the experimental optimization process.
• The strong THz field induced intervalley scattering in high-conductivity silicon, demonstrating nonlinear response under extreme field conditions.
• The source is suitable for applications in table-top X-ray generation and extreme nonlinear THz science due to its high energy and low peak frequency.