Pulse sequences for efficient multi-cycle terahertz generation in periodically poled lithium niobate.

Koustuban Ravi,Franz X Kärtner,Damian N Schimpf

doi:10.1364/oe.24.025582

Abstract

The use of laser pulse sequences to drive the cascaded difference frequency generation of high energy, high peak-power and multi-cycle terahertz pulses in cryogenically cooled (100 K) periodically poled Lithium Niobate is proposed and studied. Detailed simulations considering the coupled nonlinear interaction of terahertz and optical waves (or pump depletion), show that unprecedented optical-to-terahertz energy conversion efficiencies > 5%, peak electric fields of hundred(s) of mega volts/meter at terahertz pulse durations of hundred(s) of picoseconds can be achieved. The proposed methods are shown to circumvent laser induced damage limitations at Joule-level pumping by 1µm lasers to enable multi-cycle terahertz sources with pulse energies >> 10 milli-joules. Various pulse sequence formats are proposed and analyzed. Numerical calculations for periodically poled structures accounting for cascaded difference frequency generation, self-phase-modulation, cascaded second harmonic generation and laser induced damage are introduced. The physics governing terahertz generation using pulse sequences in this high conversion efficiency regime, limitations and practical considerations are discussed. It is shown that varying the poling period along the crystal length and further reduction of absorption can lead to even higher energy conversion efficiencies >>10%. In addition to numerical calculations, an analytic formulation valid for arbitrary pulse formats and closed-form expressions for important cases are presented. Parameters optimizing conversion efficiency in the 0.1-1 THz range, the corresponding peak electric fields, crystal lengths and terahertz pulse properties are furnished.

Highlights

Multi-cycle or narrowband terahertz pulses in the frequency range of 0.1 to 1 THz have garnered interest as drivers of compact particle acceleration [1, 2], coherent X-ray generation [3] and electron beam diagnostics
An impediment to the widespread deployment of these applications has been the inadequate development of accessible sources of narrowband terahertz radiation (hundred(s) of picoseconds pulse duration) with simultaneously high pulse energy (> 10 milli-joules) and peak powers (> 100 Mega-Volt per meter (MV/m) peak electric fields)
It is worth pointing out that recently we proposed another set of approaches employing terahertz driven cascaded parametric amplification which yield similar performance [32]

Summary

Introduction

Multi-cycle or narrowband terahertz pulses in the frequency range of 0.1 to 1 THz have garnered interest as drivers of compact particle acceleration [1, 2] , coherent X-ray generation [3] and electron beam diagnostics. With the rapid scaling of laser pulse energies, laser driven approaches employing second order nonlinear processes such as difference frequency generation (DFG) or optical rectification (OR) are promising. Scaling this approach to high terahertz pulse energies will require achieving high optical-to-terahertz energy conversion efficiencies (or conversion efficiency for short) as well as the development of high energy optical lasers. We describe approaches to improve conversion efficiencies for multi-cycle terahertz generation, which are still relatively low at the sub-percent level This problem must be distinguished from broadband or single-cycle source development where percent level conversion efficiencies have been demonstrated [7, 8, 9]. We only survey work pertinent to multi-cycle or narrowband sources

Methods

Results

Conclusion