Abstract-Terahertz Acceleration Technology Towards Compact Light Sources

Arya Fallahi

https://arxiv.org/abs/1906.06186

The concepts in this thesis comprise three groups focusing on: (1) fast electron sources, (2) THz injectors, and (3) THz linacs. First, the feasibility of ultrafast, high-yield electron emitters based on nanostructured cathodes is demonstrated. Benefitting from field enhancement effects, namely tip-enhancement and plasmonic enhancement, laser-induced field emission is realized over large, dense and highly uniform field emitter arrays. The theoretical principles of these field emitter arrays are studied and their suitability for pico-Coulomb charge production over femtosecond time-scales is confirmed. In the framework of THz injectors, two ground-breaking concepts including ultrafast single-cycle THz guns and segmented THz electron accelerator and manipulator (STEAM) devices are developed and tested. The possibility of using transient fields to realize ultrahigh acceleration gradients close to 0.5 GeV/m is confirmed. Specifically, a STEAM device capable of performing multiple high-field operations on the 6D-phase-space of ultrashort electron bunches is demonstrated. With this single device, powered by few-micro-Joule, single-cycle, 0.3 THz pulses, we demonstrated record THz-acceleration of >30 keV, streaking with <10fs resolution, focusing with >2 kT/m strength, compression to ~ 100 fs as well as real-time switching between these modes of operation. Travelling wave THz linacs based on dielectric-loaded metallic waveguides operating under few-cycle excitations are proposed. Based on this concept, keV-level energy gain through a linear accelerator using optically-generated THz pulses is demonstrated. Moreover, the possibility of electron acceleration to tens of MeV with millijoule level THz pulses is theoretically shown. The final goal of the above studies is a fully THz-driven compact light source facility, whose start-to-end simulation is fulfilled in this thesis.

Abstract-High efficiency terahertz generation in a multi-stage system

Lu Wang, Arya Fallahi, Koustuban Ravi, and Franz Kärtner

Fig. 2 Schematic illustration of the simulated geometry: the dark thick arrows represent the polarization direction of both pump and terahertz beams. The polarization direction is aligned with the extraordinary optical axis of PPLN. The origin of the cylindrical coordinate is at the center of the beam. r and z represent the transverse and propagation directions, respectively.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-23-29744&origin=search

We describe a robust system for laser-driven narrowband terahertz generation with high conversion efficiency in periodically poled Lithium Niobate (PPLN). In the multi-stage terahertz generation system, the pump pulse is recycled after each PPLN stage for further terahertz generation. By out-coupling the terahertz radiation generated in each stage, extra absorption is circumvented and effective interaction length is increased. The separation of the terahertz and optical pulses at each stage is accomplished by an appropriately designed out-coupler. To evaluate the proposed architecture, the governing 2-D coupled wave equations in a cylindrically symmetric geometry are numerically solved using the finite difference method. Compared to the 1-D calculation which cannot capture the self-focusing and diffraction effects, our 2-D numerical method captures the effects of difference frequency generation, self-phase modulation, self-focusing, beam diffraction, dispersion and terahertz absorption. We found that the terahertz generation efficiency can be greatly enhanced by compensating the dispersion of the pump pulse after each stage. With a two-stage system, we predict the generation of a 17.6 mJ terahertz pulse with total conversion efficiency ηtotal = 1.6% at 0.3 THz using a 1.1 J pump laser with a two-lines spectrum centered at 1 μm. The generation efficiency of each stage is above 0.8% with the out-coupling efficiencies above 93.0%.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Segmented terahertz electron accelerator and manipulator (STEAM)

Dongfang Zhang, Arya Fallahi, Michael Hemmer, Xiaojun Wu, Moein Fakhari, Yi Hua, Huseyin Cankaya, Anne-Laure Calendron, Luis E. Zapata, Nicholas H. Matlis, Franz X. Kärtner

https://www.nature.com/articles/s41566-018-0138-z

Acceleration and manipulation of electron bunches underlie most electron and X-ray devices used for ultrafast imaging and spectroscopy. New terahertz-driven concepts offer orders-of-magnitude improvements in field strengths, field gradients, laser synchronization and compactness relative to conventional radiofrequency devices, enabling shorter electron bunches and higher resolution with less infrastructure while maintaining high charge capacities (pC), repetition rates (kHz) and stability. We present a segmented terahertz electron accelerator and manipulator (STEAM) capable of performing multiple high-field operations on the six-dimensional phase space of ultrashort electron bunches. With this single device, powered by few-microjoule, single-cycle, 0.3 THz pulses, we demonstrate record terahertz acceleration of >30 keV, streaking with <10 fs resolution, focusing with >2 kT m–1 strength, compression to ~100 fs as well as real-time switching between these modes of operation. The STEAM device demonstrates the feasibility of terahertz-based electron accelerators, manipulators and diagnostic tools, enabling science beyond current resolution frontiers with transformative impact.

Abstract-Designing chirped aperiodically poled structures for high-energy single-cycle terahertz generation

Alireza Yahaghi, Koustuban Ravi, Arya Fallahi, and Franz Kärtner

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-34-3-590&origin=search

We introduce a collinear scheme for the highly efficient generation of broadband single or few-cycle high-power terahertz (THz) pulses using optical rectification. For this purpose, two concepts are introduced and thoroughly analyzed. The first concept is the generation of chirped broadband terahertz pulses using chirped aperiodically poled electro-optic crystals. The second concept involves the compression of the terahertz pulses extracted from the aperiodically poled structure using a chirped mirror. An illustrative design employing cryogenically cooled aperiodically poled lithium niobate crystals and the appropriate chirped mirrors is presented. It is shown that the presented design allows the optical pulse to be re-used in subsequent generation stages, resulting in optical-to-THz conversion efficiencies in excess of 5% for terahertz radiation centered around ∼0.3  THz$\sim 0.3\mathrm{THz}$. Analytic solutions and numerical calculations are presented. In order to design a crystal with the optimum conversion efficiency, we take advantage of binary optimization techniques. This work paves the way for the generation of broadband terahertz radiation with several mJ of pulse energy using large-area aperiodically poled structures.

Abstract-Terahertz-driven linear electron acceleration

Kyung-Han Hong, Ravi Koustuban, Arya Fallahi, Gustavo Moriena, R. J. Miller, Ronny Huang, Franz Kaertner,

http://dspace.mit.edu/handle/1721.1/100509?show=full

The cost, size and availability of electron accelerators are dominated by the achievable accelerating gradient. Conventional high-brightness radio-frequency accelerating structures operate with 30–50 MeV m[superscript −1] gradients. Electron accelerators driven with optical or infrared sources have demonstrated accelerating gradients orders of magnitude above that achievable with conventional radio-frequency structures. However, laser-driven wakefield accelerators require intense femtosecond sources and direct laser-driven accelerators suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing requirements due to the short wavelength of operation. Here we demonstrate linear acceleration of electrons with keV energy gain using optically generated terahertz pulses. Terahertz-driven accelerating structures enable high-gradient electron/proton accelerators with simple accelerating structures, high repetition rates and significant charge per bunch. These ultra-compact terahertz accelerators with extremely short electron bunches hold great potential to have a transformative impact for free electron lasers, linear colliders, ultrafast electron diffraction, X-ray science and medical therapy with X-rays and electron beams.

Abstract-Toward a terahertz-driven electron gun

W. Ronny Huang

, Emilio A. Nanni

, Koustuban Ravi

, Kyung-Han Hong

, Arya Fallahi

, Liang Jie Wong

,Phillip D. Keathley

, Luis E. Zapata

 & Franz X. Kärtner

http://www.nature.com/articles/srep14899

Femtosecond electron bunches with keV energies and eV energy spread are needed by condensed matter physicists to resolve state transitions in carbon nanotubes, molecular structures, organic salts, and charge density wave materials. These semirelativistic electron sources are not only of interest for ultrafast electron diffraction, but also for electron energy-loss spectroscopy and as a seed for x-ray FELs. Thus far, the output energy spread (hence pulse duration) of ultrafast electron guns has been limited by the achievable electric field at the surface of the emitter, which is 10 MV/m for DC guns and 200 MV/m for RF guns. A single-cycle THz electron gun provides a unique opportunity to not only achieve GV/m surface electric fields but also with relatively low THz pulse energies, since a single-cycle transform-limited waveform is the most efficient way to achieve intense electric fields. Here, electron bunches of 50 fC from a flat copper photocathode are accelerated from rest to tens of eV by a microjoule THz pulse with peak electric field of 72 MV/m at 1 kHz repetition rate. We show that scaling to the readily-available GV/m THz field regime would translate to monoenergetic electron beams of ~100 keV.