Limitations to THz generation by optical rectification using tilted pulse fronts

Terahertz (THz) generation by optical rectification (OR) using tilted-pulse-fronts is studied. One-dimensional (1-D) and 2-D spatial models, which simultaneously account for (i) the nonlinear coupled interaction of the THz and optical radiation, (ii) angular and material dispersion, (iii) absorption, iv) self-phase modulation and (v) stimulated Raman scattering are presented. We numerically show that the large experimentally observed cascaded frequency down-shift and spectral broadening (cascading effects) of the optical pump pulse is a direct consequence of THz generation. In the presence of this large spectral broadening, the phase mismatch due to angular dispersion is greatly enhanced. Consequently, this cascading effect in conjunction with angular dispersion is shown to be the strongest limitation to THz generation in lithium niobate for pumping at 1 micron. It is seen that the exclusion of these cascading effects in modeling OR, leads to a significant overestimation of the optical-to-THz conversion efficiency. The simulation results are supported by experiments

Terahertz-driven linear electron acceleration

The cost, size and availability of electron accelerators is dominated by the achievable accelerating gradient. Conventional high-brightness radio-frequency (RF) accelerating structures operate with 30-50 MeV/m gradients. Electron accelerators driven with optical or infrared sources have demonstrated accelerating gradients orders of magnitude above that achievable with conventional RF structures. However, laser-driven wakefield accelerators require intense femtosecond sources and direct laser-driven accelerators and suffer from low bunch charge, sub-micron tolerances and sub-femtosecond timing requirements due to the short wavelength of operation. Here, we demonstrate the first linear acceleration of electrons with keV energy gain using optically-generated terahertz (THz) pulses. THz-driven accelerating structures enable high-gradient electron or proton accelerators with simple accelerating structures, high repetition rates and significant charge per bunch. Increasing the operational frequency of accelerators into the THz band allows for greatly increased accelerating gradients due to reduced complications with respect to breakdown and pulsed heating. Electric fields in the GV/m range have been achieved in the THz frequency band using all optical methods. With recent advances in the generation of THz pulses via optical rectification of slightly sub-picosecond pulses, in particular improvements in conversion efficiency and multi-cycle pulses, increasing accelerating gradients by two orders of magnitude over conventional linear accelerators (LINACs) has become a possibility. These ultra-compact THz accelerators with extremely short electron bunches hold great potential to have a transformative impact for free electron lasers, future linear particle colliders, ultra-fast electron diffraction, x-ray science, and medical therapy with x-rays and electron beams

All-Optical Electron Acceleration with Ultrafast THz Pulses

This thesis discusses a series of advances toward - and resulting in -the demonstration of the first ultrafast THz-driven electron gun, a technology with the potential to deliver unprecedented electron beam quality to scientists studying matter at the ultrafast and ultrasmall scale via electron diffraction or x-ray imaging. In Part 1, we discuss various advances in generation of high energy pulsed THz radiation, a spectral regime uniquely effective at accelerating electrons but historically lacking in effcient sources. In particular, through various improvements to thegrating-based tilted pulse front (TPF) technique, we demonstrate a record conversion effciency of 1%. We also implement echelon-based TPF, achieving 3.5x higher effciency than grating-based TPF for short (~100 fs) pulses. Finally, we reuse the residual pump to obtain a recycled effciency around half to a quarter that of the original. This reduced effciency can be linked spatio-spectral distortions in the residual pump, and we visualize these distortions to better understand the asymmetric dynamics of the THz generation process. In Part 2, we discuss the design, testing, and commissioning of an electron gun driven exclusively by THz radiation. The accelerating structure, capable of broadband, dispersionless THz propagation and sub-wavelength confinement, is analyzed through electromagnetic simulations and experimental characterization. We also characterize the accelerated electrons in absolute charge and spectrum as a functionof emission phase and THz energy, while showing that the behavior matches well with theory and simulation. Our first-version THz gun delivers near 1 keV electrons accelerated by field strengths surpassing that of the best operational RF guns. The gun also delivers narrowband electron spectra which can already be used for low-energy electron diffraction

All-optical electron acceleration with ultrafast THz pulses

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2017.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 203-214).This thesis discusses a series of advances toward - and resulting in - the demonstration of the first ultrafast THz-driven electron gun, a technology with the potential to deliver unprecedented electron beam quality to scientists studying matter at the ultrafast and ultrafast scale via electron diffraction or x-ray imaging. In Part 1, we discuss various advances in generation of high energy pulsed THz radiation, a spectral regime uniquely eective at accelerating electrons but historically lacking in ecient sources. In particular, through various improvements to the grating-based tilted pulse front (TPF) technique, we demonstrate a record conversion eciency of 1%. We also implement echelon-based TPF, achieving 3.5x higher eciency than grating-based TPF for short (~100 fs) pulses. Finally, we reuse the residual pump to obtain a recycled eciency around half to a quarter that of the original. This reduced eciency can be linked to spatio-spectral distortions in the residual pump, and we characterize these distortions to better understand the asymmetric dynamics of the THz generation process. In Part 2, we discuss the design, testing, and commissioning of an electron gun driven exclusively by THz radiation. The accelerating structure, capable of broad-band, dispersionless THz propagation and sub-wavelength confinement, is analyzed through electromagnetic simulations and experimental tests. We also characterize the accelerated electrons in absolute charge and spectrum as a function of emission phase and THz energy, while showing that the behavior matches well with theory and simulation. Our first-version THz gun delivers near 1 keV electrons accelerated by field strengths surpassing that of the best operational RF guns. The gun also delivers narrowband electron spectra which can already be used for low-energy electron diffraction.by Wenqian Ronny Huang.Ph. D

All-optical electron acceleration with ultrafast THz pulses

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2017.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 203-214).This thesis discusses a series of advances toward - and resulting in - the demonstration of the first ultrafast THz-driven electron gun, a technology with the potential to deliver unprecedented electron beam quality to scientists studying matter at the ultrafast and ultrafast scale via electron diffraction or x-ray imaging. In Part 1, we discuss various advances in generation of high energy pulsed THz radiation, a spectral regime uniquely eective at accelerating electrons but historically lacking in ecient sources. In particular, through various improvements to the grating-based tilted pulse front (TPF) technique, we demonstrate a record conversion eciency of 1%. We also implement echelon-based TPF, achieving 3.5x higher eciency than grating-based TPF for short (~100 fs) pulses. Finally, we reuse the residual pump to obtain a recycled eciency around half to a quarter that of the original. This reduced eciency can be linked to spatio-spectral distortions in the residual pump, and we characterize these distortions to better understand the asymmetric dynamics of the THz generation process. In Part 2, we discuss the design, testing, and commissioning of an electron gun driven exclusively by THz radiation. The accelerating structure, capable of broad-band, dispersionless THz propagation and sub-wavelength confinement, is analyzed through electromagnetic simulations and experimental tests. We also characterize the accelerated electrons in absolute charge and spectrum as a function of emission phase and THz energy, while showing that the behavior matches well with theory and simulation. Our first-version THz gun delivers near 1 keV electrons accelerated by field strengths surpassing that of the best operational RF guns. The gun also delivers narrowband electron spectra which can already be used for low-energy electron diffraction.by Wenqian Ronny Huang.Ph. D

High field, high efficiency THz pulse generation by optical rectification

Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 51-55).The great difficulty of producing high intensity radiation in the terahertz (THz) spectral region by conventional electronics has stimulated interest in development of sources based on photonics. Optical rectification in lithium niobate is an attractive approach, because it supports high generation efficiencies, uses low cost, bulk LN crystals, and is powered by common Yb-doped lasers at wavelengths of around 1 Pm. In this work, a theoretical framework for THz generation by optical rectification is developed. Several novel methods for optimizing the generation efficiency are shown, including pump beam imaging, pump pulse optimization, cryogenic cooling, and THz antirefiection coating. Finally, experimental results will be presented showing a THz generation efficiency of 3.7%, which is 10x higher than current state-of-the-art. The generated few-cycle THz pulses can be used for coherent control of electrons, setting the stage for compact, table-top accelerators.by Wenqian Ronny Huang.S.M

Temperature dependent refractive index and absorption coefficient of congruent lithium niobate crystals in the terahertz range

Optical rectification with tilted pulse fronts in lithium niobate crystals is one of the most promising methods to generate terahertz (THz) radiation. In order to achieve higher optical-to-THz energy efficiency, it is necessary to cryogenically cool the crystal not only to decrease the linear phonon absorption for the generated THz wave but also to lengthen the effective interaction length between infrared pump pulses and THz waves. However, the refractive index of lithium niobate crystal at lower temperature is not the same as that at room temperature, resulting in the necessity to re-optimize or even re-build the tilted pulse front setup. Here, we performed a temperature dependent measurement of refractive index and absorption coefficient on a 6.0 mol% MgO-doped congruent lithium niobate wafer by using a THz time-domain spectrometer (THz-TDS). When the crystal temperature was decreased from 300 K to 50 K, the refractive index of the crystal in the extraordinary polarization decreased from 5.05 to 4.88 at 0.4 THz, resulting in ~1° change for the tilt angle inside the lithium niobate crystal. The angle of incidence on the grating for the tilted pulse front setup at 1030 nm with demagnification factor of −0.5 needs to be changed by 3°. The absorption coefficient decreased by 60% at 0.4 THz. These results are crucial for designing an optimum tilted pulse front setup based on lithium niobate crystals

Broadband terahertz generation with a stair-step echelon

A method to overcome limitations of conventional broadband terahertz generation techniques is presented. A stair-step echelon allows for the creation of superior tilted-pulse-frontsto yield larger frequencies and bandwidths, energy conversion efficiencies exceeding 5%