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Lithium niobate (LN) doped with MgO for terahertz THz LINAC - [email protected]

THz pulse generation
The THz pulse that is used in the accelerating structure is generated with optical rectification of 700 fs, 1.03 μm pulses in cryogenically cooled stoichiometric lithium niobate (LN) doped with 1% MgO. LN was chosen because it exhibits multiple advantages with low THz absorption, large bandgap, a high damage threshold and a high effective nonlinear coefficient, that is, large deff. For a high efficiency of conversion in LN, a tilted-pulse-front pumping scheme23 is required. The THz pulse is centred at 0.45 THz with a broad spectrum ranging from 0.2 to 0.8 THz. A THz pulse energy of 10 μJ is produced from 1.2 mJ of NIR, which is slightly lower than the peak conversion efficiency of THz generation23 due to a larger spot size in the LN (decreased fluence) for improved transport of the THz beam. The THz beam has excellent Gaussian mode content, which allows for low-loss coupling. The THz beam was characterized spatially by a pyroelectric detector array (Spiricon Pyrocam IV, Ophir Photonics).

Pump source
The pump source for THz generation is a Yb:KYW chirped pulse regenerative amplifier (RGA) producing 1.5-mJ pulses with 1 kHz repetition rate at a centre wavelength of 1,030 nm and bandwidth of 2.1 nm. The dielectric grating compressor following the RGA compresses the pulses to a near-transform-limited 700-fs pulse duration (full width at half maximum). The seed for the RGA was a mode-locked Yb-doped fibre oscillator emitting 70 fs, 0.2 nJ pulses at 80 MHz (ref. 34) amplified to 1.6 nJ by a Yb-doped fibre amplifier. After losses through the optical elements in the pulse front tilting set-up including a diffraction grating, the impinging pump energy into the (LN) crystal was 1.2 mJ.

Ultraviolet photoemitter
About 2% of the available 1,030 nm NIR pump energy was used to generate the ultraviolet photoemitter pulses by fourth harmonic generation from 1.03 μm based on two-stage second harmonic generation in a 10-mm-long type I LBO crystal for infrared to green and in a 0.5-mm-long type-I BBO crystal for the green to ultraviolet, respectively. A BG-39 band-pass filter was used to remove the fundamental NIR component after the first stage. The ultraviolet pulses have a duration of ∼350 fs and are focused onto the photocathode with a beam waist of ∼200 μm. Since both the THz and ultraviolet pulses are produced from the same sub-picosecond NIR laser, the timing jitter between them is negligible.

Electro-optic sampling
Electro-optic sampling was used to determine the temporal and spectral properties of the THz pulse and dispersion induced from the quasi-optical elements in the THz beamline. Optical synchronization between the THz pulse and the mode-locked Yb-doped fibre seed oscillator (80 MHz, 70 fs, 1,030 nm) was ensured as explained in the Pump source section of Methods. Birefringence was induced in a 200-μm thick, 110-cut ZnTe crystal. A quarter-wave plate followed by a polarizer converts the field-induced birefringence to an intensity modulation, and the intensity modulation is measured using a balanced detector with a delay scan.

Radially polarized THz beam
The THz pulse generated by optical rectification is linearly polarized, which is not compatible with the TM01 mode used in the accelerating structure. A segmented half waveplate was used to convert the linearly polarized light to radially polarized light35. Each segment of the waveplate imparts the appropriate rotation to the polarization to transition from a linear to a radially polarized beam, which couples well in the far field to the TM01 mode of the accelerating structure. A segmented λ/2 waveplate with eight segments of ∼8-mm-thick quartz designed for operation at 0.45 THz was used.

THz LINAC
The THz pulse accelerates electrons in a circular waveguide consisting of a quartz capillary inserted into a hollow copper cylinder36, see Fig. 1a. The inner diameter of the copper waveguide is 940 μm with a dielectric wall thickness of d=270 μm. This results in a vacuum space with a radius of rv=200 μm. The dielectric constant of the quartz capillary is nominally ɛ=4.41. The operational mode of the LINAC is a travelling TM01 mode, see Fig. 4a. The dispersion relation for the operating mode is shown in Fig. 4b. At the centre frequency of the THz pulse (450 GHz), the group velocity is vg/c=0.46 and the phase velocity is vp/c=0.505.

Terahertz-driven linear electron acceleration
Emilio A. Nanni, Wenqian R. Huang, Kyung-Han Hong, Koustuban Ravi, Arya Fallahi, Gustavo Moriena, R. J. Dwayne Miller & Franz X. Kärtner
Nature Communications volume 6, Article number: 8486 (2015)
doi:10.1038/ncomms9486