S. P. Jamison

Single-mode waveguide propagation and reshaping of sub-ps terahertz pulses in sapphire fibers

Waveguide propagation of sub-ps terahertz pulses in single-crystal sapphire fibers is reported. An incident THz pulse of approximately 0.6 ps duration undergoes considerable reshaping due to the absorptive and dispersive waveguide propagation, resulting in transmitted chirped pulse durations of 10–30 ps. Good agreement between theory and experiment is obtained by analyzing the propagation in terms of the single HE11 waveguide mode. The dominance of the single HE11 mode, despite the fiber dimensions allowing for multimode propagation, is attributed to the free-space to waveguide coupling.

Upconversion of a relativistic Coulomb field terahertz pulse to the near infrared

We demonstrate the spectral upconversion of a unipolar subpicosecond terahertz (THz) pulse, where the THz pulse is the Coulomb field of a single relativistic electron bunch. The upconversion to the optical allows remotely located detection of long wavelength and nonpropagating components of the THz spectrum, as required for ultrafast electron bunch diagnostics. The upconversion of quasimonochromatic THz radiation has also been demonstrated, allowing the observation of distinct sum- and difference-frequency mixing components in the spectrum. Polarization dependence of first and second order sidebands at ωopt±ωTHz, and ωopt±2ωTHz, respectively, confirms the χ(2) frequency mixing mechanism.

Role of misalignment-induced angular chirp in the electro-optic detection of THz waves.

A general description of electro-optic detection including non-collinear phase matching and finite transverse beam profiles is presented. It is shown theoretically and experimentally that non-collinear phase matching in ZnTe (and similar materials) produces an angular chirp in the χ(2)-generated optical signal. Due to this, in non-collinear THz and probe arrangements such as single-shot THz measurements or through accidental misalignment, measurement of an undistorted THz signal is critically dependent on having sufficient angular acceptance in the optical probe path. The associated spatial walk-off can also preclude the phase retardation approximation used in THz-TDS. The rate of misalignment-induced chirping in commonly used ZnTe and GaP schemes is tabulated, allowing ready analysis of a detection system.

Demonstration of sub-luminal propagation of single-cycle terahertz pulses for particle acceleration

The sub-luminal phase velocity of electromagnetic waves in free space is generally unobtainable, being closely linked to forbidden faster than light group velocities. The requirement of sub-luminal phase-velocity in laser-driven particle acceleration schemes imposes a limit on the total acceleration achievable in free space, and necessitates the use of dispersive structures or waveguides for extending the field-particle interaction. We demonstrate a travelling source approach that overcomes the sub-luminal propagation limits. The approach exploits ultrafast optical sources with slow group velocity propagation, and a group-to-phase front conversion through nonlinear optical interaction. The concept is demonstrated with two terahertz generation processes, nonlinear optical rectification and current-surge rectification. We report measurements of longitudinally polarised single-cycle electric fields with phase and group velocity between 0.77c and 1.75c. The ability to scale to multi-megavolt-per-metre field strengths is demonstrated. Our approach paves the way towards the realisation of cheap and compact particle accelerators with femtosecond scale control of particles.Controlled generation of terahertz radiation with subluminal phase velocities is a key issue in laser-driven particle acceleration. Here, the authors demonstrate a travelling-source approach utilizing the group-to-phase front conversion to overcome the sub-luminal propagation limit.

Generation of longitudinally polarized terahertz pulses with field amplitudes exceeding 2 kV/cm

We demonstrate the generation of near-single cycle longitudinally polarized terahertz radiation using a large-area radially biased photoconductive antenna with a longitudinal field amplitude in excess of 2 kV/cm. The 76 mm diameter antenna was photo-excited by a 0.5 mJ amplified near-infrared femtosecond laser system and biased with a voltage of up to 100 kV applied over concentric electrodes. Amplitudes for both the transverse and longitudinal field components of the source were measured using a calibrated electro-optic detection scheme. By tightly focusing the radiation emitted from the photoconductive antenna, we obtained a maximum longitudinal field amplitude of 2.22 kV/cm with an applied bias field of 38.5 kV/cm.

First results from the Daresbury Compton backscattering x-ray source (COBALD)

Polarized X-ray pulses at 0.6 Å have been generated via head-on collision of a laser pulse from the high-field laser facility at Daresbury with a 30 MeV electron bunch in the ALICE energy recovery linear accelerator. The angular distribution of the backscattered X rays was obtained in single-shot using a scintillation screen. The temporal profile of the X ray yield as a function of the time delay between the laser pulse and electron bunch was measured and agreed well with that expected from the collision point dependence of the laser-electron beam longitudinal overlap.

Status of the ALICE Energy Recovery Linac

The ERLP energy recovery linac prototype at Daresbury Laboratory has been rebranded, and is now called ALICE (Accelerators and Lasers In Combined Experiments). This paper gives an overview of the project and its status, outlines some of the challenges experienced during the commissioning of the photoinjector and superconducting accelerating systems, and presents our photoinjector gun commissioning results. An outline is given of the planned photon science program for ALICE, and its under‐pinning role in the EMMA NS‐FFAG project.

Revealing carrier-envelope phase through frequency mixing and interference in frequency resolved optical gating

We demonstrate that full temporal characterisation of few-cycle electromagnetic pulses, including retrieval of the carrier envelope phase (CEP), can be directly obtained from Frequency Resolved Optical Gating (FROG) techniques in which the interference between non-linear frequency mixing processes is resolved. We derive a framework for this scheme, defined Real Domain FROG (ReD-FROG), for the cases of interference between sum and difference frequency components and between fundamental and sum / difference frequency components. A successful numerical demonstration of ReD-FROG as applied to the case of a self-referenced measurement is provided. A proof-of-principle experiment is performed in which the CEP of a single-cycle THz pulse is accurately obtained and demonstrates the possibility for THz detection beyond optical probe duration limitations inherent to electro-optic sampling.

Inverse Compton Backscattering Source driven by the multi 10 TW-Laser installed at Daresbury

Inverse Compton is a promising method to implement a high brightness, ultra-short, energy tunable X-ray source at accelerator facilities. We have developed an inverse Co Compton ba mpton backscattering X-ray source driven by the multi 10 TW-L backscattering Laser installed at Daresbury (COBALD). Hard X-rays, with spectral peak ranging from 15 to 30 keV, depending on the scattering geometry, will be generated through the interaction of a laser pulse with an electron bunch delivered by the energy recovery linear accelerator prototype (ERLP) at Daresbury. X-ray pulses containing 9×107 photons per pulse will be created from head on collisions, with a pulse duration comparable to that of the incoming electron bunch. For transverse collisions 8×106 photons per pulse will be generated, where the laser pulse transit time defines the X-ray pulse duration. The peak spectral brightness is predicted to be ~ 1021 photons / s / mm2 / mrad2 / 0.1% ΔE/E, which is comparable to fourth generation synchrotron light sources.

Dielectric terahertz waveguides

Summary form only given. We believe we report the first demonstration of waveguide propagation of sub-ps THz pulses in a dielectric fiber. The experimental arrangement employed for coupling of free-space single-cycle THz radiation to the fiber is similar to that used for investigations of THz propagation in metallic waveguides. Briefly, the THz radiation is focused by a silicon lens to a waist diameter of /spl ap/200 /spl mu/m with the fiber entrance face placed at the waist position. An identical arrangement is used at the exit face of the sapphire fiber.