Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan

National Taiwan University, Taiwan

National Central University, Taiwan

National Chung Cheng University, Taiwan

Chih-Hao Pai, Li-Chung Ha, Yen-Mu Chen, Hsu-Hsin Chu, Jiunn-Yuan Lin, Jyhpyng Wang, Szu-yuan Chen

Production of intense ultrashort mid-infrared pulses from a laser-wakefield electron accelerator

• Methods for the generation of sub-ps mid-infrared pulses
• Laser-wakefield electron accelerator operated in the bubble regime
• Experimental setup and tomographic measurement
• Generation of intense ultrashort mid-infrared pulses in the bubble regime

Outline

Generation of sub-ps MIR pulses

• Free-electron lasers:

• Frequency conversion in nonlinear crystals or gas media:

[1] Fuji et al., Opt. Lett. 32, 3330 (2007) [2] Imahoko et al., Appl. Phys. B 87, 629 (2007)

• This work:

Laser-wakefield electron accelerator operated

in the bubble regime

Pukov et al., Appl. Phys. B 74, 355 (2002)

Laser-wakefield electron accelerator operated

in the bubble regime

Pukov et al., Appl. Phys. B 74, 355 (2002)

energy: 50 MeV±10％, divergene: 4 mrad

duration: ~10 fs (PIC simualtion)

Phys. Rev. E 75, 036402 (2007)

monoenergetic electron beam

200 mJ, 42 fs

4x10¹⁹ cm^-3 plasma density

Laser-wakefield electron accelerator operated

in the bubble regime

spectral broadening

Faure et al., Phys. Rev. Lett. 95, 205003 (2005)

SLM: spatial light modulator

OAP: off-axis parabolic mirror

Diagnostic tools

1. LANEX imaging system for electron beam
2. Interferometry for plasma density measurement

Experimental setup for production of electron beam

SLM: spatial light modulator

OAP: off-axis parabolic mirror

Diagnostic tools

1. LANEX imaging system for electron beam (replaced by (c))
2. Interferometry for plasma density measurement

(c) MIR grating spectrometer

Diagnoses for MIR pulse (1): spectrometer

SLM: spatial light modulator

OAP: off-axis parabolic mirror

Diagnostic tools

1. LANEX imaging system for electron beam (replaced by (d))
2. Interferometry for plasma density measurement

(c) MIR grating spectrometer

4. Pyroelectric detector

Diagnoses for MIR pulse (2): energy & beam profile

SLM: spatial light modulator

OAP: off-axis parabolic mirror

Diagnostic tools

1. LANEX imaging system for electron beam (replaced by (e))
2. Interferometry for plasma density measurement

(c) MIR grating spectrometer

4. Pyroelectric detector
5. Ge-wafer photo-switch

Diagnoses for MIR pulse (3): temporal profile

1. The machining beam ionizes and heats up selected regions.
2. Plasma heating leads to hydrodynamic expansion.
3. Several nanoseconds later the ionized region is evacuated.
4. Characteristics of final products as functions of pump-pulse

positions in the gas jet can be measured.

Scanning the interaction length for tomographic measurement

intensity of the

machining pulse

Phys. Plasmas 12, 070707 (2005)

Phys. Rev. Lett. 96, 095001 (2006)

Setup of the machining beam for tomographic measurement

focal spot:

20 μm×1.3 mm

function of the knife-edge: setting the interaction length

machining pulse

variable position

knife-edge or SLM

pump pulse

gas jet

cylindrical

lens pair

Self-injection of the monoenergetic electron beam and rapid growth of the MIR pulse occurs in the same region.

Dependence of MIR energy on interaction length

pump pulse energy: 205 mJ

pump pulse duration: 42 fs

plasma density: 4.1x10¹⁹ cm^-3

self-injection regions

of electrons

The spectral profile of the MIR pulse suggests that the MIR pulse is produced by the strong spectral broadening of the pump pulse in the bubble regime.

Dependence of MIR spectra on interaction length

pump pulse energy: 205 mJ

pump pulse duration: 42 fs

plasma density: 4.1x10¹⁹ cm^-3

Dependence of MIR spectra on interaction length

The spectral profile of the MIR pulse suggests that the MIR pulse is produced by the strong spectral broadening of the pump pulse in the bubble regime.

The Raman satellite is related to the modulational instability of the pump pulse in the early stage of the bubble regime evolution.

​

The MIR pulse energy increases with plasma density faster than the emergence of the monoenergetic electron beam. This is consistent with the bubble-regime model as the strong spectral broadening and self-compression is the cause of bubble formation.

Dependence of MIR energy on plasma density

pump pulse energy: 205 mJ

pump pulse duration: 42 fs

The MIR pulse has a lower pump energy threshold than that of the monoenergetic electron beam. This is consistent with the bubble-regime model as the strong spectral broadening and self-compression is the cause of bubble formation.

plasma density: 4.1x10¹⁹ cm^-3

Dependence of MIR energy on pump energy

The MIR pulse is linearly polarized with the same polarization as the pump pulse. This is consistent with the bubble-regime model since the spectral broadening by phase modulation should preserve the pump laser polarization.

Polarization of the MIR pulse

polarization axis

of the pump pulse

​

1. coherent transition radiation from the electron bunch passing the plasma-vacuum boundary

(2) Cherenkov-type emission from the electron bunch or the plasma wave

​

Both are radially polarized.

The data rule out the possibility of other mechanisms

Ref: Leemans et al., Phys. Rev. Lett.

91, 074802 (2003)

The MIR pulse is a flattop distribution with its diameter determined by the clear aperture of the ZnSe vacuum window.

​

The angular divergence of the MIR pulse is larger than the collection angle (8°) and the total MIR pulse energy should be considerably larger than 3 mJ.

MIR pulse energy vs. iris radius

pump pulse energy: 205 mJ

pump pulse duration: 42 fs

plasma density: 4.1x10¹⁹ cm^-3

radius of the ZnSe

vacuum window

Ge-wafer photo-switch

MIR pulse

excitation

pulse

pinhole

Ge-wafer photo-switch

MIR pulse

excitation

pulse

pinhole

Ge-wafer photo-switch

MIR pulse

excitation

pulse

pinhole

Temporal profile of the MIR pulse

Ge-wafer photo-switch

pump pulse: 205 mJ/42 fs

excitation pulse: 500 μJ/38 fs

plasma density: 4.1x10¹⁹ cm^-3

temporal profile

pulse duration

X ps

4.6 ps

9.8 ps

5-mm Ge window

5-mm Ge window

X＜0.6 ps

Experimental data suggest that the MIR pulse is produced by the strong spectral broadening of the pump pulse in a laser- wakefield electron accelerator operated in the bubble regime.

Production of an intense MIR pulse with at least 3-mJ pulse energy and ultrashort pulse duration from a laser-wakefield electron accelerator is demonstrated. The output energy is one order of magnitude larger than that of the most intense free electron lasers, and three order of magnitude larger than that of conventional wave mixing.

Summary

Thanks for your attention.

Picture of experimental chamber

Picture of experimental chamber

machining beam

main beam

(1) MIR

(2) electron

beam