Yen-Yu Chang, Li-Chung Ha, Yen-Mu Chen

Chih-Hao Pai

Investigator

Jypyng Wang, Szu-yuan Chen, Jiunn-Yuan Lin

Contributing Students

Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan

National Central University, Taiwan

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

• Methods for the generation of sub-ps mid-IR 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

: frequency

: wavelength

mid-IR: 5 – 40 μm; far-IR: (25-40) – (200-350) μm

Terahertz wave(兆赫波): 0.1 – 10 THz (THz＝10¹² Hz)

Spectrum of electromagnetic wave

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 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

3-dimensional particle-in-cell simulation

• Code: VORPAL
• Laser pulse:

energy: 205 mJ

central wavelength: 810 nm

pulse length: 42 fs

beam size: 8 μm in FWHM

peak laser intensity: 6×10¹⁸ W/cm2

linearly polarized in z direction

• Uniform plasma density: 4.1×10¹⁹ cm^-3

L_ramp＝500 μm, flattop＝1.6 mm

flattop

• Size of window:

L_x＝64 μm

L_y＝ L_z＝ 100 μm

• Size of gird:

2560 cells in X

250×250 cells in Y and Z

• 4 particles per cell

1. The pump pulse undergoes phase modulation imposed by the plasma wave and relativistic self-phase modulation. As a result, the laser spectrum broadens.
2. The laser pulse with its pulse duration longer than the plasma period breaks up into a pulse train.
3. As a result of spectrum broadening, the laser pulse in the bubble undergoes longitudinal self-compression.
4. As the laser intensity gets higher and higher, a plasma bubble is formed. When the plasma bubble evolves into a certain shape and amplitude, a monoenergetic electron beam can be generated.
5. Since most of the photons in the laser pulse stay in the descending slope of the plasma bubble, the spectrum of the laser pulse is mainly broadened toward the long wavelength side.

Simulation-evolution of plasma wave and laser pulse

intensity profile of the laser pulse

Simulation-MIR spectrum and temporal profile

The duration of the MIR pulse is about 20 fs from the simulation, which indicates that the laser peak power may reach 0.4 TW.



The maximum MIR pulse energy is 7 mJ. The spectrum shows a continuous distribution extended from the shorter wavelength side and the trend agrees well with the experimental data.



intensity profile of the MIR pulse covering 2－20 μm

The MIR pulse is trapped by the plasma bubble, which enables the MIR pulse to propagate through the plasma.



MIR pulse

covering 6－10 μm

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.

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

Simulation-monoenergetic electron beam

The duration of the MIR pulse is about 20 fs from the simulation, which indicates that the laser peak power may reach 0.4 TW.



The maximum MIR pulse energy is 7 mJ. The spectrum shows a continuous distribution extended from the shorter wavelength side and the trend agrees well with the experimental data.



intensity profile of the MIR pulse covering 2－20 μm

The MIR pulse is trapped by the plasma bubble, which enables the MIR pulse to propagate through the plasma.



Picture of experimental chamber

Picture of experimental chamber

machining beam

main beam

(1) MIR

(2) electron

beam