Double Optical Gating
for attosecond pulse generation
Why attosecond pulses?
Attosec.
10-18 s
femtosec. picosec.
10-15 s
10-12 s
Time
Electron
dynamics
Vibration
Rotation
Why attosecond?
Electron dynamics timescale
1Ǻ
150 as
 1 a.u of time is 24 attosecond
 Attosecond light is soft x-ray
1.825
 ( fs ) 
E (eV )
E  73eV
Conventional laser is limited to fs
Transform-limited Gaussian pulse
 ( fs )E (eV )  1.83
Visible light: 400-700 nm=1.33 eV, 2.7 fs
The bandwidth is too narrow
High order harmonic generation
Discovery by Rhodes & L’Huillier,1987
20 fs laser
(hv=1.5 eV)
Ar gas
1014 W/cm2
Harmonic order
Photon energy (eV)
XUV
(hv~100 eV)
Laser harmonic generation
Intensity
Pertubative
Nonpertubative (plateau)
1 3 5 7 9 11…
Harmonic order
Three steps in one laser cycle
Proposed by Corkum & Kulander,1993
U
Laser field
Attosecond x-ray
Ion
Attosecond
electron
wave packet
1. Electron emission (tunneling ionization)
2. Acceleration (in E field of laser)
3. Attosecond emission (recombination)
Attosecond revolution
5
10
4
Pulse duration (fs)
10
3
10
Nonpertubative
interaction
(HHG)
2
10
1
10
0
10
-1
10
1960 1970 1980 1990 2000 2010
80 as
Year
Corkum & Chang, Optics & Photonics News, October (2008).
Atto pulse generated by few-cycle lasers
Demonstrated by Krauz, 2001
Single isolated pulse
80 as (2008)
Pump laser:
3.3 fs, ~0.5 mJ
EL
85 eV
135 eV
Time
Attosecond pulse train
Attosecond research at KSU: 2001
 0 attoseconds
 0 photons
Our goals:
1. Generate 24 attosecond isolated pulses
2. With multi-cycle lasers (10-20 fs)
150 as
 1 a.u of time is 24 attosecond
 Bandwidth: 73 eV
1.8
 ( fs ) 
E (eV )
E  73eV
Attosecond pulse train and HHG
Gas
Multi-cycle laser
Atto Pulse Train
Intensity
Half Cycle
1
3
5
7
9
11…
Harmonic order
Extraction of single pulse by gating
Necessary conditions:
 Gatewidth equals to pulse spacing
 Ground state population available in the gate
 Carrier-envelope phase locking
Our approach:
Double optical gating (DOG)=
polarization gating + two color gating.
Polarization gating
Multi-cycle laser
Gas
XUV pulse train
Half cycle
Corkum, Opt. Lett. 19,
1870 (1994)
Chang, Phys. Rev. A
70, 043802 (2004)
Gas
Half cycle
Single pulse
Two-color gating
Laser: w
Gas
XUV pulse train
Half cycle=1.3 fs
Paul et al, Science 292, 1689 (2001)
Laser: w +2w
Gas
Full cycle=2.6 fs
Mauritsson et al, PRL97, 013001(2006)
Double Optical Gating
Chang, PRA 76, 051403(R) (2007)
Kansas Light Source
 Grating based CPA: 3 mJ, 25 fs
 Hollow-core fiber: 1 mJ, 6 fs
Shan et al, US patent No. 7,050,474, (2006)
 Grating based CPA: 3 mJ, 25 fs
 Hollow-core fiber:
1 mJ, 6 fs
Shan et al, US patent No. 7,050,474, (2006)
CE phase locking of grating-based CPA
Stretcher
Pump laser
G1
G2
PZT
AOM
Amplifier
fs oscillator
Computer
f-to-2f interferometer
Locking electronics
Grating
Compressor
f-to-2f interferometer
 Separation of two feedback loops
 Long locking time
CEP drift introduced by a grating pair
d  1m
S
S

G
G
Chang, Applied Optics 45,
8350(2006)
CE
2
2
G tan[ (w0 )]
d
CE Phase(radian)
CE phase locked by controlling gratings
3
2
1
0
-1
-2
-3
Phase drift CE(RMS)=167 mrad
PZT displacement (m)
0
200
400
600
800
1000
Time (s)
2
1
0
-1
PZT displacement LPZT(sd)=0.52 m
-2
0
200
400
600
800
1000
Time (s)
Li et al. Opt. Express 14, 11468 (2006), licensed to Femtolasers GmbH.
Demonstration of DOG with 9 fs laser
XUV spectra generated with gating
Supercontinua generated with DOG
and corresponding attosecond pulses
Gilbertson et al, APL 92, 071109 (2008)
Carrier envelope phase
CE
Envelope
Electric Field
Effects of carrier-envelope phase
on double optical gated spectrum
Mashiko et al, PRL 100, 103906 (2008)
Simulated effects of CE phase
6
4
2
Intensity (Arb. units)
8
Time domain
0
360
-2
-1
0
1
2
0
270
180 eg)
(d
e
90
as
h
p
CE
Tme (cycle)
Chang, PRA 76, 051403(R) (2007)
Summary
CE phase locking of grating based CPA
• Separation of feedback loops.
• High laser energy.
Double optical gating:
• Isolated as pulse (140 as),
• Longer driving laser (~10 fs).