prospects of thz pulse generation with mj- l l e d 100 mv
TRANSCRIPT
Department of ExperimentalPhysicsPhysics
http://physics.ttk.pte.hu
Prospects of THz Pulse Generation with mJ-L l E d 100 MV/ El t i Fi ldLevel Energy and 100 MV/cm Electric Field
J A Fülöp1,2,* Z Ollmann3 L Pálfalvi3 G Almási1,3J.A. Fülöp , , , Z. Ollmann , L. Pálfalvi , G. Almási , ,J. Hebling1,3
1High-Field THz Research Group HAS HungaryHigh Field THz Research Group, HAS, Hungary2Attosecond Light Pulse Source of the Extreme Light Infrastructure, Hungary
3Department of Experimental Physics, University of Pécs, Hungary
*e mail: fulop@fizika ttk pte hue-mail: [email protected]
Workshop on THz Sources for Time Resolved Studies of Matter, ANL, Argonne, July 30-31, 2012
Applications of High-Energy THz Pulses
0,6
0,8
1,01,0
ampl
itude
(a. u
.)
.)
Emax
0 1 2 30,0
0,2
0,4
0 0
0,5
Spe
ctra
l a
Frequency (THz)
tric
field
(a. u
.
-0,5
0,0El
ect
Linear THz spectroscopy (E ≈ 100 V/cm → 10 fJ pulse energy)
-2 0 2 4 6 8time (ps)
Linear THz spectroscopy (Emax ≈ 100 V/cm → 10 fJ pulse energy) graphene, nanotubes, molecular magnets, etc.
N li TH t (E 100 kV/ J l )Nonlinear THz spectroscopy (Emax ≈ 100 kV/cm → µJ pulse energy)THz pump – THz probe measurements of dynamics (Hoffmann et al., PRB, 2009)
Manipulation and acceleration of charged particlesManipulation and acceleration of charged particles(Emax ≈ 100 MV/cm → 10 mJ pulse energy)VUV–XUV pulse generation by Thomson-scattering, X-ray free electron laser, …
Optical Rectification
[ ][ ]2
THz2
232
22eff
2THz
THz 44sinh2
THz
LLeILd L α
εωη α ⋅⋅= −
Conversionefficiency
[ ]THz3
THz2
0 4Lcnnv αεdepends on:
THz frequency: 2THzTHz ωη ∝
h
Material parameters → FOM
THzTHzη
phTHz
grvis vv =Phase matching → velocity matching:
L << 12222 ILdeffω
η eff LdFOM
22
Figure of merit (FOM):
αTHzL << 1 320 cnn THzv
ffTHz ε
η =
228 Ideffω
THzv
ffNA nn
FOM 2=
24 ffdαTHzL >> 1 322
0
8cnn
d
THzTHzv
effTHz αε
ωη = 22
4
THzTHzv
effA nn
dFOM
α=
Materials for Optical Rectification
Material deff( /V) ( 1)
FOM*( 2 2/V2)
grnmn800 THn
grmn μ55.1
THzα(pm/V) (cm-1) (pm2cm2/V2)
CdTe 81.8 3.24 2.81 4.8 11.0
nm800 THzn μ
GaAs 65.6 4.18 3.59 3.56 0.5 4.21
G P 24 8 3 67 3 34 3 16 0 2 0 72GaP 24.8 3.67 3.34 3.16 0.2 0.72
ZnTe 68.5 3.13 3.17 2.81 1.3 7.27
GaSe 28.0 3.13 3.27 2.82 0.5 1.18
sLiNbO3sLN 100K
168 2.25 4.96 2.18 174.8
18.248.6sLN 100K 8 8 6
DAST 615 3.39 2.58 2.25 50 41.5
Velocity matching condition: THzgrNIR
phTHz
grNIR nnvv =⇒=
* for L = 2 mm
Tilted-Pulse-Front Pumping (TPFP)
phTHz
grvis cos vv =⋅ γ
~100 fs typicalHebling et al., Opt. Express, 2002
Tilted-Pulse-Front Pumping vs. Line Focusing
Stepanov et al., Opt. Express, 2005 Hoffmann & Fülöp, J. Phys. D, 2011
THz Pulse Generation by Optical Rectification in LiNbO Using Tilted Pulse Front Pumping
Large nonlinear coefficient (deff = 168 pm/V)
LiNbO3 Using Tilted Pulse Front Pumping
Velocity matching by tilting the pump pulse front
Hebling et al Opt E press 2002phgr Hebling et al., Opt. Express, 2002
Highest THz pulse energy from a table-top source0 25 μJ Stepanov et al Opt Express 2005
phTHz
grvis cos vv =⋅ γ
0.25 μJ Stepanov et al., Opt. Express, 200510 μJ Yeh et al., Appl. Phys. Lett., 200750 μJ Stepanov et al., Appl. Phys. B, 2010
125 J Fülö t l O t L tt 2012125 μJ Fülöp et al., Opt. Lett., 2012
Further increase of THz energy is in sightti l diti – optimal conditions
(pump pulse duration & wavelength, crystal temperature & length)Fülöp et al., Opt. Express, 2011
– by increasing the pumped area in optimized setupsPálfalvi et al., Appl. Phys. Lett., 2008Fülöp et al., Opt. Express, 2010
Effective Interaction Length
-20 -10 0 10 20
Pump propagation distance, ζ [mm]
ps]
εdnPulse front tilt:
2
dura
tion,
τ [p (a)
λελγddtan
gnn
−=
1
Pum
p pu
lse
d
2Ld
materialdispersion
angular dispersionLiNbO3
[%]
0 τ0 = 50 fsτ = 350 fs
P
GVD parameter:
dispersiondispersionLiNbO3λp = 800 nm
Fp = 5.1 mJ/cm2
Ωpm = 1 THz
( )⎥⎦
⎤⎢⎣
⎡−⎟
⎠⎞
⎜⎝⎛==
−
2
221
λλελ
λ dnd
ddn
cdvd
D g
4
(b) Ln ef
ficie
ncy
[ τ0 350 fs τ0 = 600 fs
⎦⎣ ⎠⎝ λλλ ddcd2
(b) Leff
Hz
gene
ratio
n
Martínez et al JOSA A 1984-5 0 5
0
THz propagation distance, z [mm]
TH
Martínez et al., JOSA A, 1984Hebling, Opt. Quantum Electron., 1996Fülöp et al., Opt. Express, 2010
Effective Interaction Length
Limiting effect Possible solution
Change of pump pulse duration insidethe medium
Use longer Fourier-limited pump pulsesUse materials requiring smaller pulse-front tiltfront tilt
Absorption at THz frequenciesCool the crystal to reduce α
Absorption coefficient αMulti-photon absorption (MPA) of thepump
Cool the crystal to reduce αUse longer pump wavelength tosuppress low-order MPA
Use materials requiring smaller pulse-
Walk-offfront tiltUse large pump beam (and energy):→ Optimized imiganig systemp g g y→ Contatc grating (no imaging)
Optimization of the Pump Pulse Length
3.0LiNbO3
Fülöp et al., Opt. Express, 20113.0
2.5 300 K
[MV
/cm
] LiNbO3L ≤ 10 mmIp, max = 40 GW/cm2
λp = 1064 nm2.5
300 K 100 K 10 K
2.8 MV/cm
[MV
/cm
]
1 5
2.0
ric fi
eld
[ p
absorption 1 5
2.0
ric fi
eld
[
2.3 MV/cm
1.0
1.5
eak
elec
tr absorption coefficient of
LiNbO3:1.0
1.5
eak
elec
tr
0.5
240 kV/cm
1.0 MV/cmpe T [K] α [1/cm]
10 0.350.5
240 kV/cm
1.0 MV/cmpe
0 500 1000 15000.0
pump pulse duration [fs]
100 2.1
300 160 500 1000 1500
0.0
pump pulse duration [fs]
experimental value: 110 kV/cm Hoffmann et. al., Phys. Rev. B., 2009
Optimization of the Pump Pulse Length
300 K100 K
25 300 K100 K
10
y [
%]
100 K 10 K
15
20
y [
mJ]
100 K 10 K
5
effic
ienc
y
2.0%5
10
THz
ener
g
0 500 1000 15000
pump pulse duration [fs]0 500 1000 1500
0
pump pulse duration [fs]p p p [ ] p p p [ ]
τp = 500 fsEp = 200 mJI 40 GW/cm2
THz energy ≈ 25 mJ
THz field = 2.8 MV/cm (unfocused)
10 MV/cm level using imagingIp, max = 40 GW/cm2 → 10 MV/cm level using imaging
→ 100 MV/cm level using focusing
Optimization of the Pump Pulse Length
Frequency at the spectral peak
1,5 LiNbO3
L ≤ 10 mm 300 K 100 K 10 K
0 [T
Hz] Ip, max = 40 GW/cm2
lp = 1064 nm1,0
uenc
y, Ω
0
Phase matching adjusted to the
0,5
peak
freq
u
spectral peak
0 200 400 600 800 1000
p
pump pulse duration [fs]
New Appilcation Possibilities for THz Pulses with High Electric Field
TH
High Electric Field
THz beam
AccelerationAccelerationPlettner et al., Phys. Rev. Spec. Top. – Accel. and Beams 9, 111301 (2006)
Beam deflection, focusingPl tt t l Ph R S T A l d B 12 101302 (2009)
1 GV/m = 10 MV/cm peak field strength is needed
Plettner et al., Phys. Rev. Spec. Top. – Accel. and Beams 12, 101302 (2009)
New Appilcation Possibilities for THz Pulses with High Electric Field High Electric Field
Enhancement of HHGEnhancement of HHGE. Balogh et al., PRB, 2011K. Kovács et al., PRL, 2012
Longitudinal compression of electron bunches→ single-cycle MIR…XUV pulse generationHebling et al., arXiv:1109.6852Hebling et al., presentation on Monday
Electron undulationH bli t l Xi 1109 6852Hebling et al., arXiv:1109.6852
Proton acceleration(40 → 100 MeV requires 30 mJ THz energy)(40 → 100 MeV requires 30 mJ THz energy)→ hadron therapyPálfalvi et al., in preparation
Towards mJ Level THz Pulses: Experiment
In collaboration with S. Klingebiel, F. Krausz, S.
1000 experiment 1.6 power fitcalculation
KarschMPQ, Garching
100
calculation square law
y [
μJ]
Further possibilities:
• Optimal pump duration10
THz
ener
gy
T =300 K Optimal pump duration• Cooling the LiNbO3• Contact grating
1
T T =300 Klp = 1030 nm
tp = 1.3 ps (not Fourier-limited)1 10 100
1
pump pulse energy [mJ]
Fülöp et al., Opt. Lett. 2012:125 J
Comparison with earlier results:
Stepanov et. al., Appl. Phys. B, 2010:E 50 J 125 µJ
0.25%ETHz = 50 µJ η = 0.05%
×2.5
×5
THz Beam Profiles
1.0(a)
1.0(a)
Contact gratingNo limit for lateral
THzLNcrystal
4
0
2
-2
x [mm]
( )1.0
(a)
0 5
(a)
y[a
rb.u
.]
0 5
(a) (b)
y[a
rb.u
.]size
No imaging optics
Pálf l i t l APL 2008
-4
2
(c)pump
0 5
(a) (b) (c)
y[a
rb.u
.]
THzLNcrystal
pump
0.5
THz
inte
nsity
THzLNcrystal
pump
0.5
THz
inte
nsityPálfalvi et al., APL, 2008
Ollmann et al., APB,submitted
THzLNcrystal
pump
0.5
THz
inte
nsity
(a) -8-40480.0
T
x [mm]THz
(a) -8-40480.0
T
x [mm]
Non-optimized
Optimized THz
(a) -8-40480.0
T
x [mm]
pump
THz
2
m] 8E-4
1E-3
THz generation efficiency [arb. u.]
4
02
x [mm]Optimized
Grating image is tangential to pump
THz
2
m] 8E-4
1E-3
THz generation efficiency [arb. u.]
4
02
x [mm]
-2
0
ξ [m
m
0
2E-44E-4
6E-4
-4
0-2
pulse front
Longer focal length lens
-2
0
ξ [m
m
0
2E-44E-4
6E-4
-4
0-2
LN crystal-2 0 2
ζ [mm]4-4(b)
length lens
Fülöp et al., Opt. Express, 2010
LN crystal-2 0 2
ζ [mm]4-4(b)
Optimizing the TPFP Setup Containing Imaging
LiNbO3, λp = 800 nm
Fülöp et al Opt Express 2010
90 θi optimal angle of incidence
Fülöp et al., Opt. Express, 2010
60
ncid
ence
θLittrow
θLittrow+10θLittrow-10o
30
angl
e of
in
1200 1600 2000 24000
2200 1/mm
1/d (1/mm)
1800 1/mm
1/d (1/mm)
1 MV/cm field and 0 1% efficiency reached by applying our above results1 MV/cm field and 0.1% efficiency reached by applying our above resultsHirori et al., Appl. Phys. Lett., 2011
Design of Contact Grating on LiNbO3
Contact grating for THz generation proposed:
Index matching by glass:
R f ti i d t hi li id (RIML)
Pálfalvi et al., APL, 2008
Nagashima et al., Japan J. Appl. Phys., 2010
Oll t l APB b itt dRefractive index matching liquid (RIML):
Diffraction efficiency for order -1
Ollmann et al., APB, submitted
RIML for BK7, d = 350 nm pitch grating
THz Energy vs. Pump Energy
105
103
104
J)
LN (tilted pulse front)
101
102
LN (Yeh, 2008)ergy
(n
1
100
10 ( ) LN (Yeh, 2007) LN (Stepanov, 2005) LN (Stepanov, 2008)ul
se e
ne
10-2
10-1( p )
LN (Fülöp, 2012) ZnTe (Blanchard, 2007) ZnTe (Löffler, 2005)TH
z pu
10-4
10-3
( , )
ZnTe (collinear)
10 100 1000 10000 10000010
pump pulse energy (μJ)
Reconsidering Semiconductors for THz Generation
Materi-al
λp = 800 nm λp = 1064 nm λp = 1560 nmMPA γ MPA γ MPA γ MPA: lowest-order
ZnTe 2PA collinear 2PA 22.4° 3PA 28.6°
GaSe 2PA 16.5° 2PA 25.9° 3PA 30.2°
multi-photon pumpabsorptionγ : pulse-front tilt angle
1 Ω0 = 1 THz
LiNbO3 3PA 62.7° 4PA 63.4° 5PA 63.8°γ : pulse-front tilt angle
Fülöp et al
100
101]
Ω0 1 THzFülöp et al.,Opt. Express, 2010
Advantages:
10-2
10-1
cien
cy [%
Saturation in LiNbO :
Advantages:Pump spot diameter can be increased
4
10-3
Effi
c
ZnTe, 1560 nm, 80 K (β3 = 3.99x10-4 cm3 GW-2)
ZnTe, 1064 nmGaSe 1560 nm
in LiNbO3:Hoffmann et al., Opt. Express,
increased
Smaller tilt angle, resulting in longer
0,1 1 10 100
10-4
Pump intensity [GW cm-2]
GaSe, 1560 nm LN, 1064 nm
p2007effective length
Design of Contact Grating on ZnTe
Ollmann et al., in preparation
0.8sinusoidal profile 1 4
0.7pump wavelength:
1460 nm
sinusoidal profile
1.3
1.4
μm]
0.5
0.6 1560 nm 1700 nm
ffici
ency
1.2
g pe
riod
[μ
0.4E
f1.1 G
ratin
g
0 10 20 30 40 50 60
0.3
Angle of incidence AOI [°]
1.0
Angle of incidence, AOI [ ]
Pump: 1.5 µm wavelength, 5 cm beam diameter→ THz energy = 2.2 mJgy→ THz intensity = 630 GW/cm2 (focused)→ THz field ≈ 20 MV/cm (focused)
Summary
Single-cycle THz pulse generation with 100 MV/cm field and ~10 mJ energy is feasible around ~1 THzf b OR d b ffi i t DPSS lfrequency by OR pumped by efficient DPSS lasers
Optimization of the TPFP technique2.0
2.5
3.0
300 K 100 K 10 K
2.8 MV/cm
ld
[MV/
cm]
2.3 MV/cm
→ pump pulse duration: 500 fs optimal for LN→ low crystal temperature→ optimal crystal length
0 0
0.5
1.0
1.5
240 kV/cm
1.0 MV/cmpeak
ele
ctric
fie
p y g→ contact grating setup for LN and ZnTe
(extension of beam size)
P li i i t l lt
0 500 1000 15000.0
pump pulse duration [fs]
100
1000 experiment 1.6 power fit calculation square law
J]Preliminary experimental results:125 μJ energy, 0.25% efficiency (non optimised!)
New application possibilities: 1
10
100
THz
ener
gy
[μJ
New application possibilities:
→ nonlinear THz spectroscopy→ manipulation of electrons and ions
1 10 1001
pump pulse energy [mJ]
p
Acknowledgement: Hungarian Scientific Research Fund (OTKA), grant numbers 78262 and 101846, SROP-4.2.1.B-10/2/KONV-2010-0002, and hELIos ELI_09-01-2010-0013.