Intense Pulsed Terahertz Sources & their Applications
International Research School IMPACT 2016, Cargèse (Corsica), France, 23 August – 2 September 2016
J. A. Fülöp
Institute of Physicswww.physics.ttk.pte.hu
MTA-PTE High-Field THz Research Group, University of Pécs, Pécs, HungaryELI-ALPS, ELI-Hu Nkft., Szeged, Hungary
Outline
Introduction
– THz spectral range
– THz control of matter
Technology of intense pulsed THz sources
– Photoconductive antennas
– Optical rectification & difference-frequency generation in:SemiconductorsLiNbO3
Organic crystals
– Laser plasma
Application examples
Summary
The THz spectral range
12:00
frequency (Hz)
108 109 1010 1011 1012 1013 1014 1015 1016 1017
radio microwave infrared UV X-rayTHz
visible
Frequency 𝜈 0.1 – 30 THz
Wavenumber 𝜆−1 3.3 – 1000 cm–1
Wavelength 𝜆 10 – 3000 μm
Photon energy h𝜈 0.4 – 123 meV
THz pulse
Generated by a laser-driven table-top source
Single- or nearly-single-cycle waveform
Electric field directly measureable
0 1 2 30.0
0.5
1.0
Spe
ctr
al am
plit
ude (
arb
. u.)
Frequency (THz)
-2 0 2 4
-0.5
0.0
0.5
1.0
Ele
ctr
ic fie
ld (
arb
. u.)
Time (ps)
Applications of THz pulses
Linear THz spectroscopyEmax ≈ 100 V/cm | 10 fJ pulse energySpectroscopy of graphene, nanotubes, molecular magnets, hydrated molecules, etc.
Nonlinear THz spectroscopyEmax ≈ 100 kV/cm | µJ pulse energyTHz pump—THz / optical / X-ray / etc. probe measurements of dynamics
Manipulation and acceleration of charged particlesEmax ≈ 10 – 100 MV/cm | (multi)-mJ pulse energyacceleration of proton & relativistic electron beams, X-ray free electron laser, etc.
Resonant control of matter by THz
Ionic motion
– Molecular rotation & vibration
– Lattice vibration in solids (phonons)
Spin control
– Spin waves
Bound & free electrons
– Internal excitations of electron-hole pairs, Cooper pairs, etc.
Kampfrath et al., Nat. Photon., 2013
F = 𝑞E + 𝑞v × B
T = p × ET = 𝛍 × B
Non-resonant control of matter by THz
Ponderomotive energy:
work of THz field within a half-cycle on an electron
𝑊p =𝑒2𝐸max
2
4𝑚∗𝜔2
Field ionization
Impact ionization
THz-induced phase transitions
Particle acceleration & manipulation
Kampfrath et al., Nat. Photon., 2013
Peak electric field vs. carrier frequency & pulse energy
Assumptions:
Single-cycle pulse
Focusing with F# = 1
Brunner et al., Workshop on High-Field THz Science, 2012, Pécs, Hungary
Cutting-edge pulsed THz sources
Method Peak electricfield
[MV/cm]
Pulse energy
[μJ]
Accelerator-based
440[Wu, 2013]
600[Wu, 2013]
Photoconductiveantenna*
0.14[Ropagnol, 2013]
3.6[Ropagnol, 2013]
Laser plasma* 8[Oh, 2014]
7[Oh, 2013]
Opticalrectification*
40[Vicario, 2014]
900[Vicario, 2014]
*Pumped by femtosecond lasers[Wu, 2013] Wu et al., Rev. Sci. Instrum.,
2013[Ropagnol, 2013] Ropagnol et al., Appl.
Phys. Lett., 2013
[Oh, 2013] Oh et al., New J. Phys., 2013[Oh, 2014] Oh et al., Appl. Phys. Lett.,
2014[Vicario, 2014] Vicario et al., Opt. Lett.,
2014
Free-electron laser (FEL)
Terahertz radiation is generated by relativistic electrons moving and transversally accelerating inside the undulator
𝐁 =0
𝐵0 sin 𝑘u𝑧0
THz FEL @ ELBE/HZDR, Dresden
Parameters of the FEL radiation
Wavelength range 18 – 250 μm U100-FEL with undulator U100
Pulse energy 0.01 – 2 μJ depending on wavelength
Pulse length 1 – 25 ps depending on wavelength
Repetition rate 13 MHz 3 modes:• cw• macropulsed > 100 μs, < 25 Hz• single-pulse switched at kHz/Hz www.hzdr.de
Femtosecond-laser driven pulsed THz sources
0.1 1 10 100
1
10
100
1000
OR, LN
PCA
OR, LN
OR, DSTMS
OR, GaAs
LPAOR, LN
DFG, GaSe
plasma
DFG, GaSe
OR, OH1
TH
z p
uls
e e
nerg
y (J)
Frequency (THz)
OR, DAST
OR, ZnTe
plasma
Photoconductive antenna (PCA)
THz pulse
DC bias voltage
Photoconductive switch
fs laser pulse
+
-50 m5-10
m
semiconducting
substrate
lithographically
deposited
metal leads
Ropagnol et al., Appl. Phys. Lett., 2013
Photoconductive antenna (PCA)
Radiated field: Iph: photocurrent td
IdtE
ph
THz
Laser pulse duration→ switch-on time
Photoexcited carrier lifetime → switch-off time
Lee, Principles of terahertzscience and technology, Springer, 2009
Time-domain THz spectroscopy (TDTS)
fs laser pulse
THz pulse
variable delay
Jepsen et al., Laser Photonics Rev., 2011
Linear response
Polarization
Wave equation
Nonlinear response
Polarization
Wave equation
Optical medium with linear / nonlinear response
𝐏 𝐄 = 𝜀0𝜒1 𝐄 = 𝐏 1
𝛻2𝐄 −1
𝑣2
𝜕2𝐄
𝜕𝑡2= 0
𝐏 𝐄
= 𝜀0 𝜒 1 𝐄 + 𝜒 2 𝐄𝐄 + 𝜒 3 𝐄𝐄𝐄 + ⋯
= 𝐏 1 + 𝐏 2 + 𝐏 3 + ⋯
= 𝐏 1 + 𝐏NL
𝛻2𝐄 −1
𝑣2
𝜕2𝐄
𝜕𝑡2=
1
𝜀0𝑐2
𝜕2𝐏NL
𝜕𝑡2
Optical rectification (OR)
Special case of difference-frequency generation (DFG) with𝜔1 ≈ 𝜔2 ⟹ 𝜔3 small
𝜔1: depleted, 𝜔2, 𝜔3: amplified
c(2)w1
w3 = w1 - w2
w2
ħw1
ħw2
ħw3
𝐸 𝑡 =1
2 𝐸1𝑒
𝑖𝜔1𝑡 +1
2 𝐸2𝑒
𝑖𝜔2𝑡 + c.c.
𝑃 2 𝑡 = 𝜀0𝜒2 𝐸𝐸
=1
4𝜀0𝜒
2 𝐸12𝑒𝑖2𝜔1𝑡 + 𝐸2
2𝑒𝑖2𝜔2𝑡 + 2 𝐸1 𝐸2𝑒
𝑖 𝜔1+𝜔2 𝑡
Optical rectification: phase matching
wwwc dEEP
=
0
20NL
0=-= ww kkkk
0g wvv =
-
=
-=
0g
0g
11
ww
vvcnnk
w
==
c
n
vk
g
g
0
0
10
w
ww
w
Velocity matching:
Phase matching:
Nonlinear polarization:
THz frequency optical frequencies
Approximation:
THz refractive index optical group index
=
For a broadband (fs) laser pulse: intra-pulse DFG between all possiblecombinations of spectral components
Optical rectification: efficiency
Efficiency of THz generation for phase-matched conditions:R.L. Sutherland, Handbook of nonlinear optics, Marcel Dekker, 1996
2
2
32
0
222
4
4sinh
2exp
2
-=L
L
L
cnn
ILd
THz
THz
THz
THzv
eff
THz
w
THzL << 1
THzL >> 1
32
0
2222
cnn
ILd
THzv
eff
THz
w =
322
0
228
cnn
Id
THzTHzv
eff
THz
w =
THzv
eff
NAnn
LdFOM
2
22
=
22
24
THzTHzv
eff
Ann
dFOM
=
Figure-of-merit (FOM) for optical rectification (including THz absorption THz ):Hebling et al., J. Opt. Soc. Am. B, 2008
Materials for optical rectification: semiconductors
Materialdeff
[pm/V]ng
@ 800 nm (1.55 µm)nTHz
αTHz
[cm-1]
FOMfor L = 2 mm[pm2cm2/V2]
CdTe 81.8 (2.81) 3.24 4.8 11.0
GaAs 65.6 4.18 (3.56) 3.59 0.5 4.21
GaP 24.8 3.67 (3.16) 3.34 0.2 0.72
ZnTe 68.5 3.13 (2.81) 3.17 1.3 7.27
GaSe 28.0 3.13 (2.82) 3.27 0.5 1.18
Velocity matching condition:
Hebling et al., JOSA B, 2008
THzlasergTHzplaserg wwww nnvv ==
Optical rectification: zinc-blende crystals
Lee, Principles of terahertz science and technology, Springer, 2009
Collinear velocitymatching in ZnTe at ~0.8 µm pump
Nagai et al., Appl. Phys. Lett., 2004
𝐿c =𝜋𝑐
Ω𝑛 Ω − 𝑛g 𝜔
Results with ZnTe[Blanchard et al., Opt. Express, 2007]
– THz energy: 1.5 µJ
– THz generation efficiency: 3.1×10-5
Reason for low efficiency: strong two-photon absorption of the 0.8-µm pump
Coherence length for OR:
THz generation by DFG in GaSe
DFG between two detunedpulses
Birefringent phase matchingabove the phonon frequency
Peak electric fields up to100 MV/cm
Center frequencies continuouslytunable from 10 to 72 THz
DFG: Sell et al., Opt. Lett., 2008
OPA: Junginger et al., Opt. Lett., 2010
Time-domain THz spectroscopy (TDTS)
fs laser pulse
THz pulse
variable delay
Jepsen et al., Laser Photonics Rev., 2011
Time-domain THz spectroscopy (TDTS)
Lee, Principles of terahertz science and technology, Springer, 2009
Materials for optical rectification: LN
Materialdeff
[pm/V]ng
@ 800 nm (1.55 µm)nTHz
αTHz
[cm-1]
FOMfor L = 2 mm[pm2cm2/V2]
CdTe 81.8 (2.81) 3.24 4.8 11.0
GaAs 65.6 4.18 (3.56) 3.59 0.5 4.21
GaP 24.8 3.67 (3.16) 3.34 0.2 0.72
ZnTe 68.5 3.13 (2.81) 3.17 1.3 7.27
GaSe 28.0 3.13 (2.82) 3.27 0.5 1.18
sLiNbO3 @ 300 K@ 100 K
168 2.25 (2.18) 4.96 174.8
18.248.6
Velocity matching condition:
Hebling et al., JOSA B, 2008
THzlasergTHzplaserg wwww nnvv ==
Phase matching by tilting the pump pulse front
Enables velocity matching if n() > ng(w), e.g. in LiNbO3
= vv w cos0g
Hebling et al., Opt. Express, 2002
Optical pulse with tilted intensity front
Intensity / pulse front:loci of intensity maxima at the same time instant
Pulse propagates perpendicular to phase fronts
Pulse-front-tilt angle (𝛾): angle between pulse & phase fronts
Fülöp & Hebling, Applications of tilted-pulse-front excitation, In: Recent optical and photonic technologies, K. Y. Kim (Ed.), INTECH, Croatia, (2010)http://www.intechopen.com/articles/show/title/applications-of-tilted-pulse-front-excitation
Setup for tilted-pulse-front pumping (TPFP)
Fülöp et al., Opt. Express, 2010
d
dtan
gn
n-= Pulse-front-tilt (𝛾) is linked to angular
dispersion ( d𝜀 d𝜆):
→ Imaging required to restore pulseduration & intensity
THz generation with a (line) focused beam
Cherenkov geometry
Low efficiency
Emission geometry disadvantageous for applications
Cherenkov geometry & TPFP
Stepanov et al., Opt. Express, 2005 Hoffmann & Fülöp, J. Phys. D, 2011
Dispersion curve of phonon-polariton and NIR light in LN
J. Hebling et al., Appl. Phys. B 78, 593 (2004)
Phonon-polariton: mixed EM and lattice excitation in crystals
Effective light velocity can be changed by changing the tilt angle
Angle 1: low frequency, broadband THz generation
Angle 2: higher frequency narrower band THz pulse generation
Tunable THz generation by TPFP
Frequency tuning by the tilt angleJ. Hebling et al., Appl. Phys. B 78, 593 (2004)
Tunable THz pulse generation by TPFP
Measured spectra at T=10 KPeak frequency and THz
energy vs. tilt angle
Larger tuning range and narrower spectra can be expected fromless absorpbing materials with higher phonon frequency (e.g. GaSe, GaP, etc.)
Tunable THz pulse generation by two-beam excitation
A. G. Stepanov et al., Opt. Express 12, 4650 (2004)
𝛼 = 1.9°
𝛼 = 1.3°
𝛼 = 0.35°
Tunable THz pulse generationby two-beam excitation
A. G. Stepanov et al., Opt. Express 12, 4650 (2004)
Shaped THz waveform generation by TPFP
Optical pulse sequence THz pulse sequence
K.-L. Yeh et al., Opt. Commun. 281, 3567 (2008)Pump Laser
HRPR
Grating
Liquid crystal mask filter amplitude and/or phase of each frequency component
Grating
Input: Single beam,Single fs pulse
Output: Single beamwith specified waveform
T. Feurer et al., Science 299, 374 (2003)
Shaped THz waveform generation by TPFP
Pulse front tilt & angular dispersion
-
==
-
2
221
d
nd
d
dn
cd
vdD
g
-5 0 50
2
4
(b) Leff
THz propagation distance, z [mm]
TH
z g
en
era
tio
n e
ffic
ien
cy [%
]
0
1
2
-20 -10 0 10 20
0 = 50 fs
0 = 350 fs
0 = 600 fs
Pump propagation distance, [mm]
Pu
mp
pu
lse
du
ratio
n,
[ps]
2Ld
(a)
GVD parameter:
Martínez et al., J. Opt. Soc. Am. A, 1984Hebling, Opt. Quantum Electron., 1996Fülöp et al., Opt. Express, 2010
materialdispersion
angular dispersion
d
dtan
gn
n-=
Pulse front tilt:
LiNbO3
λp = 800 nm
Fp = 5.1 mJ/cm2
Ωpm = 1 THz
Limitations in TPFP
Limiting effect Possible solution
Change of pump pulse duration insidethe medium owing to pulse-front tilt
Use longer Fourier-limited pump pulses
Use materials requiring smaller pulse-front tilt
Absorption at THz frequencies
Absorption coefficient α
Multi-photon absorption (MPA) of the pump
Cool the crystal to reduce α
Use longer pump wavelength tosuppress low-order MPA
Walk-off
Use materials requiring smaller pulse-front tilt
Use large pump beam (and energy):
→ Optimized imiganig system
→ Contatc grating (no imaging)
τp = 500 fs
Ep = 200 mJ
Ip, max = 40 GW/cm2
THz energy ≈ 25 mJ
THz field = 2.8 MV/cm (unfocused)
10 MV/cm level using imaging
100 MV/cm level using focusing
0 500 1000 15000
5
10
effic
ien
cy [%
]
pump pulse duration [fs]
300 K
100 K
10 K
2.0%
Optimization of the pump pulse duration (LiNbO3)
0 200 400 600 800 1000
0,5
1,0
1,5
300 K
100 K
10 K
pump pulse duration [fs]
pe
ak f
req
ue
ncy,
0
[T
Hz]
Phase matchingadjusted to thespectral peak
absorption coefficientof LiNbO3:
T [K] α [1/cm]
10 0.35
100 2.1
300 16Fülöp et al., Opt. Express, 2011
Enhancement of THz generation at low temperature
RT: 300 K CT: 23 K
Calculation: ηCT/ηRT ≈ 3 to 4 for785-fs pump pulses
Pump parameters:
pulse duration: 785 fs wavelength: 1030 nm
1 100.1
1
10
100
CT, large A, lens ( EOS)
RT, large A, lens
y = ax1.75
y = ax1.51
y = ax1.79
y = ax1.25
TH
z e
ne
rgy [J]
pump energy [mJ]
186 J
68.3 J
1 10
pump intensity [GW/cm2]
0 10 20 300
2
4
6
CT, large A, lens
RT, large A, lens
effic
ien
cy [x10
-3]
pump energy [mJ]
0.62%
0.23%
0 5 10 15 20 25
pump intensity [GW/cm2]
2.4
2.6
2.8
CT
/RT
en
ha
nce
men
t fa
cto
r
Fülöp et al., Opt. Express, 2014
Pump spot size: 1.0 cm2
>0.4-mJ THz pulses efficiently generated (room T)
Pump parameters:
pulse duration: 785 fs wavelength: 1030 nm
0 50 100 150 2000.0
0.2
0.4
0.6
0.8
p = 1.3 ps
(Fülöp et al., Opt. Lett., 2012)
RT, small A, telescope 0.77%
effic
ien
cy [%
]
pump fluence [mJ/cm2]
0.26%
0 50 100 150 200 250
pump intensity [GW/cm2]
1 10 100
1E-3
0.01
0.1
1
p = 1.3 ps
(Fülöp et al., Opt. Lett., 2012)
RT, small A, telescope
y = ax1.53
y = ax1.40
y = ax1.67
436 J
TH
z e
ne
rgy [m
J]
pump energy [mJ]
10 100
pump intensity [GW/cm2]
Fülöp et al., Opt. Express, 2014
Pump spot size: 0.3 cm2
Characterization of high-energy THz pulses (low T)
-10 -5 0 5 10-0.2
0.0
0.2
0.4
0.6 WTHz
= 26 J
WTHz
= 77 J
WTHz
= 163 J
ele
ctr
ic fie
ld [M
V/c
m]
time [ps]
0.45 MV/cm
0.65 MV/cm
0.26 MV/cm
0.0 0.2 0.4 0.6 0.8 1.00.0
0.2
0.4
0.6
0.8
1.00.46 THz0.14 THz
0.19 THz
am
plit
ud
e [arb
. u.]
frequency [THz]
0.25 THz
WTHz
= 26 J
WTHz
= 77 J
WTHz
= 163 J
calculated
Fülöp et al., Opt. Express, 2014
THz energy vs. pump energy (LiNbO3)
10-2
10-1
100
101
102
10-4
10-3
10-2
10-1
100
101
102
103
0.1% short pump pulses
LN (Stepanov, 2005)
LN (Yeh, 2008)
LN (Yeh, 2007)
LN (Stepanov, 2008)
TH
z e
ne
rgy [J]
pump energy [mJ]
room temp.
10-2
10-1
100
101
102
10-4
10-3
10-2
10-1
100
101
102
103
Long pump pulses
LN (Fülöp, 2012)
LN (Huang, 2013)
LN (Vicario, 2013)
LN (Fülöp, 2014)
0.1% Short pump pulses
LN (Stepanov, 2005)
LN (Yeh, 2008)
LN (Yeh, 2007)
LN (Stepanov, 2008)
785 fs
0.77%
0.62%
1.3 ps
0.25%
TH
z e
nerg
y [J]
pump energy [mJ]
680 fs
3.8%
1.2%
room temp.
cryog. temp.
1%
Requires index-matching Difficult manufacturing:
trapezoidal instead of binary profile →
Directly on crystalOllmann et al., Appl. Phys. B, 2012
With dielectric multilayersTsubouchi et al., Opt. Lett., 2014
LN
LiNbO3 based contact gratings
500 nm
350 nm • Al2O3 + Ta2O5 multilayers on
LiNbO3
• 71% diffraction efficiency• 0.41 µJ THz pulse energy• 1.5x10-4 conversion efficiency
Scitech Precision Ltd. (UK)
Limitations of tilted-pulse-front pumping in LiNbO3
Imaging errors at large spot sizes
Fülöp et al., Opt. Express, 2010
Limited interaction length due to large angular dispersion (γ≈63°)
Fülöp et al., Opt. Express, 2011
Nonlinear interaction between pump & THz
Ravi et al., Opt. Express, 2014Lombosi et al., New J. Phys., 2015
→ It is challenging to increase the THz energy & field strength further
→ Noncollinear geometry leads to THz pulse & beam distortions, disadvantageous for applications
Conventional tilted-pulse-frontpumping (TPFP)Hebling et al., Opt. Express, 2002
PumpTHz
LNgrating
Reconsidering semiconductors for THz generation
Collinear phase matching atcommon pump laser wavelengths
Small nonlinear coefficient
Strong two-photon absorption(2PA) at such pumpwavelengths
→Free carrier absorptionof THz
→Limited useful pumpintensity
→Low efficiency for THz generation
0.01 0.1 1 10 100
1 pJ
1 nJ
1 mJ
10-6
10-5
10-4
LN (TPFP)
Stepanov, 2005
Yeh, 2007
Stepanov, 2008
Yeh, 2008
Fülöp, 2012
Huang, 2013
Vicario, 2013
Fülöp, 2014
0.1%
ZnTe (collinear, 0.8 m)
Löffler, 2005
Blanchard, 2007
TH
z e
nerg
y
Pump energy [mJ]
1%
2PA
1 J
Solution: longer pump wavelength
→Allows for higher pump intensity and more efficient THz generation
→Requires tilted-pulse-front pumping
Fülöp et al., Opt. Express, 2010 Blanchard et al., Appl. Phys. Lett., 2014
Material ZnTe GaP GaAs LN
deff [pm/V] 68.5 24.8 65.6 168
Tilted-pulse-front pumping of semiconductors
0 2 4 6 8 10
0.0
0.2
0.4
0.6
0.8
1.0
0 2 4 6 8 100
1
2
3
Pump propagation distance, z (mm)
Pum
p p
uls
e d
ura
tion, F
WH
M (
ps)
LN, = 1 m
ZnTe, = 1.7 m
GaP, = 1.7 m
100 fs pump
Phase matching @ 1 THz
Effic
iency,
(%
)
THz propagation distance, z cos() (mm)
Small tilt angle for semiconductors (𝜸 ≲ 𝟑𝟎°)
Large interaction length for THz generationcompensates for smaller nonlinear coefficient
More uniform crystal length across thepumped area
Advantageous for the implementation of a contact grating [Fülöp et al., Optica, 2016]
ZnTe contact-grating THz source
[Polónyi et al., Opt. Express, 2016]
PumpTHz
Conventional TPFP (with imaging)
Hebling et al., Opt. Express, 2002
ZnTe contact-gating THz source
TPFP with contact grating (no imaging)
Pálfalvi et al., Appl. Phys. Lett., 2008
Collinear geometry possible(with symmetrically propagating diffraction orders m = ±1)
Bakunov et al., J. Opt. Soc. Am. B, 2014
THz energy easily increased byusing larger pumped area
Excellent THz beam quality
LiNbO3grating imaging
Fülöp et al., Optica, 2016
Design aspects
Large interaction length forefficient THz generation
→ 1.7 µm pump wavelength (no 3PA)
→ Phase matching at 1 THz (smallabsorption coefficient α ≲ 5 cm-1)
High diffraction efficiency forefficient pumpingOllmann et al., Opt. Commun., 2014
→ Binary grating profile with ~50% filling factor
→ Period: 1.275 μm, profile depth: 0.4 μm
Manufacturing
Electron-beam lithography+ reactive ion etchingScitech Precision Ltd. (UK)
Closely fitting the design profile
Substrate quality critical
ZnTe contact grating THz source: design & fabrication
0 1 2 3 4
3.2
3.4
3.6
Re
fractive
index, n
Frequency (THz)
0
5
10
15
20
Absorp
tion
coe
ffic
ient,
(1/c
m)ZnTe
Fülöp et al., Optica, 2016
THz sourceZnTe
Blanchard, et al., Opt. Express, 2007
GaAsBlanchard, et al., Appl. Phys. Lett.,
2014
ZnTe contact grating
Fülöp et al., Optica, 2016
Multi-photonabsorption
2PA 3PA 2PA 3PA 2PA 3PA
THz gen. efficiency 3.1×10-5 5×10-4 3×10-3
THz energy 1.5 µJ 0.6 µJ 3.9 µJ
ZnTe contact grating THz source: Experimental results
ZnTe contact grating THz source: Experimental results
Fülöp et al., Optica, 2016
THz energy vs. pump energy: semiconductors & LN
10-2
10-1
100
101
102
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
103
10-6
10-5
10-4
LN (TPFP)
Stepanov, 2005
Yeh, 2007
Stepanov, 2008
Yeh, 2008
Fülöp, 2012
Huang, 2013
Vicario, 2013
Fülöp, 2014
0.1%
Semiconductors (TPFP) ZnTe (collinear, 0.8 m)
/ ZnTe, 1.45 m / 1.7 m Löffler, 2005
GaP, 1.7 m Blanchard, 2007
GaAs (Blanchard, 2014), 1.8 mT
Hz e
nerg
y [J]
Pump energy [mJ]
1%
2PA
3PA
4PA
IR-pumped semiconductors can deliver efficiencies similar to LN
Prospects of increasing the efficiency
1 mJ THz pulse energy is feasible from a 5-cm size contact grating with 200 mJ pump energy, efficiently delivered by novel 1.7-2.5 µm infrared sources …
… such as a Holmium laser [Malevich et al., Opt. Lett., 2013]
0 2 4 6 80
1
2
Eff
icie
ncy, (
%)
Crystal length, L (mm)
1 THz, RT, ZnTe, = 28.2°
1 THz, CT, ZnTe, = 26.6°
2 THz, RT, ZnTe, = 29.6°
2 THz, CT, ZnTe, = 28.2°
2 THz, RT, GaP, = 21.4°
0.32% 0.72% 1.17%
0 2 4 6 80
1
2
Pe
ak e
lectr
ic f
ield
(a
rb.
u.)
Crystal length, L (mm)
ZnTe, RT, 1 THz
ZnTe, CT, 1 THz
ZnTe, RT, 2 THz
ZnTe, CT, 2 THz
GaP, RT, 2 THz
Fülöp et al., Optica, 2016Polónyi et al., Opt. Express, 2016
Materials for optical rectification: organic crystals
Materialdeff
[pm/V]ng
@ 800 nm (1.55 µm)nTHz
αTHz
[cm-1]
FOMfor L = 2 mm[pm2cm2/V2]
CdTe 81.8 (2.81) 3.24 4.8 11.0
GaAs 65.6 4.18 (3.56) 3.59 0.5 4.21
GaP 24.8 3.67 (3.16) 3.34 0.2 0.72
ZnTe 68.5 3.13 (2.81) 3.17 1.3 7.27
GaSe 28.0 3.13 (2.82) 3.27 0.5 1.18
sLiNbO3 @ 300 K@ 100 K
168 2.25 (2.18) 4.96 174.8
18.248.6
DAST 615 3.39 (2.25) 2.58 50 41.5
Velocity matching condition:
Hebling et al., JOSA B, 2008
THzlasergTHzplaserg wwww nnvv ==
Organic materials
Collinear phase matching for 1.2 –1.6 µm pump wavelength
Best suited for the 1 – 20 THz range
Extremely large nonlinearcoefficient
High efficiency up to 3% possible
Pumped by OPA or Cr:forsteritelaser
Often complicated THz spectrumbecause of phonon absorptionbands
Limited crystal size (~1 cm)
Chemicalstructure of DAST (4-N, N-dimethylamino-4’-N’-methyl-stilbazoliumtosylate)Walther et al., Opt. Lett., 2000
Organic materials: phase matching
Vicario et al., Opt. Express, 2015
www.rainbowphotonics.com
Organic materials: bandwidth
Tuning curve ofa 1-mm DSTMSTHz generator
www.rainbowphotonics.com
Coherence length for HMQ-TMS
THz spectra for different pumpwavelengths in HMQ-TMS
Organic materials: tuning
Vicario et al., Sci. Rep., 2015
Organic materials: scaling up the energy
Partitioned crystal (DSTMS)
0.9 mJ pulse energy
Peak field up to40 MV/cm and 14 T
Up to 3% conversion efficiency
Pumped by Cr:forsterite laser(1.25 μm wavelength)
Vicario et al., Opt. Lett., 2014Vicario et al., Phys. Rev. Lett., 2014
THz generation in gas plasma
THz radiation by asymmetric photocurrent Sensitive to the relative phase (φ) between
ω and 2ω fieldsKarpowicz et al., J. Mod. Opt., 2009
Kim et al., IEEE J. Quantum Electron, 2012
Hoffmann & Fülöp, J. Phys. D, 2011
800 nm
800 nm + 400 nm
THz generation in gas plasma
Extremely broad spectra up to ~100 THz
Up to 7 μJ pulse energy, 8 MV/cm peakelectric field
Energy scaling
– Longer pump wavelength[Clerici et al., Phys. Rev. Lett., 2013]
– Longer filament→ off-axis phase matching[Oh et al., New J. Phys., 2013]
– Plasma sheet by cylindrical focusing[Oh et al., New J. Phys., 2013]
Broadband THz detection in gas
THzAC
232 EEII ww c
Karpowicz et al., Appl. Phys. Lett., 2008
Applications of THz pulses
Linear THz spectroscopyEmax ≈ 100 V/cm | 10 fJ pulse energySpectroscopy of graphene, nanotubes, molecular magnets, hydrated molecules, etc.
Nonlinear THz spectroscopyEmax ≈ 100 kV/cm | µJ pulse energyTHz pump—THz / optical / X-ray / etc. probe measurements of dynamics
Manipulation and acceleration of charged particlesEmax ≈ 10 – 100 MV/cm | (multi)-mJ pulse energyacceleration of proton & relativistic electron beams, X-ray free electron laser, etc.
THz pump – THz probe spectroscopy
Commercial-grade nonlinear THz spectroscopy systemdeveloped at University of Pécs
Manipulation and acceleration of charged particlesup to 10 mJ pulse energy, Epeak ≈ 100 MV/cm
• Enhancement of high-harmonic generation (HHG)E. Balogh et al., Phys. Rev. B, 2011K. Kovács et al., Phys. Rev. Lett., 2012
• Electron undulationHebling et al., arXiv:1109.6852
• Longitudinal compression and acceleration of relativistic electron bunches→ single-cycle MIR…X-ray pulse generationHebling et al., arXiv:1109.6852Wong et al., Opt. Express, 2013Nanni et al., Nat. Commun., 2015
• Proton acceleration→ hadron therapy40 MeV to 100 MeV accel. requires 30 mJ THz energyPálfalvi et al., Phys. Rev. ST Accel. Beams, 2014Sharma et al., Phys. Plasmas, 2016
Applications of high-energy THz pulses
1 GV/m = 10 MV/cm peak field strength is needed
THz beam
AccelerationPlettner et al., Phys. Rev. ST Accel. Beams 9, 111301 (2006)
Beam deflection, focusingPlettner et al., Phys. Rev. ST Accel. Beams 12, 101302 (2009)
Electron acceleration
Possibility of a THz proton postaccelerator
By THz evanescentwave:Pálfalvi et al., Phys. Rev. ST Accel. Beams, 2014
… for laser-generated proton beams
0 20 40 60 80 10040
45
50
55 E0 = 0.7 MV/cm
w/2 = 0.25 THz
d = 100 m
outp
ut
energ
y (
MeV
)
ordinal number of the proton (i)
1.stage
2.stage
3.stage
4.stage
5.stage
Suitable THz source:
More than 0.4 mJ THz pulse energyby tilted-pulse-front pumping of LiNbO3
Fülöp et al., Opt. Express, 2014
Summary
Intense THz sources now cover the entire THz spectral rangefrom 0.1 to 100 THz and beyond
Perspectives by optical rectification:
– LiNbO3: ~10 MV/cm, multi-mJ, low frequency
– Semiconductors: ~20 MV/cm, multi-mJ, low & medium frequency,extremely compact & robust contact-grating technology
– Organic crystals: ~100 MV/cm, multi-mJ, higher frequency
Intense THz sources and nonlinear (pump-probe) spectroscopic tools enabled resonant and non-resonantcontrol of matter
New range of applications: charged-particle maipulation and acceleration
