High Efficiency THz Generation in Novel Organic Nonlinear … · 2018-12-10 · Nonlinear Optics...
Transcript of High Efficiency THz Generation in Novel Organic Nonlinear … · 2018-12-10 · Nonlinear Optics...
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Peter GünterNonlinear Optics Laboratory
Swiss Federal Institute of Technology ETH Zürich, Switzerland
• Generation of THz Radiation in Organic Materials• DAST and DSTMS Crystals• THz Generation in DSTMS • Applications in Spectroscopy and Imaging• Conclusions
High Efficiency THz Generation in Novel Organic Nonlinear Optical Crystals
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Electronics, THz Waves and Lasers
Inorganic NLO
DAST
30 33000.3Wavelength (m) Reststrahlen Band
DSTMS
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Motivation
New system
DSTMS/DSTMS
DAST/DAST
Terahertz photonics:Established technology but expensive and bulky
and often restricted to < 3 THz
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Nonlinear Optics
• Deals with the interactions between optical waves and electric fields in nonlinear optical materials
• Can be used for controlling optical beams with other optical waves or electrical signals
• Largest Effects in Highly Polarizable Polar Materials
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d
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Difference-frequency generation
»
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Requirements for Efficient Generation
Organic crystals satisfy all these conditions particularly well!
• Large nonlinear susceptibility (2)
• Low absorption• Velocity-matching between
the optical and the THz pulse
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Figure of Merit NLO
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Electro-optic materials
r(pm/V)
n n3r(pm/V)
n3r/(pm/V)
(nm)
GaAsZnTe
1.24
3.52.9
5098
13 4 1020800
LiNbO3 30 2.2 320 30 10 633
Sn2 P2 S6 180 2.8 4000 230 17 1318
EO polymer
100 1.6 400 3 140 1318
DSTMS 18053
2.52.2
2800560
5.25.2
540110
7201318
Sem
icon
d.Fe
rroe
l.O
rgan
ic.
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Molecular Orbitals
Benzene
Para-nitro-aniline
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Organic Nonlinear Optical Thin Films
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Organic molecules for (2) applications
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Oriented gas-model
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.
nonlinear optics electro-optics
f3
g(
dijj
cos
sin2 riii
cos3
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Engineering strategies
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Engineering strategies II
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NLO Stilbazolium Salts
Counter anions and SHG intensities:SO3C
HH2C
SO3HN
SO3NNN
SO3
SO3
SO3
NN CH3
SO3
SO3
SO3
NH2
DAST
DSDMS
DSTMS
DSNS-1
DSNS-2
DSANS
DSSS
DSPAS
DSMO
Z. Yang et.al.,Cryst. Growth and Design 7, 83 (2007)
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The Organic EO Crystal DAST(4-N,N-dimethylamino-4’-N’-methyl stilbazolium tosylate)
Highly birefringent (n1 -n2 = 0.7 @ 800 nm)Electro-optic coefficient r11 = 77 pm/V
(ZnTe: 4 pm/V, GaAs: 1.5 pm/V)
Producer: Rainbow Photonics AG
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DAST Crystal Structure (PGS: 2)
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Refractive indices of DAST
800 1000 1200 1400 1600
1.6
1.8
2.0
2.2
2.4
2.6
Wavelength (nm)
n1 DASTn2 DASTn3 DAST
n1
n3
n2
n ~ 0.9
N+
H3C
H3C CH3
-O3S CH3
N
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Organic EO/THz Materials
Small Lattice Polarizability leads to:• Similar Refractive Indices at Optical,
Electrical and THz Frequencies• Mainly Electronic EO Effect• Fast EO Response• Small Mismatch of Electrical and Optical
Waves in Travelling Wave EO Modulators• Phase Matched THz Generation
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THz Optical Rectification
NLO fs pulses
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THz Spectrum (Theory)
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Source Term
0 2.1012 4.1012 6.1012 8.1012 1.10130
1.1013
2.1013
3.1013
Frequency [Hz]
FWHM 200 fsFWHM 150 fsFWHM 100 fs
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THz Spectrum (Theory)
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Energy of THz-Waves Generated in DAST
Reference: T. Taniuchi et al., Nonlinear Optics, Quantum Optics, 34, 57 (2005)
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THz Spectrum (Theory)
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Velocity-matching
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DAST: Dispersion of Refractive Indices
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TO Phonon in DAST (1.1 THz)
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Velocity-matching in DAST
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Effective generation length in DAST
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THz Pulse Amplitude (
= 1350 nm)
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DAST Derivatives
How to bridge the gap?
Behavior of the TO phonon at 1.1 THz?
Modify the chemical structure of DAST without compromising its high nonlinearity
- Reduction of the absorption coefficient
Derivatives: - DSMOS- DASC- DANS
- DSXYS- DSTMS- …
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Stilbazolium Salts I
X Abbreviation
OH DSHS
OCH3 DSTMS *
NH2 DSAS
N(CH3 )2 DSMAS
CH3 DAST
*: 4-N,N-Dimethylamino-4'- N'-methyl-STilbazolium 2,4,6-
triMethylbenzeneSulfonate
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DSTMS
BULK 33 x 33 x 2 mm3 THIN 6 x 5 x 0.03 mm3
NN CH3
SO3
Z. Yang et al, Adv. Funct. Mater. 17 (2007)
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Emitted THz FieldSource: 0.45 mm, FWHM = 100 fs
• Increased amplitude near the resonances around 1 THz and 3 THz flatter THz spectrum than for DAST
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Set up THz Generation and Detection
THz Pulse Generation by Optical Rectification,Detection : THz-induced Lensing .
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Terahertz-Induced Lensing
• THz signal on the detector crystal induces a refractive index change that follows the field distribution:
• Refractive index change acts as a lens
n(x,y)n3
2rETHz(x,y)
ETHz =0 ETHz 0
Aperture: detect central intensity
ETHz =0
ETHz 0Probe PD
EO Crystal
nn0
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THz-induced lensing (TIL)
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Read-out of THz-induced lensing
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THz Temporal Waveform
•
= 1125 nm, pump pulse energy E = 48 J
-3 -2 -1 0 1 2 3 4
-4000
-2000
0
2000
4000
Time [ps]
ETHz [V/cm]
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Pump Energy Dependence of ETHz
•
= 1125 nm
0 10 20 30 40 500
2000
4000
6000
8000
10000
Pump Pulse Energy [μJ]
TH
z F
ield
, Pea
k T
o P
eak
[V/c
m]
ΔEpump
ΔETHz
Efficiency: ΔEpumpΔETHz
(Deviation ~Epump2 due to
two-photon absorptionof pump beam)
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THz Generation in DSTMS
• 0.3 to 12 THz (Small Resonances at 0.65/1.02/2.7/5.1/8.05 THz)
• Dynamic Range: 3 103 (65 dB Spectral Power)• Scan Time: 20 s
1.5 m Laser:- 70 fs- 100 MHz- Pavg =150 mW2009
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Summary: DSTMS Crystals
A. Schneider, M. Stillhart, P. Günter,Opt. Express 14, 5376-5384 (2006)
DSTMS Detector
DSTMS(organic)
ZnTe*(inorganic)
Velocity- matching
Wavelength m :1400-1700 nm 750-850 nm
(2)
(Generation)
r (Detection)
490 pm/V
47 pm/V
137 pm/V
4.2 pm/V
Telecom Lasers!
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TeraSys 4000 (0.3 - 4 THz)
Rainbow Photonics Ltd
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Rainbow Photonics TeraSys 4000
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THz Absorption of 3 Explosives
0.5 1.0 1.5 2.00
20
40
60
Frequency [THz]
–1 ]
SemtexTNTEmulgit LWC Al
47
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Reflection Mode
X-Y-scanning
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TeraKit with Imaging Option
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THz-Image of Semtex
Semtex hidden in Teflon
10 mm
Size: 25 x 25 mm
No explosiveExplosiveRed paper sticker in upper right corner.
(a) (b)
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THz-Application: Mail Inspection
v=6 m/s
Envelope C475
*106
Pulse/s
Detection E(t)
THz-
radi
atio
n
Velocity (C4): 50msNumber of pulses (C4):
375’000
51
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THz imaging: Bacillus cereus spores
- Particle size ~1 micron- ~ Monolayer- Scanned area: 7.5 x 15 mm
- 0.25 x 0.25 mm/pixel
Bacillary, anaerobic and spore forming bacteriaFood poisoningClosely related to B. anthracis!
THz-Image 10 mm
52
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THz Images
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Application: THz Imaging
• Most dielectrics are transparent• Conductors are opaque
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UHMWPE Sample
UHMWPE plate with three different defects used for terahertz pulse imaging.
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Terahertz Pulse Imaging
Terahertz pulses transmitted through an 8 mm thick UHMWPE plate in a region with a 1 mm thick hole and in a defect-free region, respectively.
Longitudinal resolution≈
10-50 μm
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Terahertz reflection tomography
Terahertz pulses reflected from a 4 mm thick UHMWPE plate with a 1 mm thick defect.
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Phase image (fixed time-delay)
Phase image acquired by terahertz pulse imaging. Left panel: UHMWPE plate without any defects. Right panel: UHMWPE plate with defects.
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THz Wave Technology Offers
• Coherent Radiation with Amplitude and Phase Information
• High Spectral Resolution (up to 0.005 THz (0.17 cm-1)
• Wide Spectrum for Spectral Fingerprint Selectivity (particularly with DSTMS)
• 2 (and 3)- Dimensional Images• Spatial Resolution of Less Than 0.1 mm
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Conclusions
• Efficient THz generation in DSTMS crystals (high nonlinearity, velocity-matching)
• Broadband THz waves (0.3 - 12 THz) generated/detected (up to 20 THz possible)
• Compact turn-key operated THz spectrometer / Imager realized
• Terahertz-induced lensing is an attractive technique for the detection of THz pulses
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Collaborators
Dr. A. Schneider Dr. M. Stillhart (RP)Dr. F. BrunnerDr. G. PoberajDr. B. Ruiz (RP)Dr. C. Medrano (RP)Dr. T. Bach (RP)
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DAST Crystal
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Optical Properties of DSTMS
Theoretical: Three harmonic oscillators model
Optical properties in the THz spectral range:
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Organic (2) crystals
A. Conventional molecular crystals (COANP, DAN, MNA…): moderate EO n3r < 40 pm/V• Large-scale integration possible
B. State-of-the-art: organic salts such as DAST, DSTMS: very high EO n1
3r11 > 400 pm/V• EO waveguides possible, but very difficult on-chip integration
• New materials combining both high electro- optic activity and good processability needed
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DAST: EO and NLO Properties
Large EO and NLO coefficients:
r11 = 62 ± 8 pm/V @ 1300 nm
d11 = 1010±110 pm/V @ 1318 nmd11 = 290±15 pm/V @ 1542 nm
r11 = 400±150 pm/V @ 820 nm
+ Superior EO & NLO Properties and Stability- Difficult to process using standard techniques
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Set up THz Generation and Detection
THz Pulse Generation by Optical Rectification,Detection via THz-induced Lensing .
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Distinct Absorption Lines
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THz Wave Technology offers
• Coherent Radiation with Amplitude AND Phase Information
• High Spectral Resolution (up to 0.005 THz (0.17 cm-1)
• Wide Spectrum for Spectral Fingerprint Selectivity (particularly with DAST)
• 2 (and 3)- Dimensional Images• Spatial Resolution of Less Than 0.1 mm
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Chemical Structure of DSTMS
DAST: X=H, DSTMS[1] : X=CH3
DSTMS: 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate
Chromophore
Counter-Ion
[1] Z. Yang, Adv. Funct. Mater. 17, 2018 (2007)
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Materials at = 800 nm
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Rainbow Photonics Ltd