Nemlineáris optika és spektroszkópia a távoli infravörös (THz) tartományon High Power THz Generation, THz Nonlinear Optics, and THz Nonlinear Spectroscopy CLEO2008 invited talk
Hebling János PTE Kísérleti Fizika Tanszék
THz-es sugárzás, T-sugarak
THz imaging Based on transmission through or reflection off objects Metals reflect, various materials absorb Sensitivity to material type & composition
THz transmission reveals contents
THz reflection reveals contents
THz imaging of metal, ceramic, & explosives inside the sole of a shoe
Detection inside containers, shoes, etc.
THz fingerprinting Organic molecules & crystals show distinct THz absorption Crystal lattice vibrations at THz frequencies
RDX molecules in crystal unit cell THz intermolecular vibrations THz spectra of common explosives
THz spectra reveal specific organic materials
MDMA, Metamfetamin, aszpirin
K. Kawase et al: Opt. Express 11, 2549 (2003)
Töltésmozgás bakteriorodopszinban G. I. Groma et al.: Proc. Nat. Acad. Sci. 105, 6888 (2008)
Nagyintenzítású THz-es impulzusok előállítása Szabadelektron lézer Lézerrel keltett plazma Optikai egyenirányítás, átalakítási hatásfoka függ: • ωTHz (η ~ ω2THz) • fázisillesztés → sebességillesztés
gr ph vvis = vTHz
• anyagi paraméterek 2 ⎡α THz L ⎤ sinh 2ω d L I 4 ⎥⎦ ⎢⎣ ⎡− α THz L ⎤ ⋅ exp = ⋅ 2 ⎥⎦ ⎡α L ⎤ 2 ⎢⎣ ε 0 nv2 nTHz c 3 THz 4 ⎥⎦ ⎢⎣ 2
ηTHz
αTHzL << 1 αTHzL >> 1
2 eff
2
ηTHz =
ηTHz =
2ω 2 d eff2 L2 I
ε 0 nv2 nTHz c 3 8ω 2 d eff2 I
2 c3 ε 0 nv2 nTHzα THz
FOM SA ≡ FOM LA ≡
d eff2 L2 2 n NIR nTHz
4d eff2 2 2 n NIR nTHzα THz
FOM értékek 2 mm kristályhosszt feltételezve Material
deff (pm/V)
CdTe
81.8
GaAs
65.6
GaP
gr 800 nm
n
nTHz
n1gr.55μm αTHz
(cm-1)
FOM (pm2cm2/V2)
3.24
2.81
4.8
11.0
4.18
3.59
3.56
0.5
4.21
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
sLiNbO3 sLN 100K
168
2.25
4.96
2.18
17 3.0
18.2 124
DAST
615
3.39
2.58
2.25
50
41.5
Velocity matching condition:
gr ph gr v NIR = vTHz ⇒ n NIR = nTHz
Fotórefrakció elnyomása Z-scan mérés Magnéziummal történő szennyezés egy küszöbszint felett (amely 5% kLN és 0.6% szLN esetén) megszünteti a fotórefrakciót L. Pálfalvi et al.: Appl. Phys. Lett 80, 2245 (2002) L. Pálfalvi et al.: J. Opt. A: Pure Appl. Opt. 5, 280 (2003) L. Pálfalvi et al.: J. Appl. Phys. 95, 902 (2004) L. Pálfalvi, et al.: Appl. Phys. B 78, 775 (2004)
LN THz-es abszorpciója és törésmutatója FT spektrométer: Bruker 113V Kongruens és sztöchiometrikus LN Hőmérséklet és Mg koncentráció függés L. Pálfalvi et al.: J. Appl. Phys. 97, 123505 (2005)
Velocity matching by tilting of the pump pulse front J. Hebling et al: Optics Exp. 10, 1161 (2002)
THz radiation excited along the tilted pulse front propagates perpendicularly to this front → velocity matching condition: gr ph vis THz
v ⋅ cos γ = v
Experimental results Appl. Phys. Lett. 83, 3000 (2003), Appl. Phys. B 78, 593 (2004)
THz pulse energy at ( 77/300 K ) : 400/100 pJ
Amplitude FFT (r. e.)
peak electric field: 7 kV/cm, photon conversion efficiency: 3.4 %
EOS signal (r. u)
5
1.0 0.8 0.6 0.4 0.2 0.0
0
1
0
2
Frequency (THz)
3
-5 -2
0
2
4
Time (ps)
6
8
Scaling-up the THz pulse energy Opt. Express 13, 5762 (2005) 0
100
200
300
400
500
Pump energy (μJ)
5
Tilted pulse Line focus
4 3
1
Tilted pulse
2 1 0
0.1 Pump energy (μJ)
0
100
200
300
ETHz = 240 nJ, ηph = 12 %, electric field > 100 kV/cm
400
500
-4
10
Efficiency (10 )
THz pulse energy (nJ)
100
High power THz systems at MIT
1 MHz APL 93, 141107 (2008) Epump (mJ) ETHz (μJ)
1 kHz Opt. Com. 281, 3567 (2008)
10 Hz APL 90, 171121 (2007)
0.014
5.6
18
0.25
3.0
10
Paver (mW)
0.25
3.0
0.1
Ppeak (MW)
0.05
3
5
150
200
6
300
>400
2
18
45
I (MW/cm2) F (kV/cm) ηph (%)
Nonlinear THz transmission, Nonlinear optics
1 kHz
THz pump – THz probe ITHz = 150 MW/cm2,
E=250 kV/cm
THz nonlinear spectroscopy • Semiconductor nanostructures or bulk semiconductors at low temperature. Free electron laser, not time resolved or 10 ps resolution. Intensity: <1 W/cm2 – 10 kW/cm2. S. D. Ganichev and W. Prettl: Intense Terahertz Excitation of semiconductors, Oxford Uni. Press 2006
• Single-cycle THz pulse ionization of Rydberg states, E ≤ 20 kV/cm (I=1 MW/cm2). R. P. Jones et al., Phys. Rev. Lett. 70, 1236 (1993)
• Ionization of impurity Rydberg sates in GaAs, E ≤ 70 kV/cm, P. Gaal et al., Phys. Rev. Lett. 96, 187402 (2006)
• Investigation of polaron internal motion in GaAs, E = 20 kV/cm, P. Gaal et al., Nature 450, 1210-1213 (2007)
• At MIT nonlinear THz transmission and THz pump – THz probe set up with ITHz = 150 MW/cm2
THz nonlinear optics • Extra large second order nonlinearity on the microwave range (below lattice resonance). Phys. Rev. Lett. 26, 387 (1971) (56 GHz) • Resonance enhancement of THz second order nonlinearity. CO2 pumped CH3F laser, 40 ns, 2 mJ, 15 kV/cm. Phys. Rev. B 33, 6954 (1986) • Theory of third order nonlinearity below lattice resonance. Progr. Quant. Electr. 4, 271(1976)
• Third harmonic generation by free carrier anharmonicity in semiconductors. Phys. Rev. B 33, 6962 (1986)
• At MIT observation of second- and third order nonlinearity using singlecycle THz pulses with ITHz = 150 MW/cm2
Szabadelektron abszorpció telítődése n- típusú germániumban 8
12
Ref. 100% 33% 7%
Absorption (cm-1)
10
Electric field (a. u.)
6
4
8 6 4 2
2
0
0.5
1.0
1.5
Frequency (THz)
2.0
0
-2
0
2
Time (ps)
4
6
J. Hebling et al.: Phys. Rev. Lett. (beküldve) Telectron-Tlattice 1
10
100
1000
5
250
4
-1
150
3
100
2
-1
Ge absorption (cm )
GaAs absorption (cm )
200
50 1 0 1E-3
0.01
0.1
THz energy (μJ)
1
0
A. Mayer, F. Keilmann: Phys. Rev. B 33, 6962 (1986), CO2 pumped CH3F laser 40 ns, 2 mJ, 15 kV/cm α ~ μ (~1/m*) EΔ=EL+0.18 eV
Abszorpció feléledése n-típusú germániumban 15 14 13 12 11 10 9 8
ln(α)
7 6 5 4 3
2 -2
-1
0
1
2
Probe delay (ps)
3
4
5
Ütközési ionizáció, alagút ionizáció C. Zener: Proc. Roy. Soc. A 145, 523 (1934) L. Esaki: Phys. Rev. 109, 1234 (1958) InSb undoped 450 um 80K 35
eff absorption (cm-1)
30
00 deg 20 deg 30 deg 45 deg
25 20 15 10 5 0 -10
0
10
20
30
probe delay (ps)
40
50
Lattice anharmonicity THz gen. saturation, THz harmonic generation? 2
Electric field (a. u.)
red: 9.5 x Ip blue: Ip x 9.5
1
0
-1
-2 0
1
2
3
4
Time (ps)
5
6
7
J. Hebling et al.: IEEE J. Sel. Top. Quant. Elect. 14, 343 (2008)
1
Amplitude spactrum
red: 9.5 x Ip blue: Ip, x 9.5
0.1
0.01
0.00
0.74
1.48
2.22
2.96
3.70
4.44
5.18
5.92
Frequency (THz)
G. D. Boyd: Phys. Rev. B 7, 5345 (1973), LiNbO3: do = 40, deo = 181, dm = 5,870 pm/V LiTaO3: dm = 18,620 pm/V, BaTiO3: dm = 97,000 pm/V
Self-phase modulation in the THz source sLN at 10 K
Ampl. spectr., Ratio
J. Hebling et al.: IEEE J. Sel. Top. Quant. Elect. 14, 343 (2008)
Electric Field (a. u.)
0.4
0.2
5 4 3 2 1 0
0
1
2
Frequency (THz)
0.0
-0.2
0
3
6
Time (ps)
9
12
3
Self-phase-modulation in 2 mm thick external sLN at 100 K Ultrafast Phenomena 2008 Tóth György: poszter
Normalized electric field strength
1.0
0.5
0.0
-0.5
measured original measured transmitted
-1.0
2
simulated, n2=0 m /W
-1.5
-12
sim., n2=5.4*10 1
2
3
4
Time (ps)
5
2
cm /W
6
Ref 4 * LN Transm
T = 100 K
Spectra (a.u.)
1.0
1.2
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0 0.0
0.5
1.0
1.5
Frequency (THz)
2.0
0.0
External transmission
1.2
Tilted-pulse-front THz generation set up without imaging Appl. Phys. Lett. 92, 171107 (2008) There is no limit for lateral size!
Limitations of nonlinear pulse propagation description ∂A << k 0 A ∂z
∂A << ω 0 A ∂t
Slowly-evolving-wave approximation:
∂A << k 0 A ∂z
k0 − ω0 ⋅
Spectral amplituide (a. u.)
Electric field strenght (a. u.)
Slowly-varying-envelope approximation:
1.0
dk << k 0 dω
Phys. Rev. Lett. 78, 3282 (1997)
1 kHz
0.5 0.0
Direct sensing of anharmonic potential!
x 200
-0.5 -1.0
-2
0 Time (ps)
2
4
1.0 0.8 0.6 0.4 0.2 0.0
0
1 2 Frequency (THz)
3
High energy THz group at MIT Keith A. Nelson
János Hebling
Matthias Hoffmann
Harold H. Hwang
Ka-Lo Yeh
Collaborators G. Almási, B. Bartal, J. A. Fülöp, L. Pálfalvi, Gy. Tóth Department of Experimental Physics, University of Pécs, Hungary
Idea of tilted-pulse-front THz exc., new set-up, simulations
A. G. Stepanov and J. Kuhl Max-Planck-Institute for Solid State Research, Stuttgart, Germany
First experimental demonstration
I. Z. Kozma and E. Riedle Lehrstuhl für BoiMolekulare Optik, LMU München, Germany
Scaling-up the THz energy
K. Polgár and Á. Péter Institute for Solid State Physics, Budapest, Hungary
Mg doped sLN crystals
Nonlinearity of free electrons
200
-1
Absorption coefficient (cm )
Linear (red) and nonlinear (blue) “absorption” spectrum of GaAs
100
0
-100
n-type GaAs (Si doped) [100], 0.4 mm 15 -3 8.6x10 cm
-200 1
2
Frequency (THz)
3
4
Abszorpció feléledése Si-ban, szennyezés ionizálása n- típusú (P szennyezés) Si, 0.8 mm Probe delay (ps) -2
-1
0
1
2
3
4
5
E0 10
0.4E0
-1
Absorption coefficient (cm )
0.2E0 8 Time (ps) -2
6
-1
0
1
2
3
4
2
Electric field (a. u.)
14xProbe 4
2
0
-2
0
Pump
THz pump – probe, n-type Ge
-1
Absorption α [cm ]
12
probe delay 5.74 ps 3.93 ps 2.61 ps 1.47 ps 0.50 pa Linear absorption
10 8 6 4 2 0 0.0
0.5
1.0
1.5
Frequency (THz)
2.0
EΔ=EL+0.18 eV
Ge
EL=EΓ+0.36 eV
GaAs
EL=EX+0.89 eV
Si
THz pump – THz probe measurement Reference
Germanium Time (ps)
Time (ps) -2
Probe delay (ps)
-2
-1
0
1
2
Ref.
-2 -2
-1
-1
0
0
1
1
2
2
3
3
4
4
5
5
Ge
-1
0
1
2
red: Ref
blue: 2.5xGe, -60.8 ps
o
4
45
4
25 %
56%
2
2
0
0
-2
-2
-3
4
-2
-1
0
1
2
3
-3
-1
0
1
2
3
-1
0
1
2
3
o
4
100 %
2
2
0
0
-2
-2
-2
-2
0
o
60
6%
-3
o
30
-1
0
1
Time (ps)
2
3
-3
-2
Time (ps)
THz absorption smallest for sLN with 0.6% Mg doping level 50
sLN, 100 K 0.0% Mg 0.6% Mg 1.5% Mg 4.0% Mg
-1
α (cm )
40 30 20 10 0
1
2
3 ν (THz)
4
5
Electric field strenght (a. u.) Spectral amplituide (a. u.)
1.0
1 kHz
0.5 0.0 x 200
-0.5 -1.0
-2
0 Time (ps)
2
4
1.0 0.8 0.6 0.4 0.2 0.0
0
1 2 Frequency (THz)
3
Shaped THz waveforms Optical pulse sequence Æ THz pulse sequence
HR
PR
Pump Laser
K.-L. Yeh et al, Opt. Commun. (2008) Versatile optical pulse shaper now installed
