Nemlineáris optika és spektroszkópia a távoli infravörös (THz) tartományon

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1 Nemlineáris optika és spektroszkópia a távoli infravörös (THz) tartományon High Power THz Generation, THz Nonlinear Optics, and THz Nonlinear Spectroscopy CLEO8 invited talk Hebling János PTE Kísérleti Fizika Tanszék

2 THz-es sugárzás, T-sugarak

THz imaging Based on transmission through or

reflection reveals contents THz imaging of metal,

3 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.

4 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

5 MDMA, Metamfetamin, aszpirin K. Kawase et al: Opt. Express 11, 549 (3)

6 Töltésmozgás bakteriorodopszinban G. I. Groma et al.: Proc. Nat. Acad. Sci. 15, 6888 (8)

7 Nagyintenzítású THz-es impulzusok előállítása Szabadelektron lézer Lézerrel keltett plazma Optikai egyenirányítás, átalakítási hatásfoka függ: ω THz (η ~ ω THz ) fázisillesztés sebességillesztés anyagi paraméterek η THz ω d = ε n n v eff THz L I exp α 3 c THz L v = gr vis v ph THz sinh α THz L 4 α THzL 4 α THz L << 1 α THz L>> 1 ω d η THz = ε n n η = THz ε n v v eff L THz 8ω d n c I 3 I eff THzαTHz c 3 d eff FOM SA nnir FOM LA n NIR 4d n n eff THz L THz α THz

8 Material FOM értékek mm kristályhosszt feltételezve gr gr d eff n 8 nm n n THz 1.55μm (pm/v) THz (cm -1 ) α FOM (pm cm /V ) CdTe GaAs GaP ZnTe GaSe slinbo 3 sln 1K DAST Velocity matching condition: v = v n = gr NIR ph THz gr NIR n THz

9 Fotórefrakció elnyomása Z-scan mérés Magnéziummal történő szennyezés egy küszöbszint felett (amely 5% kln és.6% szln esetén) megszünteti a fotórefrakciót L. Pálfalvi et al.: Appl. Phys. Lett 8, 45 () L. Pálfalvi et al.: J. Opt. A: Pure Appl. Opt. 5, 8 (3) L. Pálfalvi et al.: J. Appl. Phys. 95, 9 (4) L. Pálfalvi, et al.: Appl. Phys. B 78, 775 (4) 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, 1355 (5)

10 Velocity matching by tilting of the pump pulse front J. Hebling et al: Optics Exp. 1, 1161 () THz radiation excited along the tilted pulse front propagates perpendicularly to this front velocity matching condition: gr ph v vis cos γ = vthz

12 Experimental results Appl. Phys. Lett. 83, 3 (3), Appl. Phys. B 78, 593 (4) THz pulse energy at ( 77/3 K ) : 4/1 pj peak electric field: 7 kv/cm, photon conversion efficiency: 3.4 % EOS signal (r. u) 5 Amplitude FFT (r. e.) Frequency (THz) Time (ps)

13 Scaling-up the THz pulse energy Opt. Express 13, 576 (5) Pump energy (μj) 1 THz pulse energy (nj) 1 1 Tilted pulse Line focus Tilted pulse Efficiency (1-4 ).1 Pump energy (μj) E THz = 4 nj, η ph = 1 %, electric field > 1 kv/cm

14 High power THz systems at MIT 1 MHz 1 khz 1 Hz APL 93, (8) Opt. Com. 81, 3567 (8) APL 9, (7) E pump (mj) E THz (μj) P aver (mw) P peak (MW) I (MW/cm ) 15 F (kv/cm) 6 3 >4 η ph (%) 18 45

15 Nonlinear THz transmission, Nonlinear optics 1 khz THz pump THz probe I THz = 15 MW/cm, E=5 kv/cm

16 THz nonlinear spectroscopy Semiconductor nanostructures or bulk semiconductors at low temperature. Free electron laser, not time resolved or 1 ps resolution. Intensity: <1 W/cm 1 kw/cm. S. D. Ganichev and W. Prettl: Intense Terahertz Excitation of semiconductors, Oxford Uni. Press 6 Single-cycle THz pulse ionization of Rydberg states, E kv/cm (I=1 MW/cm ). R. P. Jones et al., Phys. Rev. Lett. 7, 136 (1993) Ionization of impurity Rydberg sates in GaAs, E 7 kv/cm, P. Gaal et al., Phys. Rev. Lett. 96, 1874 (6) Investigation of polaron internal motion in GaAs, E = kv/cm, P. Gaal et al., Nature 45, (7) At MIT nonlinear THz transmission and THz pump THz probe set up with I THz = 15 MW/cm

17 THz nonlinear optics Extra large second order nonlinearity on the microwave range (below lattice resonance). Phys. Rev. Lett. 6, 387 (1971) (56 GHz) Resonance enhancement of THz second order nonlinearity. CO pumped CH 3 F laser, 4 ns, mj, 15 kv/cm. Phys. Rev. B 33, 6954 (1986) Theory of third order nonlinearity below lattice resonance. Progr. Quant. Electr. 4, 71(1976) Third harmonic generation by free carrier anharmonicity in semiconductors. Phys. Rev. B 33, 696 (1986) At MIT observation of second- and third order nonlinearity using singlecycle THz pulses with I THz = 15 MW/cm

18 Szabadelektron abszorpció telítődése n- típusú germániumban Electric field (a. u.) Absorption (cm -1 ) Ref. 1% 33% 7% Frequency (THz) Time (ps)

19 J. Hebling et al.: Phys. Rev. Lett. (beküldve) 5 T electron -T lattice GaAs absorption (cm -1 ) Ge absorption (cm -1 ) 1E THz energy (μj)

20 A. Mayer, F. Keilmann: Phys. Rev. B 33, 696 (1986), CO pumped CH 3 F laser 4 ns, mj, 15 kv/cm α ~ μ (~1/m*) E Δ =E L +.18 ev

21 Abszorpció feléledése n-típusú germániumban ln(α) Probe delay (ps)

22 Ütközési ionizáció, alagút ionizáció C. Zener: Proc. Roy. Soc. A 145, 53 (1934) L. Esaki: Phys. Rev. 19, 134 (1958) InSb undoped 45 um 8K 35 3 deg deg 3 deg 45 deg eff absorption (cm-1) probe delay (ps)

23 Lattice anharmonicity THz gen. saturation, THz harmonic generation? Electric field (a. u.) 1-1 red: 9.5 x I p blue: I p x Time (ps)

24 J. Hebling et al.: IEEE J. Sel. Top. Quant. Elect. 14, 343 (8) 1 Amplitude spactrum.1 red: 9.5 x I p blue: I p, x Frequency (THz) G. D. Boyd: Phys. Rev. B 7, 5345 (1973), LiNbO 3 : do = 4, deo = 181, dm = 5,87 pm/v LiTaO 3 : dm = 18,6 pm/v, BaTiO 3 : dm = 97, pm/v

25 Self-phase modulation in the THz source sln at 1 K J. Hebling et al.: IEEE J. Sel. Top. Quant. Elect. 14, 343 (8) Electric Field (a. u.).4.. Ampl. spectr., Ratio Frequency (THz) Time (ps)

26 Self-phase-modulation in mm thick external sln at 1 K 1. Ultrafast Phenomena 8 Tóth György: poszter Normalized electric field strength measured original measured transmitted simulated, n = m /W sim., n =5.4*1-1 cm /W Time (ps)

27 1. 1. T = 1 K Ref 4 * LN Transm Spectra (a.u.) External transmission Frequency (THz).

28 Tilted-pulse-front THz generation set up without imaging Appl. Phys. Lett. 9, (8) There is no limit for lateral size!

29 Limitations of nonlinear pulse propagation description Slowly-varying-envelope approximation: A z << k A A t << ω A Slowly-evolving-wave approximation: A z << k A dk k << dω ω k Electric field strenght (a. u.) Spectral amplituide (a. u.) khz x - 4 Time (ps). 1 3 Frequency (THz) Phys. Rev. Lett. 78, 38 (1997) Direct sensing of anharmonic potential!

30 High energy THz group at MIT Keith A. Nelson János Hebling Matthias Hoffmann Harold H. Hwang Ka-Lo Yeh

31 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

32 Nonlinearity of free electrons Linear (red) and nonlinear (blue) absorption spectrum of GaAs Absorption coefficient (cm -1 ) 1-1 n-type GaAs (Si doped) [1],.4 mm 8.6x1 15 cm Frequency (THz)

33 Abszorpció feléledése Si-ban, szennyezés ionizálása n- típusú (P szennyezés) Si,.8 mm Probe delay (ps) E 1.4E.E Absorption coefficient (cm -1 ) Electric field (a. u.) 4 Time (ps) xProbe Pump -

34 THz pump probe, n-type Ge Absorption α [cm -1 ] probe delay 5.74 ps 3.93 ps.61 ps 1.47 ps.5 pa Linear absorption Frequency (THz)

35 E Δ =E L +.18 ev Ge E L =E Γ +.36 ev GaAs E L =E X +.89 ev Si

36 THz pump THz probe measurement Reference Germanium Time (ps) Time (ps) Ref. - Ge -1-1 Probe delay (ps)

37 red: Ref blue:.5xge, -6.8 ps 4 45 o 5 % 4 3 o 56% o 6 % 4 o 1 % Time (ps) Time (ps)

38 THz absorption smallest for sln with.6% Mg doping level α (cm -1 ) sln, 1 K.% Mg.6% Mg 1.5% Mg 4.% Mg ν (THz)

40 khz x - 4 Time (ps) 1 3 Frequency (THz) Spectral amplituide (a. u.) Electric field strenght (a. u.)

41 Shaped THz waveforms Optical pulse sequence THz pulse sequence HR PR Pump Laser K.-L. Yeh et al, Opt. Commun. (8) Versatile optical pulse shaper now installed

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