THz Physik: Grundlagen und Anwendungen

Transkript

1 THz Physik: Grundlagen und Anwendungen Inhalt: 1. Einleitung 2. Wechselwirkung von THz-Strahlung mit Materie 3. Erzeugung von THz-Strahlung 3.1 Elektronische Erzeugung 3.2 Photonische Erzeugung 3.3 Nachweis von THz-Strahlung 3.4 Nichtlineare Methoden 4. Erzeugung ultrakurzer Pulse (Grundlagen) 5. THz-Optik 6. THz-Zeitbereichs-Spektroskopie 6.1 Kompleer Brechungsinde 6.2 Fouriertransformation 7. Anwendungen

2 Waveguide THz-TDS Eperimental apparatus Microwave approach THz form time-domain funnel using spectroscopy commercial metal system shim beam adjust waist opening between and curvature two parabolic to optimize mirrors coupling THz adiabatic spot size compression appro. 9 of µm the in THz the wave center M. Theuer, D. R. Grischkowsky, and R. Beigang, tm- Technisches Messen, Vol. 78, No. 5, pp (2011)

3 Plasmonen Festkörper Oberfläche Nanopartikel (lokalisierte Plasmonen)

4 Surface plasmon polariton: EM wave at metal-dielectric interface EM Welle koppelt an die Plasma-Oszillationen der Oberflächenladungen z k k t i d d z e E t z E,0,, z k k t i m m z e E t z E,0 ),, ( Für propagierende gebundene Wellen - k real - k z imaginär z

5 SPP-Dispersion: k m d c m d 1/ 2 ck d Radiative modes ' m > 0) real k real k z p p Quasi-bound modes d < ' m < 0) imaginary k real k z 1 d z Dielectric: d Metal: m = m ' + m " Bound modes (' m < d ) real k imaginary k z Re k

6 Anregung von SPPs

7 Anregung von SPPs (Kretschmann)

8 Anregung von SPPs (periodische Struktur) Gittereinkopplung Wood s anomaly

9 Terahertz und Plasmonik THz spectral range: 100 GHz < n < 10 THz 3 mm < l < 30 µm THz plasmonics Sub wavelength holes Propagation on surfaces and wires Structured media periodic/random T. W. Ebbesen et al.,nature 391, 667 (1998) Agrawal et al., New Journal of Physics (2005) G. Torosyan et al., APL 85, 3372 (2004) Abul K. Azad et al., Appl. Phys. Lett. 86, (2005) Wang & Mittleman, NATURE, VOL 432, 376 (2004) K. J. Chau et al., PRL 94, (2005)

10 Periodische metallische StruKturen 1- or 2-dimensional array of conducting rods z y z y Metallic photonic crystals with large metal filling factor Field: Periodicity: E y-z z y Field: E Periodicity: -y

11 Transmissionsspektren 2 E THz 1.5 mm THz in THz THz out Intensity [a.u.] 1 Typical properties: 1. periodic minima Wood s anomalies 2. variation of profiles Fano profiles 3. low frequency cut-off eff. Plasma frequency Frequency [THz] Ecitation and propagation of surface plasmon polaritons at the frequencies of Wood s anomalies

12 Wood sche Anomalien und Anregung von SPPs Ecitation of surface plasmon polaritons if: k SPP k sina m g a k THz Inverse grating vector: Perpendicular incidence: 2 g ˆ d c n SPP m d n SPP c m d ( 1 sina ) k - mg g - k d g + k + mg angle a: Ecitation frequencies are missing in the transmitted spectra Surface plasmon polaritons propagate in two directions: k SPP ˆ

13 Relative Intesnity [Arb. units] Surface plasmon polaritons FFT FFT Emission and splitting 4 Absorption Absorption Intensity [a.u.] Emission Emission d = 1.5 mm m = n Profile? n [THz] G. Torosyan, C. Rau, B. Pradarutti and R. B., Appl. Phys. Lett. 85, 3372 (2004)

14 Fano Profile und Woodsche Anomalien Superposition of resonant and non-resonant waves Fano profile M. Sarrazin et al. PRB 67, (2003) Resonant part: diffraction of resonantly ecited surface plasmons c n SPP m d Non-resonant part: resonant part non-resonant part direct transmission of incoming THz pulse - depends on gap properties - skin depth 1 µm Variation of Fano profiles by changing the gap and keeping the diameter of the cylinders constant

15 I [a.u.] Intensity [a.u.] Fano Profile close contact Variation of distance between cylinders (constant diameter) 200 µm gap Normalized to m = 2 SPP frequency mm Periode 1500 m Periode 1505 m Periode 1510 m Periode 1520 m Periode 1550 m Periode 1600 m Periode 1700 m mm gap width n [THz] n/n

16 I [a.u.] Rel. Phase [rad] Fano Profile Variation of distance between cylinders (constant diameter) close contact 200 µm gap Spectral phase for m = 2, mm mm n [THz] Frequency [THz]

17 Transmission

18 Transmission

19 Propagation von SPPs Ecitation with narrow band THz pulse Determination of propagation speed G. Torosyan et al., APL 85, 3372 (2004)

20 Propagation von SPPs

21 Propagation von SPPs

22 Propagation von SPPs

23 Propagation von SPPs auf Drähten NATURE VOL NOVEMBER 2004

24 Two dimensions 2-dimensional array of conducting rods z y z y Field: Periodicity: E y-z spheres 5 cm 3 cm

25 Resonant modes in cylinder gaps TE modes TM modes + Cubic array + 0 Zero field - Heagonal array Resonant structures within the space between the cylinders

26 Resonant transmission 6 layers heagonal band gap model Transmission wave vector

27 Resonant transmission 5 layers cubic band gap model Transmission wave vector

28 Resonant transmission Allowed frequency band: resonant transmission Wavelength high field strength inside Frequency in band gap: no transmission

29 Non resonant transmission Thin layers: Transmission also in band gaps cubic Multiple scattering between cylinders: heagonal

30 Transmission and thickness Large non resonant part broadband short time duration resonant transmission only narrow band long lasting oscillations

31 Spectral Intensity [arb. units] Electric Field [arb. units] Transmission ps THz in THz out 20 mm FFT Delay [ps] Frequency [THz]