Intense Terahertz Generation and Spectroscopy of Warm Dense Plasmas

1. Intense Terahertz Generation and Spectroscopy of Warm Dense Plasmas Kiyong Kim University of Maryland, College Park

2. Collaborators: Kishore Yellampalle George Rodriguez Toni Taylor Jim Glownia LOS ALAMOS NATIONAL LABORATORY

3. Outline: • Background: • - Terahertz (TH) science. • Intense THz generation: • - Two-color photoionization. • THz spectroscopy: • - Warm dense plasmas.

4. Phenomena at terahertz (THz) frequencies: 1 THz = 1012 Hz =1 ps = 300 m = 0.004 eV = 33.3 cm-1 molecules Rydberg atoms Gaseous and solid-state plasmas Semiconductor nanostructures Figure courtesy of Klaas Wynne Biomolecules & proteins

5. Photo courtesy: ALS Strong THz sources: Large facility THz sources* FEL Linacs Synchrotrons Photo courtesy: DESY Stanford, UCSB, FELIX SLAC, JLab,BNL ALS (BNL) Free electron lasing synchrotron radiation Coherent synchrotron radiation * M. S. Sherwin etal., DOE-NSF-NIH Workshop on Opportunities in THz Science

6. Intense THz generation: Two-color photoionization

7. ? Four-wave mixing * THz =  +  -2 2 plasma THz pulse Lens SHG But the third order nonlinearity originating from bound electrons of ions ((3)ions) and free electrons ((3)free-electrons) via ponderomotive or thermal effects is too small to explain the measurements. * M. Kress et al, Opt. Lett. 29, 1120 (2004); T. Bartel et al, Opt. Lett. 30, 2805 (2005); X. Xie et al, Phys. Rev. Lett. 96, 075005 (2006). (3)plasma = (3)ions + (3)free-electrons Two-color photoionization: 

8. THz e- e- e- Directional quasi- DC current e- e- e- e- e- Current surge  THz generation THz generation mechanism:  2  BBO crystal

9. THz energy measurement: THz energy vs pressure THz energy vs laser energy • ETHz ~ 5 J/pulse with Kr (C.E. > 10-4) K. Y. Kim et al., Nature Photonics doi:2008.153 (2008).

10. THz spectrum measurement: Field autocorrelations Fourier-transform spectra (a´) (a) (b´) (b) (c´) (c) THz generation up to 75 THz (= 4 m)

11. THz spectroscopy: Warm Dense Matter

12. Drude model r (THz) Optical pump pulse AC (0)  THz 0 Electrical conductivity measurements of WDM: Optical probe WDM 0 Measure probe reflectivity From the reflectivity, one can measure the electrical conductivity at the probe frequency. With THz probing, one can measure quasi-DC conductivity directly. H. M. Milchberg et al., Phys. Rev. Lett. 61, 2364 (1988). A. Ng et al., Phys. Rev. Lett. 72, 3351 (1994). A. N. Mostovych et al., Phys. Rev. Lett. 79, 5094 (1997).

13. THz probe D ~ 1 mm Pump pulse To single-shot THz diagnostic THz conductivity measurements of WDM: Target (Aluminum) The quasi-DC electrical conductivity can be directly determined from THz probe reflectivity measurements.

14. Breakdown of Drude model Experimental results I: THz reflectivity for various pump energies Possible pseudogap formation at the Fermi energy ??? K. Y. Kim et al., Phys. Rev. Lett. 100, 135002 (2008).

15. Conductor-to-insulator-like transition Experimental results II: THz reflectivity vs delay Room temp. Al: r= 4.1  107-1m-1 = 3.7 1017 s-1[-1m-1] = 1.1  10-10[s-1]

16. Resistivity saturation Experimental results III: THz reflectivity vs intensity

17. Summaries: • THz generation via two-color photoionization: • – Generated intense (>5 J), super-broadband TH radiation (>75 THz). • – Developed a transient photocurrent model. • – Potential application for nonlinear THz optics and spectroscopy. • THz spectroscopy for WDM: • – Directly measured the quasi-DC electrical conductivity of warm dense aluminum. • – Complements optical and x-ray diagnostics for WDM studies.

18. Backup slides:

19. P.D. P.D. Experimental setup: B-dot probe & 3 measurement THz spectrum measurement THz energy measurement Pyroelectric detector BBO d B-dot probe 3 filter P.D. THz pulse Si window BBO Plasma

20. Photo courtesy: the Star Tiger Strong THz field science*: • THz pump experiments • THz pumping of metals, insulators, and correlated electron materials. • Coherent band-gap distortion & phase transition. • THz-pump optical-probe experiments. • THz coherent control • Rapid THz imaging • Biomedical and security imaging ETHz > 1 MV/cm Strong THz sources • High magnetic field effects • 1 MV/cm  0.3 T • Pulsed electron spin resonance • THz spintronics • Nonlinear THz Optics • THz 2nd, 3rd nonlinear effects. • Extreme nonlinearity with ponderomotive energy> photon energy • THz-optical nonlinear mixing * M. S. Sherwin etal., DOE-NSF-NIH Workshop on Opportunities in THz Science

21.  = 0  = /2 Electron drift velocity Plasma current model I: Laser field 2 field  field  : relative phase  : photoionization phase  ( = 800 nm) and 2( = 400 nm) lasers with relative intensity of I = 1015 W/cm2 and I2= 2  1014 W/cm2 (assuming 20% efficiency of frequency doubling) K. Y. Kim et al., Opt. Express 15, 4577 (2007).

22. Plasma current model II: The nonlinearity arises from extremely nonlinear tunneling ionization localized near the laser peaks.* * Laser field: * Ionization rate: Ea:atomic field * Plasma current: * THz field: for Ea > E >> E2and Ng >> Ne * The function f(E) is highly nonlinear, not necessarily quadratic dominant.

23. Quasi-DC current Simulation results I: ADK tunneling ionization and subsequent classical electron motion in the laser field are considered. Simulation with  = 0 Simulation with  =  /2 I = 1015 W/cm2, I2= 2  1014 W/cm2, 50 fs (FWHM) Assumptions:No rescattering effect, No electron-ion or electron-neutral collisional processes, No space charge effect, No electron transport.

24. CCD P ZnTe THz imaging 4.4 mm Experimental setup I: Electro-optic THz detection Balanced detector Laser pulse or WP QWP ZnTe P Pellicle THz pulse BBO (Type I) Si window Air plasma Max. 8% conversion efficiency with polarization An amplified Ti:sapphire laser system delivering 815 nm, 50 fs, 25 mJ pulses at a 10 Hz repetition rate was used.

25. Strong THz absorption by water vapor in air Experimental result I: THz spectrum THz waveform Detection bandwidth is limited by dispersion and absorption in our 1-mm thick ZnTe crystal.

26. d   0  As d 0, THz yield  0 Current model : Four-wave mixing : Experimental result II: To check the validity of our plasma current model, we studied  dependence of THz yield BBO   = (nn2)d/c

27. Anti-correlation of THz and THG 3 measurements: Simulation Experiment 2polarization angle K. Y. Kim et al., Nature Photonics (submitted).

28. WDM: warm (0.1~100 eV) dense (0.1~10 times the solid density) matter which is a strongly coupled (ekBT) and Fermi degenerate (F ~ kBT) plasma. Brown dwarfs Jupiter Laser-heated solids WDM NASA NASA Warm Dense Matter (WDM): WDM lies between a solid state and an ideal plasma state. It is too hot to be described by solid-state physics and too dense to be depicted by the classical plasma theory.

29. Single-shot THz detection: Chirped spectral interferometric technique * Spectrometer Chirped optical pulse CCD Electro-optic crystal (ex. ZnTe) Polarizer ETHz (t) THz field Polarizer Pellicle beam combiner Optical pulse THz pulse Delay (time) * K. Y. Kim et al., Appl. Phys. Lett. 88, 041123 (2006); Z. Jiang et al., Appl. Phys. Lett. 72, 1945 (1998);

30. (a) (b) 0.2 0 Difference spectrum (a.u.) (c) Spectrum (a.u.) 2 -0.2 1 Freq (THz) 0 0 1 2 800 820 840 Wavelength (nm) Imaging spectrometer CCD Experimental setup: THz generation pulse Chirped optical probe Polarizer ZnTe Optical pump Teflon Al target Polarizer ZnTe

31. Experimental setup: Al disk Laser-ablated spots Gratings ZnTe Sample Pellicle

32. Experimental result IV: THz generation from ablation Aluminum Transient current e- + Optical pump pulse + e- + e- THz waveform 1 ps Coherent THz generation from a current surge in the laser-produced plasma

33. THz propagation simulation: To determine the THz skin depth, we solve the Helmholtz equation. At 1ps: Te ~ 0.9 eV, ~ 2.6 g/cm3, r ~ 1016 s-1 At 10 ps: Te ~ 0.6 eV, ~ 1.6 g/cm3, r ~1015 s-1 K. Y. Kim et al., Phys. Rev. Lett. 100, 135002 (2008).