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

4. Biomolecules & proteins Figure courtesy of Klaas Wynne Rydberg atoms molecules Semiconductor nanostructures Gaseous and solid-state plasmas Phenomena at terahertz (THz) frequencies: 1 THz = 10 12 Hz =1 ps = 300  m = 0.004 eV = 33.3 cm -1

5. Strong THz sources: FELSynchrotronsLinacs Stanford, UCSB, FELIXSLAC, JLab,BNLALS (BNL) Free electron lasing * M. S. Sherwin et al., DOE-NSF-NIH Workshop on Opportunities in THz Science Photo courtesy: DESY Photo courtesy: ALS Large facility THz sources* Coherent synchrotron radiation synchrotron radiation

6. Intense THz generation: Two-color photoionization

7. Lens SHG THz pulse Two-color photoionization: * 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). Four-wave mixing *  THz =  +  - 2  22 plasma 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.  (3) plasma =  (3) ions +  (3) free-electrons

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

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

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

11. THz spectroscopy: Warm Dense Matter

12. 00 rr  Drude model Optical pump pulse WDM Electrical conductivity measurements of WDM: Optical probe 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). AC  (  0 ) 00  (  THz )  THz 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.

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

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

15. Experimental results II: THz reflectivity vs delay Conductor-to-insulator-like transition Room temp. Al:  r = 4.1  10 7  -1 m -1 = 3.7  10 17 s -1  [  -1 m -1 ] = 1.1  10 -10  [s -1 ]

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

17. 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. Summaries:

18. Backup slides:

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

20. Strong THz field science*: Nonlinear THz Optics THz 2nd, 3rd nonlinear effects. Extreme nonlinearity with ponderomotive energy > photon energy THz-optical nonlinear mixing Rapid THz imaging Biomedical and security imaging High magnetic field effects 1 MV/cm  0.3 T Pulsed electron spin resonance THz spintronics Strong THz sources E THz > 1 MV/cm Photo courtesy: the Star Tiger * M. S. Sherwin et al., DOE-NSF-NIH Workshop on Opportunities in THz 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

21. Plasma current model I: Electron drift velocity Laser field  field 2  field  ( = 800 nm) and 2  ( = 400 nm) lasers with relative intensity of I  = 10 15 W/cm 2 and I 2  = 2  10 14 W/cm 2 (assuming 20% efficiency of frequency doubling)  = 0  =  /2  : relative phase  : photoionization phase K. Y. Kim et al., Opt. Express 15, 4577 (2007).

22. * Laser field: * Ionization rate: E a : atomic field * Plasma current: * THz field: for E a > E  >> E 2  and N g >> N e * The function f(E  ) is highly nonlinear, not necessarily quadratic dominant. The nonlinearity arises from extremely nonlinear tunneling ionization localized near the laser peaks.* Plasma current model II:

23. Simulation results I: Simulation with  = 0 ADK tunneling ionization and subsequent classical electron motion in the laser field are considered. Simulation with  =  /2 Quasi-DC current I  = 10 15 W/cm 2, I 2  = 2  10 14 W/cm 2, 50 fs (FWHM) Assumptions: No rescattering effect, No electron-ion or electron-neutral collisional processes, No space charge effect, No electron transport.

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

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

26. Experimental result II: As d  0, THz yield  0 Current model : Four-wave mixing : To check the validity of our plasma current model, we studied  dependence of THz yield BBO   =  (n   n 2  )d/c  d   0

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

28. 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. WDM : warm (0.1~100 eV) dense (0.1~10 times the solid density) matter which is a strongly coupled ( e   k B T) and Fermi degenerate (  F ~ k B T ) plasma. WDM Brown dwarfs NASA JupiterLaser-heated solids NASA

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

30. Experimental setup: 800820840 -0.2 0 0.2 Difference spectrum (a.u.) Wavelength (nm) 012 0 1 2 Spectrum (a.u.) Freq (THz) (c) (a) (b) Al target ZnTe THz generation pulse Chirped optical probe Optical pump Polarizer Teflon Imaging spectrometer CCD Polarizer

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

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

33. THz propagation simulation: To determine the THz skin depth, we solve the Helmholtz equation. At 1ps: T e ~ 0.9 eV,  ~ 2.6 g/cm 3,  r ~ 10 16 s -1 At 10 ps: T e ~ 0.6 eV,  ~ 1.6 g/cm 3,  r ~10 15 s -1 K. Y. Kim et al., Phys. Rev. Lett. 100, 135002 (2008).