Ultra-broadband THz-mid-infrared pulses from two-color photo-induced gas plasmas:

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1 Ultra-broadband THz-mid-infrared pulses from two-color photo-induced gas plasmas: Current challenges for spectroscopy and coherent detection Mark D. Thomson, Volker Blank and Hartmut G. Roskos, Johann Wolfgang Goethe-Universität, Frankfurt, Germany 1

2 Outline Two-color plasma-thz emitter o Ultra-broadband optical pump pulses o Spectra o Beam profile o Propagation simulations Coherent detection o Limitations of EO detection >100 THz o Spectrogram detection o Deconvolution algorithm Non-linear transmission measurements o Undoped GaAs z-scan Conclusion 2

3 Two-color plasma-thz Emission mechanism Cook et al, Opt. Lett. 25, 1210 (2000) Kim et al, Opt. Express. 15, 4577 (2007) 3

4 Two-color plasma-thz geometries Direct focussing Mach-Zehnder Correction optics Xie et al, Phys. Rev. Lett. 96, (2006) Houard et al, Opt. Lett. 33, 1195 (2008) Filamentation geometry ( (3) ) Fuji et al, Opt. Lett. 32, 3330 (2007) Wang et al, Laser Phys. Lett. 7, 517 (2010) Oishi et al, Opt. Express 14, 7230 (2006) Dai et al, Phys. Rev. Lett. 103, (2009) Additional frequency tripling crystal Petersen et al, Opt. Lett. 35, 1962 (2010) Baiz et al, Opt. Lett. 36, 187 (2011) 4

5 Ultra-broadband plasma-thz emission FTIR detection Optical THz Thomson et al, Opt. Express 18, (2010) FT J THz = 360 nj 5

6 Ultra-broadband plasma-thz emission Predicted peak fields vs. chirp Moderate focusing: GDD: 450-µm Si blocker: 130 fs 2 6

7 THz dependence on BBO orientation BBO geometry Modelling Pulse plane wave, SVAA Plasma-THz: Sub-cycle tunnel ionisation and PC polarisation, L=1-mm, N 2 (1 bar) 77

8 Plasma-THz beam profiles 150-fs pump pulses 8

9 Plasma-THz beam profiles ~20-fs pump pulses 9

10 Spatio-temporal optical-plasma propagation Air, 150 fs J = 40 µj J = 400 µj 10

11 Electro-optic detection Conventional EO sampling Pulse duration factor Phase-matching factor

12 EO detection efficiency vs frequency Pulse duration factor Gaussian: For > 100 THz, require pulses below 10 fs Phase-matching factor Ultra-thin crystals still possess null ranges Thick crystals wash out PM oscillations but strong low-freq. response can be troublesome

13 Spectrogram methods Quasi-cw Un-chirped High frequency resolution Low rel. conversion efficiency Require ps-opt. pulse Chirped pulse XFROG High frequency resolution Low rel. conversion efficiency Phase correction required Moderate time and frequency resolution Complete recovery of pulse possible Requires deconvolution algorithm

14 SFG spectrogram Numerical examples: Ideal ZnTe, 50 µm 150 fs opt. pulses

15 SFG spectrogram Numerical examples: Incl. efficiency factors ZnTe, 50 µm 150 fs opt. pulses

16 Experimental setup

17 Experimental SFG spectrogram 150 fs optical pulses, thin ZnTe ZnTe <110> Nominal 25 µmthick on 500 µm <100> substrate

18 XFROG spectrogram <20 fs optical pulses, GaP GaP <100> 100 µm

19 XFROG spectrogram <20 fs optical pulses, GaP THz pulse width currently dominated by dispersion of Si wafer following plasma

20 Deconvolution algorithm Arbitrary phase-matching/non-linear dispersion 20

21 Deconvolution algorithm Synthetic test spectrogram 21

22 Deconvolution algorithm Convergence and retrieval accuracy 22

23 Non-linear transmission measurements: z-scan 23

24 Non-linear transmission measurements: Semi-insulating GaAs z-scan 24

25 Conclusions Plasma-THz generation o Pulse duration directly after plasma on the order of 10 fs o Peak fields of at least 10 MV/cm could be achieved o Still significant scope to better understand and optimize emission characteristics o May prove to be the source of choice for certain THz-mid-IR nonlinear spectroscopies Sum-frequency detection o Possibility for single-shot spectral measurements Spectrogram methods o Relaxes requirements optical pulse duration compared to EO sampling o Deconvolution can allow for complete recovery of complex pulse envelope 25