Nonlinear optical compression of high-power, 10 µm CO 2 laser pulses in gases and semiconductors Jeremy Pigeon Sergei Tochitsky Chan Joshi Neptune Laboratory UCLA 08/04/2016 AAC 2016

Motivation for high power CO 2 lasers Challenge – high repetition rate source requires sub-ps pulses, exceeds bandwidth of active medium *D.J. Haberberger, et. al., Nature Phys. 8, (2012) Requires the development of a 0.1 – 1 TW CO 2 laser -Laser driven electrostatic shock acceleration of monenergetic (ΔE/E < 10%) ions* -Studies of critical density laser-plasma interaction using gas targets -X-ray generation via high harmonic generation -E x-ray ~ λ 2 -Generation of broadband light in the molecular fingerprint region Figure: A. Schliesser, et. al. Nature Photonics 6, (2012)

Amplification of picosecond pulses in a CO 2 laser – broadening mechanisms Pressure Broadening 1/ν coll <τ transition Δν pressure ≈ 3.5 GHz/atm Field Broadening Rabi frequency Δν field ≈ Ω R = 1.38x10 7 μ√(I) ≈ GW/cm 2 Strategy for ps amplification mJ – J Low pressure, high field µJ – mJ High pressure, low field Seed D.J. Haberberger, S. Ya. Tochitsky, and C. Joshi. 15 terawatt picosecond CO 2 laser system. Opt. Express 18(17), (2010). Dipole moment Δν total = Δν pressure + Δν field

UCLA Neptune Laboratory’s High Repetition Rate CO 2 Laser System System parameters – 1 Hz repetition rate – 20 GW peak power – 3 or 200 ps pulse duration – 10.6 μm wavelength – Can synchronously amplify two wavelengths in vicinity of 10 μm S. Ya. Tochitsky, et. al. Amplification of Multi-Gigawatt 3ps Pulses in Atmospheric CO 2 Laser Using ac Stark Effect Opt. Express 20(13), 13762–13768 (2012). 200 mJ 3 ps Master Oscillator 8 atm Booster Amplifier 25 mJ 3 ps Plasma Shutter 3-pass TEA Amplifier 10 atm Regenerative Amplifier 4 mJ 3 ps PC WP Thin Film Polarizer Front End Amplification of picosecond pulses in CO 2 -Pressure broadening -Field broadening

C. Sung, S. Ya. Tochitsky and C. Joshi. Guiding of 10 um laser pulses by use of hollow waveguides. Eleventh AAC Workshop, (2004) AgI Ag a No Waveguide a=500  m a=200  m Silica hollow glass waveguide (HGW) - a=500  m, L=84 cm - Avalanche ionization limits the usable pressure to < 200 torr (Threshold ~ λ -2 ) - No SPM was observed with GW/cm 2 guided intensities over 84 cm -Recent measurements of n 2 of Xe at 10 µm show that nonlinearity is ~ 3 times smaller in the mid-IR than the near-IR Self-phase modulation of 10 µm pulses in Xe- filled hollow glass waveguide Coupling condition for EH 11 mode : w 0 /a = 0.64

Introduction to mid-IR nonlinear pulse compression in semiconductors Technical challenges – Highly nonlinear (χ (3) ) ~1000x that of silica – Negative group velocity dispersion Modulational instability Self-compression Supercontinuum generation – Large χ (2) Cascaded nonlinearities – Dynamic Franz-Keldysh effect Linear properties of semiconductors can change with field strength – Commercially available gratings are too dispersive D.J. Haberberger, S. Ya. Tochitsky, and C. Joshi. 15 terawatt picosecond CO 2 laser system. Opt. Express 18(17), (2010).

Supercontinuum generation in 10 µm pumped GaAs Spectrum after 67 mm of GaAs Steinmeyer, Günter, and Julia S. Skibina. "Supercontinua: Entering the mid- infrared." Nature Photonics 8.11 (2014): Spectrum after 30 mm of GaAs

Self-phase modulation and compression of 10 µm pulses in GaAs and NaCl 20 GW, 1 Hz CO 2 laser Nonlinear medium 100 GW, 1 Hz CO 2 laser Dispersive element 20 GW, 3 ps CO 2 pulses 7 mm GaAs (AR coated) 7 cm NaCl slab Stretcher Monochromator HCT Detector Diode laser CS 2 Kerr cell Beam combiner Polarizer Analyzer Polarizations Streak Camera 45 0 T ~ 50%

Measurements of spectral broadening and pulse compression ~1 ps Optimal compression Nominal pulse train Spectral Measurements Streak camera measurements Compression by a factor of ~ 3 is observed

Picosecond, mid-IR pulses via multi-wave mixing compression -Has been used to convert CW light to 0.1 – 1 ps pulse trains in negative GVD fibers -Transform limited pulses can be obtained by balancing nonlinearity and dispersion -L D /L NL ≈ 9 J. Fatome, S. Pitois, and G. Millot, IEEE J. Quantum Electron., 42, 1038 (2006). Can we produce picosecond pulses without high pressure lasers or a broadband seed?

Multiple four-wave mixing compression χ (3) medium Δλ ~ 40 nm Spectral density λ λ J.J. Pigeon, S. Ya. Tochitsky, C. Joshi., Optics Letters 40(24), 5730–5733 (2015).

Spectral results for mid-IR, multi-wave mixing compression ExperimentSimulation (1D GNLSE)

Temporal results for mid-IR, multi-wave mixing compression ExperimentSimulation (1D GNLSE)

Production of transform limited pulses by using a NaCl plate as a pulse compressor -Replacing the last 134 mm of GaAs with 200 mm of NaCl balances the effects of nonlinearity and dispersion (NaCl has comparable dispersion as GaAs but much smaller n 2 )

Cascaded quadratic nonlinearities in mid- IR pumped GaAs -GaAs is known for its χ (2) nonlinearity -d eff = 40 pm/V ))) -Cascading SHG -10 % of Kerr nonlinearity -Difference frequency generation? n 2 eff = n 2 Kerr - n 2 casc Figure from: S. Ya. Tochitsky, et. al., J. Opt. Soc. Amer. B., Vol. 24 No. 9, (2007). -Changing the beat frequency from 106 GHz (Δk DFG = 0) to 882 GHz (Δk DFG ≠0) results in 40% reduction in nonlinearity J.J. Pigeon, S. Ya. Tochitsky, C. Joshi., Optics Letters 40(24), 5730–5733 (2015).

Conclusions Acknowledgements This work was supported by U.S. Department of Energy grant DE-SC – We have applied our high-repetition rate, picosecond CO 2 laser system to investigate various schemes of nonlinear pulse compression at a wavelength of 10 µm. – Self-phase modulation in gas filled waveguides was limited by low nonlinearities and avalanche ionization at longer wavelengths. – The large nonlinearity and negative group velocity dispersion of semiconductors makes these materials more suitable for supercontinuum generation than for pulse compression. – Multiple four-wave mixing compression provides a method to generate high- power, picosecond pulse trains at 10 µm and should work with the ~ 3 ns pulses available from mode-locked TEA CO 2 lasers