Нанооптика та фотоніка
Cubic nonlinear optical phenomena in copper oxides thin films
V. Rudenko1, S. Mulenko2, N.Stefan3, V. Liakhovetskyi1,
O. Tverdokhlibova1, A. Brodin4, M. Brodyn1
1 Institute of Physics NAS of Ukraine, 46 Nauky Blvd, Kiev-28, Ukraine
E-mail: val@iop.kiev.ua
2 G. V. Kurdyumov Institute for Metal Physics NAS of Ukraine, 36 Academician Vernadsky Blvd., Kiev 142, Ukraine
3National Institute for Lasers, Plasma and Radiation Physics, PO Box MG-54, RO-77125, Magurele, Romania
4National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, 37 Peremohy av., Kyiv, 03056, Ukraine

In recent years, investigations of cubic optical nonlinearity of metal oxides have shown that these materials are promising for optical applications, i.e. commutation and communication. For example, we reported large cubic nonlinear optical susceptibility of Fe2O3 (20 ns; λ=532 nm, 1064 nm) and PdO (9 ns; λ=532 nm) thin films [1, 2].
Here, we report about investigations of thin copper oxide (Cu2O) films, utilizing Z-scan technique with nanosecond laser excitation (9 ns and 15 ns at the wavelengths of λ=532 nm and λ=1064 nm, accordingly).
Thin copper oxide films were synthesized by reactive pulsed laser deposition (RPLD) in an ambient oxygen pressure atmosphere at pressures of P(O2)=1.0Pa, 3.0Pa and 5.0Pa, with substrate temperatures Ts=293K and 800K. These films demonstrated a substantial nonlinear response under nanosecond laser radiation. Films synthesized at P(O2)=1.0Pa, 3.0Pa Ts=293K and at P(O2)=1.0Pa, Ts=800K demonstrated negative refractive nonlinearity (Reχ(3) ~ -6.58×10-5 esu), consistent with the thermal mechanism of refractive index change when exposed to nanosecond laser pulses at a wavelength of 532 nm. At the same time, synthesized films at P(O2)=5.0Pa TS=293K and P(O2)=3.0Pa, 5.0Pa Ts=800K demonstrated the highest self-focusing as high as  Reχ(3) ~ 2.37×10-5 esu, which indicates a Kera character of refractive index changes. 
Investigations of Cu2O films were carried out with λ=1064 nm radiation, whose which wavelength is significantly higher than the fundamental absorption edge. Films synthesized with P(O2)=1.0Pa, 3.0Pa and Ts=293K demonstrated exhibited positive refractive nonlinearity (Reχ(3) ~ 1.7×10-5 esu), which is typical for the nonlinear optical Kerr effect, whereas those thin films synthesized with P(O2)=5.0Pa Ts=293K and P(O2)=1.0Pa Ts=800K displayed negative susceptibility (Reχ(3) ~ -2.5×10-5 esu), signed suggesting that it may be related to thermal processes under nanosecond laser illumination.
Nonlinear absorption displayed variations depending on the details of synthesis. In particular, the films produces with Ts=800K displayed two-photon absorption, whereas in films obtained with Ts=293K absorption was relatively lower or even changed sign, transforming thereby into saturable absorption.
Here, it was shown that copper oxide thin films demonstrated promising results for nonlinear optical applications in wide spectral and pulse width intervals. 

1.	S.A. Mulenko, V.I. Rudenko, V.R. Liakhovetskyi, A.M. Brodin, N. Stefan, Large third-order optical nonlinearities in iron oxide thin films synthesized by reactive pulsed laser deposition // Optical Materials.- 2016 - 60.- P.123-127. 
2.	M.S. Brodyn, V.I. Volkov, V.I. Rudenko, V.R. Liakhovetskyi, A.O. Borshch. Large third-order optical nonlinearity in PdO thin films // Journal of Nonlinear Optical Physics & Materials.- 2017 - 26. - 1750037.