Femtosecond time-resolved absorption processes in lithium niobate crystals

Abstract

Femtosecond pump pulses are strongly attenuated in lithium niobate owing to two-photon absorption; the relevant nonlinear coefficient ␤ p ranges from ϳ3.5 cm/ GW for p = 388 nm to ϳ0.1 cm/ GW for 514 nm. In collinear pump-probe experiments the probe transmission at the double pump wavelength 2 p = 776 nm is controlled by two different processes: A direct absorption process involving pump and probe photons ͑␤ r Ӎ 0.9 cm/ GW͒ leads to a pronounced short-duration transmission dip, whereas the probe absorption by pump-excited charge carriers results in a long-duration plateau. Coherent pump-probe interactions are of no importance. Hot-carrier relaxation occurs on the time scale of Շ0.1 ps. © 2005 Optical Society of America OCIS codes: 160.3730, 320.7110, 190.4180. Lithium niobate ͑LiNbO 3 ͒ is a wide-gap ͑ϳ4 eV͒ ferroelectric optical material that is of prime importance for nonlinear optical and holographic applications such as frequency conversion, optical filtering and switching, and data storage.1-3 Large 2 coefficients, outstanding photorefractive properties, robustness, and availability make the material promising for realization of devices.High intensities and correspondingly short pulses are required for efficient nonlinear optical processes and ultrafast holography. However, although LiNbO 3 is well investigated in the continuous and microsecond-nanosecond ranges, 4-7 almost nothing is known about its nonlinear response in the femtosecond range. It is unknown, in particular, which effects will govern the nonlinear propagation of femtosecond pulses. Free-carrier nonlinearity and absorption, Kerr nonlinearity, two-photon absorption (TPA), and photorefractive nonlinearity can compete in this range.A crucial advantage of using femtosecond pulses as compared with picosecond and longer pulses is that the complicated processes, caused by repopulation and reexcitation of energy levels, can be separated from the direct electron excitation processes. 8,9 The reported results on the TPA coefficient of LiNbO 3 , obtained with nanosecond and subnanosecond pulses, are highly controversial 10 ; they have a range of more than 1 order of magnitude at ϳ530 nm. Thus for practical purposes and also for physics it is highly important to explore femtosecond time-resolved absorption processes in LiNbO 3 , which is the topic of this work.The conditions of our collinear pump-probe experiments are as follows: A single axially symmetric pump pulse of wavelength p = 388-776 nm, extracted from a Ti:sapphire femtosecond laser (CPA-2010, Clark-MXR plus an optical parametric amplifier system), is incident normally onto the YZ face of a LiNbO 3 sample; the polar c axis is parallel to the Z axis. The pulse width and beam diameter (at the half-intensity) are ϳ0.24 ps and ϳ0.6 mm, respectively, for 388 nm. Peak intensity I p 0 ranges from ϳ1 to ϳ 276 GW/ cm 2 . The maximum pump fluence is approximately 59 mJ/ cm 2 . The input polarization vector of the pump pulse is either parallel or perpendicular to the c axis. Thickness d for three inves...

Femtosecond holography in lithium niobate crystals

“…11 In our experiment, the photorefractive effect is obviously negligible; otherwise a permanent grating would exist. Furthermore, as the measured is constant on a picosecond-to-nanosecond time scale, there is also no obvious contribution from a free hot carrier grating.…”

“…9,10 At these intensity levels, the photorefractive effect is still the dominant nonlinear effect; however, holographic recording with even more intense pulses will reveal other nonlinear material responses. The enhanced temporal resolution obtained with femtosecond pulses and accumulated knowledge about two-photon absorption 11,12 enable us to draw conclusions about the participating nonlinear processes.…”

“…An instantaneous response can be due to either pump-induced changes in the refractive index because of the Kerr effect or twocolor two-photon absorption (involving one pump photon and one probe photon). 11,12 Longer-lasting effects can arise from charge separation and the electro-optic nonlinearity (photorefractive effect) as well as from index and absorption changes at 776 nm attributed to spatial modulation of the concentration of excited carriers.…”

“…For the longer-lasting effects, the value c is explained within the experimental accuracies without any fit parameter by an absorption grating resulting from the spatially modulated carriers that we characterized earlier. 11,12 The instantaneous response is the combination of the two-color two-photon absorption and the Kerr effect. The two-color two-photon absorption is known from earlier measurements.…”

“…12 The main reason for the attenuation of the pump light is two-photon pump absorption, and the absorption coefficient is ␤ p 2I p , with ␤ p Ϸ 3.5 cm/ GW at 388 nm. 11 The values of p and c are plotted on logarithmic scales as functions of the pump pulse intensity in According to coupled-wave theory of volume grating, 14 diffraction efficiency depends, for Ӷ 1, quadratically on the product of effective grating thickness L and the grating amplitude, ⌬n for index gratings and ⌬␣ for absorption gratings. This is true In Fig.…”