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...
The propagation of high-power femtosecond light pulses in lithium niobate crystals ͑LiNbO 3 ͒ is investigated experimentally and theoretically in collinear pump-probe transmission experiments. It is found within a wide intensity range that a strong decrease of the pump transmission coefficient at wavelength 388 nm fully complies with the model of two-photon absorption; the corresponding nonlinear absorption coefficient is  p Ӎ 3.5 cm/ GW. Furthermore, strong pump pulses induce a considerable absorption for the probe at 776 nm. The dependence of the probe transmission coefficient on the time delay ⌬t between probe and pump pulses is characterized by a narrow dip ͑at ⌬t Ӎ 0͒ and a long ͑on the picosecond time scale͒ lasting plateau. The dip is due to direct two-photon transitions involving pump and probe photons; the corresponding nonlinear absorption coefficient is  r Ӎ 0.9 cm/ GW. The plateau absorption is caused by the presence of pump-excited charge carriers; the effective absorption cross section at 776 nm is r Ӎ 8 ϫ 10 −18 cm 2 . The above nonlinear absorption parameters are not strongly polarization sensitive. No specific manifestations of the relaxation of hot carriers are found for a pulse duration of Ӎ0.24 ps.
Spatial gratings are recorded holographically by two femtosecond pump pulses at 388 nm in lithium niobate ͑LiNbO 3 ͒ crystals and read out by a Bragg-matched, temporally delayed probe pulse at 776 nm. We claim, to our knowledge, the first holographic pump-probe experiments with subpicosecond temporal resolution for LiNbO 3 . An instantaneous grating that is due mostly to the Kerr effect as well as a long-lasting grating that results mainly from the absorption caused by photoexcited carriers was observed. The Kerr coefficient of LiNbO 3 for our experimental conditions, i.e., pumped and probed at different wavelengths, was Ϸ1.0 ϫ 10 −5 cm 2 /GW. © 2005 Optical Society of America OCIS codes: 050.7330, 190.3270, 190.4380, 190.7110. Ferroelectric lithium niobate ͑LiNbO 3 ͒ crystals are one of the most investigated materials for widespread and promising applications in nonlinear optics, e.g., for parametric amplification and second-harmonic generation. 1 LiNbO 3 also shows photorefractive properties, 2 which are characterized by a change in its refractive index that results from an optically induced separation of electrons and the linear electrooptic effect. The ability to record holograms makes LiNbO 3 crystals attractive for many applications such as holographic data storage, optical information processing, phase conjugation, and wavelength filters. 3,4 Pulses with temporal durations of tens of picoseconds and peak intensities of 0.1 to 1 GW/ cm 2 were previously used to investigate the photorefractive effects in various materials, e.g., bismuth silicate ͑Bi 12 SiO 20 ͒, 5 barium titanate ͑BaTiO 3 ͒, 6 and potassium niobate ͑KNbO 3 ͒.7 Femtosecond pulses were used for nonvolatile spectral holography in LiNbO 3 , 8 but pump-probe experiments that allow its ultrafast material response to be studied have not been conducted to our knowledge. In this Letter we investigate holograms recorded in LiNbO 3 with femtosecond pulses at 388 nm. Grating-recording experiments have been conducted extensively in LiNbO 3 at low intensities ͑Շ1 W/cm 2 ͒ with continuous-wave lasers and at high intensities (up to Ϸ10 MW/ cm 2 ) by nanosecond pulses.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.The experimental setup is illustrated in Fig. 1. Two pump pulses at p = 388 nm are incident symmetrically onto a 70 m thick LiNbO 3 : Fe sample (c Fe Ϸ 5.6ϫ 10 19 cm −3 and c Fe 2+ / c Fe 3+ Ϸ 0.01) with a recording angle 2 p = 8°outside the crystal. The grating vector is oriented perpendicularly to the crystal's polar axis. The pump pulses are polarized parallel to the polar axis of the crystal and have the same intensity with peak values I p up to Ϸ170 GW/ cm 2 per pulse inside the sample...
Volume holographic gratings are recorded and retrieved in two commercially available glasses: Schott Foturan and Hoya PEG3. These materials are photoetchable, which describes their major application, but they also allow storage of volume holograms without any chemical etching. The samples are illuminated with ultraviolet light at a wavelength of 325 nm and thermally processed to achieve a maximum diffraction efficiency of Ϸ9% for a 1-mm-thick sample. The two glasses show similar behavior; the diffraction efficiencies in Foturan tend to be slightly larger, whereas PEG3 tends to have weaker light scattering.
The finite dimension of the incident beam used to read out volume holographic gratings has interesting effects on their filtering properties. As the readout beam gets narrower, there is more deviation from the ideal response predicted for monochromatic plane waves. In this paper we experimentally explore beam-width-dependent phenomena such as wavelength selectivities, angular selectivities, and diffracted beam profiles. Volume gratings in both reflection and transmission geometries are investigated near 1550 nm. Numerical simulations utilizing the technique of Fourier decomposition provide a satisfactory explanation and confirm that the spread of spatial harmonics is the main contributing factor.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.