| Abstract [eng] |
In the regime of nonlinear optics, the magnitude of the refractive index of the medium changes with increasing light intensity - an effect which can be described by a constant called nonlinear refractive index or n2. It is a highly important property of all optical materials. One way to measure it experimentally is to analyse phase front variations which occurs when medium is excited with high intensity laser radiation. Such an evaluation method is used for example in z-scan measurements. z-scan technique is distinguished by its high sensitivity and is relatively simple both in experimental execution and in nonlinear refractive index value determination from measured data. However both very high beam shape and pulse temporal envelope quality is required as it is crucial when determining peak axial irradiance value associated with self-focusing inflicted phase shift; it is also important to evaluate correctly other nonlinear process influence or minimize it choosing pump radiation parameters (pulse duration, intensity, repetition rate) accordingly. The main purpose of this work was to determine n2 values of various transparent materials using femtosecond pulses in infrared where research analysing materials nonlinear properties are scarce. For that purpose standard z-scan system was constructed employing "He-Topas" idler wave of optical parametric amplifier (OPA) pumped by Ti:sapphire laser. Experiments were conducted at 1700 nm and 1800 nm with 51 fs and 69 fs pulse durations respectively. Nonlinear refractive index values for measured samples were evaluated relative to fused silica value estimated by T. Kato et al. at 1550 nm (https://doi.org/10.1364/OL.20.000988) . Relative refractive index values for investigated samples are thus as follows: n21700(FS) = (2,79 ± 0,27)·10-20 m2/W, n21800(FS) = (2,75 ± 0,33)·10-20 m2/W, n21700(MgF2) = (1,12 ± 0,19)·10-20 m2/W, n21700(Al2O3) = (3,64 ± 0,50)·10-20 m2/W, n21700(BK7) = (3,70 ± 0,50)·10-20 m2/W, n21800, o-pol(BBO) = (6,60 ± 0,95)·10-20 m2/W, n21700(ZnSe-1) = (120 ± 18,45)·10-20 m2/W, n21700(ZnSe-2) = (118 ± 10,11)·10-20 m2/W. To my knowledge this is the first time BBO nonlinear refractive index value was evaluated above 1500 nm. |
