Abstract
We present the use of a power limiting apparatus to evaluate ultrafast optical nonlinearities of transparent liquids (water and ethanol) in the femtosecond filamentation regime. The setup has been previously employed for the same purpose, however, in a longer pulsewidth (> 20 ps) regime, which leads to an ambiguous evaluation of the critical power for self-focusing. The uncertainty originates from the existence of a threshold power for optical breakdown well below the critical power for self-focusing within this timeframe. Contrarily, using the proposed apparatus in the femtosecond regime, we observe for the first time a unique optical response, which features the underlying physics of laser filamentation. Importantly, we demonstrate a dependence of the optical transmission of the power limiter on its geometrical, imaging characteristics and the conditions under which a distinct demarcation for the critical power for self-focusing can be determined. The result is supported by numerical simulations, which indicate that the features of the observed power-dependent optical response of the power limiting setup are physically related to the spontaneous transformation of the laser pulses into nonlinear conical waves.
Highlights
We present the use of a power limiting apparatus to evaluate ultrafast optical nonlinearities of transparent liquids in the femtosecond filamentation regime
Thereby, we observe unique features on the optical response of the setup when the pinhole is placed at various positions with respect to the focal plane of the imaging lens and we demonstrate that the critical power for self-focusing, among with other nonlinearities, can be reliably evaluated by this technique in the femtosecond regime
The technique has been utilized in the past only in longer pulse regimes (> 1 ps) leading to optical breakdown inside the samples, which typically occurs at optical pulse powers lower than the critical power for self-focusing
Summary
We present the use of a power limiting apparatus to evaluate ultrafast optical nonlinearities of transparent liquids (water and ethanol) in the femtosecond filamentation regime. The result is supported by numerical simulations, which indicate that the features of the observed power-dependent optical response of the power limiting setup are physically related to the spontaneous transformation of the laser pulses into nonlinear conical waves. By placing an imaging lens after the nonlinear medium, one can observe limited transmission through a pinhole placed at the focus of that lens at high input powers In their original paper, the authors have employed their setup using nanosecond and picosecond pulses at an optical wavelength of 1.06 μm to study the nonlinear response of CS2. Thereby, we observe unique features on the optical response of the setup when the pinhole is placed at various positions with respect to the focal plane of the imaging lens and we demonstrate that the critical power for self-focusing, among with other nonlinearities, can be reliably evaluated by this technique in the femtosecond regime. We discuss a comparison between experimental observations and numerical simulations related to the beam size in the far-field and its transformation into a nonlinear conical wave[6,7,8] for the well-established case of water
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