Phase sensitive pump probe measurement of the complex nonlinear susceptibility of silicon across the direct band edge

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Abstract

The nonlinear response of materials, an increasingly important aspect of light-matter interaction, can be challenging to measure in highly absorbing materials. Here, we introduce an interferometric technique that enables a direct measurement of the nonlinear complex permittivity in a bulk medium from reflectivity alone. We demonstrate the utility of pump-probe supercontinuum (SC) spectral interferometry in reflection by measuring time-dependent variations in the complex dielectric function (, ) over the visible wavelength range in bulk silicon. Transient phase shifts in the reflected SC due to a near infrared pump pulse allow us to track modifications to , whereas changes in are derived from transient fluctuations in the reflected SC probe amplitude. The ultrafast response is attributed to effective two-photon absorption () and Kerr (2) coefficients. We observe the onset of strong two-photon absorption as the two-photon energy is tuned through the direct band edge of silicon (1 = 3.4 eV). This technique allows straightforward spectroscopic measurements of the (3) nonlinear response at the surface of absorbing materials.

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Keywords

Spectroscopy, Optical physics and communications and Condensed matter

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