Phase retrieval and compression of low-power white-light pulses

We characterize and compress sub-nJ visible white-light continuum (WLC) pulses generated by self-phase modulation in yttrium aluminium garnet. The spectral phase is retrieved by spectrally resolving the transient reflectivity from an optically-excited transition metal oxide. This measured phase is compensated by applying the appropriate distortion to a deformable mirror. By comparing the response of two different materials we show that the white-light pulses can be compressed to approximately 10 fs duration. The use of ultra-broadband pulses generated via self-phase modulation has become an important tool in physics and chemistry . The large frequency content of these pulses can be exploited to measure temporal dynamics over a broad frequency range simultaneously, drastically reducing acquisition times and increasing experimental information. Additionally, the increased bandwidth supports pulse durations significantly shorter than the generating pulse, improving the temporal resolution. However, a key pre-requisite for the application of these pulses is an accurate measurement and control of their spectral phase so that, through suitable dispersive delay lines (pulse compressors) that adjust the relative group delay (GD) of the different frequency components of the pulse, a near transform-limited (TL) pulse can be achieved. One of the simplest and most powerful methods of generating broadband WLC pulses is by focusing a μJ-level energy pulse in a bulk transparent material. A complex interplay between numerous nonlinear effects, including temporal self-phase modulation, spatial self-focusing, filamentation and plasma formation results in a broad, smooth and stable spectrum, making it an ideal source for broadband spectroscopy. However, the nature of the WLC generation mechanism results in long pulses with significant temporal chirp, which is challenging to correct over a broad spectral range. In addition, pulse energies are low, with typical spectral energy densities of 10 pJ/nm, making their temporal characterization difficult. Many nonlinear optical techniques, such as frequency-resolved optical gating, nonlinear spectral interferometry and their variants ∗ Email: , can be used for full characterization in amplitude and phase of ultrashort light pulses. However the application of these methods to WLC pulses is difficult, due to their ultrabroad bandwidth, which requires [email protected]