Łukasz Kornaszewski

Significance: Two-photon excitation (TPE) of retinal fluorophores with infrared light allows monitoring metabolic transformation of the retina. TPE imaging can provide information about biochemical events that precede structural damage to the retina and thereby accelerate the development of therapies against blinding diseases. While subcellular TPE of endogenous retinoids in the retina and retinal pigment epithelium of mice and nonhuman primates has been achieved, TPE imaging of the human retina has not been fully realized due to concerns about safety and identity of molecular signals. We therefore improved TPE efficiency by controlling temporal properties of the excitation light, performed extensive modeling of nonlinear damage mechanisms, and identified molecular components of individual RPE granules in mouse models of retinal diseases employing phasor analyses.

Experimental demonstration of semiconductor saturable absorber‐free mode‐locked optically pumped semiconductor disk laser is presented. The origin of pulsed operation is attributed to the intensity dependent Kerr lens effect arising in the semiconductor gain medium. Achieved results represent a novel method to mode‐lock this type of laser opening new application opportunities. The laser worked stably in both hard and soft aperture configurations. No semiconductor saturable absorber was used in the laser cavity and the operation was self‐starting. The laser was mode‐locked at 210 MHz repetition rate with 1.5 W average output power and 930 fs pulse width at 985 nm. A record high 6.8 kW peak power was achieved. Measured data is presented along with a discussion of the Kerr lens effect in the cavity.

The generation of tailored femtosecond pulses with fully engineered intensity and phase profiles is demonstrated using second-harmonic generation of an Er:fiber laser in an aperiodically poled lithium niobate crystal in the undepleted pump regime. Second-harmonic pulse shapes, including Gaussian, stepped, square, and multiple pulses have been characterized using cross-correlation frequency-resolved optical gating and have been shown to agree well with theory.

Mid-infrared methane sensing is demonstrated using a photonic bandgap fiber-based gas cell and broadband idler pulses from a periodically-poled lithium niobate femtosecond optical parametric oscillator as the light source for Fourier transform infrared spectroscopy.

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