HOLOGRAPHY: Holograph captures femtosecond pulses
Researchers at the California Institute of Technology (Caltech; Pasadena, CA) have created a holographic system that captures ultrafast femtosecond-laser pulses as they interact with air, water, and other media.
Researchers at the California Institute of Technology (Caltech; Pasadena, CA) have created a holographic system that captures ultrafast femtosecond-laser pulses as they interact with air, water, and other media.1 The technique captures 3-D laser-induced plasma formation and nonlinear index changes with a 150 fs time resolution and recovers amplitude and phase information, both with spatial resolution of 4 µm-exceptional results compared to streak cameras (which provide only 1-D information), previous pulsed holograms (which achieved only nanosecond temporal resolution), and fluorescence techniques (which cannot provide a time-resolved image).
Using laser pulses with 150 fs duration to probe changes in the material properties induced by a high-energy pump pulse, the technique uses a CCD camera to capture in-line holograms that are numerically reconstructed to produce 3-D images. Dramatic differences in femtosecond pulse propagation were observed depending on the nonlinear properties of the different media investigated, which included water, air, and carbon disulfide (CS2). By reconstructing the light field at different axial positions, 3-D information was obtained. Additionally, a time sequence of four holograms was captured with a single laser shot. The phase information recovered from the holograms allowed the researchers to identify nonlinear index changes (both positive and negative) in the surrounding media.
Positive and negative index changes
Positive index changes were attributed to the Kerr nonlinearity, an effect in which the change in index of refraction of the media is proportional to the intensity of the laser pulse. This Kerr effect acts as a self-induced lens, causing the beam to focus. If the pulse intensity exceeds a computed amount, filaments can be produced that ionize the surrounding medium. Negative index changes are attributed to plasma generation.
In the experimental setup, an 800 nm Ti:sapphire laser is used to generate an ultrafast event and record it. The laser pulses have a 150 fs pulse duration and 2 mJ maximum energy. The approximately Gaussian pulse with 5 mm full-width half-maximum diameter is split into two, with a major portion of the energy going into the pump beam. A delay line synchronizes the arrival of the pump and probe pulses, and the pump pulse is focused with an achromatic lens optimized for the different media used in the experiment. The probe pulse propagates in a direction perpendicular to the pump pulse and captures the interaction of the pump with the surrounding media. A CCD camera captures an in-line hologram of the image that is focused by additional lenses and numerically reconstructed to retrieve the phase and amplitude changes induced in the probe as it traverses the material. The 150 fs pulse duration is the limit of the temporal resolution of the system, while the numerical aperture of the hologram limits the spatial resolution of the system to 4 µm.
Because of strong nonlinearities in the focal region for the system, it is difficult to perform spatial filtering on the amplified femtosecond pulses. To combat the presence of spatial noise, the researchers first record an image of the probe pulse only (the pump pulse is blocked) and use it as a background that can be subtracted. The final hologram is therefore a cleaner image showing only the modulation due to the presence of the pump beam.
A series of equations were used to perform object reconstruction using the holographic data. To make sure that the phase information was correct, the researchers performed an additional experiment that favorably compared their holographic technique to an interferometric technique. Additional equations were used to recover the nonlinear index changes in the vicinity of the pump pulse using the phase information in the hologram. Comparisons of pulse propagation in air, water, and CS2 produced unique results (see figure). For example, the femtosecond pulse ionizes the air and water medium, but does not ionize the surrounding CS2 medium.
“Holography was critical in this experiment because we wanted to capture at the femtosecond scale simultaneously the amplitude and phase image that resulted from the optical nonlinearity,” said researcher Demetri Psaltis. “This method could be applied to many problems in ultrafast optics to visualize and understand the interaction of the light pulse with the medium,” added Martin Centurion, another researcher.
1. M. Centurion, Y. Pu, and D. Psaltis, J. Applied Physics 100, 063104 (2006).