Alexander Gaeta, of the School of Applied and Engineering Physics at Cornell University, conducts research focusing on using light for imaging and other processing typically done today with electrons. Using electrons ultimately caps the frequency of processing, and Gaeta argues that using light instead can put sampling speeds into the TeraHertz regime, or on the order of Gigasamples per second. This presents useful applications in non-destructive, contact-free testing especially in biological specimens.
Gaeta began by describing how spatial diffraction works within a prism or lens when considering whether the incoming light is treated as a ray or a wave. The most interesting situation, Gaeta noted, is considering light going through a lens. Whether considered as a ray or a wave, the light ultimately focuses at a single point in space known as the focal point. Gaeta uses the wave approximation when working with lasers. The electric field of the pulse from the laser is composed of a fast varying component and a slow varying component that depends on the distance traveled through a lens. The resulting diffraction equation depends on the wavelengths of the wave and the index of refraction of the lens.
Gaeta’s work instead looks at laser pulses that travel as a Gaussian down a single mode fiber optic cable instead of through a lens. This time the electric field’s slow varying component depends on time because different frequencies travel at different speeds within the cable. This results in a spreading over time that yields a dispersion equation that has the same form as the diffraction equation when sending light through a lens in air, showing the mechanisms governing each phenomenon are the same. After making some calculations, Gaeta argues a prism applies a linear shift in the phase of the incoming pulse while a lens applies a quadratic shift. With this in mind, Gaeta and his team searched for how to apply the quadratic phase shift. The answer was to find a way to alter the index of refraction within a material quadratically as a function of time.
Gaeta’s solution is a technique called parametric wave mixing. In addition to the incoming signal, a pump pulse is pumped into a non-linear optic. The reaction of the pump pulse creates the appropriate index of refraction profile to produce a quadratic phase shift onto the signal beam and also produces an additional outgoing stream of photons called an idler. The idler’s electric field is proportional to the incoming signal, the pump pulse and the susceptibility of the non-linear optic.
Gaeta mentions the practical imaging applications his group is working on, but then mentions the use of this technology to cloaking. From Harry Potter to Klingons, cloaking has piqued the interest of human minds across the ages. Gaeta and his group created a pump pulse that after it interacted with the signal in the non-linear optic, the signal focused at two points in time separated by about forty picoseconds. As the signal continued to travel in time, the signal stretched back to its original length but the frequencies were switched in order of appearance due to the effects of dispersion. Using the idlers as a way to test if the detector had detected an event, Gaeta and his group proved their detectors could not detect the event when it occurred within the time between the two points of where the signal focused.
While Gaeta and his group are nowhere near the time or volume scale seen in science fiction, using light rather than electrons provides a way to non-destructively image samples and perhaps lead the way to the ever coveted cloaking technology.
