Attosecond time-scale feedback control of coherent X-ray generation

High-harmonic generation is an extreme, high-order, nonlinear process that converts intense, ultrafast, visible and infrared laser light pulses coherently into the soft X-ray region of the spectrum. We demonstrate that by optimizing the shape of an ultrafast laser pulse, we can selectively enhance this process by promoting strong constructive interference between X-ray bursts emitted from adjacent optical cycles. This work demonstrates that coherent control of highly nonlinear processes in the strong-held regime is possible by adjusting the relative timing of the crests of an electromagnetic wave on a sub-optical cycle, attosecond time scale. (C) 2001 Elsevier Science B.V. All rights reserved.