Sub-lambda gratings, surface plasmons, hotter electrons and brighter x-ray sources- enhanced absorption of intense, ultrashort laser light by tiny surface modulations

Abstract

We observe near 100 % absorption of light in intense ultrashort laser plasma interaction in a metal coated (Au on glass) sub-lambda grating structure under suitable conditions and the subsequent 'hot' electron generation from the grating plasma. In the low intensity regime we determine the conditions in which a monochromatic infrared light (lambda = 800nm corresponding to the central wavelength of the ultrashort laser that we used in subsequent experiments) efficiently excites surface plasmon in the grating. Then we study how the surface plasmon resonance condition changes when we excite them using low intensity ultrashort pulses. We look at the reflectivity of light varying the incident light intensity over a wide range (2x10e12Wcm-2-2x10e15Wcm-2). The reflectivity of grating with the resonance condition satisfied is the lowest over the whole range of intensity. We compare the data with those obtained from highly polished (lambda/10) Au mirror target under identical conditions. At high intensities we look at the hard x-ray emission from both the targets with and without the resonance condition. The hard X-ray spectrum shows a bimaxwellian i.e two temperature hot electron distribution with a hotter component present under the resonance condition while in all other cases it shows the presence of only one low temperature component.