title: Contrasting levels of absorption of intense femtosecond laser pulses by solids.
abstract: The absorption of ultraintense, femtosecond laser pulses by a solid unleashes relativistic electrons, thereby creating a regime of relativistic optics. This has enabled exciting applications of relativistic particle beams and coherent X-ray radiation, and fundamental leaps in high energy density science and laboratory astrophysics. Obviously, central to these possibilities lies the basic problem of understanding and if possible, manipulating laser absorption. Surprisingly, the absorption of intense light largely remains an open question, despite the extensive variations in target and laser pulse structures. Moreover, there are only few experimental measurements of laser absorption carried out under very limited parameter ranges. Here we present an extensive investigation of absorption of intense 30 femtosecond laser pulses by solid metal targets. The study, performed under varying laser intensity and contrast ratio over four orders of magnitude, reveals a significant and non-intuitive dependence on these parameters. For contrast ratio of 10(-9) and intensity of 2 × 10(19)W cm(-2), three observations are revealed: preferential acceleration of electrons along the laser axis, a ponderomotive scaling of electron temperature, and red shifting of emitted second-harmonic. These point towards the role of J × B absorption mechanism at relativistic intensity. The experimental results are supported by particle-in-cell simulations.
authors.affiliation: Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Mumbai 400005, India.
authors.forename: Prashant Kumar
authors.initials: PK
authors.lastname: Singh
authors.affiliation: Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Science, Beijing 100190, China.
authors.forename: Y Q
authors.initials: YQ
authors.lastname: Cui
authors.affiliation: Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Mumbai 400005, India.
authors.forename: Amitava
authors.initials: A
authors.lastname: Adak
authors.affiliation: Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Mumbai 400005, India.
authors.forename: Amit D
authors.initials: AD
authors.lastname: Lad
authors.affiliation: Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Mumbai 400005, India.
authors.forename: Gourab
authors.initials: G
authors.lastname: Chatterjee
authors.affiliation: Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Mumbai 400005, India.|UM-DAE Centre for Excellence in Basic Sciences, Mumbai 400098, India.
authors.forename: P
authors.initials: P
authors.lastname: Brijesh
authors.affiliation: SUPA, Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK.|Key Laboratory for Laser Plasmas (Ministry of Education), Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China.|Collaborative Innovation Center of IFSA, Shanghai Jiao Tong University, Shanghai 200240, China.
authors.forename: Z M
authors.initials: ZM
authors.lastname: Sheng
authors.affiliation: Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Mumbai 400005, India.
authors.forename: G Ravindra
authors.initials: GR
authors.lastname: Kumar