Attosecond metrology of multi-PHz currents in solids

Command over electron motion by synthesized electric fields forms the basis for modern computing, information and signal processing. The rate of computation (electronics in general) is adjudged by the speed at which currents (electronic motion) can be turned on and off through an electronic component per se a transistor. Electronic motion in conventional electronics is primarily incoherent due to very high scattering rate of electrons implying operation of electronics only in a binary mode i.e. high and low currents. In this thesis measurement and control of electric currents induced by optical driving laser pulses in the conduction band of a dielectric material (SiO2) extending in frequency up to ~ 8 PHz is presented. Driving electrons with ultrashort laser pulses with duration shorter than their quantum dephasing times (~ few fs) inside bulk materials essentially make the electron motion coherent. Electronics operating with underlying coherent electron motion carries way more information as the conserved phases of electrons during its operation can be all sources of information. Human endeavour towards light induced electric currents goes back to 1900s when K. F. Braun the inventor of rectifying diode made pioneering efforts in this direction. Ultrafast laser pulses have the capability to induce electronic motion inside solids at a very high oscillating frequency; unfortunately interaction of laser pulses with solids have been eclipsed for a long time by the material damage on exposure to laser pulses. However, lately with advanced fabrication techniques ultrathin samples are readily available and technological innovation have pushed the pulse duration of laser pulses down to only few fs; both of which are important to mitigate damage. Generation of EUV radiation extending in frequency upto 10 PHz by optically induced currents inside SiO2 was demonstrated a couple of years back, however underlying motion of electrons, whether is was vertical transitions along the energy hierarchy of SiO2 or lateral motion (scattering) along the dispersion profile of electronic bands was not clear. Its found that motion of electrons in the energy landscape of conduction bands (intraband current) which is the same as in conventional electronics is the cause behind observation of EUV radiation. The scattering dependent emission of EUV radiation from SiO2 is shown to bear significantly different temporal features compared to re-collision based generation of coherent EUV emission from noble gases. This thesis presents the capabilities of direct attosecond probing, confinement and waveform control of the intraband currents inside solids and establishes a new platform for multi-PHz coherent electronics and novel routes for fundamental study of electron dynamics and structure of condensed matter on the atomic scale. Properties of electrons at such frequencies, such as dynamic conductivity and current density have been measured, a switch in the dynamic conductivity has been shown to occur at a timescale close to 30 as. Phase coherence of intraband currents which is also directly linked to the CEP of isolated EUV attosecond pulses is also presented here for the first time. As an outcome of this work generation of first isolated attosecond pulses from a bulk solid is demonstrated. Ultrafast quantum interferences evolving on a time scale faster than 1 fs in a novel system of Xe atoms are studied by performing pump-probe spectroscopic measurements utilizing optical attosecond pulses, these experiments form a firm ground for studying quantum coherences in more complex systems with the unique tool of optical attosecond pulses.