Lightwave-Driven Electronic Phenomena in Solids Observed by Attosecond Transient Absorption Spectroscopy

Abstract

Steering electronic motion in solid state materials by light with unprecedented speed is the ultimate goal in the field of ultrafast physics and devices. Recent trends toward this goal are promoting extensive exploration of various lightwave-driven electric phenomena whose characteristic response occurs on a time scale comparable to the sub-cycle of lightwaves. Clarifying the comprehensive dynamics of lightwave-driven electric phenomena and pioneering its application to electronic functionality in solid state devices, require development of various diagnostic techniques with attosecond temporal resolution. Currently, attosecond transient absorption spectroscopy based on an isolated attosecond pulse (IAP) source is one of the most promising techniques. In this chapter, we will review a new scheme for lightwave-pulse-pump and IAP-probe attosecond transient absorption spectroscopy for solids. This scheme utilizes a quantum interference effect that appears in IAP absorption, which is induced by the simultaneous transition from the lightwave-coupled valence and conduction bands to a high-energy conduction band. We discuss the scheme based on an intuitively comprehensible approach that approximates a semiconductor band structure as a multi-eigenstate system in a theoretical formulation of the optical Bloch equation. The validity of the scheme is demonstrated by our recent experimental observation of third-order polarization in a wide-gap GaN semiconductor, which is well reproduced by the numerical simulation. This first observation of electronic oscillation beyond 1 PHz in a solid state system clearly shows the potential of future petahertz signal processing technology based on ordinary semiconductor devices.