Lauren J. Borja

Attosecond band-gap dynamics in silicon

Electron transfer from valence to conduction band states in semiconductors is the basis of modern electronics. Here, attosecond extreme ultraviolet (XUV) spectroscopy is used to resolve this process in silicon in real time. Electrons injected into the conduction band by few-cycle laser pulses alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ~450-attosecond step rise time provides an upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band-gap modifications due to lattice motion, which occurs on a time scale of 60 ± 10 femtoseconds, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field–induced electron tunneling. Excited electrons in semiconducting silicon are tracked on a time scale faster than the lattice vibrations. [Also see Perspective by Spielmann] Watching electrons dart through silicon The ultimate speed limit in electronic circuitry is set by the motion of the electrons themselves. Schultze et al. applied attosecond spectroscopy to glimpse this motion in a sample of silicon, the semiconducting building block of modern integrated circuits (see the Perspective by Spielmann). The technique distinguished the electron dynamics—which proceed faster than a quadrillionth of a second after laser excitation—from the comparatively slower lattice motion of the silicon atomic nuclei. Science, this issue p. 1348; see also p. 1293

Direct and simultaneous observation of ultrafast electron and hole dynamics in germanium

Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical pump-probe spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by ultrafast transient absorption spectroscopy in the extreme ultraviolet at the germanium M4,5 edge. We decompose the spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8 × 1020 cm−3. Separate electron and hole relaxation times are observed as a function of hot carrier energies. A first-order electron and hole decay of ∼1 ps suggests a Shockley–Read–Hall recombination mechanism. The simultaneous observation of electrons and holes with extreme ultraviolet transient absorption spectroscopy paves the way for investigating few- to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions.

Extreme ultraviolet transient absorption of solids from femtosecond to attosecond timescales [Invited]

High-harmonic generation (HHG) produces ultrashort pulses of extreme ultraviolet radiation (XUV), which can be used for pump–probe transient absorption spectroscopy in metal oxides, semiconductors, and dielectrics. Femtosecond transient absorption on iron and cobalt oxides identifies ligand-to-metal charge transfer as the main spectroscopic transition, rather than metal-to-metal charge transfer or d–d transitions, upon photoexcitation in the visible. In silicon, attosecond transient absorption reveals that electrons tunnel into the conduction band from the valence band under strong-field excitation, to energies as high as 6 eV above the conduction band minimum. Extensions of these experiments to other semiconductors, such as germanium, and other transition metal oxides, such as vanadium dioxide, are discussed. Germanium is of particular interest because it should be possible to follow both electron and hole dynamics in a single measurement using transient XUV absorption.

Ultrafast carrier thermalization and trapping in silicon-germanium alloy probed by extreme ultraviolet transient absorption spectroscopy

Semiconductor alloys containing silicon and germanium are of growing importance for compact and highly efficient photonic devices due to their favorable properties for direct integration into silicon platforms and wide tunability of optical parameters. Here, we report the simultaneous direct and energy-resolved probing of ultrafast electron and hole dynamics in a silicon-germanium alloy with the stoichiometry Si0.25Ge0.75 by extreme ultraviolet transient absorption spectroscopy. Probing the photoinduced dynamics of charge carriers at the germanium M4,5-edge (∼30 eV) allows the germanium atoms to be used as reporter atoms for carrier dynamics in the alloy. The photoexcitation of electrons across the direct and indirect band gap into conduction band (CB) valleys and their subsequent hot carrier relaxation are observed and compared to pure germanium, where the Ge direct (ΔEgap,Ge,direct=0.8 eV) and Si0.25Ge0.75 indirect gaps (ΔEgap,Si0.25Ge0.75,indirect=0.95 eV) are comparable in energy. In the alloy, comparable carrier lifetimes are observed for the X, L, and Γ valleys in the conduction band. A midgap feature associated with electrons accumulating in trap states near the CB edge following intraband thermalization is observed in the Si0.25Ge0.75 alloy. The successful implementation of the reporter atom concept for capturing the dynamics of the electronic bands by site-specific probing in solids opens a route to study carrier dynamics in more complex materials with femtosecond and sub-femtosecond temporal resolution.

Simultaneous generation of sub-5-femtosecond 400 nm and 800 nm pulses for attosecond extreme ultraviolet pump-probe spectroscopy

Few-cycle laser pulses with wavelengths centered at 400 nm and 800 nm are simultaneously obtained through wavelength separation of ultrashort, spectrally broadened Vis-NIR laser pulses spanning 350-1100 nm wavelengths. The 400 nm and 800 nm pulses are separately compressed, yielding pulses with 4.4 fs and 3.8 fs duration, respectively. The pulse energy exceeds 5 μJ for the 400 nm pulses and 750 μJ for the 800 nm pulses. Intense 400 nm few-cycle pulses have a broad range of applications in nonlinear optical spectroscopy, which include the study of photochemical dynamics, semiconductors, and photovoltaic materials on few-femtosecond to attosecond time scales. The ultrashort 400 nm few-cycle pulses generated here not only extend the spectral range of the optical pulse for NIR-XUV attosecond pump-probe spectroscopy but also pave the way for two-color, three-pulse, multidimensional optical-XUV spectroscopy experiments.

Attosecond kinetics of photoexcited germanium

Attosecond transient reflectivity is developed to observe the photoexcitation dynamics in germanium. Attosecond time-resolved measurements of the dielectric function reveal a few-femtosecond collective electronic response time, which renormalizes the Coulomb interaction between the excited carriers.

Ultrafast transient absorption at the Germanium M 4,5 -edge to measure electron and hole dynamics

Extreme ultraviolet (XUV) transient absorption at the germanium M4,5-edge simultaneously measures electron and hole dynamics over 1.5 ps with few-femtosecond resolution. In the analysis, time-dependent density functional theory (TD-DFT) will be compared with experimental data.