Applications of Ultrafast Terahertz Pulses for Intra-Excitonic Spectroscopy of Quasi-2D Electron-Hole Gases

Excitons are of fundamental interest and of importance for opto-electronic applications of bulk and nano-structured semiconductors. This paper discusses the utilization of ultrafast terahertz (THz) pulses for the study of characteristic low-energy excitations of photoexcited quasi 2D electron-hole (e-h) gases. Optical-pump THz-probe spectroscopy at 250-kHz repetition rate is employed to detect characteristic THz signatures of excitons and unbound e-h pairs in GaAs quantum wells. Exciton and free-carrier densities are extracted from the data using a two-component model. We report the detailed THz response and pair densities for different photoexcitation energies resonant to heavy-hole excitons, light-hole excitons, or the continuum of unbound pairs. Such experiments can provide quantitative insights into wavelength, time, and temperature dependence of the low-energy response and composition of optically excited e-h gases in low-dimensional semiconductors

Contactless terahertz probes of correlations and dynamics in low-dimensional electron-hole gases

Confinement of carriers in nanostructures entails strong modifications of their physical properties, offering a well-defined laboratory for investigating the complex many-body interactions between charge, lattice and spin degrees of freedom [1]. In particular, electron-hole (e-h) gases photoexcited into low-dimensional semiconductors are characterized by new optical transitions and strongly enhanced Coulomb interactions. Terahertz radiation offers a unique tool to measure low-energy excitations and transport properties in nanostructures without the need for an electrical contact. Along this path, we have recently developed a new, sensitive scheme to probe time-varying Coulomb correlations in confined carrier plasmas. Here, we discuss experiments that utilize this pulsed terahertz source to probe the dynamical interplay of bound and unbound e-h pairs on a picosecond timescale [2]. A new low-energy oscillator is observed directly after resonant creation of heavy-hole excitons in GaAs quantum wells. This peak arises from transitions between the hydrogen-like exciton bound states, most notably the 1s-2p level transition. The terahertz field probes excitons in a large range of in-plane momenta K in contrast to the usual restriction of interband probes close to K ~ 0. Owing to the strongly correlated motion of electrons and holes, charge-neutral excitons are electrically insulating up to a frequency that matches the separation between their lowest internal states. Above-bandgap excitation at elevated temperatures however induces unbound e-h pairs which represent a conducting ionized gas with a Drude-like response. The distinct responses of these extreme phases enable us to follow in time a metal-insulator transition that occurs upon formation of excitons out of a gas of unbound e-h pairs, as well as its reverse process of ionization. These are dynamical transitions which occur on different timescales. Ionization of excitons can occur within only a few picoseconds, and depends on the phonon occupation. Exci

Contactless terahertz probes of correlations and dynamics in low-dimensional electron-hole gases

Confinement of carriers in nanostructures entails strong modifications of their physical properties, offering a well-defined laboratory for investigating the complex many-body interactions between charge, lattice and spin degrees of freedom [1]. In particular, electron-hole (e-h) gases photoexcited into low-dimensional semiconductors are characterized by new optical transitions and strongly enhanced Coulomb interactions. Terahertz radiation offers a unique tool to measure low-energy excitations and transport properties in nanostructures without the need for an electrical contact. Along this path, we have recently developed a new, sensitive scheme to probe time-varying Coulomb correlations in confined carrier plasmas. Here, we discuss experiments that utilize this pulsed terahertz source to probe the dynamical interplay of bound and unbound e-h pairs on a picosecond timescale [2]. A new low-energy oscillator is observed directly after resonant creation of heavy-hole excitons in GaAs quantum wells. This peak arises from transitions between the hydrogen-like exciton bound states, most notably the 1s-2p level transition. The terahertz field probes excitons in a large range of in-plane momenta K in contrast to the usual restriction of interband probes close to K ~ 0. Owing to the strongly correlated motion of electrons and holes, charge-neutral excitons are electrically insulating up to a frequency that matches the separation between their lowest internal states. Above-bandgap excitation at elevated temperatures however induces unbound e-h pairs which represent a conducting ionized gas with a Drude-like response. The distinct responses of these extreme phases enable us to follow in time a metal-insulator transition that occurs upon formation of excitons out of a gas of unbound e-h pairs, as well as its reverse process of ionization. These are dynamical transitions which occur on different timescales. Ionization of excitons can occur within only a few picoseconds, and depends on the phonon occupation. Exci

A comprehensive dataset for the thermal conductivity of ice Ih for application to planetary ice shells

Contemporary models representing the thermal conductivity of ice Ih as a function of temperature are based on data from published experiments that span over a century. Each model is derived using specific datasets with distinct experimental setups, temperature ranges, and uncertainties. Model errors introduced by inaccurate digitization and biased datapoints are challenging to trace due to a lack of transparency of the primary data. This dataset is a collection of published thermal conductivity data for ice Ih, including both tabulated and digitized data, presented in the original units. Specific samples or pressure conditions are noted where applicable. The dataset also includes a survey of published thermal conductivity models, providing the valid temperature range, accuracy and uncertainty (where noted in the original publication), and the primary data sources. Importantly, the dataset includes notes that were contained in the original publication or subsequent publications that provide additional context for the data. This dataset is used to derive a new thermal conductivity model which best represents the thermal conductivity of ice Ih for temperatures greater than 30 K. Statistics are provided to evaluate the fit of each thermal conductivity model in the survey of published models to the comprehensive dataset presented here. This dataset is constructed in support of the work “New insights into temperature-dependent ice properties and their effect on ice shell convection for icy ocean worlds” [1]

Ultrafast terahertz probes of transient conducting and insulating phases in an electron-hole gas

Many-body systems in nature exhibit complexity and self-organization arising from seemingly simple laws. The long-range Coulomb interaction between electrical charges generates a plethora of bound states in matter, ranging from the hydrogen atom to complex biochemical structures. Semiconductors form an ideal laboratory for studying many-body interactions of quasi-particles among themselves and with lattice vibrations and light. Oppositely charged electron and hole quasi-particles can coexist in an ionized but correlated plasma, or form bound hydrogen-like pairs called excitons which strongly affect physical properties. The pathways between such states however remain elusive in near-visible optical experiments that detect a subset of excitons with vanishing center-of-mass momenta. In contrast, transitions between internal exciton levels which occur in the far-infrared at terahertz (10 s) frequencies are in dependent of this restriction suggesting their use as a novel pro be of pair dynamics. Here, we employ an ultrafast terahertz probe to directly investigate the dynamical interplay of optically-generated excitons and unbound electron-hole pairs in GaAs quantum wells. Our observations witness an unexpected quasi-instantaneous excitonic enhancement, reveal formation of insulating excitons on a hundred picosecond timescale and manifest conditions under which excitonic populations prevail

Dynamics of Mixed Clathrate-Ice Shells on Ocean Worlds

International audienceThe habitability of oceans within icy worlds depends on material and heat transport through their outer ice shells. Previous work shows a methane clathrate layer at the upper surface of the ice shell of Titan thickens the convecting region, while on Pluto a clathrate layer at the base of the ice shell hinders convection. In this way, the dynamics of clathrate-ice shells may be essential to the thermal evolution and habitability of ocean worlds. However, studies to date have not addressed the dynamics that determine the location of clathrates within the ice shell. Here, we show that, in contrast to previous studies, clathrates accumulating at the base of the ice shell are entrained throughout the shell. Clathrates are stiffer than ice. As a result, entrainment slows convection and thickens the conductive lid across a range of ocean worlds, potentially preserving sub-ice oceans but limiting avenues for material transport into them