1,720,987 research outputs found
Ultrafast Charge Dynamics in Bulk Zinc Oxide under Intense Photoexcitation
No full textThe photo-induced charge dynamics of textbook wide-bandgap semiconductor ZnO have been investigated on the picosecond time-scale. We performed optical Pump-THz Probe experiments in order to measure the dielectric constant of the material after high-fluence photo-excitation of charge carriers. The technique allows access to both carrier lifetime and scattering rates, and it provides direct access to the intrinsic dielectric function changes upon excitation. A complex dynamic is unveiled in the high-fluence pumping regime, where the relaxation time is in the hundreds of picoseconds range and increases with increasing Pump fluence, while the onset of photoconductivity takes place in a few picoseconds. The plasma frequency and the relaxation time dependence on the Pump fluence are discussed
Optical second harmonic on oxide interfaces
No full textDiscontinuities are the fundamentals of any interface-based device. The rich physics of oxide interfaces lies in the electronic correlations at the boundaries. There, the intrinsic symmetry of the bulk constituent materials is inherently broken. Optical second harmonic generation is, thus, an ideal tool to investigate the manifestations of several effects related to the presence of the interface. Two examples, addressing different issues in different systems, are reviewed here in order to highlight the potential richness of this technique when applied to interface-related phenomena in oxides
Optical Second Harmonic Generation on LaAlO3/SrTiO3 Interfaces: A Review
No full textAs we approach the limits of semiconductor technology, the development of new materials and technologies for the new era in electronics is compelling. Among others, perovskite oxide hetero-structures are anticipated to be the best candidates. As in the case of semiconductors, the interface between two given materials can have, and often has, very different properties, compared to the corresponding bulk compounds. Perovskite oxides show spectacular interfacial properties due to the the rearrangement of charges, spins, orbitals and the lattice structure itself, at the interface. Lanthanum aluminate and Strontium titanate hetero-structures (LaAlO3/SrTiO3) can be regarded as a prototype of this wider class of interfaces. Both bulk compounds are plain and (relatively) simple wide-bandgap insulators. Despite this, a conductive two-dimensional electron gas (2DEG) is formed right at the interface when a LaAlO3 thickness of n≥4 unit cells is deposited on a SrTiO3 substrate. The 2DEG is quite thin, being confined in only one or at least very few mono-layers at the interface, on the SrTiO3 side. A very intense and long-lasting study was triggered by this surprising discovery. Many questions regarding the origin and characteristics of the two-dimensional electron gas have been (partially) addressed, others are still open. In particular, this includes the interfacial electronic band structure, the transverse plane spatial homogeneity of the samples and the ultrafast dynamics of the confined carriers. Among a very long list of experimental techniques which have been exploited to study these types of interfaces (ARPES, XPS, AFM, PFM, …and many others), optical Second Harmonic Generation (SHG) was found to be suitable for investigating these types of buried interfaces, thanks to its extreme and selective interface-only sensitivity. The SHG technique has made its contribution to the research in this field in a variety of different and important aspects. In this work we will give a bird’s eye view of the currently available research on this topic and try to sketch out its future perspectives
Complex Permittivity of Ionic Liquid Mixtures Investigated by Terahertz Time-Domain Spectroscopy
No full textIonic liquids are salts found in their liquid state at ambient temperature. The physicochemical properties of ionic liquids can be tailored by selecting constituent cation and anion from numerous available ions. The physicochemical properties can be further tuned by mixing different neat ionic liquids. Reported data of ionic liquid mixtures reveal that frequently investigated properties such as density, viscosity, and thermal stability follow corresponding mixing laws. Complex permittivity in the interval of terahertz frequencies is of great importance to understand the molecular interactions and the solvation dynamics which drive the macroscopic properties of ionic liquids; however, to the best of our knowledge, there are few reports about the mixing behavior of complex permittivity in ionic liquid mixtures. In this contribution, binary mixtures of 1-butyl-3-methylimidazoulium iodide ([C4C1im]I) and 1-butyl-3- methylimidazoulium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) are investigated in the terahertz spectral range, and the resulting low-energy spectra are analyzed in order to clarify the mixing laws at play. The results show that the complex permittivity of mixtures of [C4C1im]I and ([C4C1im][NTf2] obeys a linear mixing law
Monitoring the interfacial electric field in pure and doped SrTiO3 surfaces by means of phase-resolved optical second harmonic generation
No full textOxides and new functional materials such as oxide-based hetero-structures are very good candidates to achieve the goal of the next generation electronics. One of the main features that rules the electronic behavior of these compounds is the interfacial electric field which confines the charge carriers to a quasi-two-dimensional space region. The sign of the confined charge clearly depends on the electric field direction, which is however a very elusive quantity, as most techniques can only detect its absolute value. Even more valuable would be to access the sign of the interfacial electric field directly during the sample growth, being thus able to optimize the growth conditions directly looking at the feature of interest. For this aim, solid and reliable sensors are needed for monitoring the thin films while grown. Recently optical second harmonic generation has been proposed by us as a tool for non-invasive, non-destructive, real-time, in-situ imaging of oxide epitaxial film growth. The spatial resolution of this technique has been exploited to obtain real-time images of the sample under investigation. Here we propose to exploit another very important physical property of the second harmonic wave: its phase, which is directly coupled with the electric field direction, as shown by our measurements
Coherent THz Hyper-Raman: Spectroscopy and Application in THz Detection
No full textRecently we have demonstrated a new nonlinear optical effect in the THz interval of frequencies. The latter is based on the use of femtosecond optical pulses and intense, sub-ps, broadband terahertz (THz) pulses to generate a THz-optical four- and five-wave mixing in the investigated material. The spectrum of the generated signal is resolved in time and wavelength and displays two pronounced frequency sidebands, Stokes and anti-Stokes, close to the optical second harmonic central frequency 2 ω L , where ω L is the optical central frequency of the fundamental beam, thus resembling the spectrum of standard hyper-Raman scattering, and hence we named this effect ‘THz hyper-Raman’—THYR. We applied this technique to several crystalline materials, including α-quartz and gallium selenide. In the first material, we find that the THYR technique brings spectroscopic information on a large variety of low-energy excitations that include polaritons and phonons far from the Γ-point, which are difficult to study with standard optical techniques. In the second example, we show that this new tool offers some advantages in detecting ultra-broadband THz pulses. In this paper we review these two recent results, showing the potentialities of this new THz technique
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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