Session S28: Superlattices and Nanostructures (Wires, Dots, etc.): Optical Properties II

Design of uniaxial metallodielectric metamaterials having large optical nonlinearities

We describe how the intrinsically large optical nonlinearities of metals could be accessed and increased by fabrication of uniaxial homogenized composites comprising alternating metal and dielectric layers of subwavelength thickness. Such composites are predicted to exhibit effective third-order nonlinear susceptibilities orders of magnitude larger than those intrinsic to the metallic component. The enhancement is due to a resonance effect, and is limited to the direction perpendicular to the layer interfaces. We illustrate our predictions with calculations for several metallodielectric systems, including those consisting of copper and titanium dioxide components.   

Spectroscopic and thermal studies of L-arginine doped Potassium Dihydrogen Phosphate crystals

We have used IR transmission and Raman spectroscopy to study the active doping of potassium dihydrogen phosphate (KDP) crystals with L-arginine amino acid. In the present investigation, pure and doped KDP crystals were grown by slow evaporation solution method. Although the dominant bands observed in the infrared absorption spectra correspond to KDP crystals, the existence of vibrational lines at 1401 cm$^{-1 }$(CH$_{2})$, 1637 cm$^{-1}$ (COO$^{-})$, 1716 cm$^{-1}$ (NH$_{3}^{+})$, and 3127 cm$^{-1}$ (NH$_{3}^{+})$ indicate that successful doping was achieved. This affirmation is further corroborated by the FT-Raman data, where strong lines are observed in the 2800 cm$^{-1}$ -- 3100 cm$^{-1}$ region, which is associated with C-H stretching modes of amino acids. The crystal structure and the thermal stability of the samples were also examined by powder X-ray diffraction and thermogravimetric techniques, respectively. Thermogravimetric analysis demonstrates a decrease of the thermal stability with increasing doping amount. An increase of second harmonic generation efficiency was found with more L-Arginine doping.   

Optical properties in the visible range of Co clusters capped by Pd under hydrogen

Co clusters with mean size of 1.8 nm were deposited to form a 25 nm thick cluster assembled film on glass, capped by a continuous 15 nm Pd film. The light transmission and reflection, in the visible range (400 to 1000 nm), were measured when the sample was exposed to different hydrogen pressures up to 120 Torr. Measurements on 15 nm continuous Pd film were done for comparison. Electrical resistance of the films was also measured as an independent parameter to determine hydrogen absorption by the samples. In both samples the transmission and resistance of the films increase, reaching saturation at 30 Torr hydrogen pressure. Increase of the light transmission and electrical resistance on the pure Pd film indicates absorption of hydrogen in the bulk of the film. Smaller relative change of the resistance and reflection of light on the Co cluster sample capped by Pd indicates that hydrogen absorption is limited to the Pd capping layer only. This work is supported by the Fund for Scientific Research-Flanders (FWO), by the Flemish Concerted Action (GOA), and by the Belgian Interuniversity Poles of Attraction (IAP) programs.   

Novel optical signatures of sub-3 nm rare earth sesquioxide nanocrystals.

Europium and terbium based sesquioxide nanomaterials, known for their characteristic red and green luminescence, respectively, have recently garnered much research attention due to their size-dependent optical properties. Here, we present systematic investigation of the size-dependent optical properties Eu$_{2}$O$_{3}$, Tb$_{2}$O$_{3}$, and Gd$_{2}$O$_{3}$:Eu$^{3+}$ / Tb$^{3+ }$nanocrystals (NCs) in the size range of 1-3 nm in diameter. We observe a new luminescence peak at 620 nm in Eu$_{2}$O$_{3}$ and Gd$_{2}$O$_{3}$:Eu$^{3+}$ NCs, which represents modulation of the $^{7}$F$_{2}$ transition in Eu$^{3+}$ ion. Intensity modulation with respect to the 612 nm is observed as a function of nanocrystal size. For the Tb$_{2}$O$_{3}$ NCs, a new luminescence signature at 548 nm characterizes modulation of the $^{7}$F$_{5}$ transition in Tb$^{3+}$ ion. In addition, we probe the effect of NC size on the luminescence efficiencies of the doped and pure sesquioxide NCs. The concentration quenching effect, which leads to low luminescence efficiencies in bulk, pure sesquioxides, is explored in sub-3 nm sesquioxides.   

Far-infrared Magneto-Spectroscopic Studies of Ca$_{3}$Co$_{4}$O$_{9}$ Thin Films and Single Crystals

In recent years, the 2D-layered cobaltates have emerged as promising p-type thermoelectric materials due to their unique combinations of high thermo-coefficient and good metallic transport properties. These systems show high thermoelectric figure of merit and are ideal candidates as the materials of choice at elevated temperatures. We have carried out far- infrared magneto-spectroscopic studies of Ca$_{3}$Co$_{4}$O$_{9} $ thin films in Faraday geometry as a function of frequency, magnetic field and temperature with the emphasis on the coupling between the lattice, the charge and the spin degrees of freedom. Far infrared transmission reduces at low frequencies in the presence of magnetic field corresponding to negative magneto-resistance. Below 20K, hysterisis occurs. However, the spectral responses to magnetic field and temperature are different. This indicates that the negative magneto-resistance is due to reduced magnetic scattering when Co spins become aligned. Further infrared studies will be performed with magnetic field parallel to the CoO$_{2}$ layers. A good understanding of our infrared results should shed light on the origin of high thermo-power in these 2D-layered cobaltates.   

Ultrafast Time Resolved Transient Absorption and Photoluminescence (PL) Studies of In$_{0.2}$Ga$_{0.8}$As/GaAs Quantum Wells in High Magnetic Fields

We investigate the temporal dynamics of dense magneto-plasmas excited by intense femtosecond laser pulses in In$_{0.2}$Ga$_{0.8}$As/GaAs multiple quantum wells in high magnetic fields. To fully fill the Landau levels (LLs), we pump to carrier densities near 10$^{13}$/cm$^{2}$ using 150 fs pulses. Time-resolved transient absorption experiments probe the occupancy of each e1h1 LL, revealing a dramatic decrease in decay times above the zero field e1h1 transition dynamics. Our PL results reveal evidence for multiple short bursts of emission pulses at the highest fields (17.5T) from in-plane PL emission compared with zero field emission. In addition, qualitatively different temporal dynamics from in-plane and out-of-plane collection geometries are observed. Our results are analyzed in the context of ultrafast cooperative emission mechanisms from dense electron-hole plasmas.   

Magneto-photoluminescence studies of CdTe/CdSe/ZnS nanoparticles

Recently, the effect of the magnetic flux on the excitonic energy has received much attention. Optical Aharonov-Bohm was observed on negatively charged exciton in InGaAs/GaAs quantum ring as well as neutral exciton in type-II InP-GaAs heterostrcture. In this talk we'll present our magneto-photoluminescence studies on the optical properties of CdTe/CdSe/ZnS system. The nanoparticles that were grown by chemical method have size of about 6 nm and the band alignment between the core (CdTe) and the shell (CdSe) is a type--II band alignment. The addition of ZnS layer is to passivate the surface of CdSe and to enhance the light emitting efficiency. Magneto-photoluminescence experiment was performed at T=1.4 K with a 14 T superconducting magnet in conjunction with a green diode laser and a monochromator. Oscillation on the peak energy of the photoluminescence spectra as well as oscillation in the integrated intensity as a function of magnetic field were observed and are attributed to the optical Aharonov-Bohm-like effect.   

Absorption and photocurrent of semiconductor quantum wells: a multi-band NEGF study

Interband photoexcitation of carriers in semiconductor quantum wells is exploited in various optoelectronic devices such as photodetectors or quantum well solar cells. For a quantitative prediction of the photocurrent, realistic models for the (sub)bandstructure as well as for the transport properties are required. In the present approach, the derivation of optical properties based on a multi-band empirical tight-binding Hamiltonian is combined with the non-equilibrium Green's function formulation of quantum transport. The photocurrent is calculated in presence of elastic and inelastic electron-phonon scattering from the self-consistent Born approximation of the self energy for carrier-light interaction, while the absorption is obtained from the transverse interband polarization function. Since the absorption as experimentally observed is governed by the excitonic contribution, the inclusion of this feature into the calculation of photocurrent and interband polarization via the respective many-body corrections is discussed.   

Magneto-infrared study on 2-dimensional electrons and holes in GaSb-InAs-AlSb coupled quantum wells.

InAs-AlSb heterostructures have been a subject of interests for their unusual type-II band alignment between InAs and AlSb. The spatially separated 2-dimensional electron and hole gases are confined in different layers and in equilibrium with each other at the InAs/GaSb interface. This unique circumstance has led to several predicted many-body effects, as well as possible applications as infrared detectors and sources. In the past, magneto-infrared studies on InAs-AlSb single quantum wells have revealed a range of phenomena arising from the electron-hole binding. We have carried out an infrared optical study up to 33T on a series of GaSb-InAs-AlSb coupled quantum well structures, in which the electrons and holes are separated by a thin barrier and the Fermi level is tuned by the thickness of the GaSb cap layer. In addition to the electron cyclotron resonance (CR), another transition has been observed at the fields higher than 13 T. The linewidth of the CR shows the oscillatory behavior with the filling factor that is consistent with the electron densities obtained from the transport measurements. The transition energy of this line is close to the energy difference between the lowest Landau level (LL) in the InAs layer and the highest LL in the GaSb layer, which suggests that the it might result from an excitonic transition across the barrier.   

Infrared dielectric properties and optical magnetoconductivity of CaRuO$_3$/CaMnO$_3$ superlattices

Spectroscopic ellipsometry and magneto-reflectivity in the far-infared spectral range is used to study the electronic properties of $\rm [(CaRuO_3)_N|(CaMnO_3)_{10}]_6$ superlattices (SLs). The nonlinear regression procedure is employed to extract the dynamical conductivity and dielectric permittivity of bare SLs within the effective medium approximation with a mixture of the ruthenate and manganite layers. We find that the infrared conductivity of the SLs decreases with decreasing individual ruthenate layer thickness, so that the effective number of conducting electrons per Ru atoms remains independent of $N$ and is comparable with the bulk value even for ultrathin $\rm CaRuO_3$ layers (N = 4-10 unit cells). This implies no major charge transfer effects between non-Fermi liquid metal $\rm CaRuO_3$ and antiferromagnetic insulator $\rm CaMnO_3$. While the low-energy electrodynamics of the SLs is governed by $\rm CaRuO_3$ layer behavior, we find a negative magnetoresistivity at temperatures below $\sim$ 150 K, which correlates with the Neel temperature of the AFM state in $\rm CaMnO_3$. The magnetoresistivity effect is discussed to be due to the strong spin dependent scattering from the interface.   

Observation of coherent high-wavevector acoustic vibrations in a bulk material using time-resolved X-ray diffraction

We report on the observation of high-wavevector acoustic phonons in bulk InP that originate from folded phonons in a GaInAs/AlInAs superlattice. Synchrotron time-resolved X-ray diffraction is used to probe the evolution of the laser-generated acoustic phonons. Due to the short wavelength, X-ray diffraction gives access high-wavevector components of the acoustic wave-packet in a bulk material. Experiments show a bulk excitation at a wavevector $q=2 \pi/D$, where $D$ is the superlattice period, which propagates into the substrate at the speed of sound. These results are supported by time-resolved dynamical diffraction calculations in which the strain is included as a perturbation from the perfect crystal.   

Fr\"{o}hlich phonon modes in PbSe and PbS colloidal quantum dots

The measured infrared absorption of colloidal PbSe and PbS quantum dots in hexane is shown to be dominated by absorption near the Fr\"{o}hlich mode frequency position. However, in both sets of quantum dots the mode is at a higher frequency than calculated from the bulk dielectric constant, and shifts to still higher frequency with decreasing diameter of dot. This behavior is shown to be consistent with a decreasing contribution of the near-infrared and visible exciton absorption to the dielectric constant at far infrared frequencies as the particle size decreases and the band gap increases. The unique presence of Fr\"{o}hlich mode absorption in a wide range of dot sizes suggests that the mechanical boundary condition of the quantum dot is a ``soft'' one in which vibrational amplitudes do not go to zero at the boundary.   

Coherent defect-assisted multiphonon intraband carrier relaxation in semiconductor quantum dots

A new defect-assisted mechanism of multiphonon intraband carrier relaxation in semiconductor quantum dots, where the carrier is found in a coherent superposition of the initial, final, and defect states, is proposed. It is shown that this mechanism is capable of explaining the observed trends in temperature dependences of the intraband relaxation rates for PbSe and CdSe colloidal nanocrystal quantum dots.   

Raman-Brillouin electronic density in GaAs/AlAs superlattices.

Raman-Brillouin scattering by acoustic phonons is an accurate experimental method to characterize vibrational states of nanostructures and understand their optical properties. Many nanoscaled systems such as quantum dots, wires, wells and membranes have been studied by the means of this technique. We present here a theoretical and experimental study of the Raman-Brillouin scattering in GaAs/AlAs superlattices. Within third order perturbation theory, we describe the Raman- Brillouin scattering process by introducing a Raman-Brillouin Electronic Density (RBED). The RBED is constructed by combining the superlattice electronic states according to their optical transition rates. This approach is useful when numerous intermediate electronic states are involved in the scattering process. It has been proven to successfully interpret the Raman- Brillouin scattering in quantum dots[1] and membranes[2]. We calculate the RBED for specific GaAs/AlAs superlattices and study the dependence of the Raman-Brillouin spectra on the GaAs/AlAs layer thickness ratio and incident photon energy. Comparison with experiments will be discussed. [1] J.R. Huntzinger et al, Phys. Rev. B 74, 115308 (2006) [2] A. Mlayah, J.R. Huntzinger and N. Large, Phys. Rev. B 75, 245303 (2007)   

Measured and Calculated Properties of \textit{n}- and \textit{p}-type PbTe-Based Materials for High-Performance Thermoelectrics

Recent advances in PbTe and other material systems for thermoelectric applications are based on nanostructuring, with a specific goal of substantially reducing lattice thermal conductivity while maintaining good electrical properties. Our work has focused on developing PbTe/PbSe$_{1-x}$Te$_{x}$ ($x\sim $0.02-0.04) nanodot superlattices (NDSLs) for improved thermoelectric performance. In this presentation we will compare the electrical and thermal properties of $n$- and $p$-type NDSLs to baseline homogeneous PbTe, and also compare the electrical data to calculations performed using the Boltzmann transport equation with the relaxation time approximation. Compared to PbTe at the same carrier concentration, NDSL samples generally have reduced mobilities ($\sim $25-35{\%}), the same Seebeck coefficients, and substantially reduced ($\sim $4-6x) lattice thermal conductivities, resulting in a large increase in \textit{ZT}. Specifically, a 300-K in-plane power factor of $\ge $25 $\mu $W/cm-K$^{2}$ has been repeatedly achieved for both $n$- and $p$-NDSLs, while the cross-plane lattice thermal conductivity has been measured as $\sim $0.35-0.4 W/m-K using various techniques. We will also present recent power generation results where 9.9 W/cm$^{2}$ was obtained from a 1 mm$^{2}$, 100-$\mu $m-thick stand-alone $n$-NDSL thermoelement, at a $\Delta $T of 202 K.