Ferroelastic modulation and the Bloch formalism.

The key to the development of advanced materials is to understand their electronic structure-property relationship. Utilization of this understanding to design new electronic materials with desired properties led to modern epitaxial growth approaches for synthesizing artificial lattices, which for almost half a century have become the mainstay of electronic and photonic technologies. In contrast to previous scalar modulation approaches, we now study synthetic crystal lattices that have a tensor artificial modulation and develop a theory for photons and conduction band states in these lattices in a regime with an unusual departure from the familiar consequences of translational symmetry and Bloch's theorem. This study reveals that a nonmagnetic crystal lattice modulated by a purely geometrical orientational superlattice potential can lead to localized states or to spiral states for electrons and photons, as well as weakly or strongly localized states that could be used to markedly slow down the propagation of light and for optical energy storage applications.

Ultra-low threshold polariton condensation.

We demonstrate the condensation of microcavity polaritons with a very sharp threshold occurring at a two orders of magnitude pump intensity lower than previous demonstrations of condensation. The long cavity lifetime and trapping and pumping geometries are crucial to the realization of this low threshold. Polariton condensation, or "polariton lasing" has long been proposed as a promising source of coherent light at a lower threshold than traditional lasing, and these results indicate some considerations for optimizing designs for lower thresholds.

Electronic Raman scattering as an ultra-sensitive probe of strain effects in semiconductors.

Semiconductor strain engineering has become a critical feature of high-performance electronics because of the significant device performance enhancements that it enables. These improvements, which emerge from strain-induced modifications to the electronic band structure, necessitate new ultra-sensitive tools to probe the strain in semiconductors. Here, we demonstrate that minute amounts of strain in thin semiconductor epilayers can be measured using electronic Raman scattering. We applied this strain measurement technique to two different semiconductor alloy systems using coherently strained epitaxial thin films specifically designed to produce lattice-mismatch strains as small as 10(-4). Comparing our strain sensitivity and signal strength in Al(x)Ga(1-x)As with those obtained using the industry-standard technique of phonon Raman scattering, we found that there was a sensitivity improvement of 200-fold and a signal enhancement of 4 × 10(3), thus obviating key constraints in semiconductor strain metrology.

"Quantum coaxial cables" for solar energy harvesting.

Type II core-shell nanowires based on III-V and II-VI semiconductors are designed to provide the highly desirable but not readily available feature--efficient charge separation--and concurrently address the different material challenges specific for a few key renewable energy applications: including hydrogen generation via photoelectrochemical water splitting, dye-sensitized solar cells, and conventional solar cells. They also open up new avenues for studying novel physics and material sciences in reduced dimensionality of very unusual quasi-one-dimensional systems. A first-principles density function theory within the local density approximation (LDA) is used for the electronic structure calculation and a valence-force-field method for the structural relaxation, and empirical corrections to the LDA errors are applied.

From 1D chain to 3D network: tuning hybrid II-VI nanostructures and their optical properties.

In an effort to make semiconductor nanomaterials with tunable properties, we have deliberately designed and synthesized a family of novel organic-inorganic hybrid nanocomposites based on II-VI semiconductors with structures ranging from one-dimensional (1-D) chain to two-dimensional layer (2-D) to three-dimensional (3-D) framework. All nanostructures exhibit strong quantum confinement effect (QCE), while possessing a perfectly periodic arrangement. The optical absorption experiments show that all compounds generate a very large blue shift in the absorption edge (1.0-2.0 eV) due to the strong QCE. More significantly, their band edge shift and optical properties can be tuned by changing the dimensionality of inorganic motifs as well as overall crystal structures. Raman studies reveal that not only do these structures have distinctly different vibrational signatures from those of the II-VI host semiconductors, but they also differ significantly from each other as a result of changes in dimensionality. The crystal structures of these nanocomposite materials have been characterized by single crystal and/or powder X-ray diffraction methods. [ZnTe(pda)] (1; pda = propanediamine) is composed of 1-D chains of [ZnTe] with pda chelating to Zn atoms. [ZnTe(N(2)H(4))] (2; N(2)H(4) = hydrazine) and [ZnTe(ma)] (3; ma = MeNH(2) = methylamine) are two-dimensional (2-D) layered structures containing [ZnTe] slabs and terminal hydrazine (2) or methylamine (3) molecules. The crystal structures of [CdSe(en)(0.5)] (4; en = ethylenediamine) and [CdSe(pda)(0.5)] (5) are 3-D networks containing [CdSe] slabs bridged by bidentate organic diamine molecules. Crystal data for 1: Orthorhombic, space group Pbcm, a = 9.997(2), b = 6.997(1), c = 10.332(2) A, Z = 4. For 2: Monoclinic, space group P2(1), a = 4.2222(6), b = 6.9057(9), c = 7.3031(10) A, beta = 98.92(8) degrees, Z = 2. For 3: Orthorhombic, Pbca, a = 7.179(1), b = 6.946(1), c = 18.913(4) A, Z = 8. For 4: Orthorhombic, Pbca, a = 7.0949(3), b = 6.795(3), c = 16.7212(8) A, Z = 8. For 5: Orthorhombic, Cmc2(1), a = 20.6660(12), b = 6.8900(4), c = 6.7513(4) A, Z = 8.