In-plane Optical Anisotropy Research Articles

In-plane optical anisotropy in self-assembled Ge quantum dots induced by interfacial chemical bonds

In-plane optical anisotropy has been observed in self-assembled Ge quantum dots (QDs). It is found that the photoluminescence (PL) spectrum polarized along [110] exhibits different features compared to that corresponding to [11¯0]. Besides, the polarized PL spectrum is able to reveal a detailed fine structure much more pronounced than that in the unpolarized spectrum. It is shown that the observed optical anisotropy is a result of the inherent property of the type-II band alignment of Ge QDs embedded in Si matrix. The light emission arises from the recombination of electrons and holes across the interface, and it thus reflects the anisotropic nature of the interfacial chemical bonds.

Strain-induced in-plane optical anisotropy in (001) GaAs∕AlGaAs superlattice studied by reflectance difference spectroscopy

In-plane optical anisotropy (IPOA) in (001) GaAs∕AlGaAs superlattice induced by uniaxial strain has been investigated by reflectance difference spectroscopy (RDS). Uniaxial strain on the order of 10−4 was introduced by bending a strip sample with a stress apparatus. The IPOA of all interband transitions shows a linear dependence on strain. The birefringence and dichroism spectra induced by strain are obtained by RDS on the basis of a three-phase model, which is in good agreement with the reported results.

In-plane optical anisotropy for the charge/orbital ordered state in half-doped layered manganites

We have investigated the in-plane anisotropy of the electronic response for the charge/orbital ordering (CO–OO) phase in half-doped layered manganites. It has been suggested that the CE-type magnetically ordered CO–OO phase might show the electronic anisotropy in the MnO 2 plane due to the quasi-one-dimensional electron hopping along the ferromagnetic chain. To address this issue, we measured optical spectra for a single domain of layered manganites Eu 1/2Ca 3/2MnO 4 with polarization dependence. It was found that the optical conductivity along the chain direction exhibits smaller optical gap and lower energy distribution of the spectral weight than along the stripe (interchain) direction. This sizable anisotropy, which is identified below the CO–OO transition temperature, is attributed mainly to the e g orbital ordering.

Quantum electrodynamical torques in the presence of Brownian motion

Quantum fluctuations of the electromagnetic field give rise to a zero-point energy that persists even in the absence of electromagnetic sources. One striking consequence of the zero-point energy is manifested in the Casimir force, which causes two electrically neutral metallic plates to attract in order to reduce the zero-point energy. A second, less well-known, effect is a torque that arises between two birefringent materials with in-plane optical anisotropy as a result of the zero-point energy. In this paper, we discuss the influence of Brownian motion on two birefringent plates undergoing quantum electrodynamical (QED) rotation as a result of the system's zero-point energy. Direct calculations for the torque are presented, and preliminary experiments are discussed.

Strong in-plane optical anisotropy of asymmetric (001) quantum wells

It is well known that asymmetry in the (001) direction can induce in-plane optical anisotropy (IPOA) in (001) quantum wells (QWs). In this letter, asymmetry is introduced in (001) GaAs∕AlGaAs QWs by inserting 1 ML (monolayer) of InAs or AlAs at interfaces. Strong IPOA, which is comparable to that in the InGaAs∕InP QWs with no common atom, is observed in the asymmetric GaAs∕AlGaAs QW by reflectance difference spectroscopy.

Optical anisotropy and strain evolution of GaAs surfaces at the onset of the formation of InAs quantum dots

By using reflectance difference spectroscopy we have studied the in-plane optical anisotropy of GaAs surfaces covered by ultrathin InAs layers. The strain evolution of the GaAs surface with the InAs deposition thickness can be obtained. It is found that the optical anisotropy and the surface tensile strain attain maximum values at the onset of the formation of InAs quantum dots (QDs) and then decrease rapidly as more InAs QDs are formed with the increase of InAs deposition. The origin of the optical anisotropy has been discussed.

Evolution of wetting layer of InAs∕GaAs quantum dots studied by reflectance difference spectroscopy

For the InAs∕GaAs quantum-dot system, the evolution of the wetting layer (WL) with the InAs deposition thickness has been studied by reflectance difference spectroscopy (RDS) in combination with atomic force microscopy and photoluminescence. One transition related to the light hole in the WL has been observed clearly in RDS, from which its transition energy and in-plane optical anisotropy (OA) are determined. The evolution of WL with the InAs dot formation and ripening has been discussed. In addition, the remarkable changes in OA at the onsets of the dot formation and ripening have been observed, implying the mode transitions of atom transport between the WL and the dots.

Study on in-plane optical anisotropy of Si0.75Ge0.25∕Si∕Si0.5Ge0.5 asymmetric superlattice by reflectance difference spectroscopy

Si 0.75 Ge 0.25 ∕ Si ∕ Si 0.5 Ge 0.5 trilayer asymmetric superlattices were prepared on Si (001) substrate by ultrahigh vacuum chemical vapor deposition at 500 °C. The nonlinear optical response caused by inherent asymmetric interfaces in this structure predicted by theories was verified by in-plane optical anisotropy in (001) plane measured via reflectance difference spectroscopy. The results show Si0.75Ge0.25∕Si∕Si0.5Ge0.5 asymmetric superlattice is optically biaxial and the two optical eigen axes in (001) plane are along the directions [110] and [−110], respectively. Reflectance difference response between the above two eigen axes can be influenced by the width of the trilayers and reaches as large as ∼10−4–10−3 in 15-period 2.7nm-Si0.75Ge0.25∕8nm-Si∕1.3nm-Si0.5Ge0.5 superlattice when the normal incident light wavelength is in the range of 500–1100 nm, which is quite remarkable because the optical anisotropy does not exist in bulk Si.

Spectroscopic ellipsometry study of optical anisotropy inGd5Si2Ge2and comparison with reflectance difference spectra

The complex dielectric functions of single crystals of ${\mathrm{Gd}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$ were obtained using spectroscopic ellipsometry (SE) in the photon energy range of $1.5--5.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ at room temperature. Reflectance difference (RD) spectra for the $a\text{\penalty1000-\hskip0pt}b$ and $b\text{\penalty1000-\hskip0pt}c$ planes of single crystals of ${\mathrm{Gd}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$ were derived from these dielectric functions and compared to those obtained from reflectance difference spectroscopy (RDS) at near-normal incidence. The two experimental RD spectra from SE and RDS agreed well. The in-plane optical anisotropy of the sample is mainly due to intrinsic bulk properties because of its larger magnitude $(4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2})$ compared to surface-induced optical anisotropies, with a magnitude of only about ${10}^{\ensuremath{-}3}$ for a typical cubic material.

Effects of Giant Optical Anisotropy in R-plane GaN/AlGaN Quantum Wells by Valence Band Mixing

Investigation of optical anisotropy spectra in the R-plane (i. e., the [1012]-oriented layer plane) of GaN/Al0.2Ga0.8N quantum wells with different widths is studied. The optical matrix elements in the wurtzite quantum wells are calculated using the k·p finite difference scheme. The calculations show that the valence band mixing effect produces giant in-plane optical anisotropy in [1012]-oriented GaN/Al0.2Ga0.8N quantum wells with a narrow width. The nature of the in-plane optical anisotropy is found to be dependent on the well width. Specifically, it is found that the anisotropy changes from x′-polarization to y′-polarization as the well width increases. DOI: 10.2529/PIERS060801054904

Torque on birefringent plates induced by quantum fluctuations

We present detailed numerical calculations of the mechanical torque induced by quantum fluctuations on two parallel birefringent plates with in-plane optical anisotropy, separated by either vacuum or a liquid (ethanol). The torque is found to vary as $\mathrm{sin}(2\ensuremath{\theta})$, where $\ensuremath{\theta}$ represents the angle between the two optical axes, and its magnitude rapidly increases with decreasing plate separation $d$. For a $40\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ diameter disk, made out of either quartz or calcite, kept parallel to a barium titanate plate at $d\ensuremath{\simeq}100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the maximum torque (at $\ensuremath{\theta}=\ensuremath{\pi}∕4$) is of the order of $\ensuremath{\simeq}{10}^{\ensuremath{-}19}\phantom{\rule{0.3em}{0ex}}\mathrm{N}\phantom{\rule{0.2em}{0ex}}\mathrm{m}$. We propose an experiment to observe this torque when the barium titanate plate is immersed in ethanol and the other birefringent disk is placed on top of it. In this case the retarded van der Waals (or Casimir-Lifshitz) force between the two birefringent slabs is repulsive. The disk would float parallel to the plate at a distance where its net weight is counterbalanced by the retarded van der Waals repulsion, free to rotate in response to very small driving torques.

Interface-Related In-Plane Optical Anisotropy of Quantum Wells Studied by Reflectance-Difference Spectroscopy

The in-plane optical anisotropy of three groups of GaAs/AlGaAs quantum well structures has been studied by reflectance-difference spectroscopy (RDS). For GaAs/Al0.36Ga0.64As single QW structures, it is found that the optical anisotropy increases quickly as the well width is decreased. For an Al0.02Ga0.98As/AlAs multiple QW with a well width of 20nm, the optical anisotropy is observed not only for the transitions between ground states but also for those between the excited states with transition index n up to 5. An increase of the anisotropy with the transition energy, or equivalently the transition index n, is clearly observed. The detailed analysis shows that the observed anisotropy arises from the interface asymmetry of QWs, which is introduced by atomic segregation or anisotropic interface roughness formed during the growth of the structures. More, when the 1 ML InAs is inserted at one interface of GaAs/AlGaAs QW, the optical anisotropy of the QW can be increased by a factor of 8 due to the enhanced asymmetry of the QW. These results demonstrate clearly that the RDS is a sensitive and powerful tool for the characterization of semiconductor interfaces.

In-plane optical anisotropy in InxGa1−xN∕GaN multiple quantum wells induced by Pockels effect

We have investigated the crystal orientation dependence of optical properties in InxGa1−xN∕GaN multiple quantum wells. The spectral peaks and intensity of the microphotoluminescence signal for different crystal orientations were found to have six-fold symmetry. Quite interestingly, the refractive index, obtained from the interference pattern, also varies with the crystal orientation. The 60° periodic anisotropy of electronic transitions as well as optical parameters was interpreted in terms of the Pockels effect induced by the strong built-in field in nitride heterojunctions. The linear dependence of the change of the refractive index on electric field is consistent with the prediction of the Pockels effect. Our result provides an alternative solution to improve the designs of photonic and electronic devices based on nitride semiconductors.

Effect of Fluorination of the Hydrophilic Heads on Morphology and Molecular Structure of Langmuir Monolayers of Long-Chain Ethers

Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GIXD) were used to compare morphology and molecular structure of Langmuir monolayers of docosyl trifluoroethyl ether (DFEE) and docosyl ethyl ether (DEE) on water. When spread at a molecular area of 70 Å2/molecule both substances form islands of condensed phase and suspension of small 2D microcrystals attaching at the borderlines under compression. The DEE islands and the DEE monolayer at low surface pressure exhibit strong in-plane optical anisotropy, while the DFEE islands and the compact DFEE film are isotropic. The anisotropy of the DEE monolayer disappears at 10 mN/m, and above this surface pressure the monolayers with fluorinated and nonfluorinated heads are visually the same. The GIXD analysis identifies a L2‘ phase with NNN molecular tilt and NN lattice distortion in the DEE islands and DEE monolayer at low surface pressure. The maximum tilt angle at zero compression is 22.6°. The islands and the compact DFEE film consist of closely packed upright molecules forming an S phase. At high surface pressure both monolayers have the same upright molecular arrangement with centered rectangular lattice and practically the same unit cell parameters and lattice distortion. Under such conditions the DFEE and DEE molecules occupy the same area of the water surface Axy = A0 = 19.1 Å2. Since the van der Waals radius of the CF3 group is by 16% larger as compared to that of the CH3 group, the same A0 value of the two films means a significantly thinner hydration shell of the fluorinated head. Substitution of the terminal CH3 group in the DEE head by the CF3 group in the DFEE head removes the low-pressure tilted phase and causes formation of the condensed phase of upright fluorinated molecules even at no compression. The decreased headgroup hydration of DFEE could be responsible for the upright versus tilted orientation of the DFEE and DEE molecules at low surface pressure. We suppose that the mechanism of the continuous tilting transition causing erection of the DEE chains is related to an increasing dehydration of the headgroups under compression.

Investigation of GaAs/AlGaAs interfaces by reflectance-difference spectroscopy

The in-plane optical anisotropy of several GaAs/AlGaAs quantum well samples with different well widths has been measured at room temperature by reflectance-difference spectroscopy (RDS). The RDS line shapes are found to be similar in all the samples examined here, which dominantly consist of two peak-like signals corresponding to 1HH-->1E and 1LH-->1E transition. As the well width is decreased, or the 1 ML InAs layer is inserted at one interface, the intensity of the anisotropy increases quickly. Our detail analysis shows that the anisotropy mainly arises from the anisotropic interface roughness. The results demonstrate that the RDS technique is sensitive to the interface structures.

Flow orientation and super-quadratic dependence of second harmonic intensity on the film thickness in hemicyanine Langmuir–Blodgett multilayers

Abstract Substantial in-plane optical anisotropy in Y-type Langmuir–Blodgett multilayers of a hemicyanine dye interleaved with inert material arachidic acid due to dipping-induced and “substrate”-enhanced molecular orientation was demonstrated by polarized UV-visible absorption and rotation-angle second harmonic generation techniques. We gave the first direct evidence for a super-quadratic enhancement of the second harmonic intensity with increasing film thickness due to the additional in-plane polarization.

Magnetic field controlled in‐plane optical anisotropy in parabolic (Cd,Mn,Mg)Te quantum wells

In-plane optical anisotropy was measured in polarization-resolved reflectivity experiments on a CdTe-based multiple quantum well sample. Parabolic confining potentials were achieved by introduction of controlled Mg mole fraction. Addition of 6 molar % of Mn across the sample structure allowed us to control the light – heavy hole splitting by giant Zeeman effect. We observed an enhancement of the measured anisotropy when the light- and heavy hole bands were brought together by the magnetic field. A model calculation produced a phenomenological description of the observed behavior. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization properties of nonpolar GaN films and (In,Ga)N/GaN multiple quantum wells

We have grown GaN films and (In,Ga)N/GaN multiple quantum wells (MQWs) by plasma-assisted molecular-beam epitaxy on γ-LiAlO2(100) substrates. Due to the crystal symmetry of γ-LiAlO2(100), GaN films and (In,Ga)N/GaN MQWs can be realized in a nonpolar (M-plane) configuration, i.e., the c axis of the wurtzite unit cell lies in the growth plane. For compressively, anisotropically strained M-plane GaN films, the band structure of the valence band changes in such a way that the optical transmittance becomes 100% linearly polarized for two orthogonal in-plane directions, where one of these directions is parallel to the c axis of the GaN film. The photoluminescence properties of M-plane In0.1Ga0.9N/GaN MQWs exhibit a strong in-plane optical anisotropy for linear polarization with an energy-dependent polarization degree of up to 96%, which is presumably also due to a large valence-band splitting induced by the large compressive strain in the QWs.

Multisoliton Behavior of a Ferrielectric Phase in an External Electric Field: Genetic Algorithm Study

A numerical modeling of the three-layer SmC* FI phase behavior under an external electric field is performed. A discrete free energy of the model is analyzed by means of genetic algorithms. We focus our attention on the structure evolution induced by the field and its relation to the field dependence of the in-plane optical anisotropy. We show the multisoliton character of the azimuthal molecular reorientation under the field. It permits us to explain the unusual field-induced behavior of the conoscopic figures in the SmC* FI phase.

Characterization of Self-Assembled CdTe/ZnTe Quantum Dots

We present microluminescence investigations of self-assembled CdTe/ZnTe quantum dots. The dots proprieties resulting from our studies are: values of optical in-plane anisotropy parameters (electron - heavy hole exchange splitting and orientation of anisotropy) and value of effective Lande factor. Parameters giving information about in-plane anisotropy possess random distribution of values with the exchange splitting from 0 to 240 μeV. The effective Lande factor values for our dots are around g* = -3.2 with a scatter of about 18%. Some PL lines exhibit sudden jumps of energetic position, related to variation of the charge state in their neighborhood.