Exciton dephasing and absorption line shape in semiconductor quantum dots

The homogeneous broadening of exciton absorption spectral lines in semiconductor quantum dots (QDs) in the strong confinement regime is studied theoretically. It is shown that the term linear in nuclear displacements in the difference of the phonon Hamiltonians of the ground and optically excited states does not lead to the zero-phonon line (ZPL) broadening. The ZPL width is contributed by the term quadratic in nuclear displacements associated with short-living optical phonons. This contribution is estimated for CdSe nanocrystals (NCs) and found to be much less than the linewidth observed in recent experiments. We conclude that the experimentally observed linewidth is due to the longitudinal lifetime associated with the exciton relaxation to the dark state. The shape of spectral wings originating from the exciton interaction with long-living acoustic phonons is studied at various temperatures for a CdSe NC embedded in a glass matrix.     

The electronic structures for the optical absorption of Hf-O-N thin films

The local structures of Hf-O-N thin films were analyzed using an extended X-ray absorption fine structure (EXAFS) study on Hf L _III-edge and first-principles calculations. Depending on their composition and atomic configurations, Hf₄O₈ (CN: 7.0), Hf₄O₅N₂ (CN: 6.25) and Hf₄O₂N₄(CN: 5.5) were suggested as the local structures of Hf-O-N thin films. The optical band gaps of Hf-O-N thin films were compared with the calculated band gap. And to investigate the optical absorption, the effects of film compositions on the valence bands of Hf-O-N thin films were analyzed by comparing the experimental valence band with the valence band.     

Exploring the optical properties of exposed-core-based photonic-crystal fibers

Journal of Computational Electronics - A photonic-crystal fiber (PCF) based on the surface plasmon resonance (SPR) effect with low confinement loss and high sensitivity response is designed and its...     

Optical properties of quantum dots versus quantum antidots: Effects of hydrostatic pressure and temperature

The effects of hydrostatic pressure and temperature on the linear, nonlinear and total absorption coefficients (ACs) of a hydrogenic impurity in the center of spherical quantum dot (QD) and quantum antidot (QAD) have been investigated. The comparative approach is used for presenting the results of both models. Our numerical results indicate that for QD nano-systems, by increasing the pressure, the resonance peak positions (RPPs) of ACs shift towards higher energies, while for QAD nano-systems, RPPs of ACs approximately remain unchanged. Furthermore, the larger pressure leads to the smaller height of resonance peak in both models. Also, our results show that the temperature increasing imposes the opposite effect on RPPs than the pressure increasing to the both models.     

Chiral carbon dots:synthesis,optical properties,and emerging applications

Carbon dots are luminescent carbonaceous nanoparticles that can be endowed with chiral properties,making them particularly interesting for biomedical applicatio...     

Effects of spontaneous and piezoelectric polarization fields on the electronic and optical properties in GaN/AlN quantum dots: multimillion‐atom sp3d5s* tight‐binding simulations

GaN quantum dots (QDs) grown on AlN substrates are strong candidates for UV and near‐infrared applications. The wurtzite crystal symmetry in these dots induces internal fields arising from the following: (i) crystal atomicity;(ii) strained active region; (iii) piezoelectricity; and (iv) spontaneous polarization (pyroelectricity). Accurate modeling of electronic and optical properties of these QDs must capture the interplay of these atomistic and long‐range fields and the size quantization on an equal footing. In this work, single‐particle electronic structure and interband optical transition rates of a GaN/AlN QD grown along the c‐axis are studied using a coupled molecular mechanics‐atomistic 20‐band sp³d⁵s^* tight‐binding (VFF‐TB) framework. To calculate piezoelectricity, a recently reported model that takes into account both the linear and the nonlinear dependence of polarization on the strain tensors has been employed. The simulated GaN/AlN dot is realistically sized (containing ~3 million atoms) and of hexagonal disk shape having height and base length of 4.1 and 17 nm, respectively. It is found that, in contrast to the well‐studied InN/GaN systems, the pyroelectric potential in GaN/AlN dot is larger than the piezoelectric counterpart, and the effects of piezoelectric and pyroelectric fields add up. The internal fields result in a large redshift in the electronic states near the Brillouin zone center (known as quantum confined Stark effect), pronounced non‐degeneracy in the excited states, strongly suppressed optical transition, and anisotropic emission spectra. Copyright © 2014 John Wiley & Sons, Ltd.     

Progress in GaN-based quantum dots for optoelectronics applications

Our recent progress in GaN-based quantum dots (QDs) for optoelectronics application is discussed. First, we discussed an impact of the use of GaN-based QDs on semiconductor lasers, showing theoretically that reduction of threshold current by using the QDs in GaN-based lasers is much more effective compared to those in GaAs-based or InP-based lasers. Then discussed are our growth technology including self-assembling growth of InGaN QDs on sapphire substrates by atmospheric-pressure metalorganic chemical vapor deposition. Using the self-assembling growth technique, we have succeeded in obtaining lasing action in an edge-emitting laser structure with the InGaN QDs embedded in the active layer under optical excitation with the emission wavelength of 410 nm. Toward UV light wavelength emission, we have recently established self-assembled GaN QDs of high quality and high density under very low V-III ratio. We clearly observed two photoluminescence peaks from both the QDs and the wetting layer at room temperature, which clearly shows the nanostructures are formed with the Stranski-Krastanow growth mode.     

Classical and quantum mechanical transport simulations in open quantum dots

Simulations of open dot arrays are performed where the electron motion is considered in a harmonic potential and in a static magnetic field. Results concern electron trajectories and Poincaré sections from the classical calculation. The analysis of the Poincaré sections reveals a strong influence of the length of the constriction on the chaotic regions in phase space. Density probabilities from the quantum mechanical calculation show a significant correspondence to classically calculated trajectories.     

Boundary effects on the optical properties of InGaN multiple quantum wells

We examine the issues of spontaneous and piezoelectric polarization discontinuity on the optical properties of 3.0-nm-thick indium gallium nitride (InGaN) multiple quantum wells (MQWs). A quench of band-edge emission from the cap GaN layer is observed when the photoexcitation source is changed from a 355- to a 248-nm laser. The interband transitions from the InGaN wells exhibit a linear dependence on the 1) spectral blue shift of /spl sim/8.5/spl times/10/sup -18/ meV /spl middot/ cm/sup 3/ and 2) change of the internal field of /spl sim/3/spl times/10/sup -14/ meV /spl middot/ cm/sup 2/ with the injected carrier density up to N/sub inj//spl sim/10/sup 19/ cm/sup -3/ at 77 K. These observations are attributed to the redistribution of photogenerated carriers in the InGaN wells due to the polarization discontinuity at the QW interface and the surface band bending effect. By incorporating an additional boundary condition of surface Fermi-level pinning into the Poisson equation and the band-structure analysis, it is shown the emission from the InGaN-GaN MQWs is dominant by the recombination between the high-lying subbands and the screening of internal field effects.     

Effects of substrate orientation on opto-electronic properties in self-assembled InAs/GaAs quantum dots

Electronic structure and optical transition characteristics in (100), (110), and (111) oriented InAs/GaAs quantum dots (containing \({\sim }2\) million atoms) were studied using a combination of valence force-field molecular mechanics and 20-band \(sp^{3}d^{5}s^{*}\) atomistic tight-binding framework. These quantum dots are promising candidates for non-traditional applications such as spintronics, quantum cryptography and quantum computation, but suffer from the deleterious effects of various internal fields. Here, the dependence of strain and polarization fields on the substrate orientation is reported and discussed. It is found that, compared to the (100) and (110) oriented counterparts, quantum dots grown on the (111) oriented substrate exhibit a smaller splitting (non-degeneracy) in the excited \(P\) states and enhanced isotropy in the interband optical emission characteristics.     

The effect of carrier-induced change on the optical properties ofAlGaAs-GaAs intermixed quantum wells

The carrier-induced effects in the change of absorption and refractive index on the AlGaAs-GaAs intermixing modified quantum wells (QW's) have been investigated theoretically. Band-filling, bandgap shrinkage, and free-carrier absorption have been included for various carrier concentrations. The Schrodinger and the Poisson equations have been considered self-consistently. The polarized absorption coefficients are calculated using the Kane k·p method for a four band model and followed by the Kramers-Kranig transformation to obtain the refractive index change. The results obtained show a more enhanced bandgap renormalization and change of absorption, but a reduced change in refractive index for the larger intermixing extents. It is important to know the carrier-induced optical parameter changes the intermixed QW's because of their recent interests in photonics     

Immersion lens microscopy of photonic nanostructures and quantum dots

We describe recent experimental and theoretical advances in immersion lens microscopy for, both surface and subsurface imaging as applied to photonic nanostructures. We examine in detail the ability of sharp metal tips to enhance local optical fields for nanometer resolution microscopy and spectroscopy. Finally, we describe a new approach to nano-optics, that of combining solid immersion microscopy with tip-enhanced focusing and show how such an approach may lead to 20-nm resolution with unity throughput.     

Tailoring the properties of quantum dotmicropillars by ultrafast optical injection of free charge carriers

We review recent studies of cavity switching induced by the optical injection of free carriers in micropillar cavities containing quantum dots. Using the quantu...     

First principles investigations of geometric, electronic and optical properties of 5-aminotetrazole derivatives

Density functional theory (DFT) is an important computational technique to study and predict the properties of isolated molecules. It is now a leading method for electronic structure calculations in chemistry and solid state physics. In this paper, we have investigated the geometric, electronic and optical properties of six 5-aminotetrazole derivatives employing DFT. The ground state geometries were optimized at B3LYP/6-311G^?? and B3LYP/6-31G^?? level of theories. The density of states, HOMOs and LUMOs and absorption spectra of all the compounds under study have been computed and discussed. The HOMOs are delocalized on aminotetrazole moiety in all the compounds. A comprehensible intra charge transfer has been observed from aminotetrazole moiety to entire compounds. In the absorption spectra, the wavelength of maximum absorption for triplets in all the systems is red shifted relative to their corresponding singlet wavelengths of absorption maximum. The B3LYP/6-311G^?? level of theory is found to give better results than B3LYP/6-31G^?? level of theory, to reproduce previously reported experimental data. In most of the cases B3LYP/6-31G^?? level of theory overestimate more the bond lengths.     

DFT study on structural,electronic, and optical properties of cubic and monoclinic CuO

First-principles calculations were made to explore the structural, electronic, and optical properties of copper oxide (CuO) with monoclinic (m-CuO) and cubic (c-CuO) structures. We calculated the equilibrium structural parameters: lattice parameters (a, b, and c), angle \(\beta \), and volume V. The obtained results were in good agreement with the experimental data reported in the literature. The cohesive energy showed that m-CuO is more stable than c-CuO. The band structure indicated that c-CuO is an indirect band gap semiconductor with a band gap of 0.87 eV along R–G, while m-CuO has a metallic behavior. Furthermore, electrovalent and covalent bonds were observed in both c-CuO and m-CuO. The linear optical properties were calculated and analyzed along different polarization directions of the incident light. The results indicated that m-CuO possesses optical anisotropic properties. In particular, c-CuO can be used as a potential UV detector material because of its high absorption coefficient (356351.3).     

Metalorganic chemical vapor deposition of InN quantum dots and nanostructures

Using one material system from the near infrared into the ultraviolet is an attractive goal, and may be achieved with (In,Al,Ga)N. This Ⅲ-N material system, fam...     

Growth of InGaN self-assembled quantum dots and their applicationto lasers

We have successfully grown InGaN self assembled quantum dots (QD's) on a GaN layer, using atmospheric-pressure metalorganic chemical vapor deposition (MOCVD). The average diameter of the QD's was as small as 8.4 nm, and strong emission from the QD's was observed at room temperature. Next, we have investigated a structure in which InGaN QD's were stacked to increase the total QD density. InGaN QD's were formed even when the number of stacked layers was ten. As the number of layers increased, the photoluminescence (PL) intensity increased drastically. Moreover, we have fabricated a laser structure with InGaN QD's embedded into the active layer. A clear threshold of 6.0 mJ/cm² was observed in the dependence of the emission intensity on the excitation energy at room temperature under optical excitation. Above the threshold, the emission was strongly polarized in the transverse electric (TE) mode, and the linewidth of the emission spectra was reduced to below 0.1 nm (resolution limit). The peak wavelength was around 405 nm. These results indicate lasing action at room temperature