Tunable anticrossing of gated and ungated plasma resonances and enhancement of interlayer terahertz electric field in an asymmetric bilayer of density-modulated two-dimensional electron gases

We study theoretically the terahertz (THz) response of a bilayer of density-modulated two-dimensional electron gases, which we employ to model the actual double-quantum-well electron channel of a grid-gated field-effect transistor in which strong THz photoresponse was recently observed. We have shown that such a system can be driven into the anticrossing regime between gated and ungated plasma resonances by tuning the gate voltage. The amplitude of the interlayer THz electric field in the ungated (double-layered) portions of the channel increases dramatically in the anticrossing regime. This strong interlayer THz electric field may strongly affect interlayer electron tunneling which, in turn, may contribute to the physical mechanism underlying the strong THz photoresponse observed in recent experiments.     

Ultra-strong coupling effects with quantum metamaterials

We study a semiconductor-based quantum metamaterial which has the optical characteristics of a metal in two directions, but behaves like a collection of artificial atoms, whose properties can be designed-in using quantum theory, in the third. We find that it supports a type of guided collective plasma resonance (CPR) mode which exhibits efficient optical coupling and long propagation distances. Furthermore, the coupling of the CPR mode with the ‘artificial atom’ transition leads to a case of “Ultra-Strong-Coupling”, demonstrated by a record vacuum Rabi splitting of 65 meV, a sizable fraction (42%), of the bare intersubband energy.     

Field transformation approach to photonic band structure calculations

A field transformation method is introduced for the calculation of photonic band structures in periodic lattices of dielectrics. The method has the advantage of avoiding the complications due to matching boundary conditions at the interface of the constituents in a composite medium. The formalism is presented for propagation modes in which the electric field is parallel to the interfaces in both one-dimensional and two-dimensional periodic dielectric structures. Numerical calculations using the present formalism involve typically a matrix of size much smaller than that of using standard plane wave expansions.     

One-dimensional surface-plasmon gratings for the excitation of intersubband polaritons in suspended membranes

We present the observation of the strong light-matter coupling regime between intersubband transitions of semiconductor quantum wells and the plasmonic-like resonances of a one dimensional metallic grating. Polariton spectra have been recorded in transmission employing a suspended membrane sample and are consistent with theoretical calculations. This arrangement, avoiding the complexity of dispersive substrate, is particularly attractive for the development of time-resolved pump-probe experiments.     

Resonance structure of dynamic fractional Stark ladders in laser-driven biased superlattices

The resonance structure of an electronic Floquet state in a dynamic fractional Stark ladder (DFSL) is examined based on the scattering theory applied to a dressed potential resulting from renormalization of a laser-electron interaction to an original potential. Here, the DFSL is realized in laser-driven biased superlattices with a fractional matching ratio of a Bloch frequency to a laser frequency. It is revealed that, in contrast to a conventional understanding, the DFSL resonance position and lifetime tend to redshift and shorten, respectively, with an increase in strength of the laser field, and further, these show irregular changes in a limited region of the strength. The underlying physics is discussed in detail.     

Optical properties of InGaN/GaN double quantum wells with varying well thickness

The optical properties and recombination kinetics of the InGaN/GaN double quantum well (DQW) structures with different well thickness (L_w) have been studied by means of photoluminescence (PL), time-resolved PL, and cathodoluminescence (CL) measurements. With increasing quantum well thickness up to 4 nm, the PL emission energy decreases and the blueshift of the PL emission energy increases with increasing excitation density. On the other hand, the PL emission energy of the DQWs with L_w=16 nm is higher than that of the DQWs with L_w=4 nm, and is independent of the excitation density. With increasing L_w from 1 to 4 nm, the PL decay times increase. In contrast, the decay times of 16 nm DQWs are faster than those of 4 nm DQWs. These different results for 16 nm DQWs such as the blueshift of the emission energy, the decrease of the excitation density dependence, and the increase of recombination rate can be ascribed to the relaxation of the piezoelectric field. We also observed the inhomegeneity in the CL spectra of the DQWs with L_w=1 nm on 1 μm scale.     

Optical characterization of InGaAsN/GaAsN/GaAs quantum wells with InGaP cladding layers

Optical properties of InGaAsN/GaAs and InGaAsN/GaAsN/GaAs quantum well structures with InGaP cladding layers were studied by photoreflectance at various temperatures. The excitonic interband transitions of the InGaAsN/GaAsN/GaAs QW systems were observed in the spectral range above hν=E_g(InGaAsN). The confinement potential of the system with strain compensating GaAsN barriers became one step broader, thus more quantum states and larger optical transition rate were observed. A matrix transfer algorithm was used to calculate the subband energies numerically. Band gap energies, effective masses were adopted from the band anti-crossing model with band-offset values adjusted to obtain the subband energies to best fit the observed optical transition features. A spectral feature below and near the GaAs band gap energy from GaAs barriers is enhanced by the GaAs/InGaP interface space charge accumulation induced internal field.     

The effect of a strong magnetic field on the binding energy and the photoionization process in quantum-well wires

Within the effective-mass approximation, we have investigated the influence of a strong magnetic field on the ground state binding energy and the photon energy dependence of the photoionization cross-section of a shallow donor impurity in a quasi-one-dimensional rectangular quantum wire with infinite and finite potential barriers, using a variational approach. It is found that the binding energy and the photoionization cross-section as a function of photon energy were drastically dependent on the sizes of the wire, the potential well heights and the applied magnetic field.     

Optical properties of GaNAs/GaAs triple quantum well structures

Optical properties of the GaNAs/GaAs triple quantum well structures were characterized by using photoreflectance and photoluminescence spectroscopy at different temperatures. The excitonic interband transitions of the triple quantum well systems were observed in the spectral range above hν=E_g(GaN_xAs_1−x). A matrix transfer algorithm was used to match the GaN_xAs_1−x/GaAs boundary conditions and calculate the triple quantum well subband energies numerically for theoretical comparison. The internal electric field in the system was extracted from Franz-Keldysh oscillations in the photoreflectance spectra. The influences of the annealing treatment on the transition energy and the internal electric field are also analyzed.     

Role of effective mass in modulating linear and non-linear response properties of single carrier quantum dots: Interplay with system parameters

We explore the pattern of effective mass dependence of linear and non-linear optical (NLO) response of one electron quantum dots harmonically confined in two dimensions. For different combinations of transverse magnetic field strength (ω_c), harmonic confinement potential (ω₀), and anharmonic interaction, the influence of the effective mass (m^*) of the dot on linear (α), and the first (β), and second (γ) NLO responses of the system is computed through linear variational route. The investigation reveals a complicated interplay between the effective mass and the system parameters.     

Microstructural and optical properties of strain compensated InxGa1−xAs/InyAl1−yAs multiple quantum wells

Transmission electron microscopy (TEM) and photocurrent (PC) measurements were carried out to investigate the microstructural and excitonic transitions in In_0.52Ga_0.48As/In_0.55Al_0.45As multiple quantum wells (MQWs). TEM images showed that high-quality 11-period strain-compensated In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs had high-quality heterointerfaces. Based on the TEM results, a possible crystal structure for the In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs is presented, and their strains are compensated. The results for the PC data at 300 K for several applied electric fields showed that several excitonic transitions shifted to longer wavelengths as the applied electric field increased. These results indicate that the strain-compensated In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs hold promise for electroabsorption modulator devices.     

Extremely large binding energy of biexcitons in an organic-inorganic quantum-well material (C4H9NH3)2PbBr4

We have confirmed biexciton formation in an organic-inorganic hybrid quantum-well material (C₄H₉NH₃)₂PbBr₄ by photoluminescence and two-photon absorption measurements. The biexciton has extremely large binding energy, 60 meV, which to our knowledge is the largest value ever reported for a semiconductor. By analyzing the spectrum of biexciton luminescence, the biexciton gas temperature was found to be much higher than the bath temperature due to a higher local temperature arising from the large biexciton binding energy.     

Anisotropic property in interacting quantum wires embedded in (110) plane

We investigate the theoretically combined effect of spin-orbit interactions and Coulomb interaction on the ground state and transport property of a quantum wire oriented along different crystallographic directions in the (110) plane. We find that the electron’s ground state exhibits phase transition among spin density wave, charge density wave, singlet superconductivity and metamagnetism, which can be controlled by changing the crystallographic orientation, the strengths of the spin-orbit interactions and the Coulomb interaction. The ac conductance exhibits a significant anisotropic behavior and a out-of-plane spin polarization which can be tuned by an in-plane electric field.     

Simple chemical aqueous synthesis of dahlia nanoflower consisting of finger-like ZnO nanorods and observation of stable ultraviolet photoluminescence emission

In this work, we have reported the synthesis of dahlia flower-like ZnO nanostructures consisting of human finger-like nanorods by the hydrothermal method at 120 °C and without using any capping agent. Optical properties of the samples, including UV–vis absorption and photoluminescence (PL) emission characteristics are determined by dispersing the samples in water as well as in ethanol media. The quenching of PL emission intensity along-with the red shifting of the PL emission peak are observed when the samples are dispersed in water in comparison to those obtained after dispersing the samples in ethanol. It has been found that PL emission characteristic, particularly the spectral nature of PL emission, of the samples remains almost unaltered (except some improvement in UV PL emission) even after thermally annealing it for 2 h at the temperature of 300 °C. Also the synthesized powder samples, kept in a plastic container, showed a very stable PL emission even after 15 months of synthesis. Therefore, the synthesized samples might be useful for their applications in future optoelectronics devices.     

Photoluminescence of spark-processed metals

The photoluminescence (PL) properties of spark-processed (sp) metals were systematically investigated. It was found that spark-processing of Cr, Cu, Fe, Mg, and Mn essentially yielded no PL at room temperature in the visible spectral range. In contrast, sp-Ti, Ta, Al, Ni, Mo, and Zn showed strong PL, which is in many respects comparable to sp-Si. It was further found that the PL intensity of the sp metals depends exponentially on the nitrogen concentration, which is contained in the spark-processing ambient (along with oxygen). This behavior was also found for sp-Si. In contrast, the PL intensities for native oxides (containing essentially no nitrogen) is several orders of magnitude smaller than those for sp-metals and the respective PL peaks are situated at higher energies. The results are discussed in the light of the self-trapped exciton model.     

Electron Raman scattering in cylindrical quantum wires

Electron Raman scattering (ERS) is investigated in a free-standing semiconductor quantum wire of cylindrical geometry for two classes of materials CdS and GaAs. The differential cross section (DCS) involved in this process is calculated as a function of a scattering frequency and the radius of the cylinder. Electron states are considered to be confined within a free-standing quantum wire (FSW). Single parabolic conduction and valence bands are assumed. The selection rules are studied. Singularities in the spectra are found and interpreted for various radii of the cylinder.     

Electrospun nanofibers of poly(ethylene oxide)/teraamino-phthalocyanine copper(II) hybrids and its photoluminescence properties

Poly(ethylene oxide)/teraamino-phthalocyanine copper (II) (PEO/(NH₂)₄PcCu) hybrid nanofibers with a diameter of 200-300 nm were prepared by electrospinning technique. The hybrid nanofibers membrane was characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), ultraviolet-visible (UV-vis), and photoluminescence (PL), respectively. The results indicated that (NH₂)₄PcCu molecule was successfully embedded in the one-dimensional hybrid nanofibers via chemical interaction between PEO and (NH₂)₄PcCu. The PL results showed that the PEO/(NH₂)₄PcCu hybrid nanofibers had an intense emission at about 450 nm. A possible PL mechanism was proposed accordingly.     

Third-order nonlinear optical properties of Bi2S3 and Sb2S3 nanorods studied by the Z-scan technique

Bismuth sulfide (Bi₂S₃) and antimony sulfide (Sb₂S₃) nanorods were synthesized by hydrothermal method. The products were characterized by UV-vis spectrophotometer, X-ray powder diffraction (XRD) and transmission electron microscope (TEM). Bi₂S₃ and Sb₂S₃ nanorods were measured by Z-scan technique to investigate the third-order nonlinear optical (NLO) properties. The result of NLO measurements shows that the Bi₂S₃ and Sb₂S₃ nanorods have the behaviors of the third-order NLO properties of both NLO absorption and NLO refraction with self-focusing effects. The third-order NLO coefficient χ⁽³⁾ of the Bi₂S₃ and Sb₂S₃ nanorods are 6.25×10⁻¹¹ esu and 4.55×10⁻¹¹ esu, respectively. The Sb₂S₃ and Bi₂S₃ nanorods with large third-order NLO coefficient are promising materials for applications in optical devices.     

Exploring electro-optic effect of impurity doped quantum dots in presence of Gaussian white noise

We explore the profiles of electro-optic effect (EOE) of impurity doped quantum dots (QDs) in presence and absence of noise. We have invoked Gaussian white noise in the present study. The quantum dot is doped with Gaussian impurity. Noise has been administered to the system additively and multiplicatively. A perpendicular magnetic field acts as a confinement source and a static external electric field has been applied. The EOE profiles have been followed as a function of incident photon energy when several important parameters such as electric field strength, magnetic field strength, confinement energy, dopant location, relaxation time, Al concentration, dopant potential, and noise strength possess different values. In addition, the role of mode of application of noise (additive/multiplicative) on the EOE profiles has also been scrutinized. The EOE profiles are found to be adorned with interesting observations such as shift of peak position and maximization/minimization of peak intensity. However, the presence of noise and also the pathway of its application bring about rich variety in the features of EOE profiles through some noticeable manifestations. The observations indicate possibilities of harnessing the EOE susceptibility of doped QD systems in presence of noise.     

Tuning third harmonic generation of impurity doped quantum dots in the presence of Gaussian white noise

We perform a broad exploration of profiles of third harmonic generation (THG) susceptibility of impurity doped quantum dots (QDs) in the presence and absence of noise. We have invoked Gaussian white noise in the present study. A Gaussian impurity has been introduced into the QD. Noise has been applied to the system additively and multiplicatively. A perpendicular magnetic field emerges out as a confinement source and a static external electric field has been applied. The THG profiles have been pursued as a function of incident photon energy when several important parameters such as electric field strength, magnetic field strength, confinement energy, dopant location, Al concentration, dopant potential, relaxation time and noise strength assume different values. Moreover, the role of the pathway through which noise is applied (additive/multiplicative) on the THG profiles has also been deciphered. The THG profiles are found to be decorated with interesting observations such as shift of THG peak position and maximization/minimization of THG peak intensity. Presence of noise alters the characteristics of THG profiles and sometimes enhances the THG peak intensity. Furthermore, the mode of application of noise (additive/multiplicative) also regulates the THG profiles in a few occasions in contrasting manners. The observations highlight the possible scope of tuning the THG coefficient of doped QD systems in the presence of noise and bears tremendous technological importance.