Carrier-diffusion-limited remote optical addressing of single quantum dots

We present a scheme for remotely addressing single quantum dots (QDs) by means of near-field optical microscopy that simply makes use of the polarization of light. A structure containing self-assembled CdTe QDs is covered with a thin metal film presenting sub-wavelength holes. When the optical tip is positioned some distance away from a hole, surface plasmons in the metal coating are generated which, by turning the polarization plane of the excitation light, transfer the excitation towards a chosen hole and induce emission from the underlying dots. In addition, our procedure gives valuable insight into the diffusion of photo-excited carriers in the QD plane that can put limits to the addressing scheme.     

Direct observation of variations of optical properties in single quantum dots by using time-resolved near-field scanning optical microscope

We study the variations of optical properties of self-assembled In_0.5Ga_0.5As single quantum dots (QDs) in the spatial and time domains by combining a near-field scanning optical microscope with an ultrafast pulsed laser. Through the examinations of several tens of QDs, we find that the variations of photoluminescence (PL) intensity strongly depend on the condition of the initial carrier creation. The differences in quantum efficiency and those in the carrier flow rate into QDs cause the large distribution of PL intensity when the carriers are excited in the barrier layers. From the results of time-resolved PL decay measurements, we find that there are two types of QDs exhibiting quite different PL decay profiles.     

Pressure effect on optical properties of modified Gaussian quantum dots

In the present work, we intend to study the pressure effect on optical properties of spherical quantum dots by using the modified Gaussian potential. In this regard, the linear, nonlinear and total intersubband absorption coefficients and refractive index changes are investigated for different hydrostatic pressures. According to the results obtained from the present work, it is deduced that: (i) the linear, nonlinear and total refractive index changes decrease and shift towards higher energies when the pressure increases and (ii) the linear, nonlinear and total absorption coefficients increase and shift towards higher energies by increasing the pressure.     

Light emitting CdS quantum dots in PMMA: Synthesis and optical studies

Freshly prepared CdS-quantum dots (QDs) in DMF (clear pale solution) when loaded in polymethylmethacrylate (PMMA) lead to excellent optical properties. The tuning of the absorption and emission wavelengths via experimentally control parameters is considered novel and significant. The absorption band for CdS was observed at about 370 nm in polymeric matrix. The blue, green and orange light emissions from such composite solution were tuned and stabilized by simply varying the concentration of CdS, cadmium and sulphur in the final product. Photoluminescence (PL) measurement with 2% CdS loading showed band-edge emissions from the composite with only about 20-25 nm Stokes shift in emission wavelength. Observation of such optical properties indicated that the composite has narrow particle size distribution and particle diameter may well be below 10 nm. X-ray diffraction (XRD) patterns of the film with higher loading of CdS showed broad pattern for hexagonal CdS. Thermo-gravimetric analysis (TGA) of CdS/PMMA composite film revealed that it has better thermal stability than PMMA alone. Transmission electron microscopy (TEM) showed agglomerated tiny dots in nano-meter regime.     

Optical characterization of quantum dots entrained in microstructured optical fibers

We present a study of the basic optical properties of colloidal CdSe/ZnS core/shell quantum dots entrained in a microstructured optical fiber. Quantum dots suspended in heptane were pulled into the holes surrounding the solid core of a microstructured optical fiber of the holey fiber class via capillary action and are found to remain in the fiber. In this experiment, a laser coupled into the fiber photoexcited quantum dots along the length of the fiber. Quantum dot emission was observed to couple into the fiber core and propagate along the fiber. To investigate the use of such a system in fiber-based light generation or amplification, a second laser overlapping the low-energy portion of the quantum dot emission was simultaneously coupled into the fiber. We observed apparent amplification of this light when photoexciting the quantum dots well above their bandedge.     

Two-dimensional probe absorption in coupled quantum dots

We investigate the two-dimensional (2D) probe absorption in coupled quantum dots. It is found that, due to the position-dependent quantum interference effect, the 2D optical absorption spectrum can be easily controlled via adjusting the system parameters. Thus, our scheme may provide some technological applications in solid-state quantum communication.     

Pressure and temperature effects on the third-order nonlinear optical properties in GaAs quantum dots

This work is used in the density matrix formalism and the effective mass approximation to study the third harmonic generation coefficient in a GaAs disc-shaped quantum dot with parabolic confinement potential. It is discussed the strong and weak confinement regime. The results show that the third harmonic generation coefficient is strongly dependent on the excitonic pair localization. The study is extended to consider effects such as hydrostatic pressure and temperature to show that it is possible to induce a blue-shift and/or red-shift on the resonant peaks of the third harmonic generation coefficient.     

Novel effects produced on a two dimensional electron gas by introducing InAs dots in the plane of the quantum well

We show that the presence of InAs dots embedded in a host GaAs quantum well containing a two-dimensional electron gas dramatically modifies the cyclotron resonance (CR). Far-infrared CR measurements show two modes with different dispersions with applied magnetic field B. The lower-frequency mode, with a sub-linear dependence on B, is identified as a CR at low B, developing into a skipping orbit around the dot perimeters at higher B. This has not been previously observed for a system with randomly distributed scatterers. The higher-frequency mode is identified as a magnetoplasmon localised by the confining effect of the arrays of repulsive potentials due to the dots in the well.     

Synthesis of stable dispersion of ZnO quantum dots in aqueous medium showing visible emission from bluish green to yellow

Aqueous dispersion of 4-8 nm size stable ZnO quantum dots (QDs) exhibiting luminescence in the visible region have been synthesized by a simple solution growth technique at room temperature. Silica has been used as capping agent to control the particle size as well as to achieve uniform dispersion of QDs in aqueous medium. X-ray diffractometer (XRD) analysis reveals formation phase pure ZnO particles having wurzite (hexagonal) structure. Atomic force microscope (AFM) images show that the particles are spherical in shape, having average crystalline sizes ∼4, 5.5 and 8 nm for samples prepared at pH values of 10, 12 and 14, respectively. From the optical absorption studies, the band gap energy of QDs is found to be blue shifted as compared to bulk ZnO (3.36 eV) due to the quantum confinement effect and is consistent with the band gap calculated by using effective-mass approximation model. The photoluminescence (PL) observed in these QDs has been attributed to the presence of defect centers.     

Giant optical anisotropy in single InAs quantum dots

We present the experimental evidence of giant optical anisotropy in single InAs QDs. Polarization-resolved photoluminescence spectroscopy in single QDs reveals a linear polarization ratio which fluctuates, from one dot to another, in sign and in magnitude with absolute values up to 82%. We do not observe any dependence of the linear polarization on incident power and temperature.     

Electronic states of self-organized InGaAs quantum dots on GaAs (3 1 1)B studied by conductive scanning probe microscope

We have used conductive scanning probe microscope (SPM) in high vacuum and operated at 173 K in order to investigate the electronic properties of self-organized InGaAs quantum dots (QDs) grown on GaAs (3 1 1)B and (0 0 1) substrates. Ordered InGaAs quantum dot arrays on GaAs (3 1 1)B surface were fabricated by atomic-H assisted molecular beam epitaxy (H-MBE), and Si SPM tips coated with Au which warrants electrical conductivity were used to measure simultaneously both the topographic and current images of QDs surface. From the current–voltage (I–V) curves, unique and different plateau features were observed for QDs formed on GaAs (3 1 1)B and (0 0 1) substrates. The results suggested that a high degree of symmetry of InGaAs QDs on (3 1 1)B was responsible for the observed degeneracy of electronic states and artificial atom-like states. We demonstrate that this conductive SPM technique becomes a powerful tool in studies of single electron charging of individual dots.     

THz near-field optical imaging by a local source

A new THz imaging system based on a local illumination by a near-field source is presented. Model object is imaged, and sub-wavelength spatial resolution is demonstrated for the first time to our knowledge with this configuration. The contrast and the resolution are confirmed by a theoretical study based on the diffraction in near-field zone.     

Optically detected resonance spectroscopy of interface fluctuation quantum dots

We have performed optically detected resonance (ODR) spectroscopy on modulation-doped GaAs/AlGaAs quantum wells of different widths in which lateral fluctuations of the well width were purposely introduced by growth interruption at the interfaces. These monolayer fluctuations form quantum dots for which confinement and Coulomb correlation energies are comparable. By monitoring resonant changes of the dot ensemble photoluminescence induced by far-infrared (FIR) radiation in a magnetic field, we have observed cyclotron resonance (CR) of free electrons in the widest wells, as well as internal transitions of mobile and localized charged excitons. The latter, which are forbidden by magnetic translational invariance, have previously not been observed. For the narrower wells the effects of non-parabolicity and carrier localization on the CR and CR-like transitions have to be included for a proper interpretation of the measurements.     

CdSe quantum dots decorated by mercaptosuccinic acid as fluorescence probe for Cu

Water-soluble CdSe quantum dots (QDs) were synthesized using mercaptosuccinic acid (MSA) as a stabilizer. The growth process and characterization of CdSe quantum dots were determined by transmission electron microscopy (TEM), X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, Ultraviolet-visible (UV-vis) spectroscopy, and Fourier transform infrared (FT-IR) spectroscopy. Results demonstrated the MSA-capped CdSe QDs were highly crystalline and possessed good optical properties. Further, the resulting products could be used as fluorescent probes to detect Cu²⁺ ions in physiological buffer solution. The response was linearly proportional to the concentration of Cu²⁺ ion in the range 2×10⁻⁸- 3.5×10⁻⁷ mol L⁻¹ with a detection limit of 3.4 nmol L⁻¹.     

Resonant Raman scattering of optical phonons in self-assembled quantum dots

We have investigated the carrier relaxation mechanism in InGaAs/GaAs quantum dots by photoluminescence excitation (PLE) spectroscopy. Near-field scanning optical microscope successfully shows that a PLE resonance at a relaxation energy of 36 meV can be seen in all single-dot luminescence spectra, and thus can be attributed to resonant Raman scattering by a GaAs LO phonon to the excitonic ground state. In addition, a number of sharp resonances observed in single-dot PLE spectra can be identified as resonant Raman features due to localized phonons, which are observed in the conventional Raman spectrum. The results reveal the mechanism for the efficient relaxation of carriers observed in self-assembled quantum dots: the carriers can relax within the continuum states, and make transitions to the excitonic ground state by phonon emission.     

A new confinement potential in spherical quantum dots: Modified Gaussian potential

A new confinement potential for spherical quantum dots, called the modified Gaussian potential (MGP), is studied. In the present work, the following problems are investigated within the effective-mass approximation: (i) the one-electron energy spectra, (ii) wave functions, (iii) the problem of existence of a bound electron state, and (iv) the binding energy of center and off-center hydrogenic donor impurities. For zero angular momentum (l=0)$(l=0)$, the new confinement potential is sufficiently flexible to obtain analytically the spectral energy and wave functions. The results obtained from the present work show that (i) the new potential is suitable for predicting the spectral energy and wave functions, and (ii) the geometrical sizes of the quantum dot play the important roles on the energy levels, wave functions, the binding energy, and the existence of a bound electron state.     

Confined states in two-dimensional flat elliptic quantum dots and elliptic quantum wires

The energy spectrum and corresponding wave functions of a flat quantum dot with elliptic symmetry are obtained exactly. A detailed study is made of the effect of ellipticity on the energy levels and the corresponding wave functions. The analytical behavior of the energy levels in certain limiting cases is obtained.     

Pump–probe analysis of polaron decay in InAs/GaAs self-assembled quantum dots

We report on polaron decay in InAs/GaAs self-assembled quantum dots. The polarons are probed by pump–probe spectroscopy through their optical intersublevel absorption around 62 meV (20 μm wavelength). A T₁ polaron lifetime of the order of tens of picosecond is deduced from the low-temperature pump–probe measurements. We show that a long-lived component can be additionally observed on the pump–probe measurements. The spectral dependence of this long-lived component is, however, not correlated to the polaron absorption. It is thus not a signature of polaron relaxation quenching. The origin of this long-lived component is attributed to the two-phonon absorption of the bulk GaAs substrate.