Dan Dalacu

Spectroellipsometric characterization of plasma-deposited Au/SiO2 nanocomposite films

Nanocomposite films consisting of metal nanoparticles embedded in a dielectric matrix were fabricated by simultaneous sputtering of a gold target and plasma-enhanced chemical vapor deposition of hydrogenated SiO2. The optical constants of the films were determined from spectroscopic ellipsometry measurements and were modeled using the Maxwell–Garnett effective medium theory. The particle size dependence of the free electron absorption was included according to the limited electron mean free path effect using a broadening parameter A=0.16 determined from the comparison of the measured spectra with transmission electron microscopy micrographs. Using bulk interband optical constants for gold, very good agreement was obtained between the model and the measured spectra but only in the narrow particle size range ∼10–20 nm, the latter of which marks the onset of phase retardation effects. For smaller particles, the energy of the surface plasmon resonance was progressively blueshifted with respect to the predicte...

Observation of strongly entangled photon pairs from a nanowire quantum dot

A bright photon source that combines high-fidelity entanglement, on-demand generation, high extraction efficiency, directional and coherent emission, as well as position control at the nanoscale is required for implementing ambitious schemes in quantum information processing, such as that of a quantum repeater. Still, all of these properties have not yet been achieved in a single device. Semiconductor quantum dots embedded in nanowire waveguides potentially satisfy all of these requirements; however, although theoretically predicted, entanglement has not yet been demonstrated for a nanowire quantum dot. Here, we demonstrate a bright and coherent source of strongly entangled photon pairs from a position-controlled nanowire quantum dot with a fidelity as high as 0.859±0.006 and concurrence of 0.80±0.02. The two-photon quantum state is modified via the nanowire shape. Our new nanoscale entangled photon source can be integrated at desired positions in a quantum photonic circuit, single-electron devices and light-emitting diodes.

Eliminating the birefringence in silicon-on-insulator ridge waveguides by use of cladding stress

We propose and demonstrate the use of the cladding stress-induced photoelastic effect to eliminate modal birefringence in silicon-on-insulator (SOI) ridge waveguides. Birefringence-free operation was achieved for waveguides with otherwise large birefringence by use of properly chosen thickness and stress of the upper cladding layer. With the stress levels typically found in cladding materials such as SiO2, the birefringence modification range can be as large as 10(-3). In arrayed waveguide grating demultiplexers that were fabricated in a SOI platform, we demonstrated the reduction of the birefringence from 1.2 x 10(-3) (without the upper cladding) to 4.5 x 10(-5) when a 0.8-microm oxide upper cladding with a stress of -320 MPa (compressive) was used. Because the index changes induced by the stress are orders of magnitude smaller than the waveguide core-cladding index contrast, the associated mode mismatch loss is negligible.

Resonant scattering and second-harmonic spectroscopy of planar photonic crystal microcavities

The resonant modes of two-dimensional planar photonic crystal microcavities patterned in a free-standing InP slab are probed in a novel fashion using a long working distance microscope objective to obtain cross-polarized resonant scattering and second-harmonic spectra. We show that these techniques can be used to do rapid effective assays of large arrays of microcavities that do not necessarily contain resonant light-emitting layers. The techniques are demonstrated using microcavities comprised of single missing-hole defects in hexagonal photonic crystal hosts formed with elliptically shaped holes. These cavities typically support two orthogonally polarized resonant modes, and the resonant scattering and harmonic spectra are well fitted using a coherent sum of Lorentzian functions. The well-defined coherence between the two resonant features is explained in terms of a microscopic harmonic oscillator model. The relative merits of these techniques are quantitatively compared with the more commonly used cavi...

Ultraclean Emission from InAsP Quantum Dots in Defect-Free Wurtzite InP Nanowires

We report on the ultraclean emission from single quantum dots embedded in pure wurtzite nanowires. Using a two-step growth process combining selective-area and vapor-liquid-solid epitaxy, we grow defect-free wurtzite InP nanowires with embedded InAsP quantum dots, which are clad to diameters sufficient for waveguiding at λ ~ 950 nm. The absence of nearby traps, at both the nanowire surface and along its length in the vicinity of the quantum dot, manifests in excitonic transitions of high spectral purity. Narrow emission line widths (30 μeV) and very-pure single photon emission with a probability of multiphoton emission below 1% are achieved, both of which were not possible in previous work where stacking fault densities were significantly higher.

Selective-area vapour–liquid–solid growth of InP nanowires

A comparison is made between the conventional non-selective vapour-liquid-solid growth of InP nanowires and a novel selective-area growth process where the Au-seeded InP nanowires grow exclusively in the openings of a SiO(2) mask on an InP substrate. This new process allows the precise positioning and diameter control of the nanowires required for future advanced device fabrication. The growth temperature range is found to be extended for the selective-area growth technique due to removal of the competition between material incorporation at the Au/nanowire interface and the substrate. A model describing the growth mechanism is presented which successfully accounts for the nanoparticle size-dependent and time-dependent growth rate. The dominant indium collection process is found to be the scattering of the group III source material from the SiO(2) mask and subsequent capture by the nanowire, a process that had previously been ignored for selective-area growth by chemical beam epitaxy.

Nanowire waveguides launching single photons in a Gaussian mode for ideal fiber coupling

Quantum communication as well as integrated photonic circuits require single photons propagating in a well-defined Gaussian mode. However, tailoring the emission mode to a Gaussian remains an unsolved challenge for solid-state quantum emitters due to their random positioning in the host material or photonic structure. Here, we overcome these limitations by embedding a semiconductor quantum dot in a tapered nanowire waveguide. Owing to the deterministic positioning of the emitter in the waveguide, we demonstrate a Gaussian emission profile in the far field. Hence, we further couple the emission into a single-mode optical fiber with a record efficiency of 93%, thereby addressing a major hurdle for practical implementation of single photon sources in emerging photonic technologies.

Nanowire coupling to photonic crystal nanocavities for single photon sources

We demonstrate highly efficient evanescent coupling via a silica loop-nanowire, to ultra-small quantum-dot photonic-crystal cavities. It enables the tuning of both the Q-factor and the wavelength of the cavity mode independently.

External cavity InAs∕InP quantum dot laser with a tuning range of 166nm

We have studied the tuning behavior of an external cavity laser in Littrow configuration using antireflection/high-reflection coated InAs∕InGaAsP∕InP quantum dot laser diodes as the amplifying element. Adding the coatings improves the performance of the setup, and the tunability of the external cavity laser output has been increased up to 166nm. Detailed investigations have revealed that laser diode length and width influence the magnitude of the tuning range. Furthermore, the external differential quantum efficiency is systematically increasing as the external cavity laser wavelength is decreasing. These characteristics are discussed in terms of energy levels available in the inhomogeneous broadening of the self-assembled quantum dots.

Temperature dependence of the surface plasmon resonance of Au/SiO2 nanocomposite films

The optical response of Au/SiO2 nanocomposite films at temperatures spanning the transition to the liquid state has been measured. The change in the optical constants of gold from those corresponding to the crystalline material to those of the liquid occurs gradually and at temperatures below the bulk melting point. The similarity of the temperature-induced changes in the Au interband absorption edge to that observed with decreasing particle size suggests that a gradual transition to the liquid phase also occurs in small particles.