Lasing in Zn-doped GaAs nanowires on an iron film

In this work, we demonstrate optically pumped lasing in highly Zn-doped GaAs nanowires (NWs) lying on an iron film. The conically shaped NWs are first covered with an 8 nm thick Al2O3film to prevent atmospheric oxidation and mitigate band-bending effects. Multimode and single-mode lasing have been observed for NWs with a length greater or smaller than 2μm, respectively. Finite difference time domain calculations reveal a weak electric field enhancement in the Al2O3layer at the NW/iron film interface for the lasing modes. The high Zn acceptor concentration in the NWs provides enhanced radiative efficiency and enables lasing on the iron film despite plasmonic losses. Our results open avenues for integrating NW lasers on ferromagnetic substrates to achieve new functionalities, such as magnetic field-induced modulation.

Unique reflection from birefringent uncoated and gold-coated InP nanowire crystal arrays

We demonstrate unique reflective properties of light from bare and gold-coated InP nanowire (NW) photonic crystal arrays. The undoped wurtzite InP nanowire arrays are grown by selective area epitaxy and coated with a 12-nm thick Al₂O₃ film to suppress atmospheric oxidation. A nominally 10-nm thick gold film is deposited around the NWs to investigate plasmonic effects. The reflectance spectra show pronounced Fabry-Perot oscillations, which are shifted for p- and s-polarized light due to a strong intrinsic birefringence in the NW arrays. Gold-coating of the NW array leads to a significant increase of the reflectance by a factor of two to three compared to the uncoated array, which is partially attributed to a plasmon resonance of the gold caps on top of the NWs and to a plasmonic antenna effect for p-polarized light. These interpretations are supported by finite-difference-time-domain simulations. Our experiments and simulations indicate that NW arrays can be used to design micrometer-sized polarizers, analyzers, and mirrors which are important optical elements in optoelectronic integrated circuits.

Effect of Au substrate and coating on the lasing characteristics of GaAs nanowires

Optically pumped lasing from highly Zn-doped GaAs nanowires lying on an Au film substrate and from Au-coated nanowires has been demonstrated up to room temperature. The conically shaped GaAs nanowires were first coated with a 5 nm thick Al₂O₃ shell to suppress atmospheric oxidation and band-bending effects. Doping with a high Zn concentration increases both the radiative efficiency and the material gain and leads to lasing up to room temperature. A detailed analysis of the observed lasing behavior, using finite-difference time domain simulations, reveals that the lasing occurs from low loss hybrid modes with predominately photonic character combined with electric field enhancement effects. Achieving low loss lasing from NWs on an Au film and from Au coated nanowires opens new prospects for on-chip integration of nanolasers with new functionalities including electro-optical modulation, conductive shielding, and polarization control.

Index matching in multilayered organic waveguides

Index matching of guided modes in birefringent multilayered organic waveguides opens new prospects for the design of mode coupling and mode switching devices. We demonstrate index matching of guided modes in two multilayered structures, in (a) a PTCDA-Alq3-PTCDA three-layer and (b) a PTCDA-Alq3 effective medium multilayer waveguide. The optical waveguides were grown on a Pyrex substrate by organic molecular beam deposition. The occurrence of index matching was investigated both experimentally by measuring the effective refractive index dispersion of transverse electric and magnetic modes using the m-line technique and theoretically by modelling the index dispersion with a transfer matrix algorithm.

Emission dynamics of hybrid plasmonic gold/organic GaN nanorods

We studied the emission of bare and aluminum quinoline (Alq₃)/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ∼1.5 μm length and ∼250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq₃ coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron-hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq₃ spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq₃ spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

Real-time contrast-enhanced holographic imaging using phase coherent photorefractive quantum wells

We demonstrate wide-field real-time and depth-resolved contrast enhanced holographic imaging (CEHI) using the all-optical phase coherent photorefractive effect in ZnSe quantum wells. Moving objects are imaged at large depth-of-field by the local enhancement of a static reference hologram. The high refresh rate of the holographic films enables direct-to-video monitoring of floating glass beads and of living Paramecium and Euglena cells moving in water. Depth resolution is achieved by tilting the incident laser beam with respect to the normal of the cuvette. This creates double images of the objects, which are analyzed geometrically and with Fresnel diffraction theory. A two-color CEHI set-up further enables the visualization of a concealed 95 µm thick wire behind a thin layer of chicken skin.

Eliminating thermal effects in z-scan measurements of thin PTCDA films

We investigate the two-photon absorption (TPA) and nonlinear refraction of a micrometer thick 3,4,9,10-perylentetracarboxyl-dianhydride (PTCDA) film using z-scans with tightly focused 100 fs laser pulses. The PTCDA film was grown by organic molecular beam deposition on a Pyrex substrate. To study the influence of sample heating, the pulse repetition rate was varied between 4 MHz and 50 kHz with an acousto-optic pulse selector. We find that thermal effects diminish for pulse repetition times longer than 5 and 0.75 µs when using a 10x or 20x microscope lens, respectively, resulting in a TPA coefficient of 6 cm/GW and a nonlinear refractive index of 1.2 x 10⁻¹³ cm²/W at a wavelength of 820 nm.

Polarization analyzer for all the states of polarization of light using a structured polarizer

Polarization analyzers are an essential measuring tool to improve the characteristics of optical components and optimize them with respect to a useful application in optical networks. We describe an instrument of this kind, which consists of two crossed birefringent wedges and acts as a continuous structured polarizer for all the states of polarization of light. We analyze this device theoretically by using the Poincaré-sphere and the Jones-matrix method and verify our results in a number of experiments with quartz wedges and red filtered light. Different realizations of this instrument are discussed, and an application as a beam splitter for all the states of polarization is proposed.

Modeling the avalanche diode as a photon detector in quantum optical interferometers

Avalanche diodes (ADs) are widely used to count photons in quantum interferometry. In reality they do not count photons, but click once when a bunch of photons arrives in a light pulse. We model this behavior in typical quantum optical interferometers like the Hong-Ou-Mandel beam splitter and the Mach-Zehnder interferometer, and compare it with the behavior of the photon-number-resolving (PNR) detector and the Hanbury-Brown-Twiss detector in these measuring devices. Our results show that quantum interferometric measurements with biphotons could be performed with single ADs, if the noise of the diodes could be reduced. Even a single PNR detector can be used in these interferometers, if the variance of the measurement is determined, since it reveals information about biphoton interference in contrast to the single detector counting rate.

Nonlinearity in the rotational dynamics of Haidinger's brushes

Haidinger's brushes are an entoptic effect of the human visual system that enables us to detect polarized light. However, individual perceptions of Haidinger's brushes can vary significantly. We find that the birefringence of the cornea influences the rotational motion and the contrast of Haidinger's brushes and may offer an explanation for individual differences. We have devised an experimental setup to simulate various phase shifts of the cornea and found a switching effect in the rotational dynamics of Haidinger's brushes. In addition, age related macular degeneration reduces the polarization effect of the macula and thus also leads to changes in the brush pattern.

Interferometric optical path measurement of a glass wedge with single photons and biphotons

We have measured the phase structure of a glass wedge with single photons and biphotons in a Mach-Zehnder interferometer using parametric downconverted light from a Hong-Ou-Mandel particle interferometer as the source. By scanning the wedge through the focus of a microscope objective we find a doubling of the period of the interference pattern in the coincidence counts for biphotons compared to the single-photon experiment. We compare our measurement setup with classical ones and discuss some of the problems of superresolution in quantum lithography.

Beam mixing with a pinhole

After a small aperture the spatial information of a complex optical wavefront is lost, but amplitude and phase information is mixed and transferred to the smoothed wave that emerges from the pinhole. This mixing effect is described in the case of a wavefront with a phase step, which is shifted over the input plane of an optical processor with a pinhole as spatial filter in the Fourier plane. We constructed a polarizing interferometer to demonstrate this continuous phase shift and show that it can be used as a variable retardation wave plate similar to a birefringent compensator, but without crystalline wedges.

Observation of the transfer of the local angular momentum density of a multiringed light beam to an optically trapped particle

We observe the spinning and orbital motion of a microscopic particle trapped within a multiringed light beam that arises from the transfer of the spin and orbital components of the light's angular momentum. The two rotation rates are measured as a function of the distance between the particle and the axis of the trapping beam. The radial dependence of these observations is found to be in close agreement with the accepted theory.

Phase-matched third-harmonic generation in mercury-(I)-chloride

Using phase-matched third-harmonic generation we determine the effective nonlinear susceptibilities in Hg2Cl2 (Calomel) to /chi(3)eff,I/ = 4.5 x 10(-22) m2V(-2) and /chi(3)eff,II/ = 9.7 x 10(-22) m2V(-2) for type I and type II phase matching, respectively. The type III phase matching uses the same tensor components as type I and is deduced to be /chi(3)eff,III/ approximately equal to 1.5 x 10(-22) m2V(-2). The effective third-order susceptibilities of Hg2Cl2 are two orders of magnitude higher than those of CaCO3, and the tensor components chi11 - 3chi18 exceed the components of ADP by a factor of 5. These measurements demonstrate that Calomel might be a promising material to be used for nonlinear optical devices.