Elastic Gallium Phosphide Nanowire Optical Waveguides-Versatile Subwavelength Platform for Integrated Photonics.

Emerging technologies for integrated optical circuits demand novel approaches and materials. This includes a search for nanoscale waveguides that should satisfy criteria of high optical density, small cross-section, technological feasibility and structural perfection. All these criteria are met with self-assembled gallium phosphide (GaP) epitaxial nanowires. In this work, the effects of the nanowire geometry on their waveguiding properties are studied both experimentally and numerically. Cut-off wavelength dependence on the nanowire diameter is analyzed to demonstrate the pathways for fabrication of low-loss and subwavelength cross-section waveguides for visible and near-infrared (IR) ranges. Probing the waveguides with a supercontinuum laser unveils the filtering properties of the nanowires due to their resonant action. The nanowires exhibit perfect elasticity allowing fabrication of curved waveguides. It is demonstrated that for the nanowire diameters exceeding the cut-off value, the bending does not sufficiently reduce the field confinement promoting applicability of the approach for the development of nanoscale waveguides with a preassigned geometry. Optical X-coupler made of two GaP nanowires allowing for spectral separation of the signal is fabricated. The results of this work open new ways for the utilization of GaP nanowires as elements of advanced photonic logic circuits and nanoscale interferometers.

Self-assembled photonic structure: a Ga optical antenna on GaP nanowires.

Semiconductor nanowires are the perfect platform for nanophotonic applications owing to their resonant, waveguiding optical properties and technological capabilities providing control over their crystalline and chemical compositions. The vapor-liquid-solid growth mechanism allows the formation of hybrid metal-dielectric nanostructures promoting sub-wavelength light manipulation. In this work, we explore both experimentally and numerically the plasmonic effects promoted by a gallium (Ga) nanoparticle optical antenna decorating the facet of gallium phosphide (GaP) nanowires. Raman, photoluminescence and near-field mapping techniques are used to study the effects. We demonstrate several phenomena including field enhancement, antenna effect and increase in internal reflection. We show that the observed effects have to be considered when nanowires with a plasmonic particle are used in nanophotonic circuits and discuss the ways for utilization of these effects for efficient coupling of light into nanowire waveguide and field tailoring. The results open up promising pathways for the development of both passive and active nanophotonic elements, light harvesting and sensorics.

Nanoscale Electric Field Probing in a Single Nanowire with Raman Spectroscopy and Elastic Strain.

In this work we investigate the Raman response of extremely strained gallium phosphide nanowires. We analyze new strain-induced spectral phenomena such as 2-fold and 3-fold phonon peak splitting which arise due to nontrivial internal electric field distribution coupled with inhomogeneous strain. We show that high bending strain acts as a probe allowing us to define the electric field distribution with deep subwavelength resolution using the corresponding changes of the Raman spectra. We investigate the nature of the localization with respect to nanowire diameter, excitation spot position, and light polarization, supporting the experiment with 3D numerical modeling. Based on our findings we propose a research tool allowing to precisely localize the electric field in a certain subwavelength region of the nanophotonic resonator.

Anisotropic Radiation in Heterostructured "Emitter in a Cavity" Nanowire.

Tailorable synthesis of axially heterostructured epitaxial nanowires (NWs) with a proper choice of materials allows for the fabrication of novel photonic devices, such as a nanoemitter in the resonant cavity. An example of the structure is a GaP nanowire with ternary GaPAs insertions in the form of nano-sized discs studied in this work. With the use of the micro-photoluminescence technique and numerical calculations, we experimentally and theoretically study photoluminescence emission in individual heterostructured NWs. Due to the high refractive index and near-zero absorption through the emission band, the photoluminescence signal tends to couple into the nanowire cavity acting as a Fabry-Perot resonator, while weak radiation propagating perpendicular to the nanowire axis is registered in the vicinity of each nano-sized disc. Thus, within the heterostructured nanowire, both amplitude and spectrally anisotropic photoluminescent signals can be achieved. Numerical modeling of the nanowire with insertions emitting in infrared demonstrates a decay in the emission directivity and simultaneous rise of the emitters coupling with an increase in the wavelength. The emergence of modulated and non-modulated radiation is discussed, and possible nanophotonic applications are considered.

Single GaP nanowire nonlinear characterization with the aid of an optical trap.

Semiconductor nanowires exhibit numerous capabilities to advance the development of future optoelectronic devices. Among the III-V material family, gallium phosphide (GaP) is an attractive platform with low optical absorption and high nonlinear susceptibility, making it especially promising for nanophotonic applications. However, investigation of single nanostructures and their waveguiding properties remains challenging owing to typically planar experimental arrangements. Here we study the linear and nonlinear waveguiding optical properties of a single GaP nanowire in a special experimental layout, where an optically trapped structure is aligned along its major axis. We demonstrate efficient second harmonic generation in individual nanowires and unravel phase matching conditions, linking between linear guiding properties of the structure and its nonlinear tensorial susceptibility. The capability to pick up single nanowires, sort them with the aid of optomechanical manipulation and accurately position pre-tested structures opens a new avenue for the generation of optoelectronic origami-type devices.

Flexible Perovskite CsPbBr3 Light Emitting Devices Integrated with GaP Nanowire Arrays in Highly Transparent and Durable Functionalized Silicones.

The architecture of transparent contacts is of utmost importance for creation of efficient flexible light-emitting devices (LEDs) and other deformable electronic devices. We successfully combined the newly synthesized transparent and durable silicone rubbers and the semiconductor materials with original fabrication methods to design LEDs and demonstrate their significant flexibility. We developed electrodes based on a composite GaP nanowire-phenylethyl-functionalized silicone rubber membrane, improved with single-walled carbon nanotube films for a hybrid poly(ethylene oxide)-metal-halide perovskite (CsPbBr3) flexible green LED. The proposed approach provides a novel platform for fabrication of flexible hybrid optoelectronic devices.

Tailoring Morphology and Vertical Yield of Self-Catalyzed GaP Nanowires on Template-Free Si Substrates.

Tailorable synthesis of III-V semiconductor heterostructures in nanowires (NWs) enables new approaches with respect to designing photonic and electronic devices at the nanoscale. We present a comprehensive study of highly controllable self-catalyzed growth of gallium phosphide (GaP) NWs on template-free silicon (111) substrates by molecular beam epitaxy. We report the approach to form the silicon oxide layer, which reproducibly provides a high yield of vertical GaP NWs and control over the NW surface density without a pre-patterned growth mask. Above that, we present the strategy for controlling both GaP NW length and diameter independently in single- or two-staged self-catalyzed growth. The proposed approach can be extended to other III-V NWs.

XRD Evaluation of Wurtzite Phase in MBE Grown Self-Catalyzed GaP Nanowires.

Control and analysis of the crystal phase in semiconductor nanowires are of high importance due to the new possibilities for strain and band gap engineering for advanced nanoelectronic and nanophotonic devices. In this letter, we report the growth of the self-catalyzed GaP nanowires with a high concentration of wurtzite phase by molecular beam epitaxy on Si (111) and investigate their crystallinity. Varying the growth temperature and V/III flux ratio, we obtained wurtzite polytype segments with thicknesses in the range from several tens to 500 nm, which demonstrates the high potential of the phase bandgap engineering with highly crystalline self-catalyzed phosphide nanowires. The formation of rotational twins and wurtzite polymorph in vertical nanowires was observed through complex approach based on transmission electron microscopy, powder X-ray diffraction, and reciprocal space mapping. The phase composition, volume fraction of the crystalline phases, and wurtzite GaP lattice parameters were analyzed for the nanowires detached from the substrate. It is shown that the wurtzite phase formation occurs only in the vertically-oriented nanowires during vapor-liquid-solid growth, while the wurtzite phase is absent in GaP islands parasitically grown via the vapor-solid mechanism. The proposed approach can be used for the quantitative evaluation of the mean volume fraction of polytypic phase segments in heterostructured nanowires that are highly desirable for the optimization of growth technologies.

Structural and Optical Properties of Self-Catalyzed Axially Heterostructured GaPN/GaP Nanowires Embedded into a Flexible Silicone Membrane.

Controlled growth of heterostructured nanowires and mechanisms of their formation have been actively studied during the last decades due to perspectives of their implementation. Here, we report on the self-catalyzed growth of axially heterostructured GaPN/GaP nanowires on Si(111) by plasma-assisted molecular beam epitaxy. Nanowire composition and structural properties were examined by means of Raman microspectroscopy and transmission electron microscopy. To study the optical properties of the synthesized nanoheterostructures, the nanowire array was embedded into the silicone rubber membrane and further released from the growth substrate. The reported approach allows us to study the nanowire optical properties avoiding the response from the parasitically grown island layer. Photoluminescence and Raman studies reveal different nitrogen content in nanowires and parasitic island layer. The effect is discussed in terms of the difference in vapor solid and vapor liquid solid growth mechanisms. Photoluminescence studies at low temperature (5K) demonstrate the transition to the quasi-direct gap in the nanowires typical for diluted nitrides with low N-content. The bright room temperature photoluminescent response demonstrates the potential application of nanowire/polymer matrix in flexible optoelectronic devices.

Gallium Phosphide Nanowires in a Free-Standing, Flexible, and Semitransparent Membrane for Large-Scale Infrared-to-Visible Light Conversion.

Engineering of nonlinear optical response in nanostructures is one of the key topics in nanophotonics, as it allows for broad frequency conversion at the nanoscale. Nevertheless, the application of the developed designs is limited by either high cost of their manufacturing or low conversion efficiencies. This paper reports on the efficient second-harmonic generation in a free-standing GaP nanowire array encapsulated in a polymer membrane. Light coupling with optical resonances and field confinement in the nanowires together with high nonlinearity of GaP material yield a strong second-harmonic signal and efficient near-infrared (800-1200 nm) to visible upconversion. The fabricated nanowire-based membranes demonstrate high flexibility and semitransparency for the incident infrared radiation, allowing utilizing them for infrared imaging, which can be easily integrated into different optical schemes without disturbing the visualized beam.

Perovskite-Gallium Phosphide Platform for Reconfigurable Visible-Light Nanophotonic Chip.

Reduction of the wavelength in on-chip light circuitry is critically important not only for the sake of keeping up with Moore's law for photonics but also for reaching toward the spectral ranges of operation of emerging materials, such as atomically thin semiconductors, vacancy-based single-photon emitters, and quantum dots. This requires efficient and tunable light sources as well as compatible waveguide networks. For the first challenge, halide perovskites are prospective materials that enable cost-efficient fabrication of micro- and nanolasers. On the other hand, III-V semiconductor nanowires are optimal for guiding of visible light as they exhibit a high refractive index as well as excellent shape and crystalline quality beneficial for strong light confinement and long-range waveguiding. Here, we develop an integrated platform for visible light that comprises gallium phosphide (GaP) nanowires directly embedded into compact CsPbBr3-based light sources. In our devices, perovskite microcrystals support stable room-temperature lasing and broadband chemical tuning of the emission wavelength in the range of 530-680 nm, whereas GaP nanowaveguides support efficient outcoupling of light, its subwavelength (<200 nm) confinement, and long-range guiding over distances more than 20 µm. As a highlight of our approach, we demonstrate sequential transfer and conversion of light using an intermediate perovskite nanoparticle in a chain of GaP nanowaveguides.

Dopant-stimulated growth of GaN nanotube-like nanostructures on Si(111) by molecular beam epitaxy.

In this paper we study growth of quasi-one-dimensional GaN nanowires (NWs) and nanotube (NT)-like nanostructures on Si(111) substrates covered with a thin AlN layer grown by means of plasma-assisted molecular beam epitaxy. In the first part of our study we investigate the influence of the growth parameters on the geometrical properties of the GaN NW arrays. First, we find that the annealing procedure carried out prior to deposition of the AlN buffer affects the elongation rate and the surface density of the wires. It has been experimentally demonstrated that the NW elongation rate and the surface density drastically depend on the substrate growth temperature, where 800 °C corresponds to the maximum elongation rate of the NWs. In the second part of the study, we introduce a new dopant-stimulated method for GaN nanotube-like nanostructure synthesis using a high-intensity Si flux. Transmission electron microscopy was used to investigate the morphological features of the GaN nanostructures. The synthesized structures have a hexagonal cross-section and possess high crystal quality. We propose a theoretical model of the novel nanostructure formation which includes the role of the dopant Si. Some of the Si-doped samples were studied with the photoluminescence (PL) technique. The analysis of the PL spectra shows that the highest value of donor concentration in the nanostructures exceeds 5∙1019 cm-3.

Magnetization reversal in YIG/GGG(111) nanoheterostructures grown by laser molecular beam epitaxy.

Thin (4-20 nm) yttrium iron garnet (Y3Fe5O12, YIG) layers have been grown on gadolinium gallium garnet (Gd3Ga5O12, GGG) 111-oriented substrates by laser molecular beam epitaxy in 700-1000 °C growth temperature range. The layers were found to have atomically flat step-and-terrace surface morphology with step height of 1.8 Å characteristic for YIG(111) surface. As the growth temperature is increased from 700 to 1000 °C the terraces become wider and the growth gradually changes from layer by layer to step-flow regime. Crystal structure studied by electron and X-ray diffraction showed that YIG lattice is co-oriented and laterally pseudomorphic to GGG with small rhombohedral distortion present perpendicular to the surface. Measurements of magnetic moment, magneto-optical polar and longitudinal Kerr effect (MOKE), and X-ray magnetic circular dichroism (XMCD) were used for study of magnetization reversal for different orientations of magnetic field. These methods and ferromagnetic resonance studies have shown that in zero magnetic field magnetization lies in the film plane due to both shape and induced anisotropies. Vectorial MOKE studies have revealed the presence of an in-plane easy magnetization axis. In-plane magnetization reversal was shown to occur through combination of reversible rotation and abrupt irreversible magnetization jump, the latter caused by domain wall nucleation and propagation. The field at which the flip takes place depends on the angle between the applied magnetic field and the easy magnetization axis and can be described by the modified Stoner-Wohlfarth model taking into account magnetic field dependence of the domain wall energy. Magnetization curves of individual tetrahedral and octahedral magnetic Fe3+ sublattices were studied by XMCD.

Mid-IR spectroscopy of Fe:ZnSe quantum dots.

We report spectroscopic characterization of Fe:ZnSe quantum dots (for 2% of Zn/Fe molar ratio) fabricated by microemulsion hydrothermal synthesis. Mid-IR photoluminescence of the 5E↔5T2 transition of Fe2+ ions over 3.5-4.5 µm spectral range was observed in Fe:ZnSe quantum dot samples and kinetics of luminescence have been characterized at temperatures of 30-300 K under direct (2.788 µm) mid-IR excitation and indirect (0.355 µm) photoionization excitation. The radiative lifetime (τrad) was estimated from these measurements to be 48 µs while lifetime at room temperature was measured to be 440 ns. This agrees closely with the behavior of bulk material.

Energy scaling of 4.3 µm room temperature Fe:ZnSe laser.

We demonstrate a fourfold increase of the output energy of the gain-switched mid-IR Fe:ZnSe laser. Iron doping of the ZnSe polycrystalline samples was realized using a postgrowth thermal-diffusion method from the metal film. Gain-switched Er:Cr:YSGG (2.8 µm) laser pumped Fe:ZnSe lasing was studied in a Fabry-Perot cavity over a 236-300 K temperature range. The maximum output energy reached 4.7 mJ at 4.3 µm and 3.6 mJ at 4.37 µm at 236 K and 300 K and was limited only by available pump energy. The laser threshold was about 8 mJ and was practically unchanged over the studied temperature range. The laser slope efficiencies, measured with respect to the input pump energy, decreased from 19% to 16% with an increase of temperature from 236 to 300 K. The output radiation featured a Gaussian spatial profile with M(2) = 2.6.

Semiconductor disk laser pumped Cr2+:Znse lasers.

A new flexible pump source, the optically-pumped semiconductor disk laser (SDL), for the Cr(2+):ZnSe laser is reported. The SDL provides up to 6W output power at a free running central wavelength of 1.98 microm. The Cr(2+):ZnSe laser operated at an output power of 1.8W and a slope efficiency of approximately 50% with respect to absorbed pump power whilst maintaining a low output intensity noise figure of <0.14% RMS. The system required no optical isolation even under the situation of significant optical feedback.

Highly efficient, narrow-linewidth, and single-frequency actively and passively Q-switched fiber-bulk hybrid Er:YAG lasers operating at 1645 nm.

High power, highly efficient single frequency oscillation of Er:YAG fiber-bulk hybrid laser at 1645 nm is demonstrated in actively and passively Q-switched operation modes. The slope efficiencies in the active and passive Q-switched operation reached 75% and 20%, respectively, with the record output powers in the narrow-linewidth and single longitudinal mode regimes of operation.