Octave-spanning supercontinuum generation in a CMOS-compatible thin Si3N4 waveguide coated with highly nonlinear TeO2.

Supercontinuum generation (SCG) is an important nonlinear optical process enabling broadband light sources for many applications, for which silicon nitride (Si3N4) has emerged as a leading on-chip platform. To achieve suitable group velocity dispersion and high confinement for broadband SCG the Si3N4 waveguide layer used is typically thick (>∼700 nm), which can lead to high stress and cracks unless specialized processing steps are used. Here, we report on efficient octave-spanning SCG in a thinner moderate-confinement 400-nm Si3N4 platform using a highly nonlinear tellurium oxide (TeO2) coating. An octave supercontinuum spanning from 0.89 to 2.11 µm is achieved at a low peak power of 258 W using a 100-fs laser centered at 1565 nm. Our numerical simulations agree well with the experimental results giving a nonlinear parameter of 2.5 ± 0.5 W-1m-1, an increase by a factor of 2.5, when coating the Si3N4 waveguide with a TeO2 film. This work demonstrates highly efficient SCG via effective dispersion engineering and an enhanced nonlinearity in CMOS-compatible hybrid TeO2-Si3N4 waveguides and a promising route to monolithically integrated nonlinear, linear, and active functionalities on a single silicon photonic chip.

Si/Ge phototransistor with responsivity >1000A/W on a silicon photonics platform.

In this article, we report a Si/Ge waveguide phototransistor with high responsivity and low dark current under low bias voltages, due to an engineered electric field distribution. The photodetector consists of n-i-p-i-n doping regions and shows a responsivity of 606 A/W at 1 V bias, and 1032 A/W at 2.8V bias with an input optical power of -50 dBm, and dark current of 4 µA and 42 µA respectively. This is achieved by placing two p+-doped regions in the silicon slab region beneath the Ge epitaxial layer. A measured small signal -3 dB bandwidth of 1.5 GHz with a -80 dBc/Hz phase noise response at 1 KHz frequency offset were demonstrated experimentally.

A comprehensive calibration of integrated magnetron sputtering and plasma enhanced chemical vapor deposition for rare-earth doped thin films.

A new integrated deposition system taking advantage of magnetron sputtering and electron cyclotron-plasma enhanced chemical vapour deposition (IMS ECR-PECVD) is presented that mitigates the drawbacks of each fabrication system. This tailor-made system provides users with highly homogeneous and pure thin films with less undesired hydrogen and well-controlled rare-earth concentration compared to existing methods of rare-earth doping, such as metalorganic powders, sputtering, and ion implantation. We established the first comprehensive report on the deposition parameters of argon flow and sputtering power to achieve desired rare-earth concentrations in a wide composition range of terbium (Tb) doped-silicon oxide (Tb:SiOx) matrices including silicon-rich (x < 2), oxygen-rich (x > 2), and stoichiometric silicon oxide (x = 2). The deposition parameters to fabricate crystalline structure (Tb2Si2O7) in oxygen-rich samples are reported where Tb ions are optically active. IMS ECR-PECVD pushes the solubility limit of the rare-earth dopant in silicon films to 17 at.% for the desired future nanophotonic devices. Supplementary Information: The online version contains supplementary material available at 10.1557/s43578-023-01207-2.

Subwavelength grating metamaterial waveguides functionalized with tellurium oxide cladding.

We report on the design, fabrication and characterization of subwavelength grating metamaterial waveguides coated with tellurium oxide. The structures are first fabricated using a standard CMOS compatible process on a silicon-on-insulator platform. Amorphous tellurium oxide top cladding material is then deposited via post-process RF magnetron sputtering. The photonic bandstructure is controlled by adjustment of the device geometry, opening a wide range of operating regimes, including subwavelength propagation, slow light and the photonic bandgap, for various wavelength bands within the 1550 nm telecommunications window. Propagation loss of 1.0 ± 0.1 dB/mm is reported for the tellurium oxide-cladded device, compared to 1.5 ± 0.1 dB/mm propagation loss reported for the silicon dioxide-cladded reference structure. This is the first time that a high-index (n > 2) oxide cladding has been demonstrated for subwavelength grating metamaterial waveguides, thus introducing a new material platform for on-chip integrated optics.

Process-dependent mechanical and optical properties of nanostructured silicon carbonitride thin films.

Amorphous hydrogenated silicon carbonitride (a-SiCN:H) thin films were grown using electron cyclotron resonance chemical vapour deposition using a mixture of methane, nitrogen, and silane as precursors. The origin of the variation of macroscopic properties such as hardness (H), elastic modulus (E), photoluminescence (PL), and the optical band gap was investigated through their correlation with the microscopic features of a-SiCN:H thin films as a function of the process parameters, including the deposition temperature and methane gas flow rate. From a microstructural perspective, the thin films were investigated using x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, elastic recoil detection, transmission electron microscopy, and x-ray diffraction. It is verified that an increase of the substrate temperature resulted in the substitution of hydrogen atoms mainly by carbon atoms, causing the density of the silicon carbide-related structures to increase in the amorphous structure of the a-SiCN:H thin films. Hardness and elastic modulus were found to increase with the deposition temperature and decreased with an increase of the methane gas flow during the deposition, resulting in higher carbon content in the films. The observed changes are ascribed to the reduced density of the weak hydrogen terminated bonds and the variation of the relative bond density of Si-C to Si-N bonds. In addition, the thin films were depth profiled using a slow positron beam to investigate the role of vacancies. The observed increase of the positronium formation with increasing deposition temperature was found to correlate with the variation of PL, where an enhancement of the visible emission originating from carbon-related defects was observed. A set of optimized process parameters to fabricate a-SiCN:H thin films with improved visible emission and hardness properties is suggested.

Amorphous sub-nanometre Tb-doped SiO(x)N(y)/SiO2 superlattices for optoelectronics.

Amorphous sub-nanometre Tb-doped SiOxNy/SiO2 superlattices were fabricated by means of alternating deposition of 0.7 nm thick Tb-doped SiOxNy layers and of 0.9 nm thick SiO2 barrier layers in an electron-cyclotron-resonance plasma enhanced chemical vapour deposition system with in situ Tb-doping capability. High resolution transmission electron microscopy images showed a well-preserved superlattice morphology after annealing at a high temperature of 1000 °C. In addition, transparent indium tin oxide (ITO) electrodes were deposited by electron beam evaporation using a shadow mask approach to allow for the optoelectronic characterization of superlattices. Tb(3+) luminescent spectral features were obtained using three different excitation sources: UV laser excitation (photoluminescence (PL)), under a bias voltage (electroluminescence (EL)) and under a highly energetic electron beam (cathodoluminescence (CL)). All techniques displayed Tb(3+) inner transitions belonging to (5)D4 levels except for the CL spectrum, in which (5)D3 transition levels were also observed. Two competing mechanisms were proposed to explain the spectral differences observed between PL (or EL) and CL excitation: the population rate of the (5)D3 state and the non-radiative relaxation rate of the (5)D3-(5)D4 transition due to a resonant OH-mode. Moreover, the large number of interfaces (trapping sites) that electrons have to get through was identified as the main reason for observing a bulk-limited charge transport mechanism governed by Poole-Frenkel conduction in the J-V characteristic. Finally, a linear EL-J dependence was measured, with independent spectral shape and an EL onset voltage as low as 6.7 V. These amorphous sub-nanometre superlattices are meant to provide low-cost solutions in different areas including sensing, photovoltaics or photonics.

On the origin of emission and thermal quenching of SRSO:Er3+ films grown by ECR-PECVD.

Silicon nanocrystals embedded in a silicon-rich silicon oxide matrix doped with Er3+ ions have been fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition. Indirect excitation of erbium photoluminescence via silicon nanocrystals has been investigated. Temperature quenching of the photoluminescence originating from the silicon nanocrystals and the erbium ions has been observed. Activation energies of the thermally activated quenching process were estimated for different excitation wavelengths. The temperature quenching mechanism of the emission is discussed. Also, the origin of visible emission and kinetic properties of Er-related emission have been discussed in details.

High contrast blue organic light-emitting diodes using inorganic multilayers of Al and ZnSe.

High contrast blue organic light-emitting diodes were fabricated using an inorganic multilayer of NPB (700 Å)/MADN (200 Å)/Alq3 (300 Å)/LiF (10 Å)/Al (70 Å)/ZnSe (300 Å)/Al (1000 Å). The optical and electrical characteristics were measured and compared to conventional organic light-emitting devices (OLEDs) and OLEDs with polarizers. OLEDs with the metal multilayer cathodes had an improved contrast ratio of 135∶1 compared to 104∶1 for OLEDs with polarizers. In addition, the multilayer OLEDs had a low turn-on voltage of 3.5 V due to energy band fitting of ZnSe with Al and the electron transport layer.

Effect of thermal treatment on the growth, structure and luminescence of nitride-passivated silicon nanoclusters.

Silicon nanoclusters (Si-ncs) embedded in silicon nitride films have been studied to determine the effects that deposition and processing parameters have on their growth, luminescent properties, and electronic structure. Luminescence was observed from Si-ncs formed in silicon-rich silicon nitride films with a broad range of compositions and grown using three different types of chemical vapour deposition systems. Photoluminescence (PL) experiments revealed broad, tunable emissions with peaks ranging from the near-infrared across the full visible spectrum. The emission energy was highly dependent on the film composition and changed only slightly with annealing temperature and time, which primarily affected the emission intensity. The PL spectra from films annealed for duration of times ranging from 2 s to 2 h at 600 and 800°C indicated a fast initial formation and growth of nanoclusters in the first few seconds of annealing followed by a slow, but steady growth as annealing time was further increased. X-ray absorption near edge structure at the Si K- and L3,2-edges exhibited composition-dependent phase separation and structural re-ordering of the Si-ncs and silicon nitride host matrix under different post-deposition annealing conditions and generally supported the trends observed in the PL spectra.

Gain flattening coatings for improved spectral performance of asymmetric multiple quantum well lasers.

A gain flattening coating was designed and fabricated to enhance the wavelength tuning for asymmetric multiple quantum well (AMQW) lasers. After coating, a nonlasing gap in the middle range of the lasing wavelengths, which might exist for AMQW lasers that are operated without an external cavity, was overcome and the total lasing range was increased. With the coating, the tuning range of an AMQW laser, as measured without an external cavity, was increased to 85 nm from 70 nm.

Epitaxially driven formation of intricate supported gold nanostructures on a lattice-matched oxide substrate.

A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of lattice-matched MgAl(2)O(4) substrates. The nanostructures were fabricated through a synthesis route where a thin gold film dewets, liquefies, and then slowly self-assembles. The supported nanostructures are intricately shaped, crystalline, and epitaxially aligned. Simulations based on a continuum elastic theory indicate that the self-assembly is driven by strained epitaxy and minimization of the surface free energy.

The dependence of the plasmon field induced nonradiative electronic relaxation mechanisms on the gold shell thickness in vertically aligned CdTe-Au core-shell nanorods.

The dependence of the plasmon field enhancement of the nonradiative relaxation rate of the band gap electrons in vertically aligned CdTe-Au core-shell nanorods on the plasmonic gold nanoshell thickness is examined. Increasing the thickness of the gold nanoshell from 15 to 26 nm is found to change the decay curve from being nonexponential and anisotropic to one that is fully exponential and isotropic (i.e., independent of the nanorod orientation with respect to the exciting light polarization direction). Analysis of the kinetics of the possible electronic relaxation enhancement mechanisms is carried out, and DDA simulated properties of the induced plasmonic field of the thin and thick gold nanoshells are determined. On the basis of the conclusions of these treatments and the experimental results, it is concluded that by increasing the nanoshell thickness the relaxation processes evolve from multiple enhancement mechanisms, dominated by highly anisotropic Auger processes, to mechanism(s) involving first-order excited electron ejection process(es). The former is shown to give rise to nonexponential anisotropic decays in the dipolar plasmon field of the thin nanoshell, while the latter exhibits an exponential isotropic decay in the unpolarized plasmonic field of the thick nanoshell.

Exciton lifetime tuning by changing the plasmon field orientation with respect to the exciton transition moment direction: CdTe-Au core-shell nanorods.

We studied the anisotropy of the influence of plasmonic fields, arising from the optical excitation of a gold nanoshell plasmon absorption at 770 nm, on the lifetime of the bandgap state of the CdTe core in vertically aligned CdTe-Au core-shell nanorods. The previously observed decrease in the lifetime was studied as a function of the tilt angle between the long axis of the nanorod and the electric field polarization direction of the plasmon inducing exciting light. It is observed that the strongest enhancement to the exciton relaxation rate occurs when the two axes are parallel to one another. These results are discussed in terms of the coupling between the exciton transition moment of the CdTe rod and the electric field polarization direction of the gold nanoshell plasmon at 770 nm, which was determined from theoretical modeling based on the discrete dipole approximation.

Plasmon field effects on the nonradiative relaxation of hot electrons in an electronically quantized system: CdTe-Au core-shell nanowires.

The intense electromagnetic fields of plasmonic nanoparticles, resulting from the excitation of their localized surface plasmon oscillations, are known to enhance radiative processes. Their effect on the nonradiative electronic processes, however, is not as well-documented. Here, we report on the enhancement of the nonradiative electronic relaxation rates in CdTe nanowires upon the addition of a thin gold nanoshell, especially at excitation energies overlapping with those of the surface plasmon oscillations. Some possible mechanisms by which localized surface plasmon fields can enhance nonradiative relaxation processes of any quantized electronic excitations are proposed.