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arXiv2018

Amplitude dependence of resonance frequency and its consequences for scanning probe microscopy

With recent advances in scanning probe microscopy (SPM), it is now routine to determine the atomic structure of surfaces and molecules while quantifying the local tip-sample interaction potentials. Such quantitative experiments are based on the accurate measurement of the resonance frequency shift due to the tip-sample interaction. Here, we experimentally show that the resonance frequency of oscillating probes used for SPM experiments change systematically as a function of oscillation amplitude under typical operating conditions. This change in resonance frequency is not due to tip-sample interactions, but rather due to the cantilever strain or geometric effects and thus the resonance frequency being a function of the oscillation amplitude. Our numerical calculations demonstrate that the amplitude dependence of the resonance frequency is an additional yet overlooked systematic error source that can result nonnegligible errors in measured interaction potentials and forces. Our experimental results and complementary numerical calculations reveal that the frequency shift due to this amplitude dependence needs to be corrected even for experiments with active oscillation amplitude control to be able to quantify the tip-sample interaction potentials and forces with milli-electron volt and pico-Newton resolutions.

physics.app-ph↗

arXiv2018

Tuning spin torque nano-oscillator nonlinearity using He+ irradiation

We use He$^+$ irradiation to tune the nonlinearity, $\mathcal{N}$, of all-perpendicular spin-torque nano-oscillators (STNOs) using the He$^+$ fluence-dependent perpendicular magnetic anisotropy (PMA) of the [Co/Ni] free layer. Employing fluences from 6 to 20$\times10^{14}$~He$^{+}$/cm$^{2}$, we are able to tune $\mathcal{N}$ in an in-plane field from strongly positive to moderately negative. As the STNO microwave signal properties are mainly governed by $\mathcal{N}$, we can in this way directly control the threshold current, the current tunability of the frequency, and the STNO linewidth. In particular, we can dramatically improve the latter by more than two orders of magnitude. Our results are in good agreement with the theory for nonlinear auto-oscillators, confirm theoretical predictions of the role of nonlinearity, and demonstrate a straightforward path towards improving the microwave properties of STNOs.

physics.app-ph↗

arXiv2018

Top-down fabrication of high-uniformity nanodiamonds by self-assembled block copolymer masks

Nanodiamonds hosting colour centres are a promising material platform for various quantum technologies. The fabrication of non-aggregated and uniformly-sized nanodiamonds with systematic integration of single quantum emitters has so far been lacking. Here, we present a top-down fabrication method to produce 30.0$\pm$5.4 nm uniformly-sized single-crystal nanodiamonds by block copolymer self-assembled nanomask patterning together with directional and isotropic reactive ion etching. We show detected emission from bright single nitrogen vacancy centres hosted in the fabricated nanodiamonds. The lithographically precise patterning of large areas of diamond by self-assembled masks and their release into uniformly sized nanodiamonds open up new possibilities for quantum information processing and sensing.

physics.app-ph↗

arXiv2018

Laser-induced Phase-change Perovskite Photodetector Arrays for Optical Information Storage and Imaging

The reversible phase transition between orthorhombic and cubic phases is demonstrated in all-inorganic perovskite CsPbIBr2 grown by a dual-source vapor-deposition method. The phase transition is actuated via heating and moisture exposure. The different crystal structures of two phases result in distinct optoelectronic properties including optical absorption, refractive index, and carrier transport. The perovskite photodetector array for the non-volatile rewritable optical memory application is further investigated. A near-infrared (NIR) laser (λ = 1064 nm) is used to selectively heat photodetector pixels through the photothermal effect of the interfacial Ag electrodes, resulting in an optically absorbing perovskite phase for the photodetectors. The locations of specific laser-written pixels can be read out by measuring their photocurrents, and the stored optical information can also be erased by moisture exposure. Finally, a proof-of-concept optical imaging application has been demonstrated using perovskite photodetector arrays fabricated on flexible PET substrates. These results show promising applications of vapor-deposited inorganic perovskite for optical memory and image sensors, with unique potential for low-cost manufacturing of large-area and/or flexible devices.

physics.app-ph↗

arXiv2018

Virtual cathode induced in Rb2Ti2O5 solid electrolyte

Rb2Ti2O5 (RTO) has recently been demonstrated to be a solid electrolyte, producing colossal capacitance when interfaced with metals. In order to understand the mechanisms leading to such colossal equivalent permittivity (up to four orders of magnitude above state-of-the-art values), the charge distribution in RTO is a key feature to be investigated. In the present article, this charge distribution is probed using the pressure-wave-propagation method, in devices made of RTO single crystals or polycrystals sandwiched between two metallic electrodes. Remarkably enough, in both types of samples, negative charges are found to accumulate inside RTO, near the anode, while the electric field near the cathode remains zero. This proves that the ionic carriers are majoritarily negatively charged and provides an explanation for the colossal capacitance. The latter takes place only at the anode while the cathode is virtually shifted into the solid electrolyte.

cond-mat.other↗

arXiv2018

Two-dimensional mutual synchronization in spin Hall nano-oscillator arrays

Spin Hall nano-oscillators (SHNOs) utilize pure spin currents to drive local regions of magnetic films and nanostructures into auto-oscillating precession. If such regions are placed in close proximity to each other they can interact and sometimes mutually synchronize, in pairs or in short linear chains. Here we demonstrate robust mutual synchronization of two-dimensional SHNO arrays ranging from 2 x 2 to 8 x 8 nano-constrictions, observed both electrically and using micro-Brillouin Light Scattering microscopy. The signal quality factor, $Q=f/Δf$, increases linearly with number of mutually synchronized nano-constrictions ($N$), reaching 170,000 in the largest arrays. While the microwave peak power first increases as $N^2$, it eventually levels off, indicating a non-zero relative phase shift between nano-constrictions. Our demonstration will enable the use of SHNO arrays in two-dimensional oscillator networks for high-quality microwave signal generation and neuromorphic computing.

cond-mat.mes-hall↗

arXiv2018

Lattice-mismatched semiconductor heterostructures

Semiconductor heterostructure is a critical building block for modern semiconductor devices. However, forming semiconductor heterostructures of lattice-mismatch has been a great challenge for several decades. Epitaxial growth is infeasible to form abrupt heterostructures with large lattice-mismatch while mechanical-thermal bonding results in a high density of interface defects and therefore severely limits device applications. Here we show an ultra-thin oxide-interfaced approach for the successful formation of lattice-mismatched semiconductor heterostructures. Following the depiction of a theory on the role of interface oxide in forming the heterostructures, we describe experimental demonstrations of Ge/Si (diamond lattices), Si/GaAs (zinc blende lattice), GaAs/GaN (hexagon lattice), and Si/GaN heterostructures. Extraordinary electrical performances in terms of ideality factor, current on/off ratio, and reverse breakdown voltage are measured from p-n diodes fabricated from the four types of heterostructures, significantly outperforming diodes derived from other methods. Our demonstrations indicate the versatility of the ultra-thin-oxide-interface approach in forming lattice-mismatched heterostructures, open up a much larger possibility for material combinations for heterostructures, and pave the way toward broader applications in electronic and optoelectronic realms.

physics.app-ph↗

arXiv2018

Enhancement of the Monolayer WS2 Exciton Photoluminescence with a 2D-Material/Air/GaP In-Plane Microcavity

Light-matter interaction with two-dimensional materials gained significant attention in recent years leading to the reporting of weak and strong coupling regimes, and effective nano-laser operation with various structures. Particularly, future applications involving monolayer materials in waveguide-coupled on-chip integrated circuitry and valleytronic nanophotonics require controlling, directing and optimizing photoluminescence. In this context, photoluminescence enhancement from monolayer transition-metal dichalcogenides on patterned semiconducting substrates becomes attractive. It is demonstrated in our work using focussed-ion-beam-etched GaP and monolayer WS2 suspended on hexagonal-BN buffer sheets. We present a unique optical microcavity approach capable of both efficient in-plane and out-of-plane confinement of light, which results in a WS2 photoluminescence enhancement by a factor of 10 compared to the unstructured substrate at room temperature. The key concept is the combination of interference effects in both the horizontal direction using a bull's-eye-shaped circular Bragg grating and in vertical direction by means of a multiple reflection model with optimized etch depth of circular air-GaP structures for maximum constructive interference effects of the applied pump and expected emission light.

physics.app-ph↗

arXiv2018

Excitation Wavelength Dependent Reversible Photoluminescence Peak in Iodide Perovskites

Halide perovskites have indisputably exceptional optical and electronic properties, which are attractive for next-generation optoelectronic device technologies. We report on a reversible photoluminescence (PL) peak in iodide-based organic-inorganic lead halide perovskite materials under a two-photon absorption (TPA) process, while tuning the excitation wavelength. This phenomenon occurs when the incoming femtosecond (fs) laser photon energy is higher than a threshold energy. Intriguingly, this phenomenon also occurs in other kinds of iodide perovskite materials. Moreover, two more shorter wavelength peaks exhibit and become prominent when the excitation photon energy is being tuned in the high energy wavelength spectrum, while laser power is remained constant. However, the spectral PL energy window between the original material peak and the first high energy peak can vary based on the optoelectronic properties of the prepared films. The same phenomenon of reversible PL peak is also observed in various iodide based organic-inorganic halides as well as all-inorganic perovskite single crystals and polycrystals. We attribute to the reversible PL peak to the photoinduced structural deformation and the associated change in optical bandgap of iodide perovskites under the femtosecond laser excitation. Our findings will introduce a new degree of freedom in future research as well as adding new functionalities to optoelectronic applications in these emerging perovskite materials.

physics.optics↗

arXiv2018

Portable Low-cost MRI System based on Permanent Magnets/Magnet Arrays

Portable low-cost MRI systems have the potential to enable point-of-care and timely MRI diagnosis, and to make this imaging modality available to routine scans and to underdeveloped areas. With simplicity, no maintenance, no power consumption, and low cost, permanent magnets or arrays are attractive to use as a source of magnetic field to realize portability and low cost for a scanners. However, when taking the Fourier imaging approach and using linear gradient fields, homogeneous fields are required, thus either a bulky magnet is needed, or the imaging volume is too small to image an organ if the magnet is scaled down. Recently, with the progress on image reconstruction based on non-linear gradient field, field patterns without spatial-linearity can be used as spatial encoding magnetic fields to encode MRI signals for imaging. As a result, the requirements for the homogeneity of the field can be relaxed, which allows permanent magnets(arrays) with reduced sizes, reduced weight to image bigger volumes covering organs such as a head. It offers chances to construct a truly portable low-cost MRI scanner. For this exciting potential application, permanent magnets(arrays) have attracted increased attention. A magnet(array) is strongly associated with the imaging volume, reconstruction methods, and RF excitation and coils, etc. through field patterns and homogeneity. This paper offers a review on permanent magnets(arrays) of different kinds, especially those can be used for spatial encoding towards the development of portable and low-cost MRI systems. It is aimed to familiarize the readers with relevant knowledge, literature, and the latest updates of the development on permanent magnets for MRI. Perspectives on and challenges of using permanent magnets to supply a patterned magnetic field, without spatial-linearity nor high homogeneity, for reconstruction in a portable setup are discussed.

physics.med-ph↗

arXiv2018

X-ray tools for van der Waals epitaxy of bismuth telluride topological insulator films

Potential applications in spintronics and quantum computing have motivated much recent research in epitaxial films of bismuth telluride. This system is also an example of van der Waals (vdW) epitaxy where the interface coherence between film and substrate is based on vdW bonds instead of strong ionic or covalent bonds. Effects of lattice mismatch on electrical properties and film structure are more difficult to control due to the weakness of the vdW forces. Here we present a general x-ray diffraction method to investigate in-plane atomic displacements and lateral lattice coherence length in vdW epitaxy. The method is demonstrated in a series of films grown at different temperatures and pressures of additional tellurium sources, revealing strong intercorrelations between the lateral features as well as with the n/p-types of free charge carries.

physics.app-ph↗

arXiv2018

The effect of the charge pattern on the applicability of a nanopore as a sensor

We investigate a model nanopore sensor that is able to detect analyte ions that are present in the electrolyte solution in very small concentrations. The nanopore selectively binds the analyte ions with which the local concentrations of the ions of the background electrolyte (KCl), and, thus, the ionic current flowing through the pore is changed. Analyte concentration can be determined from calibration curves. In our previous study (Mádai et al. J. Chem. Phys., 147(24):244702, 2017.), we proposed a symmetric model (surface charge is negative all along the pore). The mechanism of sensing was a competition between K$^{+}$ and positive analyte ions, so increasing analyte concentration decreased K$^{+}$ current. Here we allow asymmetric charge patterns on the pore wall (positive/negative/neutral along the pore), thus, gaining an additional device function, rectification, resulting in a dual responsive device. We find that a bipolar nanopore is an efficient geometry with Cl$^{-}$ ions being the main charge carriers. The mechanism of sensing is that more positive analyte ions attract more Cl$^{-}$ ions into the pore thus increasing the current. Also they make the pore less asymmetric and, thus, decrease rectification. We use a hybrid computer simulation method, where a generalization of the grand canonical Monte Carlo method to non-equilibrium (Local Equilibrium Monte Carlo) is coupled to the Nernst-Planck equation with which the flux is computed.

physics.app-ph↗

arXiv2018

Enabling high spectral efficiency coherent superchannel transmission with soliton microcombs

Optical communication systems have come through five orders of magnitude improvement in data rate over the last three decades. The increased demand in data traffic and the limited optoelectronic component bandwidths have led to state-of-the-art systems employing hundreds of separate lasers in each transmitter. Given the limited optical amplifier bandwidths, focus is now shifting to maximize the spectral efficiency, SE. However, the frequency jitter from neighbouring lasers results in uncertainties of the exact channel wavelength, requiring large guardbands to avoid catastrophic channel overlap. Optical frequency combs with optimal line spacings (typically around 10-50 GHz) can overcome these limitations and maximize the SE. Recent developments in microresonator-based soliton frequency combs (hereafter microcombs) promise a compact, power efficient multi-wavelength and phase-locked light source for optical communications. Here we demonstrate a microcomb-based communication link achieving state-of-the-art spectral efficiency that has previously only been possible with bulk-optics systems. Compared to previous microcomb works in optical communications, our microcomb features a narrow line spacing of 22.1 GHz. In addition, it provides a four order-of-magnitude more stable line spacing compared to free-running lasers. The optical signal-to-noise ratio (OSNR) is sufficient for information encoding using state-of-the-art high-order modulation formats. This enables us to demonstrate transmission of a 12 Tb/s superchannel over distances ranging from a single 82 km span with an SE exceeding 10 bits/s/Hz, to 2000 km with an SE higher than 6 bits/s/Hz. These results demonstrate that microcombs can attain the SE that will spearhead future optical networks.

physics.app-ph↗

arXiv2018

Valley-Hall photonic topological insulators with dual-band kink states

Extensive researches have revealed that valley, a binary degree of freedom (DOF), can be an excellent candidate of information carrier. Recently, valley DOF has been introduced into photonic systems, and several valley-Hall photonic topological insulators (PTIs) have been experimentally demonstrated. However, in the previous valley-Hall PTIs, topological kink states only work at a single frequency band, which limits potential applications in multiband waveguides, filters, communications, and so on. To overcome this challenge, here we experimentally demonstrate a valley-Hall PTI, where the topological kink states exist at two separated frequency bands, in a microwave substrate-integrated circuitry. Both the simulated and experimental results demonstrate the dual-band valley-Hall topological kink states are robust against the sharp bends of the internal domain wall with negligible inter-valley scattering. Our work may pave the way for multi-channel substrate-integrated photonic devices with high efficiency and high capacity for information communications and processing.

physics.app-ph↗

arXiv2018

The effect of solidification direction with respect to gravity on ice-templated TiO2 micro-structures

Ice-templating produces materials with aligned, elongated pores via directional solidification of particle suspensions, sublimation of the solidified fluid, and sintering of the particle walls. Most ice-templating studies utilize upward solidification techniques, where solid ice is located at the bottom of the solidification mold, the liquid suspension is on top of the ice, and the solidification front advances upward, against gravity. Liquid water reaches its maximum density at 4°C; thus, liquid nearest the cold source is less dense than warmer liquid above. The lower density liquid nearest the cold source is expected to rise due to buoyancy, promoting convective fluid motion during solidification. Here, we investigate the effect of solidification direction with respect to the direction of gravity on ice-templated microstructures to study the role of buoyancy-driven fluid motion during solidification. We hypothesize that, for upward solidification, the convective fluid motion that results from a liquid density gradient occurs near the solidification front. For downward solidification, we expect that this fluid motion occurs farther away from the solidification front. Aqueous suspensions of TiO2 nanoparticles are solidified upward (against gravity, with ice on bottom and water on top), downward (water on bottom, ice on top), and horizontally (perpendicular to gravity). Microstructural investigation of sintered samples shows evidence of buoyancy-driven, convective fluid flow during solidification for samples solidified upwards (against gravity), including tilting of the wall (and pore) orientation with respect to the induced temperature gradient, ice lens defects, and radial macrosegregation. These features are not observed for downward nor horizontal solidification configurations.

physics.app-ph↗

arXiv2018

Coupling-controlled Dual ITO Layer Electro-Optic Modulator in Silicon Photonics

Electro-optic signal modulation provides a key functionality in modern technology and information networks. Photonic integration has enabled not only miniaturizing photonic components, but also provided performance improvements due to co-design addressing both electrical and optical device rules. However the millimeter-to-centimeter large footprint of many foundry-ready photonic electro-optic modulators significantly limits scaling density. Furthermore, modulators bear a fundamental a frequency-response to energy-sensitive trade-off, a limitation that can be overcome with coupling-based modulators where the temporal response speed is decoupled from the optical cavity photo lifetime. Thus, the coupling effect to the resonator is modulated rather then tuning the index of the resonator itself. However, the weak electro-optic response of silicon limits such coupling modulator performance, since the micrometer-short overlap region of the waveguide-bus and a microring resonator is insufficient to induce signal modulation. To address these limitations, here we demonstrate a coupling-controlled electro-optic modulator by heterogeneously integrating a dual-gated indium-tin-oxide (ITO) phase shifter placed at the silicon microring-bus coupler region. Our experimental modulator shows about 4 dB extinction ratio on resonance, and a about 1.5 dB off resonance with a low insertion loss of 0.15 dB for a just 4 μm short device demonstrating a compact high 10:1 modulation-to-loss ratio. In conclusion we demonstrate a coupling modulator using strongly index-changeable materials enabling compact and high-performing modulators using semiconductor foundry-near materials.

physics.app-ph↗

arXiv2018

Quantum-dot single-photon source on a CMOS silicon photonic chip integrated using transfer printing

Silicon photonics is a powerful platform for implementing large-scale photonic integrated circuits (PICs), because of its compatibility with mature complementary-metal-oxide-semiconductor (CMOS) technology. Exploiting silicon-based PICs for quantum photonic information processing (or the so-called silicon quantum photonics) provides a promising pathway for large-scale quantum applications. For the development of scalable silicon quantum PICs, a major challenge is integrating on-silicon quantum light sources that deterministically emit single photons. In this regard, the use of epitaxial InAs/GaAs quantum dots (QDs) is a very promising approach, because of their capability of deterministic single-photon emission with high purity and indistinguishability. However, the required hybrid integration is inherently difficult and often lacks the compatibility with CMOS processes. Here, we demonstrate a QD single-photon source (SPS) integrated on a glass-clad silicon photonic waveguide processed by a CMOS foundry. Hybrid integration is performed using transfer printing, which enables us to integrate heterogeneous optical components in a simple pick-and-place manner and thus assemble them after the entire CMOS process is completed. We observe single-photon emission from the integrated QD and its efficient coupling into the silicon waveguide. Our transfer-printing-based approach is fully compatible with CMOS back-end processes, and thus will open the possibility for realizing large-scale quantum PICs that leverage CMOS technology.

physics.app-ph↗

arXiv2019

Transient microscopy for measuring heat transfer in single cells

Heat transfer and dissipation exists in almost any physical, chemical or biological systems. Cells, as the basic unit of life, undergo continuous heat transfer and dissipation during their metabolism. The heat transfer and dissipation within cells related to not only fundamental cellular functions and biochemical reactions, but also several important applications including heat-induced control of biological processes and treatment of diseases. Unfortunately, thus far, we still know very little about the heat transfer and dissipation properties at cellular or subcellular level. Here, we demonstrated a methodology of transient microscopy to map the heat transfer coefficients in single cells, with a temporal resolution of ~5 us and a spatial resolution of ~250 nm (close to diffraction limit). The heat transfer coefficients of different location within single cells were obtained for the first time, the inner part of cells exhibited nonuniform heat transfer properties, and it suggested a self-consistent heat transfer regulation that responds to environmental temperature.

physics.bio-ph↗