Scalable in operando strain tuning in nanophotonic waveguides enabling three-quantum-dot superradiance.

The quest for an integrated quantum optics platform has motivated the field of semiconductor quantum dot research for two decades. Demonstrations of quantum light sources, single photon switches, transistors and spin-photon interfaces have become very advanced. Yet the fundamental problem that every quantum dot is different prevents integration and scaling beyond a few quantum dots. Here, we address this challenge by patterning strain via local phase transitions to selectively tune individual quantum dots that are embedded in a photonic architecture. The patterning is implemented with in operando laser crystallization of a thin HfO2 film 'sheath' on the surface of a GaAs waveguide. Using this approach, we tune InAs quantum dot emission energies over the full inhomogeneous distribution with a step size down to the homogeneous linewidth and a spatial resolution better than 1 µm. Using these capabilities, we tune multiple quantum dots into resonance within the same waveguide and demonstrate a quantum interaction via superradiant emission from three quantum dots.

Transmission Electron Microscope Nanosculpting of Topological Insulator Bismuth Selenide.

We present a process for sculpting Bi2Se3 nanoflakes into application-relevant geometries using a high-resolution transmission electron microscope. This process takes several minutes to sculpt small areas and can be used to cut the Bi2Se3 into wires and rings, to thin areas of the Bi2Se3, and to drill circular holes and lines. We determined that this method allows for sub 10 nm features and results in clean edges along the drilled regions. Using in situ high-resolution imaging, selected area diffraction, and atomic force microscopy, we found that this lithography process preserves the crystal structure of Bi2Se3. TEM sculpting is more precise and potentially results in cleaner edges than does ion-beam modification; therefore, the promise of this method for thermoelectric and topological devices calls for further study into the transport properties of such structures.

Materials analysis and focused ion beam nanofabrication of topological insulator Bi2Se3.

Focused ion beam milling allows manipulation of the shape and size of nanostructures to create geometries potentially useful for opto-electronics, thermoelectrics, and quantum computing. We focus on using the ion beam to control the thickness of Bi2Se3 and to create nanowires from larger structures. Changes in the material structure of Bi2Se3 nanomaterials that have been milled using a focused ion beam are presented. In order to characterize the effects of ion beam processing on the samples, we use a variety of techniques including analytical transmission electron microscopy and atomic force microscopy. The results show that while part of the material remains intact after shaping, amorphous regions form where the beam has been used to thin the sample. For wires created by thinning the material down to the substrate, the sidewalls of the wires appear intact based on diffraction images from samples cut at an angle, but thin crystalline regions remain at the wire edges. Even with the resulting defects and limitations when thinning, focused ion beam milling can be used to fabricate custom geometries of Bi2Se3 nanostructures.

Patterning Superconductivity in a Topological Insulator.

Topologically protected states in combination with superconductivity hold great promise for quantum computing applications, but the progress on electrical transport measurements in such systems has been impeded by the difficulty of fabricating devices with reliable electrical contacts. We find that superconductivity can be patterned directly into Bi2Se3 nanostructures by local doping with palladium. Superconducting regions are defined by depositing palladium on top of the nanostructures using electron beam lithography followed by in situ annealing. Electrical transport measurements at low temperatures show either partial or full superconducting transition, depending on the doping conditions. Structural characterization techniques indicate that palladium remains localized in the targeted areas, making it possible to pattern superconducting circuits of arbitrary shapes in this topological material.

Transfer of monolayer TMD WS2 and Raman study of substrate effects.

A facile transfer process for transition metal dichalcogenide WS2 flakes is reported and the effect of the underlying substrate on the flake properties is investigated using Raman spectroscopy. The flakes are transferred from their growth substrate using polymethyl methacrylate (PMMA) and a wet etch to allow the user to transfer the flakes to a final substrate using a microscope and micromanipulator combined with semi-transparent Kapton tape. The substrates used range from insulators such as industry standard high-k dielectric HfO2 and "green polymer" parylene-C, to conducting chemical vapor deposition (CVD) grown graphene. Raman spectroscopy is used first to confirm the material quality of the transferred flakes to the substrates and subsequently to analyze and separate the effects arising from material transfer from those arising from interactions with the substrate. We observe changes in the Raman spectra associated with the interactions between the substrates in the flakes. These interactions affect both in-plane and out-of-plane modes in different ways depending on their sources, for example strain or surface charge. These changes vary with final substrate, with the strongest effects being observed for WS2 transferred onto graphene and HfO2, demonstrating the importance of understanding substrate interaction for fabrication of future devices.

Monolayer WS2 Nanopores for DNA Translocation with Light-Adjustable Sizes.

Two-dimensional materials are promising for a range of applications, as well as testbeds for probing the physics of low-dimensional systems. Tungsten disulfide (WS2) monolayers exhibit a direct band gap and strong photoluminescence (PL) in the visible range, opening possibilities for advanced optoelectronic applications. Here, we report the realization of two-dimensional nanometer-size pores in suspended monolayer WS2 membranes, allowing for electrical and optical response in ionic current measurements. A focused electron beam was used to fabricate nanopores in WS2 membranes suspended on silicon-based chips and characterized using PL spectroscopy and aberration-corrected high-resolution scanning transmission electron microscopy. It was observed that the PL intensity of suspended WS2 monolayers is ∼10-15 times stronger when compared to that of substrate-supported monolayers, and low-dose scanning transmission electron microscope viewing and drilling preserves the PL signal of WS2 around the pore. We establish that such nanopores allow ionic conductance and DNA translocations. We also demonstrate that under low-power laser illumination in solution, WS2 nanopores grow slowly in size at an effective rate of ∼0.2-0.4 nm/s, thus allowing for atomically controlled nanopore size using short light pulses.

Momentum-dependent multiple gaps in magnesium diboride probed by electron tunnelling spectroscopy.

The energy gap is the most fundamental property of a superconductor. MgB(2), a superconductor discovered in 2001, exhibits two different superconducting gaps caused by the different electron-phonon interactions in two weakly interacting bands. Theoretical calculations predict that the gap values should also vary across the Fermi surface sheets of MgB(2). However, until now, no such variation has been observed. It has been suggested that two gap values were sufficient to describe real MgB(2) samples. Here we present an electron tunnelling spectroscopy study on MgB(2)/native oxide/Pb tunnel junctions that clearly shows a distribution of gap values, confirming the importance of the anisotropic electron-phonon interaction. The gap values, and their spreads found from the tunnel junction measurements, provide valuable experimental tests for various theoretical approaches to the multi-band superconductivity in MgB(2).