A DC SQUID consists of a superconducting loop with two Josephson junctions or weak links. Its operation is based on the fact that as a result of quantum interference the maximum dissipationless current I c that can flow through the SQUID is periodic in the magnetic flux Φ through the loop 7

A nanometer-size superconducting quantum interference device (nanoSQUID) is fabricated on the apex of a sharp quartz tip and integrated into a scanning SQUID microscope.A simple self-aligned fabrication method results in nanoSQUIDs with diameters down to 100 nm with no lithographic processing. An aluminum nanoSQUID with an effective area of 0.034 µm 2 displays flux sensitivity of 1.8 × 10 −6 Φ 0 / √ Hz and operates in fields as high as 0.6 T.With projected spin sensitivity of 65 µ B / √ Hz and high bandwidth, the SQUID on a tip is a highly promising probe for nanoscale magnetic imaging and spectroscopy.Imaging magnetic fields on a nanoscale is a major challenge in nanotechnology, physics, chemistry, and biology. One of the milestones in this endeavor will be the achievement of sensitivity sufficient for detection of the magnetic moment of a single electron. patterning methods; 3-11 the large in-plane size of the devices precludes bringing the SQUID loop into sufficiently close proximity to the sample (due to alignment issues) to scan it with optimal sensitivity. Recently, a terraced SQUID susceptometer was developed that is based on a multilayered lithographic process combined with FIB etching. This device includes a 600 nm pickup loop which can be scanned 300 nm above the sample surface. 12 Here we present a simple method for the self-aligned fabrication of a DC nanoSQUID on a tip with effective diameter as small as 100 nm that can be scanned just a few nm above the sample.We have fabricated several SQUID-on-tip (SOT) devices of various sizes. A quartz tube of 1 mm outside diameter is pulled to a sharp tip with apex diameter that can be controllably varied between 100 and 400 nm. The fabrication of the SOT consists of three "self-aligned" steps of thermal evaporation of Al, as shown schematically in Fig. 1a. In the first step, 25 nm of Al are deposited on the tip tilted at an angle of -100 • with respect to the line to the source. Then the tip is rotated to an angle of 100 • , followed by a second deposition of 25 nm. As a result, two leads on opposite sides of the quartz tube are formed, as shown in Fig. 1b. In the last step 17 nm of Al are evaporated at an angle of 0 • , coating the apex ring of the tip. The two areas where the leads contact the ring form "strong" superconducting regions, whereas the two parts of the ring in the gap between the leads, indicated by arrows in Fig. 1c, constitute two weak links, thus forming the SQUID. The resulting nanoSQUID requires no lithographic processing, its size is controlled by a conventional pulling procedure of a quartz tube, and it is located at the apex of a sharp tip that is ideal for scanning probe microscopy.The studies were carried out at 300 mK, well below the critical temperature T c ≈ 1.6 K of granular thin films of aluminum in our deposition system. Instead of the commonly used current 2 bias, the SOT was operated in a voltage bias mode, as shown schematically in the inset to Fig. 2. We use a low bias resistance R b of about 2 Ω and the SOT current...

Luminescence upconversion nanocrystals capable of converting two low-energy photons into a single photon at a higher energy are sought-after for a variety of applications, including bioimaging and photovoltaic light harvesting. Currently available systems, based on rare-earth-doped dielectrics, are limited in both tunability and absorption cross-section. Here we present colloidal double quantum dots as an alternative nanocrystalline upconversion system, combining the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. By tailoring its composition and morphology, we form a semiconducting nanostructure in which excited electrons are delocalized over the entire structure, but a double potential well is formed for holes. Upconversion occurs by excitation of an electron in the lower energy transition, followed by intraband absorption of the hole, allowing it to cross the barrier to a higher energy state. An overall conversion efficiency of 0.1% per double excitation event is achieved.

We describe a new type of scanning probe microscope based on a superconducting quantum interference device (SQUID) that resides on the apex of a sharp tip. The SQUID-on-tip is glued to a quartz tuning fork which allows scanning at a tip-sample separation of a few nm. The magnetic flux sensitivity of the SQUID is 1.8 μΦ(0)/√Hz and the spatial resolution is about 200 nm, which can be further improved. This combination of high sensitivity, spatial resolution, bandwidth, and the very close proximity to the sample provides a powerful tool for study of dynamic magnetic phenomena on the nanoscale. The potential of the SQUID-on-tip microscope is demonstrated by imaging of the vortex lattice and of the local ac magnetic response in superconductors.

Facile molecular self-assembly affords a new family of organic nanocrystals that, unintuitively, exhibit a significant nonlinear optical response (second harmonic generation, SHG) despite the relatively small molecular dipole moment of the constituent molecules. The nanocrystals are self-assembled in aqueous media from simple monosubstituted perylenediimide (PDI) molecular building blocks. Control over the crystal dimensions can be achieved via modification of the assembly conditions. The combination of a simple fabrication process with the ability to generate soluble SHG nanocrystals with tunable sizes may open new avenues in the area of organic SHG materials.