Chad Ropp

Nanoscale imaging and spontaneous emission control with a single nano-positioned quantum dot.

Plasmonic nanostructures confine light on the nanoscale, enabling ultra-compact optical devices that exhibit strong light-matter interactions. Quantum dots are ideal for probing plasmonic devices because of their nanoscopic size and desirable emission properties. However, probing with single quantum dots has remained challenging because their small size also makes them difficult to manipulate. Here we demonstrate the use of quantum dots as on-demand probes for imaging plasmonic nanostructures, as well as for realizing spontaneous emission control at the single emitter level with nanoscale spatial accuracy. A single quantum dot is positioned with microfluidic flow control to probe the local density of optical states of a silver nanowire, achieving 12 nm imaging accuracy. The high spatial accuracy of this scanning technique enables a new method for spontaneous emission control where interference of counter-propagating surface plasmon polaritons results in spatial oscillations of the quantum dot lifetime as it is positioned along the wire axis.

Manipulating Quantum Dots to Nanometer Precision by Control of Flow

We present a method for manipulating preselected quantum dots (QDs) with nanometer precision by flow control. The accuracy of this approach scales more favorably with particle size than optical trapping, enabling more precise positioning of nanoscopic particles. We demonstrate the ability to position a single QD in a 100 microm working region to 45 nm accuracy for holding times exceeding one hour and the ability to take active quantum measurements on the dynamically manipulated QD.

Positioning and Immobilization of Individual Quantum Dots with Nanoscale Precision

We demonstrate a technique for the precise immobilization of nanoscale objects at accurate positions on two-dimensional surfaces. We have developed a water-based photoresist that causes nanostructures such as colloidal quantum dots to segregate to a thin layer at surfaces. By combining this material with electroosmotic feedback control, we demonstrate the ability to position selected, individual quantum dots at specific locations and to immobilize them with 130 nm precision via localized UV exposure.

Flow Control of Small Objects on Chip: Manipulating Live Cells, Quantum Dots, and Nanowires

This article is on microscale flow control, on dynamically shaping flow fields in microfluidic devices to precisely manipulate cells, quantum dots (QDs), and nanowires. Compared to prior methods, manipulating microscopic and nanoscopic objects by flow control can be achieved with simpler and easy-to-fabricate devices, can steer a wider variety of objects, and enables entirely new capabilities such as placement and immobilization of specific quantum dots to desired on-chip locations with nanoscale precision. A companion article investigates flow control in the body and develops methods to shape magnetic fields to direct ferrofluids of therapeutic magnetic nanoparticles to disease locations in patients.

Nanoscale probing of image-dipole interactions in a metallic nanostructure

An emitter near a surface induces an image dipole that can modify the observed emission intensity and radiation pattern. These image-dipole effects are generally not taken into account in single-emitter tracking and super-resolved imaging applications. Here we show that the interference between an emitter and its image dipole induces a strong polarization anisotropy and a large spatial displacement of the observed emission pattern. We demonstrate these effects by tracking the emission of a single quantum dot along two orthogonal polarizations as it is deterministically positioned near a silver nanowire. The two orthogonally polarized diffraction spots can be displaced by up to 50 nm, which arises from a Young’s interference effect between the quantum dot and its induced image dipole. We show that the observed spatially varying interference fringe provides a useful measure for correcting image-dipole-induced distortions. These results provide a pathway towards probing and correcting image-dipole effects in near-field imaging applications.

Scanning localized magnetic fields in a microfluidic device with a single nitrogen vacancy center.

Nitrogen vacancy (NV) color centers in diamond enable local magnetic field sensing with high sensitivity by optical detection of electron spin resonance (ESR). The integration of this capability with microfluidic technology has a broad range of applications in chemical and biological sensing. We demonstrate a method to perform localized magnetometry in a microfluidic device with a 48 nm spatial precision. The device manipulates individual magnetic particles in three dimensions using a combination of flow control and magnetic actuation. We map out the local field distribution of the magnetic particle by manipulating it in the vicinity of a single NV center and optically detecting the induced Zeeman shift with a magnetic field sensitivity of 17.5 μT Hz(-1/2). Our results enable accurate nanoscale mapping of the magnetic field distribution of a broad range of target objects in a microfluidic device.

Fabrication of Nanoassemblies Using Flow Control

Synthetic nanostructures, such as nanoparticles and nanowires, can serve as modular building blocks for integrated nanoscale systems. We demonstrate a microfluidic approach for positioning, orienting, and assembling such nanostructures into nanoassemblies. We use flow control combined with a cross-linking photoresist to position and immobilize nanostructures in desired positions and orientations. Immobilized nanostructures can serve as pivots, barriers, and guides for precise placement of subsequent nanostructures.

Nanostructure-Induced Distortion in Single-Emitter Microscopy

Single-emitter microscopy has emerged as a promising method of imaging nanostructures with nanoscale resolution. This technique uses the centroid position of an emitters far-field radiation pattern to infer its position to a precision that is far below the diffraction limit. However, nanostructures composed of high-dielectric materials such as noble metals can distort the far-field radiation pattern. Previous work has shown that these distortions can significantly degrade the imaging of the local density of states in metallic nanowires using polarization-resolved imaging. But unlike nanowires, nanoparticles do not have a well-defined axis of symmetry, which makes polarization-resolved imaging difficult to apply. Nanoparticles also exhibit a more complex range of distortions, because in addition to introducing a high dielectric surface, they also act as efficient scatterers. Thus, the distortion effects of nanoparticles in single-emitter microscopy remains poorly understood. Here we demonstrate that metallic nanoparticles can significantly distort the accuracy of single-emitter imaging at distances exceeding 300 nm. We use a single quantum dot to probe both the magnitude and the direction of the metallic nanoparticle-induced imaging distortion and show that the diffraction spot of the quantum dot can shift by more than 35 nm. The centroid position of the emitter generally shifts away from the nanoparticle position, which is in contradiction to the conventional wisdom that the nanoparticle is a scattering object that will pull in the diffraction spot of the emitter toward its center. These results suggest that dielectric distortion of the emission pattern dominates over scattering. We also show that by monitoring the distortion of the quantum dot diffraction spot we can obtain high-resolution spatial images of the nanoparticle, providing a new method for performing highly precise, subdiffraction spatial imaging. These results provide a better understanding of the complex near-field coupling between emitters and nanostructures and open up new opportunities to perform super-resolution microscopy with higher accuracy.

Creation of multimaterial micro- and nanostructures through aqueous-based fabrication, manipulation, and immobilization

Despite the rapid growth of microfabrication technologies over the past decades, many desirable microstructures remain difficult or even impossible to create, especially when the structures are composed of multiple components that feature different materials that must be arranged in a highly specific, 3-D pattern. We have developed aqueous photoresists that can be used in combination with different techniques for nanomanipulation to create such structures. Multiphoton absorption polymerization can be used to create unsupported polymeric microstructures that can be nanomanipulated to place them in any desired position and orientation. Nanomanipulation techniques can also be used to place micro- or nanoscale components in desired locations in three dimensions, after which they can be immobilized photochemically. This toolbox of techniques offers the capability of creating a broad range of new structures and devices featuring polymeric, inorganic, metallic and biomolecular components.

Near-field probing of plasmonic nanostructures with a single quantum dot

Individual colloidal quantum dots are manipulated in a microfluidic device and used as near-field optical probes for visualizing the plasmonic mode of a silver nanowire.