Zhidong Du

Temperature mapping using molecular diffusion based fluorescence thermometry via simultaneous imaging of two numerical apertures.

We report a new optical technique to map two-dimensional temperature distributions in liquid solutions based on the thermal motion of fluorescent molecules. We simultaneously capture the fluorescence images of different numerical apertures (NAs) to resolve the temperature-dependent orientations of emission dipoles. In this work, we use two numerical apertures (2NA) prove the concept. This 2NA technique is robust against the intensity variations caused by photobleaching, unsteady illumination and nonuniform molecule distribution. Moreover, as the measured intensity of directional emission is insensitive to polarization changes, this method can be applied to polarizing materials, such as metal surfaces. Under this configuration, the 2NA technique offers another advantage of naturally filtering out the emission background that falls out of collection cones. We foresee the 2NA technique to open a new detection scheme of fluorescence thermometry.

Extending the diffusion approximation to the boundary using an integrated diffusion model

The widely used diffusion approximation is inaccurate to describe the transport behaviors near surfaces and interfaces. To solve such stochastic processes, an integro-differential equation, such as the Boltzmann transport equation (BTE), is typically required. In this work, we show that it is possible to keep the simplicity of the diffusion approximation by introducing a nonlocal source term and a spatially varying diffusion coefficient. We apply the proposed integrated diffusion model (IDM) to a benchmark problem of heat conduction across a thin film to demonstrate its feasibility. We also validate the model when boundary reflections and uniform internal heat generation are present.

Ultrafast time-resolved measurement of energy transport at the metal-liquid interface

The nanoscale light-matter interaction at metallic interfaces has many important applications, especially when it is crucial to enhance the surface-to-volume ratio and to achieve high spatial energy confinement. Here, we report an ultrafast time-resolved measurement to study photo-excited transport at the metal-liquid interfaces of colloidal gold nanoparticles (AuNPs). By using the transient absorption spectroscopy method together with the stimulated emission depletion of fluorescence molecules, we simultaneously measured the perturbations of energy states on both sides of the interfaces within a nanoscale distance. Our measurement results showed the evidence of ultrafast coupling between AuNPs and their surrounding solvent molecules at the picosecond time scale. This method can be extended to study the energy transfer mechanisms at the various interfaces for biology, chemistry, or optoelectronics.The nanoscale light-matter interaction at metallic interfaces has many important applications, especially when it is crucial to enhance the surface-to-volume ratio and to achieve high spatial energy confinement. Here, we report an ultrafast time-resolved measurement to study photo-excited transport at the metal-liquid interfaces of colloidal gold nanoparticles (AuNPs). By using the transient absorption spectroscopy method together with the stimulated emission depletion of fluorescence molecules, we simultaneously measured the perturbations of energy states on both sides of the interfaces within a nanoscale distance. Our measurement results showed the evidence of ultrafast coupling between AuNPs and their surrounding solvent molecules at the picosecond time scale. This method can be extended to study the energy transfer mechanisms at the various interfaces for biology, chemistry, or optoelectronics.

Prediction of Deterministic All-Optical Switching of Ferromagnetic Thin Film by Ultrafast Optothermal and Optomagnetic Couplings

All-optical switching (AOS) of magnetization induced by ultrafast laser pulses is fundamentally interesting and promises unprecedented speed for magnetic data storage that is three orders of magnitudes faster than the current techniques. For ferrimagnetic material, the AOS is attributed to magnetic circular dichroism and angular momentum transfer between sublattices. Recently, ferromagnetic material is demonstrated in AOS under multiple pulses. Since the magnetic field needed to flip the ferromagnetic magnetization within femtosecond timescale is unphysically high, some theories hypothesized that there exists a prolonged magnetic field beyond the pulse duration in the switching process. This is intuitively inconsistent with the phenomenological explanation based on the light-induced magnetic field arising from the inverse Faraday effect (IFE). Here, we numerically study the AOS process and provide new insights into the long-standing paradox of the duration of the induced magnetic field. We show that the prolonged magnetic field duration originates from the ultrafast optothermal and optomagnetic coupling. Moreover, we numerically studied both single- and multiple-pulse AOS under different coupling strength between spins and the thermal bath in the macroscopic Fockker-Planck and Landau-Lifshitz-Bloch model. This numerical model may provide a guide to find suitable ferromagnetic materials for AOS.

Progress on complementary patterning using plasmon-excited electron beamlets (Conference Presentation)

Maskless lithography using parallel electron beamlets is a promising solution for next generation scalable maskless nanolithography. Researchers have focused on this goal but have been unable to find a robust technology to generate and control high-quality electron beamlets with satisfactory brightness and uniformity. In this work, we will aim to address this challenge by developing a revolutionary surface-plasmon-enhanced-photoemission (SPEP) technology to generate massively-parallel electron beamlets for maskless nanolithography. The new technology is built upon our recent breakthroughs in plasmonic lenses, which will be used to excite and focus surface plasmons to generate massively-parallel electron beamlets through photoemission. Specifically, the proposed SPEP device consists of an array of plasmonic lens and electrostatic micro-lens pairs, each pair independently producing an electron beamlet. During lithography, a spatial optical modulator will dynamically project light onto individual plasmonic lenses to control the switching and brightness of electron beamlets. The photons incident onto each plasmonic lens are concentrated into a diffraction-unlimited spot as localized surface plasmons to excite the local electrons to near their vacuum levels. Meanwhile, the electrostatic micro-lens extracts the excited electrons to form a focused beamlet, which can be rastered across a wafer to perform lithography. Studies showed that surface plasmons can enhance the photoemission by orders of magnitudes. This SPEP technology can scale up the maskless lithography process to write at wafers per hour. In this talk, we will report the mechanism of the strong electron-photon couplings and the locally enhanced photoexcitation, design of a SPEP device, overview of our proof-of-concept study, and demonstrated parallel lithography of 20-50 nm features.

Electrostatic microcolumns for surface plasmon enhanced electron beamlets

Microcolumns are widely used for parallel electron-beam lithography because of their compactness and the ability to achieve high spatial resolution. A design of a large array of electrostatic microcolumns for our recent surface plasmon enhanced photoemission sources is optimized numerically. Because of the compactness, one million of microcolumns can be put within 1 cm2 area. To avoid the trade-off between resolution and throughput, each microcolumn has one beamlet and there is no crossing point between any of the beamlets. An aperture self-aligned fabrication process is developed to make the optimized microcolumns.

Optical temperature mapping around plasmonic structures using directional anisotropy in fluorescence

Optically measuring temperature fields around plasmonic structures is of great importance for their thermal management considering the strong energy dissipations along with the extraordinary abilities of light coupling. Among all the available methods, ratiometric studies are particularly desirable since they suppress the influence of trivial factors, such as temporal fluctuations in excitation and spatial non-uniform distributions of fluorescent species, and thus gives reliable temperature dependence. Here we report a new ratiometric thermometry that simultaneously captures the fluorescence images of different numerical apertures (NAs) to resolve the temperature-dependent orientations of emission dipoles. This thermometry measures fluorescent anisotropy based on the directionality of emission. We show that this thermometry can be used to measure temperature near metallic surfaces. We foresee it to trigger interests of a large community who desire simultaneous thermal characterization along with the optical imaging. Moreover, it brings out a general idea to simplify ratiometric setups if inequalities exist on the excitation side, which may reach for a larger number of researchers.

Design and fabrication of electrostatic microcolumn in multiple electron-beam lithography

Microcolumns are widely used for parallel electron-beam lithography because of their compactness and the ability to achieve high spatial resolution. A design of an electrostatic microcolumn for our recent nanoscale photoemission sources is presented. We proposed a compact column structure (as short as several microns in length) for the ease of microcolumn fabrication and lithography operation. We numerically studied the influence of several design parameters on the optical performance such as microcolumn diameter, electrode thickness, beam current, working voltages, and working distance. We also examined the effect of fringing field between adjacent microcolumns during parallel lithography operations. The microcolumns were also fabricated to show the possibility.

Optothermal response of plasmonic nanofocusing lens under picosecond laser irradiation

This work studied the optothermal response of plasmonic nanofocusing structures under picosecond pulsed laser irradiation. The surface plasmon polariton is simulated to calculate the optical energy dissipation as the Joule heating source and the thermal transport process is studied using a two temperature model (TTM). At the picosecond time scale that we are interested in, the Fourier heat equation is used to study the electron thermal transport and the hyperbolic heat equation is used to study the lattice thermal transport. For comparison, the single temperature model (STM) is also studied. The difference between TTM and STM indicates that TTM provides more accurate estimates in the picosecond time scale and the STM results are only reliable when the local electron and lattice temperature difference is negligible.

Nanoscale Thermal Transport in Plasmonic Nanofocusing Structure With Strong Nonlocality

Nanoscale optical energy concentration and focusing is crucial for many high-throughput nanomanufacturing applications, such as material processing, imaging and lithography. The use of surface plasmons has resulted in the rapid development of nanofocusing devices and techniques at spatial confinements as good as a few nanometers associated with strong nonlocal plasmonic response. However, operations of these plasmonic nanofocusing structures usually require extremely high optical energy density at nanoscale, which leads to intense structure heating and causes unreliable device functions and short device lifetimes. In many plasmonic applications, optical heating has become a very important issue, which has not been investigated intensively yet. In these structures, the ballistic transport and interface scattering of the energy carriers both become significant because the characteristic lengths of the devices are comparable to or smaller than the mean free paths of the carriers. A comprehensive model is desired to understand the heat generation and transport inside the plasmonic nanofocusing structures.This work studied the electromagnetic and optothermal responses of plasmonic nanofocusing nanostructures. At the nanometer length scale, the local optical response and diffusive thermal model are no longer sufficient to describe the device optothermal response because of the strong interactions between energy carriers and the ballistic nature of carriers. Here, we used the hydrodynamic Drude model to consider the nonlocality of plasmonic response and calculate the heat generation inside the metallic nanostructures. Starting from Boltzmann transport equation, we derived the energy transport equations for both electron and phonon systems under the relaxation-time approximations. The obtained multi-carrier ballistic-diffusive model was used to study the non-equilibrium heat transports inside the structures. We assume that the ballistic electrons originate from boundaries and the electron-photon couplings inside the structure, experiencing out-scattering only in the material. The optically-generated “hot” electrons are considered as ballistic and are treated separately from the “ordinary” electrons which are in local thermal equilibrium and have significantly lower energies. Meanwhile, the electron-phonon couplings are considered under the non-equilibrium conditions between the electron and phonon systems. Using our model, we further investigated the transient optothermal responses of a one-dimensional (1D) plasmonic nanofocusing structure. In comparison to the diffusive transport description, our multi-carrier ballistic-diffusive model can more accurately describe the optothermal responses of the plasmonic nanofocusing structures which are crucial for predicting the performance and the lifetime of the plasmonic nanofocusing devices.Copyright