Myungjae Lee

A wearable multiplexed silicon nonvolatile memory array using nanocrystal charge confinement

An ultrathin, stretchable memory skin patch that can reliably store bio-signals such as heart rate is newly developed. Strategies for efficient charge confinement in nanocrystal floating gates to realize high-performance memory devices have been investigated intensively. However, few studies have reported nanoscale experimental validations of charge confinement in closely packed uniform nanocrystals and related device performance characterization. Furthermore, the system-level integration of the resulting devices with wearable silicon electronics has not yet been realized. We introduce a wearable, fully multiplexed silicon nonvolatile memory array with nanocrystal floating gates. The nanocrystal monolayer is assembled over a large area using the Langmuir-Blodgett method. Efficient particle-level charge confinement is verified with the modified atomic force microscopy technique. Uniform nanocrystal charge traps evidently improve the memory window margin and retention performance. Furthermore, the multiplexing of memory devices in conjunction with the amplification of sensor signals based on ultrathin silicon nanomembrane circuits in stretchable layouts enables wearable healthcare applications such as long-term data storage of monitored heart rates.

A Highly Tunable and Fully Biocompatible Silk Nanoplasmonic Optical Sensor

Novel concepts for manipulating plasmonic resonances and the biocompatibility of plasmonic devices offer great potential in versatile applications involving real-time and in vivo monitoring of analytes with high sensitivity in biomedical and biological research. Here we report a biocompatible and highly tunable plasmonic bio/chemical sensor consisting of a natural silk protein and a gold nanostructure. Our silk plasmonic absorber sensor (SPAS) takes advantage of the strong local field enhancement in the metal-insulator-metal resonator in which silk protein is used as an insulating spacer and substrate. The silk insulating spacer has hydrogel properties and therefore exhibits a controllable swelling when exposed to water-alcohol mixtures. We experimentally and numerically show that drastic spectral shifts in reflectance minima arise from the changing physical volume and refractive index of the silk spacer during swelling. Furthermore, we apply this SPAS device as a glucose sensor with a very high sensitivity of 1200 nm/RIU (refractive index units) and high relative intensity change.

Two-dimensional photonic crystal bandedge laser with hybrid perovskite thin film for optical gain

We report optically pumped room temperature single mode laser that contains a thin film of hybrid perovskite, an emerging photonic material, as gain medium. Two-dimensional square lattice photonic crystal (PhC) backbone structure enables single mode laser operation via a photonic bandedge mode, while a thin film of methyl-ammonium lead iodide (CH3NH3PbI3) spin-coated atop provides optical gain for lasing. Two kinds of bandedge modes, Γ and M, are employed, and both devices laser in single mode at similar laser thresholds of ∼200 μJ/cm2 in pulse energy density. Polarization dependence measurements reveal a clear difference between the two kinds of bandedge lasers: isotropic for the Γ-point laser and highly anisotropic for the M-point laser. These observations are consistent with expected modal properties, confirming that the lasing actions indeed originate from the corresponding PhC bandedge modes.

Eco-friendly photolithography using water-developable pure silk fibroin

We report that pure silk fibroin can be a green and biofunctional photoresist for deep ultraviolet photolithography. All processes are entirely water-based, from resist solvent to resist removal, and rely on the phototendering effect that decreases the crystallinity of silk fibroin films by DUV exposure. Additionally, the potential decrease in activity of bio-dopants due to high-energy irradiation is irrelevant to our positive-tone lithographic method.

Competition between electron doping and short-range scattering in hydrogenated bilayer graphene on hexagonal boron nitride

We studied the electron doping of bilayer graphene (BLG) on hexagonal boron nitride (h-BN) by dissociative H2 adsorption. Charge transfer phenomena were investigated by the gate voltage (Vg)-dependent electrical conductivity σ(Vg) and Raman spectroscopy with respect to the H2 exposure. The shift of the charge neutrality point toward the negative Vg region was observed and the charge scattering mechanism was found with the variation of σ(Vg). The charge transfer at the interface as well as the lattice distortion of BLG due to hydrogenation were verified by Raman spectroscopy. From these results, we concluded that the electron doping and short-range scattering in the BLG exposed to high H2 pressure (11 bar) are intrinsic features, which were achieved using a van der Waals interface consisting of BLG and h-BN.

Anderson localizations and photonic band-tail states observed in compositionally disordered platform

The strength of photon localization in the band-tail states is determined by the degree of disorder and state energy. Anderson localization in random structures is an intriguing physical phenomenon, for which experimental verifications are far behind theoretical predictions. We report the first experimental confirmations of photonic band-tail states and a complete transition of Anderson localization. An optically activated photonic crystal alloy platform enables the acquisition of extensive experimental data exclusively on pure eigenstates, revealing direct evidence of band-tail states and Anderson localization transition within the band-tail states. Analyses of both experimental and simulated data lead to a comprehensive picture of photon localization that is highly consistent with theories by Anderson and others. We believe that our results provide a strong experimental foundation upon which both the fundamental understandings and application possibility of Anderson localization can be promoted significantly.

Efficient on-chip integration of a colloidal quantum dot photonic crystal band-edge laser with a coplanar waveguide

High-density photonic integrated circuits (PICs) are expected to replace their current electronic counterparts in the future. The most crucial prerequisite for realizing successful PICs is to develop a low-loss coupling technique between active and passive photonic components based on various nanoscale materials and devices. Here we propose and demonstrate an on-chip integration technique in which a high-refractive-index layer constitutes the coplanar structural backbone across the entire PIC chip. To prove the concept, patterns of a two-dimensional photonic crystal (PhC) band-edge laser and grating couplers are engraved into the backbone layer, and colloidal quantum dots (CQDs) for optical gain are selectively deposited in the PhC area by a conventional lift-off process. Using optical excitation, we observe that the CQD–PhC structure emits coherent single-mode laser light, which is subsequently coupled to and propagates through an adjacent slab waveguide in the well-defined directions corresponding to the selected band-edge point, finally emerging through the grating coupler. Our study demonstrates a simple but highly suggestive PIC platform that automatically guarantees high coupling efficiencies between the micro- and nanophotonic devices to be integrated (through high degrees of modal matching in the vertical direction) and will therefore advance the development of high-density PIC technologies.

Electrical modulation of a photonic crystal band-edge laser with a graphene monolayer

The electrical control of photonic crystal (PhC) lasers has been an attractive but challenging issue. Laser operation by electrical injection is of key importance for the viability and applicability of the PhC lasers. Another key factor is the electrical modulation of the laser output. The Fermi level of a graphene monolayer can be controlled by electrical gating, which adjusts its optical absorption. In this study, a graphene monolayer sheet is integrated on top of a two-dimensional PhC structure composed of InGaAsP multiple-quantum-wells (MQWs) in order to demonstrate the electrical modulation of a high-power (microwatt-scale) PhC band-edge laser. The introduced dielectric spacer layer presets the delicate balance between the optical gain from the MQWs and optical loss at the graphene monolayer. The proposed device is covered by an ion-gel film, which enables a low-voltage laser modulation at |Vg|≤1 V. The modulation is extensively investigated experimentally, and the obtained results are confirmed by performing numerical simulations.

Colloidal quantum dot photonic crystal lasers with M-point band-edge emission

Summary form only given. Utilizing colloidal quantum dots (CQDs) as a gain material in a passive two-dimensional photonic crystal (PC) backbone structure, we have experimentally demonstrated room-temperature lasing operation at band-edge modes. A silicon nitride PC slab on silica substrate contains a square lattice array of air-holes, which is designed to have an in-plane, M-point low index band-edge mode. The air-hole array is then infiltrated by colloidal quantum dots using simple spin-coating method.

Stimuli responsive plasmonic resonator and its sensing application

A fully biocompatible and tunable plasmonic resonator consisting of silk protein and gold nanostructure is demonstrated. The silk plasmonic absorber sensor is based on the metal-insulator-metal resonator exhibits stimuli responsive optical properties.