Research

Nanophotonics, Plasmonics, Optoelectronics, and Beyond 

- through New Functional/Optical Materials and Nanostructure Design

Major research projects

Ligand Engineering and Recrystallization of Perovskite QD Thin Film for Low-Threshold Plasmonic Lattice Laser

Solution-process perovskite quantum dots (QDs) are promising materials to be utilized in photovoltaics and photonics with their superior optical properties. Advancements in top-down nanofabrication for perovskite are thus important for practical photonic and plasmonic devices. However, different from the chemically synthesized nano/micro-structures that show high quality and low surface roughness, the perovskite QD thin film prepared by spin-coating or the drop-casting process shows a large roughness and inhomogeneity. Low-roughness and low-optical loss perovskite QD thin film is highly desired for photonic and optoelectronic devices. Here, this work presents a pressure-assisted ligand engineering/recrystallization process for high-quality and well-thickness controlled CsPbBr3 QD film and demonstrates a low-threshold and single-mode plasmonic lattice laser. A recrystallization process is proposed to prepare the QD film with a low roughness (RMS = 1.3 nm) and small thickness (100 nm). Due to the low scattering loss and strong interaction between gain media and plasmonic nanoparticles, a low lasing threshold of 16.9 μJ/cm2 is achieved. It is believed that this work is not only important to the plasmonic laser field but also provides a promising and general nanofabrication method of solution-processed QDs for various photonic and plasmonic devices.

Self-Healing Lithographic Patterning of Perovskite Nanocrystals for Large-Area Single-Mode Laser Array

Lead halide perovskites exhibit extraordinary optoelectronic performances and are being considered as a promising medium for high-quality photonic devices such as single-mode lasers. However, for perovskite-based single-mode lasers to become practical, fabrication and integration on a chip via the standard top-down lithography process are highly desired. The chief bottleneck to achieving lithography of perovskites lies in their reactivity to chemicals used for lithography as illustrated by issues of instability, surface roughness, and internal defects with the fabricated structures. The realization of lithographic perovskite single-mode lasers in large areas remains a challenge. In this work, a self-healing lithographic patterning technique using perovskite CsPbBr3 nanocrystals is demonstrated to realize high-quality and -crystallinity single-mode laser arrays. The self-healing process is compatible with the standard lithography process and greatly improves the quality of lithographic laser arrays. A single-mode microdisk laser array is demonstrated with a low threshold of 3.8 μJ/cm2. Moreover, the control of the lasing wavelength is made possible over a range of 8 nm by precise fabrication of the laser cavity dimensions. The presented strategy for standard top-down lithography fabrication of high-quality perovskite devices enables research on large-area and low scattering loss perovskite-based integrated optoelectronic circuits.

Near-Zero-Index Slabs on Bloch Surface Wave Platform for Long-Range Directional Couplers
and Optical Logic Gates

Near-zero-index materials and structures, with their extraordinary optical behaviors of phase-free propagation resulting in directional radiation, provide a possible approach for directional coupling and optical logic gates in photonic integrated circuits. However, the radiation from the near-zero-index structures is limited to a short range of a few hundreds of nanometers. Bloch surface wave (BSW), an electromagnetic surface wave that can be excited at the interface between an all-dielectric multilayer and a dielectric medium with a low loss optical mode, provides a solution to increase the propagation length. In this work, we present a nanostructured near-zero-index slabs integrated on the all-dielectric metal-free BSW platform for long-range surface wave radiation. By employing the long-range directional surface-wave radiation, a directional coupler and optical logic gates based on the BSW near-zero-index slabs are realized. The proposed directional couplers achieve long coupling distances (electric field magnitude ratio between the input slab and output slab is 0.22 with a 50-μm coupling distance), which is two orders of magnitude longer than that of conventional directional couplers based on evanescent wave coupling. By controlling the constructive/destructive interferences of the BSW between the slabs, the XOR logic gate is experimentally demonstrated with a significant extinction ratio of 27.9 dB at telecommunications wavelengths. The BSW near-zero-index logic gates and the directional coupler with long-range light propagation provide an approach to the development of photonic integrated circuits and metal-free surface wave-based applications. 

Metallic Nanowire Coupled CsPbBr3
Quantum Dots Plasmonic Nanolaser

Plasmonic nanolasers provide a valuable opportunity for expanding sub-wavelength applications. Due to the potential of on-chip integration, semiconductor nanowire (NW)-based plasmonic nanolasers that support waveguide mode attract a high level of interest. To date, perovskite quantum dots (QDs) based plasmonic lasers especially nanolasers that support plasmonic-waveguide mode are still a challenge and remain unexplored. Here, we report metallic NW coupled CsPbBr3 QDs plasmonic waveguide lasers. By embedding Ag NWs in QDs film, an evolution from amplified spontaneous emission with an FWHM of 6.6 nm to localized surface plasmon resonance (LSPR) supported random lasing is observed. When the pump light is focused on a single Ag NW, a QD-NW coupled plasmonic-waveguide laser with a much narrower emission peak (FWHM = 0.4 nm) is realized on a single Ag NW with the uniform PVP layer. The QDs serve as the gain medium while the Ag NW serves as a resonant cavity and propagating plasmonic lasing modes. Furthermore, by pumping two Ag NWs with different directions, a dual-wavelength lasing switch is realized. The demonstration of metallic NW coupled QDs plasmonic nanolaser would provide an alternative approach for ultrasmall light sources as well as fundamental studies of light-matter interactions.

Self-Patterned CsPbBr3 Nanocrystal Based Plasmonic Hot-Carrier Photodetector at Telecommunications Wavelengths

Extending the photodetection range of lead halide perovskites into the near-infrared telecommunications bands can not only enable a wide-spectrum solar energy harvesting, but pave ways for important applications in biodetection, IR imaging, and telecommunication. However, there is a lack of an effective means to apply lead halide perovskites for efficient photodetection covering a wide wavelength range in the telecommunications bands. Here, we demonstrate CsPbBr3 nanocrystal- (NC-) based photodetector operating at wavelengths in the telecommunications bands via surface plasmon-induced hot holes. The internal photoemission of the plasmon-induced hot holes from plasmonic antennas into CsPbBr3 NCs enables a photocurrent at wavelengths around 1550 nm. Moreover, a solvent treatment is also performed on drop-casted CsPbBr3 NC thin film leading to a nanoscale self-patterning with a compact and uniform morphology. This treatment benefits the carrier transportation contributing to a detectable photoresponse and minimizes the optical scattering so that the CsPbBr3 NCs can be integrated into the plasmonic structure without damping its resonant feature. Consequently, the CsPbBr3 NC-based plasmonic hot-carrier device achieves polarization discrimination and wavelength-selective photodetection without additional optical components.

Light Switching with a Metal-Free Chiral-Sensitive Metasurface at Telecommunication Wavelengths

Controlling the direction of light propagation, or light switching, enables the addressing of individual optical elements in high-density and complex photonic integrated devices. Light switching is therefore crucial to the development of photonic/plasmonic integrated circuits. Chiral-sensitive metasurfaces using metallic nanostructures have been used to realize light switching by coupling incident light of different spins to surface plasmon polaritons propagating in different directions. However, surface plasmon polaritons-based devices suffer from short propagation lengths and narrow resonance wavelength ranges resulting from ohmic losses in their metal layers. Bloch surface waves can be seen as a metal-free analogy to surface plasmon polaritons with superior properties such as low propagation losses and wide operating wavelength ranges. Here, we demonstrate a metal-free chiral-sensitive Bloch-surface-wave switching circuit consisting of a carefully arranged array of U-shaped apertures, guiding slabs, and grating couplers. By engineering the amplitude and phase of the Bloch surface wave to achieve spin-controlled unidirectional coupling, control of the propagation direction of the Bloch surface waves is realized. Very high directional selectivity is reported at the telecommunications wavelength of 1550 nm, both theoretically at 23 dB and experimentally at 13.5 dB. The ability to realize spin-controlled light switching on a chip at telecommunications wavelengths using metal-free chiral-sensitive metasurfaces should benefit the development of low-loss on-chip photonic integrated devices. 

On-chip monolithically fabricated plasmonic-waveguide nanolaser

Plasmonic-waveguide lasers, which exhibit sub-diffraction limit lasing and light propagation, are promising for the next-generation of nanophotonic devices in computation, communication, and biosensing. Plasmonic lasers supporting waveguide modes are often based on nanowires grown with bottom-up techniques that need to be transferred and aligned for use in optical circuits. Here, we demonstrate a monolithically fabricated ZnO/Al plasmonic-waveguide nanolaser compatible with the fabrication requirements of on-chip circuits. The nanolaser is designed with a plasmonic metal layer on the top of the laser cavity only, providing highly efficient energy transfer between photons, excitons and plasmons, and achieving lasing in the ultraviolet region up to 330 K with a low threshold intensity (0.20 mJ/cm2 at room temperature). This work demonstrates the realization of a plasmonic-waveguide nanolaser without the need for transfer and positioning steps, which is the key for on-chip integration of nanophotonic devices.

Spectrally Selective Photocapacitance Modulation in Plasmonic Hot-Electron Nanochannels

Metallic nanostructures, which can sustain surface plasmon resonances (SPRs) exhibiting subwavelength light confinement, have been utilized in a wide range of optical devices. Due to the coupling of incident light and SPRs resulting in strong electric field enhancement and hot electron excitation at metal- semiconductor interfaces, plasmonic structures can be used to convert resonant light into charge carriers for spectrally-selective photodetection.

Here, a hybrid nanochannel structure consisting of semiconducting vertical channels sandwiched by metallic U-shaped nanolayers is demonstrated as a narrowband wavelength selective photodetector. Incident light at a resonant wavelength is guided and trapped in the channels by the coupling of localized surface plasmon resonances (LSPRs), vertical SPRs and horizontal SPRs. This coupling efficiently confines light at the metal-semiconductor interface, resulting in a narrowband absorption. The nanochannel structure forms a microcircuit with electrical properties strongly dependent on light irradiation due to the photoabsorption and plasmon induced hot-electrons. This microcircuit provides a practical means of monitoring the large and high-spectral-selectivity electrical impedance variation without resorting to far-field optical detection, which realize the miniaturized and high-resolution photodetection.

Light Manipulation with Optical Vortex
in Plasmonic Nanofin Cavity

Surface plasmons, collective oscillations of electrons on an interface between a metal and a dielectric, strongly couple incident light to the surface of the metal. These structures have received a great amount of attention as a possible means of light manipulation.  

Here, a 3D suspended nanofin-cavity structure and a U-cavity structure with a metallic mirror were designed on the basis of the plasmonic cavities consisting of metallic nanofins. Due to the coupling of the cavities and the plasmonic hot spots on the ridges, standing-wave resonances with enhanced electric fields in the horizontal and vertical directions are supported. With different coupled modes, strong loop-turn optical flows resulting narrow-band light reflectance and vortex optical flows resulting light trapping are generated in the nanofin-cavities and the U-cavities, respectively. The characteristics of these structures provide a new way to design IR/NIR filters, optical switches, chemical and biological sensor, and other applications in the visible to the IR regions.