Ultra-compact Archimedes spiral plasmonic lens with a circular groove for low power optical trapping in the far-field region.

Particle levitation is crucial in optical trapping considering contamination and alteration of the character of the particle due to physical contact with the structure. A strong field gradient along the optical axis is required in this case. To manipulate the particle at a distance from the surface, we propose an Archimedes spiral plasmonic lens with a circular groove (CG-ASPL). The optical properties and parameters influencing the trapping performance of CG-ASPL are fully analyzed and discussed. By illuminating the structure with circular polarization and structure optimization, we can reduce the required optical power down to 2.4 mW for trapping particle of 1 µm in diameter with groove width and height of 100 and 125 nm, respectively. The particle can be stably trapped with trapping potential of 4138 kBT/W in the far-field region (1.1λ) owing to constructive interference of the scattered SPP waves. Furthermore, this structure is ultra-compact with a size of about 6.7 µm in diameter. We believe the results demonstrated in this work would be very useful for lab-on-a-chip applications and many others.

Photonic Crystal Polymeric Thin-Film Dye-Lasers for Attachable Strain Sensors.

In this report, using two-dimensional photonic crystals (PhC) and a one-dimensional PhC nano-beam cavity, we realized the development of all-polymeric dye-lasers on a dye-doped, suspended poly-methylmethacrylate film with a wavelength-scale thickness. In addition to the characterization of basic lasing properties, we also evaluated its capacity to serve as an attachable strain sensor. Through experimentation, we confirmed the stable lasing performances of the dye-laser attaching on a rough surface. Moreover, we also theoretically studied the wavelength responses of the utilized PhC resonators to stretching strain and further improved them via the concept of strain shaping. The attachability and high strain sensing response of the presented thin film PhC dye-lasers demonstrate their potential as attachable strain sensors.

Engineering surface lattice resonance of elliptical gold nanodisk array for enhanced strain sensing.

We demonstrate an elliptical gold nanodisk array (GNA) for engineering the spectral profile of surface lattice resonance (SLR). The nanodisk's shape has a great impact on SLR. Small linewidth of 20 nm at an aspect ratio of 1.17, as well as large wavelength tuning of 64 nm within 4% strain via different orientations and polarizations, are achieved experimentally. The enhanced wavelength response of 6.93 nm per 1% strain variation for elliptical GNA is 2.4 times better than that for general circular GNA. Furthermore, the strain sensing for elliptical GNA approaches is 5.7 times greater than that for circular GNA.

Compressible 1D photonic crystal nanolasers with wide wavelength tuning.

We propose and demonstrate a tunable photonic crystal nanolaser consisting of 1D periodic nanorods wrapped in deformable polydimethylsiloxane. In addition to low-threshold and long-term lasing stability, the nanolaser also displays reproducible and reliable wavelength tuning with a large tunability of 7.7 nm under 1% compression. By further associating with stretching, a very wide wavelength-tunable range of 155 nm that almost spans the entire S+C+L telecommunication bands is successfully demonstrated with a single nanolaser device.

Plasmonic behaviors of metallic AZO thin film and AZO nanodisk array.

Aluminum-doped zinc oxide (AZO) is well known as transparent conducting material for electro-optical devices, but is rarely used as plasmonic material, particularly on the localized surface plasmon resonance (LSPR) behavior of AZO nanostructure and its plasmonic devices. In this study, we systematically investigate the plasmonic behaviors of AZO thin films and patterned AZO nanostructures with various structural dimensions under different annealing treatments. We find that AZO film can possess highly-tunable, metal-like, and low-loss plasmonic property and the LSPR characteristic of AZO nanostructure is observed in the near-infrared (NIR) region under proper annealing conditions. Finally, environmental index sensing is performed to demonstrate the capability of AZO nanostructure for optical sensing application. High index sensitivity of 873 nm per refractive index unit (RIU) variation is obtained in experiment.

Effective optimization and analysis of white LED properties by using nano-honeycomb patterned phosphor film.

This study presents an approach for patterning a polydimethylsiloxane (PDMS) phosphor film with a photonic crystal nano-honeycomb structure on a blue chip package. A phosphor film with a nano-honeycomb structure was patterned and transferred using a nanosphere and used for fabricating remote white light-emitting diodes (w-LEDs). The angular correlated color temperature deviation of the remote phosphor LED could be improved by varying nano-honeycomb structure pitches (450, 750, and 1150 nm). In particular, w-LED samples with excellent color uniformity (ΔCCT ranging from 940 to 440 K) were fabricated from 750-nm w-LED samples with nano-honeycomb-patterned tops.

Enhanced power conversion efficiency in InGaN-based solar cells via graded composition multiple quantum wells.

This work demonstrates the enhanced power conversion efficiency (PCE) in InGaN/GaN multiple quantum well (MQWs) solar cells with gradually decreasing indium composition in quantum wells (GQWs) toward p-GaN as absorber. The GQW can improve the fill factor from 42% to 62% and enhance the short current density from 0.8 mA/cm2 to 0.92 mA/cm2, as compares to the typical MQW solar cells. As a result, the PCE is boosted from 0.63% to 1.11% under AM1.5G illumination. Based on simulation and experimental results, the enhanced PCE can be attributed to the improved carrier collection in GQW caused by the reduction of potential barriers and piezoelectric polarization induced fields near the p-GaN layer. The presented concept paves a way toward highly efficient InGaN-based solar cells and other GaN-related MQW devices.

Improvement of light quality by DBR structure in white LED.

This study demonstrates the application of DBR structure into the remote phosphor structure to improve the angular correlated color temperature (CCT) deviation in white light-emitting diodes (WLEDs). In the experiment, the LED device with DBR structure yielded a higher luminous efficiency than a conventional structure. The CCT deviation can be improved from 1758K to 280K in a range of -70 to 70 degree and the luminous flux increases more than 10% due to the enhancement of the light extraction of the blue light. Moreover, the reflectance of the different DBR structures is analyzed with different angles to reveal the reasons of such improvements. As the result, this LED device with DBR structure shows the great potential to use as the next generation lighting source.

Efficient hybrid white light-emitting diodes by organic-inorganic materials at different CCT from 3000K to 9000K.

The hybrid white light-emitting didoes (LED) with polyfluoren (PFO) polymer and quantum dot (QD) was investigated using dispensing method at the different correlated color temperature (CCT) for cool and warm color temperature. This result indicates that the hybrid white LED device has the higher luminous efficiency than the convention one, which could be attributed to the increased utilization rate of the UV light. Furthermore, the CIE 1931 coordinate of high quality white hybrid LED with different CCT range from 3000K to 9000K is demonstrated. Consequently, the angular-dependent CCT and the thermal issue of the hybrid white LED device were also analyzed in this study.

InN-based heterojunction photodetector with extended infrared response.

The combination of ZnO, InN, and GaN epitaxial layers is explored to provide long wavelength photodetection capability in the GaN based materials. Growth temperature optimization was performed to obtain the best quality of InN epitaxial layer in the MOCVD system. The temperature dependent photoluminescence (PL) can provide the information about thermal quenching in the InN PL transitions and at least two non-radiative processes can be observed. X-ray diffraction and energy dispersive spectroscopy are applied to confirm the inclusion of indium and the formation of InN layer. The band alignment of such system shows a typical double heterojunction, which is preferred in optoelectronic device operation. The photodetector manufactured by this ZnO/GaN/InN layer can exhibit extended long-wavelength quantum efficiency, as high as 3.55%, and very strong photocurrent response under solar simulator illumination.

Photonic crystal waveguide cavity with waist design for efficient trapping and detection of nanoparticles.

For manipulating nanometric particles, we propose a photonic crystal waveguide cavity design with a waist structure to enhance resonance characteristic of the cavity. For trapping a polystyrene particle of 50 nm radius on the lateral side of the waist, the optical force can reach 2308 pN/W with 24.7% signal transmission. Threshold power of only 0.32 mW is required for stable trapping. The total length of the device is relatively short with only ten photonic crystal periods, and the trapping can occur precisely and only at the waist. The designed cavity can also provide particle detection and surrounding medium sensing using the transmission spectrum with narrow linewidth. The simulated figure of merit of 110.6 is relatively high compared with those obtained from most plasmonic structures for sensing application. We anticipate this design with features of compact, efficient, and versatile in functionality will be beneficial for developing lab-on-chip in the future.

Hole injection and electron overflow improvement in InGaN/GaN light-emitting diodes by a tapered AlGaN electron blocking layer.

A tapered AlGaN electron blocking layer with step-graded aluminum composition is analyzed in nitride-based blue light-emitting diode (LED) numerically and experimentally. The energy band diagrams, electrostatic fields, carrier concentration, electron current density profiles, and hole transmitting probability are investigated. The simulation results demonstrated that such tapered structure can effectively enhance the hole injection efficiency as well as the electron confinement. Consequently, the LED with a tapered EBL grown by metal-organic chemical vapor deposition exhibits reduced efficiency droop behavior of 29% as compared with 44% for original LED, which reflects the improvement in hole injection and electron overflow in our design.

The aspect ratio effect on plasmonic properties and biosensing of bonding mode in gold elliptical nanoring arrays.

We investigate both numerically and experimentally the optical properties and biosensing of gold elliptical nanoring (ENR) arrays with various aspect ratios. The gold ENR exhibits a strong localized surface plasmon bonding mode in near-infrared region, whose peak wavelength is red-shifted as increasing the aspect ratio under longitudinal and transverse polarizations. Furthermore, the disk- and hole-like optical properties for longitudinal and transverse modes are observed, which cause different behaviors in field intensity enhancement. For biomolecule sensing, we find that both modes show increased surface sensitivities when enlarging the aspect ratio of gold ENR.

Photonic crystal nanofishbone nanocavity.

We propose a photonic crystal (PhC) nanofishbone (NFB) nanocavity, which confines an ultrahigh Q (~1.8 × 10(7)) transverse-magnetic (TM) mode. With thin slab thickness and only few PhC periods, the TM mode in NFB nanocavity shows higher Q, larger confinement factor and smaller mode volume than that in PhC nanobeam nanocavity, while the total etched-surface area is also significantly reduced. This PhC NFB nanocavity with very compact device size will be very beneficial for quantum cascade lasers, plasmonic nanolasers, and other applications needing high Q TM modes.

Super-high density Si quantum dot thin film utilizing a gradient Si-rich oxide multilayer structure.

A gradient Si-rich oxide multilayer (GSRO-ML) deposition structure is proposed to achieve super-high density Si quantum dot (QD) thin film formation while preserving QD size controllability for better photovoltaic properties. Our results indicate that the Si QD thin film using a GSRO-ML structure can efficiently increase the QD density and control the QD size. Its optical properties clearly promise the capability of effective bandgap engineering even though these QDs are closely formed. The Si QD thin film using a GSRO-ML structure obviously reveals better electro-optical properties than those using a [silicon dioxide/silicon-rich oxide] multilayer ([SiO2/SRO]-ML) structure owing to the better optical absorption and carrier transport properties. Therefore, we successfully demonstrate that our proposed GSRO-ML structure has great potential for application in solar cells integrating Si QD thin films.

Plasmonic coupling in gold nanoring dimers: observation of coupled bonding mode.

We investigate the optical properties of gold nanoring (NR) dimers in both simulation and experiment. The resonance peak wavelength of gold NR dimers is strongly dependent on the polarization direction and gap distance. As the gold NR particles approach each other, exponential red shift and slight blue shift of coupled bonding (CB) mode in gold NR dimers for longitudinal and transverse polarizations are obtained. In finite element method analysis, a very strong surface plasmon coupling in the gap region of gold NR dimers is observed, whose field intensity at the gap distance of 10 nm is enhanced 23% compared to that for gold nanodisk (ND) dimers with the same diameter. In addition, plasmonic dimer system exhibits a great improvement in the sensing performance. Near-field coupling in gold NR dimers causes exponential increase in sensitivity to refractive index of surrounding medium with decreasing the gap distance. Compared with coupled dipole mode in gold ND dimers, CB mode in gold NR dimers shows higher index sensitivity. This better index sensing performance is resulted form the additional electric field in inside region of NR and the larger field enhancement in the gap region owing to the stronger coupling of collective dipole plasmon resonances for CB mode. These results pave the way to design plasmonic nanostructures for practical applications that require coupled metallic nanoparticles with enhanced electric fields.

Dynamic single microparticle manipulation in the far-field region using plasmonic vortex lens multiple arms with a circular groove.

Recent development of particle manipulation has led to high demand for dynamic optical tweezer structures. However, confining and rotating a single microparticle in the far-field region with a uniform potential distribution remains a complicated task. A plasmonic vortex lens (PVL) has been proven to easily rotate the dielectric particle owing to its effect on orbital angular momentum (OAM). Here we propose and demonstrate PVL multiple arms with a circular groove (CG). The device consists of a multiple arm spiral slit that generates a plasmonic vortex (PV) and a circular groove to bring the PV from the surface to the far-field region. Numerical simulations are performed to calculate the intensity distribution of the primary ring, the optical force and potential. The primary ring size can be adjusted using different polarization directions. PVL 2-arms with a CG has primary ring sizes of 1082 nm under right-handed circular polarization (RCP) and 517 nm under left-handed circular polarization (LCP). Based on these primary ring sizes, a 1 µm polystyrene (PS) bead can be rotated under RCP with a minimum required power of 7.45 mW and trapped under LCP with a minimum required power of 11.84 mW. For PVL 4-arms with a CG under RCP illumination, we optimize the uniform potential distribution by carefully selecting the radius of the groove. Using a groove radius of 1050 nm, we obtain the potential difference between the smallest and largest depth along the x- and y-directions of only 70 k B T/W with a minimum required power of 14.86 mW. The method and design discussed here offer an efficient way to manipulate microparticles for micro-rotors, cell dynamic analysis, etc.

Lasing Emission from Soft Photonic Crystals for Pressure and Position Sensing.

In this report, we introduce a 1D photonic crystal (PhC) nanocavity with waveguide-like strain amplifiers within a soft polydimethylsiloxane substrate, presenting it as a potential candidate for highly sensitive pressure and position optical sensors. Due to its substantial optical wavelength response to uniform pressure, laser emission from this nanocavity enables the detection of a minimum applied uniform pressure of 1.6‱ in experiments. Based on this feature, we further studied and elucidated the distinct behaviors in wavelength shifts when applying localized pressure at various positions relative to the PhC nanocavity. In experiments, by mapping wavelength shifts of the PhC nanolaser under localized pressure applied using a micro-tip at different positions, we demonstrate the nanocavity's capability to detect minute position differences, with position-dependent minimum resolutions ranging from tens to hundreds of micrometers. Furthermore, we also propose and validate the feasibility of employing the strain amplifier as an effective waveguide for extracting the sensing signal from the nanocavity. This approach achieves a 64% unidirectional coupling efficiency for leading out the sensing signal to a specific strain amplifier. We believe these findings pave the way for creating a highly sensitive position-sensing module that can accurately identify localized pressure in a planar space.

Enhancing on/off ratio of a dielectric-loaded plasmonic logic gate with an amplitude modulator.

Plasmonic waveguides allow focusing, guiding, and manipulating light at the nanoscale and promise the miniaturization of functional optical nanocircuits. Dielectric-loaded plasmonic (DLP) waveguides and logic gates have drawn attention because of their relatively low loss, easy fabrication, and good compatibility with gain and active tunable materials. However, the rather low on/off ratio of DLP logic gates remains the main challenge. Here, we introduce an amplitude modulator and theoretically demonstrate an enhanced on/off ratio of a DLP logic gate for XNOR operation. Multimode interference (MMI) in DLP waveguide is precisely calculated for the design of the logic gate. Multiplexing and power splitting at arbitrary multimode numbers have been theoretically analyzed with respect to the size of the amplitude modulator. An enhanced on/off ratio of 11.26 dB has been achieved. The proposed amplitude modulator can also be used to optimize the performance of other logic gates or MMI-based plasmonic functional devices.

Efficient transportation of nano-sized particles along slotted photonic crystal waveguide.

We design a slotted photonic crystal waveguide (S-PhCW) and numerically propose that it can efficiently transport polystyrene particle with diameter as small as 50 nm in a 100 nm slot. Excellent optical confinement and slow light effect provided by the photonic crystal structure greatly enhance the optical force exerted on the particle. The S-PhCW can thus transport the particle with optical propulsion force as strong as 5.3 pN/W, which is over 10 times stronger than that generated by the slotted strip waveguide (S-SW). In addition, the vertical optical attraction force induced in the S-PhCW is over 2 times stronger than that of the S-SW. Therefore, the S-PhCW transports particles not only efficiently but also stably. We anticipate this waveguide structure will be beneficial for the future lab-on-chip development.