Dynamics of surface-plasmon lasing in planar metal gratings on semiconductor.

We investigate the dynamics of surface plasmon (SP) lasing in Au gratings fabricated on InGaAs with a period of around 400 nm, which locates the SP resonance near the semiconductor energy gap and facilitates efficient energy transfer. By optically pumping the InGaAs to reach the population inversion required for the amplification and the lasing, we observe SP lasing at specific wavelengths that satisfy the SPR condition depending on the grating period. The carrier dynamics in semiconductor and the photon density in the SP cavity was investigated from the time-resolved pump-probe measurement and the time resolved photoluminescence spectroscopy, respectively. Our results reveal that the photon dynamics is strongly correlated with the carrier dynamics and the lasing build-up is accelerated as the initial gain proportional to the pumping power increases, and this trend is satisfactorily explained using the rate equation model.

Cost-effective 400-Gbps micro-intradyne coherent receiver using optical butt-coupling and FPCB wirings.

We present a cost-effective and bandwidth-enhanced 64-Gbaud micro-intradyne coherent receiver based on hybrid integration of InP waveguide-photodetector (WG-PD) and silica planar lightwave circuit (PLC). InP waveguide-photodetector (WG-PD) arrays are simply chip-to-chip bonded and optically butt-coupled to a silica-based dual-polarization optical hybrid chip. Multiple flexible printed circuit boards are adapted for electrical RF and DC wirings, which provide low-cost integration and good RF performance of the receiver. A 3-dB bandwidth of the fabricated coherent receiver is extended to ~36 GHz by optimization of bondwire inductance between the WG-PD array and the transimpedance amplifier (TIA), even when commercial TIAs with a typical bandwidth of ~29 GHz are used. Through optimization of the silica hybrid integrated coherent receiver, 64-Gbaud DP-16QAM signal transmission over 1050-km standard single-mode fiber is successfully demonstrated below a bit error rate of 2 × 10-3. This is the threshold for a soft decision-based forward error correction, at the optical signal to noise ratio of 23.8 dB.

High-speed waveguide photodetector for 64 Gbaud coherent receiver.

A high-speed waveguide photodetector has been successfully fabricated for an integrated coherent receiver. Dual laterally tapered structures are introduced for a spot-size converter. We optimize the responsivity and the polarization-dependent loss of the spot-size converter-integrated waveguide photodetector through the beam propagation method simulation. The waveguide photodetector is designed with electrical as well as optical optimizations. The photodetector provides sufficient alignment tolerance, high responsivity of 0.73 A/W, and low polarization-dependent loss of 0.27 dB, which is in good agreement with the simulation results. By increasing the thickness of the matching layer and the n-doped upper taper, the electrical properties of the photodetector are enhanced. The photodetector has a 3 dB bandwidth of 45 GHz, providing high-speed operation. Through the electrical and optical optimizations, we successfully obtain the high-speed waveguide photodetector for a 64 Gbaud integrated coherent receiver.

Enhanced sulfurization reaction of molybdenum using a thermal cracker for forming two-dimensional MoS2 layers.

We propose a method to fabricate two-dimensional (2D) molybdenum disulfide (MoS2) layers to overcome issues in typical fabrication processes by promoting the sulfurization reaction of molybdenum (Mo). A thin sputtered-Mo layer was sulfurized using a sulfur (S) thermal cracker to form 2D MoS2 layers. The effects of key process parameters such as cracking-zone temperature (TC-zone), thickness of the sputtered-Mo layer, and Ar pressure during deposition of the Mo layer were systematically investigated. The degree of thermal treatment of evaporated S vapor is controlled by varying TC-zone. The higher TC-zone enabled easy formation of thin MoS2 layers at a low substrate temperature of 250 °C due to the greatly enhanced sulfurization reaction. The thickness of the final MoS2 layers was controlled by changing the initial thickness of the sputtered-Mo film. Ultra-thin MoS2 film about 2-layers-thick was obtained by sulfurizing a 2 Å-thick Mo film. The chemical state of the MoS2 layers largely depended on the Ar pressure during the sputtering process of the initial Mo. Lower Ar pressure enhanced MoS2 formation due to more efficient substitution of the MoS2 phase for the MoO3 phase. By using the S thermal cracker, we demonstrate a method to easily fabricate 2D MoS2 layers, excluding some problematic issues such as toxic and expensive reactants, non-vacuum conditions susceptible to contamination, and high substrate temperature.

Mid-IR spectroscopy of Fe:ZnSe quantum dots.

We report spectroscopic characterization of Fe:ZnSe quantum dots (for 2% of Zn/Fe molar ratio) fabricated by microemulsion hydrothermal synthesis. Mid-IR photoluminescence of the 5E↔5T2 transition of Fe2+ ions over 3.5-4.5 µm spectral range was observed in Fe:ZnSe quantum dot samples and kinetics of luminescence have been characterized at temperatures of 30-300 K under direct (2.788 µm) mid-IR excitation and indirect (0.355 µm) photoionization excitation. The radiative lifetime (τrad) was estimated from these measurements to be 48 µs while lifetime at room temperature was measured to be 440 ns. This agrees closely with the behavior of bulk material.

Light-soaking effects and capacitance profiling in Cu(In,Ga)Se2 thin-film solar cells with chemical-bath-deposited ZnS buffer layers.

We fabricated Cu(In,Ga)Se2 (CIGS) solar cells with chemical-bath deposited (CBD) ZnS buffer layers with different deposition times. The conversion efficiency and the fill factor of the CIGS solar cells reveal a strong dependence on the deposition time of CBD-ZnS films. In order to understand the detailed relationship between the heterojunction structure and the electronic properties of CIGS solar cells with different deposition times of CBD-ZnS films, capacitance-voltage (C-V) profiling measurements with additional laser illumination were performed. The light-soaking effects on CIGS solar cells with a CBD-ZnS buffer layer were investigated in detail using current density-voltage (J-V) and C-V measurements with several different lasers with different emission wavelengths. After light-soaking, the conversion efficiency changed significantly and the double diode feature in J-V curves disappeared. We explain that the major reason for the improvement of efficiency by light-soaking is due to the fact that negatively charged and highly defective vacancies in the CIGS absorber near the interface of CBD-ZnS/CIGS were formed and became neutral due to carriers generated by ultra-violet absorption in the buffer layer.

Strong optical modulation of surface plasmon polaritons in metal/semiconductor nanostructures.

We demonstrate strong modulation of the transmission around the surface plasmon polariton (SPP) resonance in metal/semiconductor hybrid nanostructures based on Ag film on top of InGaAs. The change in the real and imaginary parts of the refractive index due to photoexcited carriers in InGaAs generates a shift in the SPP resonance and enhanced transmission near the SPP resonance. Temporal evolution of the complex refractive index was traced by comparing the transient transmission with finite-difference time-domain (FDTD) simulations.

Lasing characteristics of InP-based InAs quantum dots depending on InGaAsP waveguide conditions.

We report the influences of a dot-in-a-well structure with a thin GaAs layer and the thickness of a waveguide (WG) on the lasing characteristics of InAs quantum dots (QDs) based on InP. The QD laser diodes (QDLDs) consist of seven-stacked InAs QDs separated by a 10 nm-thick InGaAsP (1.15 µm, 1.15Q-InGaAsP) layer, which is further sandwiched by a 800 nm-thick 1.15Q-lnGaAsP WG (reference QDLD). For comparison, the InAs QDs were inserted into the InGaAsP (1.35 µm, 1.35Q-InGaAsP) quantum well embedded in the 1.15Q-InGaAsP matrix at the active layer. And a 2 monolayer (ML)-thick GaAs layer was additionally introduced right before the QD layer (GDWELL-LDs). Lasing emission from the reference QDLD with only the 1.15Q-InGaAsP structure was not observed at room temperature (RT). However, the lasing emission from the GDWELL-LDs was clearly observed at the wavelength of 1.46 µm at RT under continuous-wave (CW) mode. The threshold current density of the GDWELL-LD with the 800 nm-thick InGaAsP WG was measured to be 830 A/cm2, which was lower than that of the GDWELL-LD with the 200 nm-thick WG (900 A/cm2). Also, the slope efficiency of the GDWELL-LD was significantly improved with increasing thickness of the InGaAsP WG.

Optimization of spot-size converter for low polarization dependent loss of waveguide photodetector.

We present an optimization of spot-size converter (SSC) of waveguide photodetector (PD) for small polarization dependent loss (PDL). Beam-propagation method simulation gives responsivity for each polarization and SSC structure. From the calculated responsivity data, optimum structure of SSC is determined that can be implemented with a sufficient process tolerance. We confirm the optimization by measuring PDL of waveguide PD designed according to the structure obtained through the simulation.

Enhanced in and out-coupling of InGaAs slab waveguides by periodic metal slit arrays.

We studied the in- and the out-coupling efficiencies of photons with a thin InGaAs slab covered by periodic gold nano-slit arrays, by measuring transmission and photoluminescence (PL) spectra. While the maximum in-coupled photons into the InGaAs slab waveguide were found at dip positions in transmission spectra, the mostly out-coupled photons were observed as peaks in PL spectra. For different periods of slit arrays and incident angles we discussed spectral positions of transmission dips and efficiency of the in-coupling influenced by the absorption coefficient of InGaAs. In PL spectra we measured overall enhanced PL intensities from the InGaAs slab covered by slit arrays compared to that of a bare InGaAs, where the peak positions are determined by the period of slit arrays as well. Our findings are important for designing semiconductors both as an optically passive waveguide and active light emitter.

Effects of High-Temperature Growth of Dislocation Filter Layers in GaAs-on-Si.

GaAs-on-Si templates with two different dislocation filter layers (DFLs) were grown at 550 °C low-temperature (LT)-DFL and 660 °C high-temperature (HT)-DFL using metal organic vapor-phase epitaxy and the effects of the growth temperature were studied. The threading dislocation density (TDD) values of LT-DFL and HT-DFL were 5.2 × 107 cm-2 and 1.5 × 107 cm-2, respectively. The 1.5 × 107 cm-2 of TDD in HT-DFL is reduced by almost one order compared to the 1.2 × 108 cm-2 of that in the control sample without DFLs. The annihilation process was mainly observed in the HT-DFL by a transmission electron microscope, resulting in a lower TDD. The 500-nm-thick GaAs bulk layer and InAs QDs were regrown on GaAs-on-Si templates and the optical properties were also evaluated by photoluminescence (PL). The highest PL peak intensity of the HT-DFL indicates that less non-radiative recombination in both the GaAs bulk and QDs occurred due to the reduced TDD. The GaAs p-i-n diodes were also fabricated to analyze the bulk leakage (JB) and the surface leakage current. The JB of HT-DFL shows the lowest value of 3.625 × 10-7 A/cm-2 at applied bias voltage of 1 V, which is 20 times lower than the JB of the control sample without DFLs. This supports that the high-temperature growth of DFL can make a good performance GaAs device on Si.

Layer-resolved release of epitaxial layers in III-V heterostructure via a buffer-free mechanical separation technique.

Layer-release techniques for producing freestanding III-V epitaxial layers have been actively developed for heterointegration of single-crystalline compound semiconductors with Si platforms. However, for the release of target epitaxial layers from III-V heterostructures, it is required to embed a mechanically or chemically weak sacrificial buffer beneath the target layers. This requirement severely limits the scope of processable materials and their epi-structures and makes the growth and layer-release process complicated. Here, we report that epitaxial layers in commonly used III-V heterostructures can be precisely released with an atomic-scale surface flatness via a buffer-free separation technique. This result shows that heteroepitaxial interfaces of a normal lattice-matched III-V heterostructure can be mechanically separated without a sacrificial buffer and the target interface for separation can be selectively determined by adjusting process conditions. This technique of selective release of epitaxial layers in III-V heterostructures will provide high fabrication flexibility in compound semiconductor technology.

The effect of hydrogen-bonds of amino acid-derived diacetylene by photopolymerization in supramolecular hydrogels.

Amino acids derived diacetylene hydrogelators 1 and 2 which have glycine or alanine moiety as head group were synthesized by several steps. The influences on the hydrogen-bonding interaction between amino acids moieties of hydrogelators in the polymerization by UV irradiation were investigated. The glycine-based hydrogel 1 resulted in the generation of stronger absorption peak at 540 nm at room temperature by UV-irradiation. Interestingly, the red color of the glycine-based diacetylene hydrogel 1 was gradually changed into blue color by decreasing temperature. Finally, the absorption band with 640 nm of glycine-based diacetylene hydrogel 1 showed at 77 K. On the other hand, the alanine-based hydrogel 2 had no significant color change at 77 K. These different color changes between the hydrogels 1 and 2 are attributed to the strong intermolecular hydrogen-bonding interaction between glycine moieties of the hydrogelator 1, as confirmed by FTIR observation. These results indicate that the hydrogen-bonding strength of the self-assembled hydrogel is a strong influence on the degree of polymerization.

A highly sensitive and selective turn-on fluorogenic and chromogenic sensor based on BODIPY-functionalized magnetic nanoparticles for detecting lead in living cells.

A new fluoro-chromogenic chemosensor based on BODIPY-functionalized Fe(3)O(4)@SiO(2) core/shell nanoparticles 1 has been prepared. Chemosensor 1 exhibits a high affinity and selectivity for Pb(2+) over competing metal ions tested. Moreover, confocal microscopy, and flow cytometry experiments established that 1 can be used for detecting Pb(2+) levels within living cell.

Lysine-functionalized silver nanoparticles for visual detection and separation of histidine and histidine-tagged proteins.

A new chromogenic chemosensor based on lysine-functionalized silver nanoparticles 1 was prepared and characterized by transmission electron microscopy (TEM), Fourier transform Raman, and ultraviolet-visible (UV-vis) spectroscopy. The color changes of nanoparticles 1 in the absence and the presence of metal ion were observed upon addition of various amino acids and proteins in aqueous solution. Among the various amino acids, the sensor 1 in the absence of metal ion shows a novel colorimetric sensor with capability to probe histidine and histidine-tagged proteins. On the other hand, the color changes of 1 in the presence of metal ions such as KCl or NiCl(2) did not occur with any amino acids. Therefore, the sensor 1 in the absence of metal ion responds selectively to histidine, a response which can be attributed to its aggregation induced by histidine with high numbers of electrostatic interactions. This highly selective sensor 1 allows a rapid quantitative assay of histidine to concentrations as low as 5.0 microM, providing a new tool for the direct measurement of histidine and histidine-tagged proteins in vitro system. Furthermore, we examined the effect of pH on absorbance (A(520)) of 1 in the presence of histidine (pH 4-12). The absorbance under basic conditions was higher than that under acidic or neutral conditions, in accord with the stronger aggregation of 1 with histidine by electrostatic interaction between the carboxylate anion of 1 and ammonium protons of histidine under basic conditions.

BODIPY-functionalized gold nanoparticles as a selective fluoro-chromogenic chemosensor for imaging Cu2+ in living cells.

A new fluoro-chromogenic chemosensor based on BODIPY-functionalized gold nanoparticles 1 is prepared. Addition of Cu(2+) ions to aqueous solutions of 1 gave an instantaneous color change along with a blue-shift of the absorption band and quenching of the emission spectrum at room temperature. The chemosensor 1 exhibits a high affinity and selectivity for Cu(2+) over competing metal ions tested. Moreover, confocal microscopy experiments establish that 1 can be used for detecting Cu(2+) levels within living cells.

A novel heterogeneous colorimetric and spectrophotometric chemosensor for detection and adsorption of Hg0 and Hg2+.

Functionalized mesoporous silica with an immobilized azobenzene-coupled receptor 1 (FMS-1) as heterogeneous "naked-eye" colorimetric and spectrophotometric chemosensor was prepared by sol-gel reaction. The optical sensing ability of FMS-1 was studied by addition of metal ions such as K+, Ca2+, Sr2+, Co2+, Cd2+, Pb2+, Zn2+, Fe3+, Cu2+ and Hg2+ in aqueous solution. Interestingly, upon the addition of Hg2+ in aqueous suspension, FMS-1 resulted in a color change from maroon to red within 10 s. On the other hand, no significant color changes were observed with the other metal ions. These findings confirm that FMS-1 can be useful as a chemosensor for selective detection of Hg2+ over a range of metal ions. Furthermore, the adsorption ability of the FMS-1 was also estimated by measuring the amount of Hg2+ and Hg0 adsorbed on the FMS-1, resulting in 95% for Hg2+ and 75% for Hg0, respectively, suggesting that the FMS-1 is potentially useful as the adsorbent for separation of Hg0 and Hg2+ in chromatography.

Facile preparation of self-assembled polymer nanotubes by proton beam irradiation.

We report the self-assembled nanotube formation and its polymerization from a sugar-based amphiphile 1 having a diacetylene functional group by proton beam irradiation. The self-assembled nanotubes were staibilized by photoreaction without the induction of any morphological changes. To further verify the polymerization of the self-assembled organic nanotube 1, we carefully measured the FT-Raman spectrum to examine changes in the diacetylene moiety of the self-assembled 1 by proton beam. After irradiation by proton beam for 30 s, the vibrational band for the diacetylene group at 1481 cm(-1) (V(C triple bond C)) completely disappeared whereas a new band appeared at 1510 cm(-1), which corresponds to the C = C stretching vibration region. This finding indicates that the irradiation method by proton beam is useful not only to polymerize self-assembled nanostructures with no morphology changes, but also to synthesize new functional nanomaterials.

A selective fluoroionophore based on BODIPY-functionalized magnetic silica nanoparticles: removal of Pb2+ from human blood.

Get the lead out: The title fluorescence receptor exhibits a high affinity and selectivity for Pb(2+) over competing metal ions in water (see picture) with an overall emission change of approximately 8-fold at the emission maximum for Pb(2+). The fluorescence receptor can remove 96 % of 100 ppb Pb(2+) from human blood, and can be useful and effective for the selective and rapid removal of Pb(2+) in vivo.

Broadband Surface Plasmon Lasing in One-dimensional Metallic Gratings on Semiconductor.

We report surface plasmon (SP) lasing in metal/semiconductor nanostructures, where one-dimensional periodic silver slit gratings are placed on top of an InGaAsP layer. The SP nature of the lasing is confirmed from the emission wavelength governed by the grating period, polarization analysis, spatial coherence, and comparison with the linear transmission. The excellent performance of the device as an SP source is demonstrated by its tunable emission in the 400-nm-wide telecom wavelength band at room temperature. We show that the stimulated emission enhanced by the Purcell effect enables successful SP lasing at high energies above the gap energy of the gain. We also discuss the dependence of the lasing efficiency on temperature, grating dimension, and type of metal.