Development of a liquid stub tuner for the KSTAR ICRF system

A liquid stub tuner was developed to transmit a MW level of RF power continuously in the KSTAR (Korean Superconducting Tokamak Advanced Research) ICRF (ion cyclotron range of frequencies) heating and current drive systems. It is made of a 9–3/16″ coaxial transmission line, and the property of the RF wavelength difference in a liquid and in a gas due to their different dielectric constants is utilized. Silicon oil with a relative dielectric constant of 2.74 was used as the liquid, because of its low vapor pressure and low dielectric loss. The liquid stub tuner components were equipped with electrostatic probes, thermocouples, and a humidity sensor to measure the RF voltages, the liquid temperature, and the humidity in the coaxial transmission line, respectively. The level of the liquid for the liquid stub tuner can be changed from 0 to 2.8 m, which corresponds to 2.0 m mechanical length variation at 30 MHz. The RF power test of the liquid stub tuner was performed with various conditioned silicon oils at 30 MHz to compare the high voltage characteristics among various conditioned silicon oils. The stand-off voltage of 43.82 kV (average) was achieved for the liquid stub tuner during the 300 s operation.     

Silver nanoparticle in situ growth within crosslinked poly(ester-co-styrene) induced by UV irradiation: aggregation control with exposure time

Silver nanoparticles (NPs) were photogenerated in situ in crosslinked poly(ester-co-styrene) resins (self-standing films and monoliths) by irradiating the samples with UV light. Addition of the silver salt solution did not interfere in the resin curing process and silver reduction was not detected during sample crosslinking. The samples were characterized by absorption spectroscopy and transmission electron microscopy. The initially broad and asymmetric surface plasmon resonance band was narrowed and blue-shifted as the exposure time to UV light was increased. Samples illuminated up to 120 min have an average particle size near 9.0 nm; a decrease to ∼5.0 nm was observed for longer exposure times up to 790 min. The asymmetric surface plasmon resonance band was due to particle aggregation; higher irradiation times led to a uniform particle distribution within the polymer matrix.     

Double plasmonic profile of tryptophan–silver nano-crystals—Temperature sensing and laser induced antimicrobial activity

Surface plasmon resonance (SPR) for spherical shaped silver nanoparticles showing double maxima at ～390 nm and ～520 nm respectively is reported. Self assembly of silver nanoparticles grown on tryptophan template leads to emergence of equal intensity double plasmon resonance (EIDPR). While for rod shaped nano-forms such double plasmon is explainable but for spherical shaped forms, such double plasmon can be explained on the basis of bidirectional formation of silver cluster in which attachment of silver at two nitrogen atom locations of tryptophan molecule seems to be obligatory. The absence of double resonance in case of silver nanoclusters formed with other amino acids or N-acetyl l-tryptophanamide (NATA), where bidirectional NH₂ attachment is not possible, validates the proposed EIDPR mechanism. Electron micrograph of EIDPR particle indicates a bi-periodic fringe pattern indicating unusual crystalline property. Apart from sensing tryptophan, the double plasmon peaks are sensitive to temperature. Furthermore, the particle can be used as a smart killing agent showing bactericidal activity only upon exposure to low power laser.     

Study of resonant modes in a 700 nm pitch macroporous silicon photonic crystal

In this study the modes produced by a defect inserted in a macroporous silicon (MP) photonic crystal (PC) have been studied theoretical and experimentally. In particular, the transmitted and reflected spectra have been analyzed for variations in the defect’s length and width. The performed simulations show that the resonant frequency is more easily adjusted for the fabricated samples by length tuning rather than width. The optimum resonance peak results when centered in the PC bandgap. The changes in the defect geometry result in small variations of the optical response of the PC. The resonance frequency is most sensitive to length variations, while the mode linewidth shows greater change with the defect width variation. Several MPS photonic crystals were fabricated by the electrochemical etching (EE) process with optical response in the range of 5.8 μm to 6.5 μm. Results of the characterization are in good agreement with simulations. Further samples were fabricated consisting of ordered modulated pores with a pitch of 700 nm. This allowed to reduce the vertical periodicity and therefore to have the optical response in the range of 4.4 μm to 4.8 μm. To our knowledge, modes working in this range of wavelengths have not been previously reported in 3-d MPS structures. Experimental results match with simulations, showing a linear relationship between the defect’s length and working frequency inside the bandgap. We demonstrate the possibility of tailoring the resonance peak in both ranges of wavelengths, where the principal absorption lines of different gases in the mid infrared are placed. This makes these structures very promising for their application to compact gas sensors.     

Refractive index sensors based on Ag-metalized nanolayer in microstructured optical fibers

We propose refractive index sensors based on Ag-metalized nanolayer in microstructured optical fibers. The surface plasmon resonance modes and the sensing properties are theoretically analyzed using finite element method (FEM). In the calculation, Drude–Lorentz model is used to describe the Metal Dielectric constant. The calculation results show that the sensitivity of Ag-metalized SPR sensor can reach 1500 nm/RIU corresponding to a resolution of 6.67 × 10^?5 RIU. Comparing with conventional detecting material-Au under the same structure, the sensitivity and 3 dB bandwidth of our device are better.     

Cross-shaped metal–semiconductor–metal plasmonic crystal for terahertz modulator

The transmission and tuning properties of a cross-shaped plasmonic crystal based on periodic metal–semiconductor–metal (MSM) structures have been investigated in the terahertz (THz) regime. According to the mode analysis, we find that the different resonance modes in the plasmonic crystal show the different changes when this device is actively controlled by the carrier injection of the MSM structures. The longitudinal modes disappear, while the horizontal mode moves to a higher frequency. The former leads to an intensity modulation at 0.5 THz and 1.1 THz when the groove depth h = 60 μm, and the later leads to a band blue-shift from 1.325 THz to 1.38 THz. These results will be applied to THz modulation and tunable filtering.     

Transmittance antiresonances,longitudinal resonances,and large photonic gaps in periodically modulated waveguides

Light transmission through complex-unit waveguides (WGs) with or without stubs attached to them periodically is studied theoretically. The transmittance through stubless WGs of widtha, with periodic dielectric contrast and two different subunits, exhibits longitudinal resonances due to bound states trapped in individual subunits that disappear when the optical lengths of the latter are sufficiently long. On the other hand, stubbed WGs can have bound states trapped in the stubs which may result in transmittance resonances in the gaps and antiresonances within the bands that can widen into new bands and gaps, respectively. The photonic gaps of the stubbed WGs, with or without dielectric contrast, can be several times larger than those of the corresponding one-dimensional quarter-wave structure for sufficiently shorta. This increase results from destructive interference between waves propagating along the main WG and those reflected from the stubs. Distributed-feedback-laser stubbed or stubless (with contrast) structures, of stub lengths ≈ a, show a much higherQthan that of conventional, weakly corrugated WGs.     

Manganese doping influence on the plasmon energy of nickel films

Manganese doping in nickel films capped with copper have been prepared by evaporation in vacuum. The films are composed of grains with an average diameter of ~ 20 nm from scanning electron microscope scans. Optical absorption is measured over a wavelength range of 190–450 nm. Two plasmon peaks are observed at 3.30 eV and 4.45 eV for a range of concentrations of films. The 4.45 eV peak is a bulk plasmon peak that is enhanced by increasing the manganese in nickel. The 3.30 eV peak is a surface plasmon peak that increases in width or strength of plasmon resonance with increasing concentration of manganese. This may be a combination effect of charge carrier concentration and dielectric screening from the reformed electronic band structure caused by manganese doping. By adding manganese into nickel, the ferromagnetic order is further destroyed as a transition into a spin glass occurs. This spin glass behavior is seen in a coercivity measurement at 4 K where the coercivity drops precipitously as the doping concentration increases.     

Bi-dipole excited plasmonic modes of an elongated gold nanorod

The first and higher-order longitudinal surface plasmon resonance (SPR) modes of an elongated gold nanorod (GNR) induced by the photoluminescence of two quantum dots (QDs) respectively located at the two ends were studied theoretically. Two configurations of a GNR combined with a symmetric or anti-symmetric bi-dipole were simulated and analyzed using the multiple multipole method. The results show that the local maxima of the radiative and nonradiative powers of the bi-dipole are at these modes. When the aspect ratio (AR) of GNR exceeds a specific value, not only the first mode but also the second, third and even fourth modes are generated. For example, for an elongated GNR (radius: 30 nm, AR=7) in water, the first, second, third and fourth modes are at 1800 nm, 930 nm, 680 nm and 600 nm, respectively. These SPR modes depend on the AR as well as the radius of GNR. The larger the AR is, the more the red-shift of these modes will be. In addition, the red-shift increases as the radius increases. Moreover, the odd modes are induced by the anti-symmetric bi-dipole, but suppressed by the symmetric one. On the contrary, the even modes are induced by the symmetric bi-dipole, but suppressed by the anti-symmetric one. In comparison with the scattering and absorption cross sections of GNR irradiated by a plane wave, the high-order modes, particularly the even modes, can be easily induced by the bi-dipole. Moreover, the mutual excitation rate of the two QDs is also enhanced through these modes of GNR.     

Fano-like electron–phonon interference in δ-doping GaAs superlattices

The interaction between electron excitations and LO phonons is studied by Raman scattering inδ-doping GaAs superlattices. The Raman spectra measured close to the E₀ +  Δ₀resonance of GaAs present Fano-like coupling of the LO phonons with the quasicontinuum single-particle electron excitations. Due to the self-consistent origin of the electron-energy spectrum in δ-doping superlattices the resonance of the Fano interference was found to be strongly dependent on the electron density as well as the excitation energy.     

Polarization detection analysis of dual-channel surface plasmon resonance sensing for silicone oils based on the D-shaped fiber with a central hole

We present and numerically characterize a dual channel surface plasmon resonance (SPR) sensor based on a D-shaped fiber with a central hole for silicone oil detections. The proposed design incorporates two metalized channels to facilitate the simultaneous detection of one group of silicone oils, which can consist of two different species. It has been demonstrated that the p-polarized input light can induce two peaks among surface plasmon resonance places, which come from the coupling between the core-guided mode and the fundamental surface plasmon polariton (SPP) modes at the D-shaped surface and around the central hole surface. However, the s-polarized input light can only induce one peak among surface plasmon resonance places, which comes from the coupling between the core-guided mode and the fundamental SPP mode around the central hole surface. The simulation results show that the characteristic responses of two channels independently correspond to the refractive index variations in the silicone oils with which they are in contact. A maximum sensitivity of 3500 nm/RIU (refractive index unit) and 4400 nm/RIU are achieved for channel A and B, respectively. This kind of sensor structure and polarization related demodulation method is promising in the simultaneous multi-analytes sensing applications in the future.     

Pump tuned wide tunable noncritically phase-matched ZnGeP2 narrow line width optical parametric oscillator

A line tunable singly resonant noncritically phase matched narrow band width ZnGeP₂ (ZGP) optical parametric oscillator pumped by the output idler radiation from a KTA OPO based on a 20 mm long KTA crystal pumped from a Q-switched Gaussian shaped Nd:YAG laser beam with a grating having grooves density 85 lines/mm has been demonstrated in the spectral ranges of 3–7 μm. The measured threshold of oscillation energy was 10 μJ. The conversion efficiency was 20.5% and slope efficiency of the ZGP OPO was 20% using a 23 mm long ZGP crystal at 26 mm cavity length. Line width of the generated infrared radiation from ZGP OPO was 37–60 nm.     

Interferometric profiling of microcantilevers in liquid

Silicon micro cantilevers are used as highly sensitive transducers for a wide range of physical, chemical and biochemical stimuli. Vibrating the cantilevers at higher-order resonant modes can achieve extra sensitivity, but the difficulty lies in determining exactly which modes are excited in the cantilever. This problem is exacerbated for cantilever sensors operating in liquid where the computational analysis of the resonance modes is very challenging. Using strobed interferometric microscopy, we are able to image the dynamic behavior of individual (100×500×1 μm³) cantilevers in an eight cantilever array over frequencies from 0–1 MHz. We show how some modifications to the interferometric microscope allow for the spatial visualization of 16 longitudinal modes of cantilevers working in liquid with nanometer-scale amplitudes. We also compare the shift in frequency response and reduction in quality factor for cantilevers resonating in liquid versus in air and simulations in vacuum. Because the resonant frequency spectrum is fairly complex and does not follow simple intuition, our work maps the actual behavior of cantilevers without having any specific knowledge of the sample and environment parameters and without the necessity of involved simulations and calculations.     

Investigation on two dimensional photonic crystal resonant cavity based bandpass filter

In this paper, different cavities, namely quasi square, tri-quarter square, square, hexagonal, circular, elliptical, diamond and annular ring based photonic crystal bandpass filters (PCBPFs) are proposed and investigated by exploiting coupling between two in-line quasi waveguides and a resonant cavity. The resonant wavelength, output efficiency and bandwidth of designed PCBPFs are studied by varying the size of the cavity. The normalized transmission spectra of PCBPF are observed using 2D finite difference time domain (FDTD) method. The photonic band gap (PBG) is calculated by plane wave expansion (PWE) method. The circular cavity based PCBPF gives better performance than others because of its circular resonating modes. The number of passbands is increased linearly while increasing the size of the cavity. The overall size of the PCBPF chip is about 11.4 μm × 11.4 μm, which is more suitable for photonic integrated circuits (PIC), wavelength division multiplexing (WDM) systems and sensing applications.     

Fabrication of gold nano-particle arrays using two-dimensional templates from holographic lithography

We report a simple and cost effective method to fabricate regular metallic particle arrays over large areas with good regularity by using holographic lithography interference for the study of localized surface plasmon. Samples of disk-shaped gold nano-particles arranged in square arrays with lattice spacing ranging from 300 nm to 600 nm were successfully fabricated on glass substrates first by sputtering a thin gold layer onto two-dimensional photoresist templates of hole arrays in square lattice obtained by the holographic method and then removing the photoresist by a lift-off procedure. The plasmonic resonance of the gold nano-particle arrays due to the change of morphology by thermal annealing was studied. The disk-shaped gold nano-particles were found to become more round shaped upon heating and blue shift of the extinction plasmonic band was observed. The results were explained with model calculations using spheroidal particles.     

Tailoring electron–phonon interaction in nanostructures

Efficient design of optoelectronic devices based on electron intersubband transitions depends critically on the knowledge of the intersubband relaxation times which in turn, depends on electron scattering with LO and acoustic phonons. In this article the intersubband scattering time associated with electron–acoustic-phonon interaction has been discussed in terms of phonon mode quantization and phonon confinement with describing the acoustic phonon dispersion relation in detail by introducing the cut-off frequency for each mode. It has been shown that the quantization of acoustic phonon modes lead to an enhancement in electron–phonon scattering time in AlGaAs quantum well structures. Based on the presented model, a new tailoring method has presented to adjust the electron–phonon scattering time in intersubband-transition-based structures while keeping the electronic properties unaltered. Also, we illustrated that for a quantum well with subband energy separation of ∼30 meV, the intersubband scattering time with acoustic-phonon-assisted transitions could be tailored from ∼120 ps to increased value of ∼400 ps or reduced value of ∼45 ps by inserting a 1 nm-thickacoustically soft or hard layers, respectively, while keeping the same the initial energy separation.     

Increased spectral sensitivity of Si photodetector by surface plasmon effect of Ag nanowires

Highly-sensitive Si photodetectors were prepared by using Ag nanowires (AgNWs). A transparent indium-tin-oxide (ITO) coating was coated on a Si substrate followed by spin-coating of AgNWs-containing solution. AgNWs having average length of 5–20 μm with a diameter of about 40–60 nm were observed in FESEM images. The haze effect of AgNWs was totally avoided because of the optimum value of diameter. The transmittance of above 85% was shown by AgNWs over a broad spectral range due to surface plasmon resonance effect. The AgNW-coated device showed an excellent rectifying ratio of 288. Under light illumination, AgNWs-coated device exhibited a significant photoresponse ratio of 5373. This advanced feature of AgNWs-templated method would be applied in broadband wavelength photodetection devices.     

Reflective plasmonic waveplates based on metal–insulator–metal subwavelength rectangular annular arrays

We propose and present a quarter-wave plate using metal–insulator–metal (MIM) structure with sub-wavelength rectangular annular arrays (RAA) patterned in the upper Au film. It is found that by manipulating asymmetric width of the annular gaps along two orthogonal directions, the reflected amplitude and phase of the two orthogonal components can be well controlled via the RAA metasurface tuned by the MIM cavity effect, in which the localized surface plasmon resonance dip can be flattened with the cavity length. A quarter-wave plate has been realized through an optimized design at 1.55 μm, in which the phase difference variation of less than 2% of the π/2 between the two orthogonal components can be obtained in an ultra-wide wavelength range of about 130 nm, and the reflectivity is up to ∼90% within the whole working wavelength band. It provides a great potential for applications in advanced nanophotonic devices and integrated photonic systems.     

Spectral characteristics of polymer micro-fiber MZI near 1550 nm

The authors have produced the polymer micro-fiber with a highly optical conductive efficiency of 83% and 89% for the pump light of 532 nm and 1550 nm, respectively. The authors constructed a Mach–Zehnder Interferometer (MZI) by a micro-manipulation method and measured the different interference spectra by micro-adjusting the path difference of the dual interference arms of MZI under a microscope. Due to the path difference, the coherent length of the corresponding spectrum continuously and slightly decreases from 20 μm, 13.5 μm, 10.6 μm to 8 μm. The relationships between this particular MZI structure and the surrounding temperature, as well as the refractive index changes can be determined via the evanescent field and the thermally induced expansion or contraction effect, which will be reflected in the interference spectrum.     

Exciton-polariton state in nanocrystalline SiC films

We studied the features of optical absorption in the films of nanocrystalline SiC (nc-SiC) obtained on the sapphire substrates by the method of direct ion deposition. The optical absorption spectra of the films with a thickness less than ~500 nm contain a maximum which position and intensity depend on the structure and thickness of the nc-SiC films. The most intense peak at 2.36 eV is observed in the nc-SiC film with predominant 3C-SiC polytype structure and a thickness of 392 nm. Proposed is a resonance absorption model based on excitation of exciton polaritons in a microcavity. In the latter, under the conditions of resonance, there occurs strong interaction between photon modes of light with λ_ph=521 nm and exciton of the 3С polytype with an excitation energy of 2.36 eV that results in the formation of polariton. A mismatch of the frequencies of photon modes of the cavity and exciton explains the dependence of the maximum of the optical absorption on the film thickness.