Angle-Resolved Plasmonic Properties of Single Gold Nanorod Dimers.

Through wet-chemical assembly methods, gold nanorods were placed close to each other and formed a dimer with a gap distance ~1 nm, and hence degenerated plasmonic dipole modes of individual nanorods coupled together to produce hybridized bonding and antibonding resonance modes. Previous studies using a condenser for illumination result in averaged signals over all excitation angles. By exciting an individual dimer obliquely at different angles, we demonstrate that these two new resonance modes are highly tunable and sensitive to the angle between the excitation polarization and the dimer orientation, which follows cos2 φ dependence. Moreover, for dimer structures with various structure angles, the resonance wavelengths as well as the refractive index sensitivities were found independent of the structure angle. Calculated angle-resolved plasmonic properties are in good agreement with the measurements. The assembled nanostructures investigated here are important for fundamental researches as well as potential applications when they are used as building blocks in plasmon-based optical and optoelectronic devices.

Spectrum evolution of cascaded mismatching long-period fiber gratings.

Two identical long-period fiber gratings (LPFGs) are difficult to produce in practice, notably for CO(2)-laser or arc-discharge induced LPFGs. So it is meaningful to study the spectrum evolution of cascaded LPFGs with some deviation, named "mismatching" LPFGs. Theory and experiment demonstrate that the upper envelope of the fringe pattern is intrinsically curved but less dramatically than the lower envelope. Bending the grating pair to introduce proper cladding-mode loss can improve the contrast of the fringe pattern at a desired wavelength, which indicates that cascaded mismatching LPFGs can be used as high-quality comb filters or fiber sensors.

Multiwavelength erbium-doped fiber laser based on a nonlinear amplifying loop mirror assisted by un-pumped EDF.

A multiwavelength erbium-doped fiber (EDF) laser based on a nonlinear amplifying loop mirror (NALM) is proposed and experimentally demonstrated. The NALM provides intensity-dependent transmissivity to equalize different-wavelength powers and the transmission can be uniquely optimized by controlling the cavity loss associated with a section of un-pumped EDF, which also enhances the output signal-to-noise ratio (SNR). Through adjusting the polarization controllers (PCs), under only 70 mW pump power, up to 62-wavelength output with channel spacing of 0.45 nm has been achieved. Also, the lasing tunability and stability are verified.

Ultrafine optical-frequency tunable Brillouin fiber laser based on fiber strain.

We present an ultrafine tunable single-frequency Brillouin fiber laser in which the fiber Brillouin gain also acts as a high-precision tunable filter. Controlling the strain on the fiber, the Stokes frequency shift may be precisely changed, and thus the lasing frequency can be tuned. Assisted by the frequency pulling effect, this method provides continuous ultrafine tuning ability for ~30% of the free spectral range without mode hopping, and can cover 271 MHz. The experiments showed that the laser achieves tuning step of 60 kHz.

Superluminal propagation at negative group velocity in optical fibers based on Brillouin lasing oscillation.

We report superluminal propagation in optical fibers using Brillouin lasing oscillation in a ring cavity. Negative group velocity propagation through a 10-m single mode fiber has been experimentally demonstrated. An advancement of 221.2 ns was observed before the input signal, which was achieved with a very high slope efficiency of 211.3 ns/dB. This indicates that this way is suitable for long-distance low-loss superluminal propagation via optical fibers. Correspondingly, the group velocity is -0.151c and the group index is -6.636-the highest group velocity ever reported for optical fibers.

Tunable polarization beam splitting based on a symmetrical metal-cladding waveguide structure.

Electrical tuning of polarization beam splitting is demonstrated in the structure of symmetrical metal-cladding waveguide by introducing optically nonlinear material into both the coupling prism and the guiding layer. Due to the anisotropy of the coupling material, different excitation conditions for TE and TM modes are obtained, which results in polarization-dependent reflections and transmissions. And the splitting effect of the two orthogonally polarized beams can be manipulated through an electrical modulation of the guiding layer properties.

Enhancement of the superprism effect based on the strong dispersion effect of ultrahigh-order modes.

It is demonstrated that the superprism effect is greatly enhanced in the configuration of a symmetrical metal-cladding waveguide owing to the strong dispersion effect of ultrahigh-order modes. The experimental result shows that a notable spatial displacement of the reflected beam as large as 0.9 mm is achieved within a variation of 0.15 nm in wavelength.

Oscillating wave displacement sensor using the enhanced Goos-Hänchen effect in a symmetrical metal-cladding optical waveguide.

An oscillating wave displacement sensor based on the enhanced Goos-Hänchen (G-H) effect in a symmetrical metal-cladding optical waveguide is proposed. Since the detected signal is irrelevant to the power fluctuation of the incident light and the magnitude of the G-H shift is enhanced to hundreds of micrometers, a 40 pm resolution is demonstrated in our experiment without employing any complicated optical equipment and servo techniques.

Measurement of the second hyperpolarizability of linear conjugated polymer based on attenuated-total-reflection technique.

Based on the attenuated-total-reflection technique, a new method has been proposed to determine the second hyperpolarizability gamma(-omega(4);omega(3),omega(2),omega(1)) of the linear conjugated polymer in the off-resonant region by means of quadratic electro-optic effect. An important feature of this method is the absence of a high-power pulse laser, resulting in more convenience and cost effectivity than other techniques.

Observation of large positive and negative lateral shifts of a reflected beam from symmetrical metal-cladding waveguides.

Both large positive and negative lateral shifts were observed for the reflected light beam on a symmetrical metal-cladding waveguide. The positive and negative shifts approach about 480 and 180 microm, respectively, which to our knowledge are the largest experimental results ever reported. The experiment also proves that the positive or the negative shift depends on sign of the difference between the intrinsic and radiative damping.

Effect of nonparallelism of guiding air-liquid layers on the reflection dip in attenuated total reflection.

The effect attributable to the nonparallelism of the guiding layer of a planar waveguide on the reflection dip of the attenuated total reflection (ATR) spectrum is investigated. It is considered that the reflected light from a nonparallel waveguide is, indeed, the superposition of a series of beams propagating in different directions. The ATR spectra are numerically calculated with various inclination values. Compared with those of the corresponding parallel ones, with the inclination of waveguides exceeding 1 microrad, the characteristics of the ATR spectra begin to differ considerably, and when the inclination reaches 10 microrad, the sensitivity of sensors based on the waveguide is reduced by 62.7% and 67.3% for the free space coupling structure and the prism coupling structure, respectively. When the inclination becomes larger than 100 microrad, the ATR phenomenon cannot be observed any longer.

Experiment and theoretical explanation of optical bistability in a single erbium-doped fiber ring laser.

Optical bistability phenomenon in a single fiber ring laser employing erbium-doped fiber (EDF) as gain medium is observed in our experiment. In the EDF amplifiers, the photons of pump light cannot completely transfer into that of signal light because of various attenuation factors. This part of loss (useless pump loss) and active spontaneous emission (ASE) can both lower the small-signal gain of the EDF, and can eventually result in the bistability phenomenon. The range of this bistability has a complicated relationship with the length of the EDF, the erbium-ion doping concentration, the cavity loss and the useless loss coefficient of the pump light.

Large positive and negative lateral optical beam shift in prism-waveguide coupling system.

In this paper, the lateral beam shift in a prism-waveguide coupling system at wavelengths ranging from visible to near infrared is theoretically examined. A simple theoretical formula is derived to analyze the behavior of the beam shift. We demonstrate that large positive and negative lateral optical beam shifts can be obtained when guided modes are excited. It is also found that the magnitude of the beam shift is closely related to the intrinsic and radiative damping. When the intrinsic damping is larger than the radiative damping, negative lateral beam shift occurs. Numerical calculations confirm the theoretical analysis and show that a beam shift of the order of millimeters is possible.

Solution to causality paradox upon total reflection in optical planar waveguide.

A dispute about the existence of an additional time associated with the Goos-Hänchen shift has recently arisen. By analyzing light propagation in an optical planar waveguide with both the zigzag-ray model and the electromagnetic theory, we show in this paper that the Goos-Hänchen time really exists, and the total time delay upon total reflection is the sum of the group delay time and the Goos-Hänchen time. The causality paradox of total reflection of a TM wave upon an ideal nonabsorbing plasma mirror is also solved with the consideration of a negative Goos-Hänchen shift.

Investigation of second-order nonlinearity in poled-polymer during photobleaching.

The second-order nonlinearity of poled-polymer during photobleaching process is monitored with second harmonic generation(SHG) intensity enhanced by surface plasmon resonance(SPR) in an attenuated total reflection (ATR) configuration. The nonlinear coefficient d(33) of the poled polymer is determined by comparing the SPR enhanced second harmonic generation(SHG) intensities with that from the calibrated quartz. Experimental results show that photobleaching process has less effect on the second-order nonlinearity of a poled cross-linked polymer than that of a poled side-chain polymer.

Time delay associated with total reflection of a plane wave upon plasma mirror.

By employing both the ray model and the electromagnetic theory in a slab optical waveguide, we show that the Goos-Hänchen time, which has been recently argued in the literature, really exists and the time associated with total reflection of a plane wave upon nonabsorbing plasma mirror is exactly the sum of the group delay time and the Goos-Hänchen time. Based on this concept, it is also indicated that the causality is preserved not only for the frustrated Gires-Tournois interferometer case but also for the case of total reflection of a plane TM wave on a nonabsorbing plasma mirror.

Optical approach to angular displacement measurement based on attenuated total reflection.

An optical approach for angular displacement measurement (ADM) based on the attenuated total reflection technique is presented. As a laser beam is incident upon a planar optical waveguide, an m line is obtained by scanning the incident angle. Theoretical analysis shows that the m line sharply shifts with a tiny variation of the thickness of the waveguided layer. And the specific schemes for ADM, which are based on the angular interrogation and the intensity measurement, are analyzed. The calculated result of sensitivity demonstrates that the intensity measurement is more efficient than the angular interrogation. Furthermore, small incident angles indicate higher sensitivity to the angular displacement than relatively large incident angles for the intensity measurement.

Construction of refractive-index profiles of planar waveguides with additional information obtained from surface plasmon resonance.

A surface plasmon resonance (SPR) is excited between a metal film and a graded-index planar waveguide. After the propagation constant of the SPR is measured, the refractive index near the surface of the waveguides, which is difficult to obtain by conventional techniques, is determined experimentally. With this nondestructive technique, combined with the inverse analytical transfer matrix method, the planar waveguide can be profiled to a high degree of accuracy.

Reflective-type configuration for nearly phase-matching cerenkov second-harmonic generation in a nonlinear-optical polymer waveguide.

A new technique for achieving efficient Cerenkov-type second-harmonic generation (SHG) in a nonlinear-optical (NLO) polymer waveguide is presented. The configuration, which can prevent the losses of light caused by relatively long-distance propagation and the multiple reflections that appear in the conventional Cerenkov technique, exhibits ease of fabrication and compactness. We experimentally observed a conversion efficiency of 1.6% W(-1) cm(-1), which to our knowledge is the highest value reported for Cerenkov SHG in polymer, by tuning both the thickness and the refractive index of the polymer film close to phase matching between a guided fundamental wave and a guided harmonic wave. The experimental results agreed well with the theoretical prediction.

Picometer displacement sensing using the ultrahigh-order modes in a submillimeter scale optical waveguide.

An improved scheme for displacement measurement using the ultrahigh-order guided modes in a symmetrical metal-cladding optical waveguide is proposed. Based on this idea together with the lock-in amplification technique, a sensor with a stable displacement resolution of 3.3 pm is experimentally demonstrated without any complicated servo system.