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We develop a theory of refractive index tuning for symmetry-protected optical bound states (SP-BICs) in high-contrast gratings (HCGs). A compact analytical formula for tuning sensitivity is derived and verified numerically. We also discover a new type of SP-BIC in HCGs that has an accidental nature with a spectral singularity, which is explained in terms of hybridization and strong coupling among the odd- and even-symmetric waveguide-array modes. Our work elucidates the physics of tuning SP-BICs in HCGs and significantly simplifies their design and optimization for dynamic applications in light modulation, tunable filtering, and sensing.

Two pulse ablation experiments were conducted on monolayer MoS2 with various substrates. The hole size variation is explained via bandgap renormalization and carrier relaxation for different pulse separations.

Multi-shot ablation studies of monolayer MoS2 and WS2 reveal the record optical robustness of these materials where optical damage results in laser-induced vacancies that greatly affect their optical properties.

Light scattering patterns are commonly used in industry to assess fiber quality. In this work, scattering patterns of Bragg fibers are studied. Optical fibers are coated with alternating layers of Silicon Nitride and Silica using Plasma Enhanced Chemical Vapor Deposition. A laser beam incident perpendicular to the fiber axis scatters off creating patterns distinct from that of uncoated fibers which exhibit continuous front lobe. Effect of variation in layer properties, polarization, wavelength and fiber symmetry is observed in the patterns. Anomalous suppression and enhancement in angular scattering pattern is explained through a scattering angle diagram. Features in the pattern are mapped to reflectance of the Bragg stack and it is found that a stopband creates the anomalous features in the scattering.

We develop a theory for Fano resonance tuning in dual-mode high-contrast gratings (HCGs). Compact analytical formulas of tuning sensitivity are derived and verified numerically, and are in good agreement with reported experiments. We show that the resonance tuning in HCGs, containing cooperative contribution from two propagating modes, is fundamentally different from that in single-mode microresonators. Our theory reveals the important role of the higher-order mode, which can possess large modal dispersion, especially in the long-wavelength limit beyond the cutoff of slab waveguides, to enable large tuning sensitivity. Our findings will simplify the design and optimization of active and passive tuning in HCG resonators.

This supplement contains detailed derivations and additional examples of tuning of Fano resonances in HCGs.

Avalanche ionization is shown to be the dominant carrier generation mechanism in the ultrafast ablation of transition metal dichalcogenides. Carrier densities reaching 22% of the total valence band population are needed for ablation to occur.

We study the performance limit of single-mode operation in low-loss large-V-number planar waveguide lasers, with transverse spatial hole burning as the dominating mechanism for transverse-mode competition. By introducing normalized variables, we develop a simple semianalytical model to describe the universal output characteristics of single-mode emission before the onset of high-order modes, which can be easily scaled to a wide range of laser configuration parameters. Our model is validated using exact numerical solutions, which show applicability beyond that of the low-loss approximation. Our analysis establishes a minimum criterion of the loss ratio between the fundamental and first higher-order mode for a robust single-mode operation. This criterion is much weaker than that of conventional wisdom based on the pure wave-propagation argument, which we attribute to a resonator enhancement effect. Our universal outcome can be denormalized to establish the relationship between single-mode extraction efficiency and optimum output coupling in a multimode laser with arbitrary modal loss ratios over a wide range of single-pass unsaturated gain and loss.

We demonstrate deposition of azimuthally uniform single- or multiple-layer thin films of silicon nitride and silica on fibers using plasma-enhanced chemical vapor deposition by continuously rotating the fibers during growth. Our fibers exhibit distinctive and uniform iridescence that strongly depends on coating configuration. We also report a non-invasive technique to measure refractive index and film thickness of coated fibers simultaneously based on Mie scattering. We found the films grown on fibers have very different characteristics from those grown on flat substrates. We deposit a 1-μm-thick SiN x film on a spheroidal microrod resonator, which is shown numerically to push the guided fundamental mode into the silica core. We demonstrate a Q factor of 2.2 × 106, indicating reasonably good thin film quality that could be further increased with improved process control. Our technique can be applied to coat whispering gallery mode microresonators with engineered (e.g., step, graded, or stratified) refractive index profiles, which are expected to enable many new applications.

Ultrashort bunches of electrons, emitted from solid surfaces through excitation by ultrashort laser pulses, are an essential ingredient in advanced X-ray sources, and ultrafast electron diffraction and spectroscopy. Multiphoton photoemission using a noble metal as the photocathode material is typically used but more brightness is desired. Artificially structured metal photocathodes have been shown to enhance optical absorption via surface plasmon resonance but such an approach severely reduces the damage threshold in addition to requiring state-of-the-art facilities for photocathode fabrication. Here, we report ultrafast photoelectron emission from sidewalls of aligned single-wall carbon nanotubes. We utilized strong exciton resonances inherent in this prototypical one-dimensional material, and its excellent thermal conductivity and mechanical rigidity leading to a high damage threshold. We obtained unambiguous evidence for resonance-enhanced multiphoton photoemission processes with definite power-law behaviors. In addition, we observed strong polarization dependence and ultrashort photoelectron response time, both of which can be quantitatively explained by our model. These results firmly establish aligned single-wall carbon nanotube films as novel and promising ultrafast photocathode material.

We report on the first demonstration of metal–insulator–semiconductor-type plasmonic lasers at the telecom wavelength (∼1.3 μm) using top-down fabricated semiconductor waveguides on single-crystalline metallic platforms formed using epitaxially grown Ag films. The critical role of the Ag film thickness in sustaining plasmonic lasing at the telecom wavelength is investigated systematically. Low-threshold (0.2 MW/cm2) and continuous-wave operation of plasmonic lasing at cryogenic temperatures can be achieved on a 150 nm Ag platform with minimum radiation leakage into the substrate. Plasmonic lasing occurs preferentially through higher-order surface-plasmon-polariton modes, which exhibit a higher mode confinement factor, lower propagation loss, and better field–gain coupling. We observed plasmonic lasing up to ∼200 K under pulsed excitations. The plasmonic lasers on large-area epitaxial Ag films open up a scalable platform for on-chip integrations of plasmonics and optoelectronics at the telecom wavelength.

We propose a minimally required modal loss contrast for single-mode oscillation in multimoded waveguide lasers. Our criterion is much weaker than conventional wisdom based on wave propagation which we attribute to a resonator enhancement effect.

The papers from this special section were presented at the CLEO 2016 Conference. The purpose of this issue is to highlight the high impact work from the conference related to Science and Innovations and Fundamental Science/QELS. The papers in this issue highlight the latest leading-edge developments in silicon photonics, fiber and semiconductor lasers, fiber communications, metamaterials, laser material processing, bio-photonics, and ultrafast optical science and technology.

Carbon nanotubes (CNTs) have many uses in energy storage, electron emission, molecular electronics, and optoelectronics. Understanding their light-matter interactions is crucial to their development. Here, we study a film of single-walled CNTs with a thickness of 1.67 μm and a 2D orientational order parameter of 0.51, measured by polarized Raman spectroscopy. The film is expected to have a work function of about 5.1 eV. In this study, ~100-fs pulses with 1.5 (ℏω) and 3 eV (2ℏω) photon energy are used to pump the CNT film while observing its electron emission in vacuum. Ultrafast pulses produce nonlinear phenomena in enhanced field emission, as the CNTs absorb strongly enough that thermally excited carriers can tunnel through the potential barrier. Through curve fitting of the power dependence for each pump energy, we find that the light at ℏω is absorbed via 5-photon absorption, and the light at 2ℏω is absorbed via a combination of 2- and 3-photon absorption. Further study reveals a space-charge limited regime with low applied bias, a photoemission regime with moderate bias, and a laser-assisted field emission regime when the bias is high enough that the photon pump is no longer important. Cross-correlation pumping with the two colors simultaneously shows 4x enhancement of the emission, with a FWHM that suggests a lifetime of ~190 fs, similar to the dephasing time of electrons in CNTs. These studies help illuminate the properties of CNTs as a nonlinear optical material and go towards a more thorough understanding of their optoelectronic properties.

Near-IR femtosecond laser ablation of monolayer tungsten diselenide WSe 2 on sapphire is demonstrated. The ablation threshold is found low (below 10 mJ/cm2), due to its low dimensionality and weak coupling to substrates. Substrate-dependent ablation threshold is predicted and will be discussed.

The PN junction was introduced in transistors by doping, resulting in high losses due to Coulomb scattering from the dopants. The MOSFET introduced carriers in the form of electrons and holes with an applied bias to the oxide barrier, resulting in carrier transfer without doping. This avoids high scattering losses and dominates the IC industries. With heterojunctions having valence-band maxima near and even above the conduction-band minimum in the formation of Type-III superlattices, very useful devices, introduced by Tsu, Sai-Halacz, and Esaki, soon followed. If the layer thicknesses are more than the carrier mean-free-path, incoherent scattering results in the formation of carrier transfer via diffusion instead of opening up new energy gaps. The exploitation of carriers without scattering represents a new and significant opportunity in what we call a Broken Gap Heterojunction FET.

An array of embedded subwavelength fibers is proposed as a flexible, large-scale and mass-producible metasurface for wavefront shaping. As a proof of concept, we numerically demonstrate beam deflector, focusing optics, high reflector and sinusoidal wavefront shaping.

We study nonlinear above-threshold photoemission (ATPE) in single-wall carbon nanotubes at two wavelengths. NIR photoemission demonstrates 5-photon ATPE, while UV ATPE is dominantly a 2nd process. Two-pulse correlation exhibits enhanced photoemission with a very short lifetime less than 200 fs.

A method is presented in which the wetting contact angle of a sessile drop is acquired aerially using confocal techniques to measure the radius and the height of a droplet deposited on a planar surface. The repeatability of this method is typically less than 0.25°, and often less than 0.1°, for droplet diameters less than 1 mm. To evaluate accuracy of this method, an instrument uncertainty budget is developed, which predicts a combined uncertainty of 0.91° for a 1 mm diameter water droplet with a contact angle of 110°. For droplets having diameters less than 1 mm and contact angles between 15° and 160°, these droplets approach spherical shape and their contact angles can be computed analytically with less than 1% error. For larger droplets, gravitational deformation needs to be considered.

We investigate numerically and analytically the propagation characteristics of gain-guided modes in index-antiguided planar waveguides. We show that the gain-guided mode has modal confinement proportional to the modal gain, and single-transverse mode propagation is very robust in such waveguides. The power evolution of the fundamental mode can be described analytically with very good accuracy. Limiting power and optimum length of the waveguide amplifiers can also be obtained analytically.

We show single-shot nonlinear absorption is enhanced by 5% when pump and probe pulses have parallel polarization. At very large pump energy, we observe a new and fast decay component of self-trapped excitons ∼0.5ps.

Mie scattering of Bragg fibers made of periodic layers of SiN x and SiO 2 is demonstrated. The scattering pattern is very sensitive to Bragg layers' configuration, making it attractive to characterize Bragg fibers non-invasively.

Two new monomeric anionic PAGs, and their corresponding polymers, were prepared and characterized. The acid generating efficiency of PAG bound polymers is in the order: NO2 PAG ≈ F4 PAG > CF3 PAG, which agrees well with the electron withdrawing power of the substituents. The NO2 PAG bound polymer showed better imaging resolution, than the CF3 PAG bound polymer. Based on the acid generating efficiency, and our previous results on F4 PAG bound polymer for EUV photoresists, these two new PAGs and PAG bound polymers have the potential for imaging via EUV lithography.

We present lasing experiments in index-antiguided (IAG) planar waveguides made of Nd:YAG (refractive index = 1.820) sandwiched by Terbium Gallium Garnet (refractive index = 1.954). For a core width of 220 μm, we observe fundamental mode oscillation with maximal 2.62 W output power and 0.109 slope efficiency. An analytical model is developed taking into account simultaneous index-antiguiding, gain guiding and thermal induced refractive focusing. For a core width of 400 μm, fundamental mode oscillation is only observed near the lasing threshold, suggesting that the maximally core width in IAG waveguides for fundamental mode operation depends on the strength of index antiguiding.

A boundary integral equation for the eigenmode of photonic crystal fibers is formulated and numerically solved using the Nystrom method. The real and imaginary parts of the propagation constant, which are related to the dispersion and the confinement loss of fibers, are obtained using a secant method. This formulation is very flexible to handle the fiber geometry, and therefore can be applied to photonic crystal fibers with novel refractive index profile and hole geometry.

We study transverse mode competition in index-antiguided planar lasers theoretically and experimentally. Robust fundamental mode operating 10 times above lasing threshold is predicted and confirmed experimentally.

We investigate femtosecond pulse transmission through BK7 in the presence of its second harmonic as a seeding pulse. Enhanced absorption is observed when the pulses are co-polarized. Such phenomenon can be used to control absorption.

This paper reports the fabrication and characterization of uniform conformal dielectric thin films on optical fibers using plasma-enhanced chemical vapor deposition. We show convincingly that, using both iridescence and scanning electron microscopy, uniform deposition on the fiber can only be obtained if it is rotated at a constant speed during deposition. Our study is important for depositing single or multiple uniform dielectric thin films on objects with cylindrical or spherical symmetry, which could enable new optical functionalities.

Iridescence is structural color, as opposed to pigment, in which the color is a result of constructive or destructive interference of light traveling through periodic structures, where the thickness of each layer plays a critical role in the color observed. Bragg fibers are man-made cylindrical structures with periodic layer coatings, modeled after the atomic lattice structure found in naturally occurring crystals. Bragg layers with even a slight deviation of micrometers in its thickness can result in a non-uniform color, which is the basis of our study. A non-invasive approach was taken to best approximate the dominant wavelength of the reflected hues through the use of microscopic imaging. The RGB values present in the images were extracted, and transformed into new coordinates using matrix operations, from which we were able to estimate the dominant wavelengths present in the Bragg fiber.

We studied the surface femtochemistry of CO/O2/Pt(111) induced with 0.3 ps laser pulses over a wide range of wavelength and fluence. Below 10 μJ/mm2, the yields depend linearly on fluence. Above 10 μJ/mm2, the yields scale nonlinearly in the fluence. From the dependence of the yields on wavelength, we determine that the nonlinear surface femtochemistry is influenced by nonthermal substrate electrons.

We demonstrated femtosecond laser ablation of PMMA using 2-color waveform synthesis. A modest and yet clear modulation in ablated area versus relative phase between the ω and 2 ω beams is observed.

We demonstrate robust continuous-wave lasing in a planar index-antiguided waveguide with a 220-µm Nd:YAG active layer. A pump-limited output of 1.5 W with a slope efficiency of 30% with respect to absorbed pump power is achieved.

We present new data that address the chemical pathway to desorption of O 2 and formation of CO 2 in CO/O 2 /Pt(111) induced by subpicosecond laser pulses. The data show conclusively that the O 2 desorbs molecularly. We also find that if the CO oxidizes by an atomic pathway then the capture of oxygen atoms by the CO is highly efficient; if the CO oxidizes by a molecular pathway then the oxygen atoms in the CO 3 ∗ transition state are inequivalent.

We have previously reported the observation of self-organized tungsten nanogratings during chemical vapor deposition of tungsten induced by a 400-nm 80-MHz laser oscillator on a wide range of substrates. We show that the growth of nanostructures begins with a thin tungsten film, followed by a rapid formation of periodic texture, when the laser power exceeds a threshold value. The threshold power is found strongly substrate dependent. The ubiquitous presence of thin films prior to nanograting growth suggests adatom diffusion induced by laser heating is vital, as the strong electronphonon coupling in tungsten is expected to turn absorbed photon energy rapidly into heat. Using a simplified 1D heat diffusion model, we estimate the critical surface temperature on various substrates at the onset of nanograting formation, based on substrate-specific threshold power and material properties. We found interesting correlation of critical temperatures: all the covalent substrates (AlN, Al2O3, quartz, silica, and glass) exhibit a common critical temperature while the ionic substrates (MgO, MgF2, and CaF2) share another yet different critical temperature. The critical temperature of covalent substrates is found higher than that of ionic substrates, indicating the former possesses larger activation energy for adatom diffusion. Based on this model, we can also extract a substrate-independent enthalpy for nanograting formation. Although the present 1D model overestimates the surface temperature, the correlation of critical temperatures among substrates and the presence of a unique enthalpy independent of substrates strongly support the role of laser heating and adatom diffusion in the formation of tungsten nanogratings.

Gain-guided and index anti-guided waveguides hold great promise to increase laser output power while maintaining single mode operation. Although lasing in GG-IAG fibers was demonstrated, slope efficiency is poor and output power is low. This work presents the first, to the best of our knowledge, lasing characteristics of a diode-pumped GG-IAG planar waveguide in a plano-plano resonator configuration. The laser waveguide is fabricated using 1% doped Nd:YAG core with diffusion bonded TGG claddings. With continuous-wave pumping, 8.5% slope efficiency is demonstrated and an output power of 1.5W achieved, the highest power reported so far from a GG-IAG waveguide laser. Under such pumping condition, however, thermal lensing is shown to override gain guiding and mode narrowing is observed.

We report comprehensive analysis of output characteristics of homogeneously broadened index-antiguided slab lasers with transverse mode competition. Robust single fundamental mode operation is achieved when the distributive modal loss due to index antiguiding dominates the output coupling loss. Maximal laser efficiency under single fundamental mode operation is investigated numerically for various combinations of single-pass gains and losses. We show analytically that an asymptotic limit of such efficiency exists that is solely determined by the loss ratio between the fundamental and 1st higher-order modes, which equals 66.7% for planar index antiguided lasers.

Desorption of OZ and formation of CO2 were induced with subpicosecond laser pulses on a Pt(111) surface dosed with coadsorbed OZ and CO. We report the fluence dependent yields obtained over a range of laser wavelengths from 267 to 800 nm, and pulse durations from 80 fs to 3.6 ps. The nonlinear dependence of the yield on fluence is different at different wavelengths. Two-pulse correlation measurements show two different time-scales relevant to the desorption. The results show that nonthermalized electrons play a role in the surface chemistry, and that an equilibrated pre-heating of the surface modes leads to enhanced desorption.

We experimentally investigate the ability of an all-optical 2R regenerator to suppress impairments caused by intrachannel four-wave mixing (IFWM) and intrachannel cross-phase modulation (IXPM), which are the two dominant nonlinear effects in 40-Gb/s pseudolinear transmission systems. The 2R regenerator suppresses amplitude fluctuations and shadow pulses induced by IFWM in 80-km transmission over standard single-mode fiber, allowing an increase of signal power by /spl sim/3 dB. However, the 2R regenerator shows little improvement for an 80-km TrueWave RS fiber, where timing jitter originating from IXPM dominates.

In this letter, we examine the spectrum of a 40-Gb/s return-to-zero (RZ) data stream, which has been broadened by self-phase modulation in a nonlinear fiber. Spectra were recorded as the residual chromatic dispersion at the input of the nonlinear fiber was varied using a tunable dispersion element. We show that spectral broadening can provide a useful measure of the residual chromatic dispersion of the data stream.

Leaky-waveguide laser amplifiers can sometimes have significant advantages for single-transverse-mode propagation in high-power waveguide laser systems. Most recent studies of such systems have either not included gain saturation, or they have assumed homogeneously broadened gain media. However, some of the most promising media for such amplifiers are inhomogeneously broadened. Results reported here include detailed numerical mode solutions for saturated inhomogeneous broadening, as well as useful analytical methods and approximations for both homogeneous and inhomogeneous broadening.

Femtosecond (fs) laser pulses, with pulse energy below single-shot damage threshold, are shown to produce deep-subwavelength periodic nanostructures over many laser shots during laser ablation or material deposition. Both morphological and structural characteristics of these nanostructures, strongly correlated to the properties of light including polarization, wavelength, power, and accumulated photon dosage, are shown. This universal phenomenon is interpreted under the theoretical framework of light-induced instability and self-organization. Strong light–matter interaction at fs time and nanometer length scales is shown responsible for the incubation effect and plasmon resonance that are critical to the spontaneous formation of these periodic nanostructures. Potential applications in emerging areas are discussed and the potential of a unified theory for this phenomenon is highlighted.

We investigate numerically and analytically the effects of gain saturation on the propagation of the fundamental mode in a gain-guided index-antiguided slab waveguide. The propagating mode adapts to gain saturation by becoming less confined, while at the same time its peak intensity increases more slowly. The modal gain coefficient is shown to scale linearly with the modal confinement in the cladding. At steady state, both the mode shape and power remain constant. We show that, at large index antiguiding, the field profile inside the core remains nearly constant during propagation, which allows an analytical description of the evolution of the optical power inside the core.