Colors of transparent submicron suspensions on approaching the Rayleigh regime

The features of scattered and transmitted light by dilute suspensions of transparent submicron particles are investigated both in the spectral and in the perceived colorimetric domains, as a function of effective particle diameter D, particle-host refractive-index mismatch m, and scattering angle θ. Our results show that the wavelength λ-dependence of the scattering and extinction cross sections remains quite similar well beyond the Rayleigh regime up to particle sizes of a few hundreds nm, but only for specific scattering angles that depend on D and m, and tend to 90° on approaching the Rayleigh regime. Close to this limit (D/λ<1), a simple criterion that relates the perceived scattering color at θ=90° and the ratio of the sample extinction coefficients at two properly selected wavelengths is demonstrated. A comparison between computed and measured data is presented.     

Compact laser transmitter delivering a long-range infrared beam aligned with a monitoring visible beam

A compact laser transmitter, which takes advantage of an optical subassembly module, was proposed and demonstrated, providing precisely aligned collinear IR and visible beams. The collimated IR beam acts as a long-range projectile for simulated combat, carrying an optical pulsed signal, whereas the visible beam plays the role of tracking the IR beam. The proposed laser transmitter utilizes IR (λ(1)=905 nm) and visible (λ(2)=660 nm) light sources, a fiber-optic collimator, and a beam combiner, which includes a wavelength division multiplexing (WDM) filter in conjunction with optical fiber. The device was built via the laser welding technique and then evaluated by investigating the characteristics of the generated light beams. The IR collimated beam produced had a Gaussian profile and a divergence angle of ~1.3 mrad, and the visible monitoring beam was appropriately collimated to be readily discernible in the vicinity of the transmitter. The two beams were highly aligned within an angle of 0.004 deg as anticipated. Finally, we performed a practical outdoor field test to assess the IR beam with the help of a receiver. An effective trajectory was observed ranging up to 660 m with an overall detectable beam width of ~60 cm.     

Subwavelength focusing of laser light by microoptics

Near-field scanning optical microscope (NSOM) measurements revealed that a linearly polarized Gaussian beam of wavelength λ?=?532?nm focused with a binary zone plate (ZP) of focal length λ, radius 7?μm, and groove depth 510?nm, fabricated in hydrogen silsesquioxane, produces a focal spot of size FWHM?=?(0.44?±?0.02)λ, with the side lobes being lower than 10% of the intensity peak in the focus. Replacing the incident 532?nm wavelength with a 633?nm wavelength resulted in a 1.8 times shorter focal length and a tighter (in terms of wavelengths) focal spot of FWHM?=?(0.40?±?0.02)λ. This value is smaller than the scalar diffraction-limited size in vacuum, FWHM?=?0.51λ. This is the smallest focal spot so far experimentally obtained for a binary phase ZP and the root-mean-square deviation of the experimental curve from a FDTD simulation is 5%. We show that the metallic pyramid-shaped cantilever with a 100-nm-hole in the tip that is used in the NSOM is only able to detect the transverse electric field component. The FDTD simulation showed such a cantilever to be over 3 times more sensitive to the transverse electric field component than to the longitudinal one. Using the Richards–Wolf (RW) formulae, the near-focus intensity distribution can be calculated with 6% error for focal lengths larger than 4λ. It is usually assumed that the Debye theory and the RW formulae are only valid for focal lengths much larger than the incident wavelength. By FDTD simulation, we showed that when illuminating the ZP by a radially (rather than linearly) polarized beam, a decrease in the focal spot transverse size did not result in a substantially reduced total volume of focus (4%).     

Side illumination fluorescence emission characteristics from a dye doped polymer optical fiber under two-photon excitation

Two-photon excited (TPE) side illumination fluorescence studies in a Rh6G-RhB dye mixture doped polymer optical fiber (POF) and the effect of energy transfer on the attenuation coefficient is reported. The dye doped POF is pumped sideways using 800 nm, 70 fs laser pulses from a Ti:sapphire laser, and the TPE fluorescence emission is collected from the end of the fiber for different propagation distances. The fluorescence intensity of RhB doped POF is enhanced in the presence of Rh6G as a result of energy transfer from Rh6G to RhB. Because of the reabsorption and reemission process in dye molecules, an effective energy transfer is observed from the shorter wavelength part of the fluorescence spectrum to the longer wavelength part as the propagation distance is increased in dye doped POF. An energy transfer coefficient is found to be higher at shorter propagation distances compared to longer distances. A TPE fluorescence signal is used to characterize the optical attenuation coefficient in dye doped POF. The attenuation coefficient decreases at longer propagation distances due to the reabsorption and reemission process taking place within the dye doped fiber as the propagation distance is increased.     

Light intensification modeling of coating inclusions irradiated at 351 and 1053 nm

Electric-field modeling provides insight into the laser damage resistance potential of nodular defects. The laser-induced damage threshold for high-reflector coatings is 13x lower at the third harmonic (351 nm) than at the first harmonic (1053 nm) wavelength. Linear and multiphoton absorption increases with decreasing wavelength, leading to a lower-third harmonic laser resistance. Electric-field effects can also be a contributing mechanism to the lower laser resistance with decreasing wavelength. For suitably large inclusions, the nodule behaves as a microlens. The diffraction-limited spot size decreases with wavelength, resulting in an increase in intensity. Comparison of electric-field finite-element simulations illustrates a 3x to 16x greater light intensification at the shorter wavelength.     

Instrumental error in chromotomosynthetic hyperspectral imaging

Chromotomosynthetic imaging (CTI) is a method of convolving spatial and spectral information that can be reconstructed into a hyperspectral image cube using the same transforms employed in medical tomosynthesis. A direct vision prism instrument operating in the visible (400-725?nm) with 0.6?mrad instantaneous field of view (IFOV) and 0.6-10?nm spectral resolution has been constructed and characterized. Reconstruction of hyperspectral data cubes requires an estimation of the instrument component properties that define the forward transform. We analyze the systematic instrumental error in collected projection data resulting from prism spectral dispersion, prism alignment, detector array position, and prism rotation angle. The shifting and broadening of both the spectral lineshape function and the spatial point spread function in the reconstructed hyperspectral imagery is compared with experimental results for monochromatic point sources. The shorter wavelength (λ<500 nm) region where the prism has the highest spectral dispersion suffers mostly from degradation of spectral resolution in the presence of systematic error, while longer wavelengths (λ>600 nm) suffer mostly from a shift of the spectral peaks. The quality of the reconstructed hyperspectral imagery is most sensitive to the misalignment of the prism rotation mount. With less than 1° total angular error in the two axes of freedom, spectral resolution was degraded by as much as a factor of 2 in the blue spectral region. For larger errors than this, spectral peaks begin to split into bimodal distributions, and spatial point response functions are reconstructed in rings with radii proportional to wavelength and spatial resolution.     

Shape transition in ZnO nanostructures and its effect on blue-green photoluminescence

We report that ZnO nanostructures synthesized by a chemical route undergo a shape transition at ～20?nm from spherical to hexagonal morphology thereby changing the spectral components of the blue-green emission. Spherically shaped nanocrystals (size range 11-18?nm) show emission in the range of 555-564?nm and the emission shifts to the longer wavelength as the size increases. On the other hand, rods and hexagonal platelets (size range 20-85?nm), which are the equilibrium morphology after the shape transition, show an emission near 465-500?nm which shifts to shorter wavelength as the size increases. The shape transition also leads to relaxation of microstrain in the system. Our analysis shows that the visible emission originates from a defect layer on the nanostructure surface which is affected by the shape transition. The change in the spectral component of the blue-green emission on change of shape has been explained as arising from band bending due to a depletion layer in smaller spherical particles which is absent in the larger particles with flat faces.     

Broadband amplified spontaneous emission double-clad fibre source with central wavelengths near 2 μm

Amplified spontaneous emission broadband light around 2?µm has been successfully generated in both double-clad Tm–silica and Tm:Ho fluoride fibres using a high-power 803?nm diode laser pump source. For the Tm–silica fibre an output power was produced greater than 40?mW and with a bandwidth (full width at half-maximum (FWHM)) of about 50?nm. For output powers greater than 90?mW, the FWHM of the output spectrum reduced to about 40?nm. The Tm:Ho fluoride fibre source produced about 2?mW output power and an FWHM of about 20?nm; this was reduced to about 10?nm as the output power was scaled up to 20?mW. The central wavelength for each system was greater than 2?µm.     

Fabrication of large arrays of high-aspect-ratio single-crystal silicon columns with isolated vertically aligned multi-walled carbon nanotube tips

This paper describes the fabrication of large arrays (10(6) units in 1?cm(2)) of 100?μm tall, single-crystal silicon columns with submicron tip cross-sections. The columns are formed using thin film deposition and growth, reactive ion etching, and deep reactive ion etching. The columns can be either slightly tapered or have pencil-like morphology with nanoscaled tip diameter (41?nm). Conformal thin film coating was used to substantially and uniformly modify the porous structure and, thus, vary by orders of magnitude the fluid permeability of the structure. Gaps between the vertical pillars were varied between 9?μm and 50?nm. Isolated 45?nm diameter, 5?μm tall plasma enhanced chemical vapour deposited multi-walled carbon nanotubes (MWNTs) were grown on the top surface of the columns using a 7?nm thick evaporated Ni film as catalyst. Field emission characterization of the resulting structure was conducted and it is in agreement with scanning electron micrographs of the MWNTs.     

单模硫系玻璃光纤的制备及其超连续谱的产生特性

硫系玻璃具有独特的红外光学性能和极高的光学非线性等特点, 使得硫系光纤成为中红外超连续谱产生的优选材料。基于二次挤压法制备了单模As-S光纤, 并利用飞秒脉冲和光参量放大器作为泵浦源研究了该光纤的中红外超连续谱的产生特性, 包括光纤长度、泵浦波长、泵浦功率对超连续谱产生的影响。结果表明该单模光纤具有较低的传输损耗和较小的材料色散分布; 相比于传统零色散点波长附近泵浦, 在正常色散区且杂质吸收峰附近泵浦也可获得脉冲的极大展宽(泵浦参数: 4.5 μm, 1 kHz, 150 fs, 光纤长度23 cm, 输出谱宽为1.5~8.7 μm@60 dB带宽); 较短长度的硫系光纤便可产生超宽频谱输出, 相反, 光纤长度越长输出频谱越窄且平坦性变差。     

Excitation wavelength dependence of water-window line emissions from boron-nitride laser-produced plasmas

We investigated the effects of laser excitation wavelength on water-window emission lines of laser-produced boron-nitride plasmas. Plasmas are produced by focusing 1064 nm and harmonically generated 532 and 266 nm radiation from a Nd:YAG laser on BN target in vacuum. Soft x-ray emission lines in the water-window region are recorded using a grazing-incidence spectrograph. Filtered photodiodes are used to obtain complementary data for water-window emission intensity and angular dependence. Spectral emission intensity changes in nitrogen Ly-α and He-α are used to show how laser wavelength affects emission. Our results show that the relative intensity of spectral lines is laser wavelength dependent, with the ratio of Ly-α to He-α emission intensity decreasing as laser wavelength is shortened. Filtered photodiode measurements of angular dependence showed that 266 and 532 nm laser wavelengths produce uniform emission.     

High performance short period superlattice digital alloy InSb/Ga(x)In(1-x)Sb laser emitting at 1.9 µm

For the first time, lasers with short period superlattice based quantum wells have successfully been fabricated and characterized on the (AlGaIn)(AsSb) material system. These new quantum wells are composed of alternating InSb/Ga(x)In(1-x)Sb layers with submonolayer thickness. Distributed-feedback lasers fabricated from the epitaxial layers emit in the wavelength range around 1.9?μm. Side mode suppression ratios over 35?dB and very low threshold currents of 23?mA were realized. The laser emission wavelength can be varied from 1.844 to 1.902?μm using grating periods between 255.0 and 263.8?nm.     

Surface plasmonic effect of nanoparticle-like silver nanostructure on the high responsivity of visible/infrared silver-based heterojunction photodetector

The fabrication of a silver (Ag) based photodetector on a silicon dioxide/p-silicon (SiO₂/p-Si) substrate using direct current (DC) magnetron sputtering is demonstrated. The proposed method deposits a nanoparticle-like Ag thin film that favours the photoconduction mechanism under light illumination at 468?nm and laser illumination at 660 and 980?nm. The thin film is characterized using scanning electron microscope (SEM), energy dispersive x-ray (EDX), x-ray diffraction (XRD), Raman scattering, photoluminescence (PL) and ultraviolet–visible (UV–VIS) analysis. Current–voltage (I–V) analysis and the calculated rectifying ratio (RR) suggests the establishment of good Schottky contacts for incident light/laser at 468, 660 and 980?nm, with good responsivity towards light and laser illumination in the forward and reverse DC bias regions. The responsivity increases as the wavelength decreases from 980?nm → 660?nm → 468?nm, with the highest responsivity of 213.7?mAW^?1 at 468?nm indicating better photoconduction at low light powers.     

Aligned silica nanowires on the inner wall of bubble-like silica film: the growth mechanism and photoluminescence

Large area, aligned amorphous silica nanowires grow on the inner wall of bubble-like silica film, which is prepared by thermal evaporation of a molten gallium-silicon alloy in a flow of ammonia. These nanowires are 10-20?nm in diameter and 0.5-1.5?μm in length. The bubble-like silica film functions as a substrate, guiding the growth of silica nanowires by a vapour-solid process. This work helps us to clearly elucidate the growth mechanism of aligned amorphous silica nanowires, ruling out the possibility of liquid gallium acting as a nucleation substrate for the growth of the aligned silica nanowires. A broad emission band from 290 to 600?nm is observed in the photoluminescence (PL) spectrum of these nanowires. There are seven PL peaks: two blue emission peaks at 430?nm (2.88?eV) and 475?nm (2.61?eV); and five ultraviolet emission peaks at 325?nm (3.82?eV), 350?nm (3.54?eV), 365?nm (3.40?eV), 385?nm (3.22?eV) and 390?nm (3.18?eV), which may be related to various oxygen defects.     

All-fibre Mach–Zehnder interferometer based on seven-core and coreless fibres for generating stable wavelength-switchable laser

In order to realize a wavelength-tuneable fibre-laser output, a ring-cavity erbium-doped fibre laser based on an all-fibre Mach–Zehnder interferometer (MZI) is proposed and experimentally tested. The MZI consists of a single-mode fibre, two segments of coreless fibre, and a seven-core fibre. For the proposed fibre laser, the length of the gain medium is 4?m and the lasing threshold is 75?mW. By adjusting the loss of the laser cavity, switchable single-wavelength laser emission is realized across the range of 1527.6–1549.9?nm and the wavelength interval is less than 2.4?nm; the peak power difference of each lasing wavelength is less than 7.9?dB. Tuneable dual- and three-wavelength laser outputs were obtained by adjusting the polarization controller. The 3-dB linewidth was less than 0.57?nm. The single- and dual-wavelength laser output power fluctuations were less than 1.4 and 1.7?dB, respectively, when monitored over a period of 30?min.     

Gradient polymer-disposed liquid crystal single layer of large nematic droplets for modulation of laser light

The light modulating ability of gradient polymer-disposed liquid crystal (PDLC) single layer of large droplets formed by nematic E7 in UV-cured polymer NOA65 is studied. Operating at relatively low voltages, such PDLC film with a of thickness 10-25?μm and droplet size up to 50?μm exhibits a good contrast ratio and is capable of producing a large phase shift for the propagating coherent light. For a linearly polarized He-Ne laser (λ=633?nm), an electrically commanded phase shift as large as π/2 can be obtained by the large-droplet region of the film. The electrically produced phase shift and its spatial profile controlled by the thickness of the gradient PDLC single layers of large nematic droplets can be useful for tunable spatial light modulators and other devices for active control of laser light.     

Absolute Frequency Stabilization of the 3.39-?m Laser on a CH4 Line

Theoretical and experimental studies on a primary standard of wavelength and frequency in the infrared are reported. The 3.39-?m He-Ne laser is tuned to the center of the absorption line of methane which is assigned to the F1(2)) component of the P(7) branch of the ?3 band. The absence of Stark and Zeeman effects under the highest fields tested, the strong absorption at 63.2° K as well as at 78°K, and the separation from other lines are excellent features for a high degree of absolute stability and reproducibility. A method of automatic frequency control of the He-Ne laser at the line-center of methane is described.     

Fabrication of a nano-cone array on a p-GaN surface for enhanced light extraction efficiency from GaN-based tunable wavelength LEDs

We report on the fabrication of a nano-cone structured p-GaN surface for enhanced light extraction from tunable wavelength light emitting diodes (LEDs). Prior to p-contact metallization, self-assembled colloidal particles are deposited and used as a mask for plasma etching to create nano-cone structures on the p-GaN layer of LEDs. A well-defined periodic nano-cone array, with an average cone diameter of 300?nm and height of 150?nm, is generated on the p-GaN surface. The photoluminescence emission intensity recorded from the regions with the nano-cone array is increased by two times as compared to LEDs without surface patterning. The light output power from the LEDs with surface nano-cones shows significantly higher electroluminescence intensity at an injection current of 70?mA. This is due to the internal multiple scattering of light from the nano-cone sidewalls. Furthermore, we have shown that with an incorporation of InGaN nanostructures in the quantum well, the wavelength of these surface-patterned LEDs can be tuned from 517 to 488?nm with an increase in the injection current. This methodology may serve as a practical approach to increase the light extraction efficiency from wavelength tunable LEDs.     

Group-index and resonant field enhancement in a symmetric double-sided grated waveguide

A numerical study has been carried out by means of the Green's function method to explore possible performance improvements of a simple grated waveguide (GWg) by the variations of its grated structure. It is shown that a GWg featuring symmetric two-sided grated structure of 16 teeth with a 60?nm groove depth and having a symmetric refractive index profile with a relatively large contrast between the grated and ungrated layers is capable of delivering largely improved device performance compared to that achieved previously with a one-sided grating of 40?nm groove depth and asymmetric index profile. The improvement is characterized by a remarkable 8-fold and 15-fold increase in the group index and the maximum field intensity, respectively, at the first resonance wavelength above the upper band edge (shorter wavelength), while relatively less improvement is found at the first resonance wavelength below the lower band edge (longer wavelength). It is shown that more than 20% further improvement can be obtained by an appropriate shifting of the two innermost adjacent grating teeth in the case of the 40?nm groove depth. Apart from that, the result also reveals an interesting and remarkable correlation between the variations of the group index and the confined energy.     

Bismuth activated alumosilicate optical fibers fabricated by surface-plasma chemical vapor deposition technology

Plasma chemical technology is experimentally applied to the fabrication of a Bi-activated alumosilicate-core pure-silica-cladding fiber preform. To the best of our knowledge, this is the first time this technology has been applied in this way. We measure gain efficiency at pumping by a 1058 nm wavelength Yb fiber laser in a piece of a newly obtained fiber 20 m in length within 100-1200 nm wavelengths band. The gain efficiency reaches as high as 0.2 dB/mW. Bi-activated alumosilicate-core pure-silica-cladding fiber that is not more than 12 m in length serves a basis for a 1 W output power fiber laser emitting at the wavelength of 1160 nm with 8% slope efficiency. We also measure the photoluminescence spectrum and kinetics of Bi centers responsible for laser emission under the excitation of 193 nm wavelength ArF laser pulses.