Characterization of photonic nanojets in dielectric microdisks

The direct imaging of photonic nanojets in different dielectric microdisks illuminated by a laser source is reported. The SiO₂ and Si₃N₄ microdisks are of height 650 nm with diameters ranging from 3 μm to 8 μm. The finite-difference time-domain calculation is used to execute the numerical simulation for the photonic nanojets in the dielectric microdisks. The photonic nanojet measurements are performed with a scanning optical microscope system. The photonic nanojets with high intensity spots and low divergence are observed in the dielectric microdisks illuminated from the side with laser source of wavelengths 405 nm, 532 nm and 671 nm. The experimental results of key parameters are compared to the simulations and in agreement with theoretical results. Our studies show that photonic nanojets can be efficiently created by a dielectric microdisk and straightforwardly applied to nano-photonics circuit.     

Compact WDM demultiplexer for seven channels in photonic crystal

We demonstrate a new device concept for wavelength division demultiplexing based on planar photonic crystal waveguides. The filtering of wavelength channels is realized by shifting the cutoff frequency of the fundamental photonic bandgap mode in consecutive sections of the waveguide. The shift is realized by modifying the size of the border holes.The proposed demultiplexer has an area equal to (16.5 μm × 6.5 μm) and thus it is verified that this structure is very small and can be integrated easily into optical integrated circuits with nanophotonic technologies. The output wavelengths of designed structure can be tuned for communication applications, around 1550 nm. The wavelengths of demultiplexer channels are λ₁ = 1.590 μm, λ₂ = 1.566 μm, λ₃ = 1.525 μm, λ₄ = 1.510 μm, λ₅ = 1.484 μm, λ₆ = 1.450 μm, λ₇ = 1.400 μm respectively. Designs offering improvement of number of the separate wavelengths (seven), miniaturization of the structure (107.25 μm²) is our aim in this work.In our structure, we consider that the 2D triangular lattice photonic crystal is composed of air holes surrounded by dielectric. Its parameters are: radius of holes (r = 0.130 μm), lattice constant (a = 0.380 μm), and index of membrane (n = 3.181:InP). The numerical model used to simulate the structure of the demultiplexer is based on the finite difference time domain (FDTD).     

Design of an omnidirectional mirror using one dimensional photonic crystal with graded geometric layers thicknesses

We propose a flexible design for one-dimensional photonic crystals (1D-PCs) with a controllable omnidirectional band gap covering the optical telecommunication wavelengths which are 0.85 μm, 1.3 μm and 1.55 μm. We used for this design the chirped grating. Chirping is applied to geometric thicknesses of layers. It takes two forms, one is linear and the other is exponential. We exploit this technique to have the suitable omnidirectional band gap covering the maximum of optical telecommunication wavelengths. With a quarter wave structure, we can have an omnidirectional band gap generating only one of these wavelengths. With graded structure, we obtain, as is reported in this paper, an omnidirectional band gap which covers the wavelengths 1.3 μm and 1.55 μm at the same time with a large bandwidth. We also achieve an omnidirectional band gap containing the wavelength 0.85 μm and which is obviously larger than that of the quarter wave stack.     

Real-space observation of photonic nanojet in dielectric microspheres

The three-dimensional real-space observation of photonic nanojet in different microspheres illuminated by a laser is reported. The finite-difference time-domain technique is used to perform the three-dimensional numerical simulation for the dielectric microspheres. The key parameters of photonic nanojet are measured by using a scanning optical microscope system. We reconstruct the three-dimensional real-space photonic nanojets from the collected stack of scanning images for polystyrene microspheres of 3 μm, 5 μm, and 8 μm diameters deposited on a glass substrate. Experimental results are compared to calculations and are found in good agreement with simulation results. The full width at half-maximum of the nanojet is 331 nm for a 3 μm microsphere at an incident wavelength of 633 nm. Our investigations show that photonic nanojets can be efficiently imaged by a microsphere and straightforwardly extended to rapidly distinguish the nano-objects in the far-field optical system.     

Optical coherence tomography based imaging of dental demineralisation and cavity restoration in 840 nm and 1310 nm wavelength regions

In this paper, a study of in-house built optical coherence tomography (OCT) system with a wavelength of 840 nm for imaging of dental caries, progress in demineralisation and cavity restoration is presented. The caries when imaged with the 840 nm OCT system showed minute demineralisation in the order of 5 μm. The OCT system was also proposed to study the growth of lesion and this was demonstrated by artificially inducing caries with a demineralisation solution of pH 4.8. The progress of carious lesion to a depth of about 50–60 μm after 60 hours of demineralisation was clearly observed with the 840 nm OCT system. The tooth samples were subjected to accelerated demineralisation condition at pH of approximately 2.3 to study the adverse effects and the onset of cavity formation was clearly observed. The restoration of cavity was also studied by employing different restorative materials (filled and unfilled). In the case of restoration without filler material (unfilled), the restoration boundaries were clearly observed. Overall, results were comparable with that of the widely used 1310 nm OCT system. In the case of restoration with filler material, the 1310 nm OCT imaging displayed better imaging capacity due to lower scattering than 840 nm imaging.     

Picosecond laser cutting and drilling of thin flex glass

We investigate the feasibility of cutting and drilling thin flex glass (TFG) substrates using a picosecond laser operating at wavelengths of 1030 nm, 515 nm and 343 nm. 50 μm and 100 μm thick AF32^®Eco Thin Glass (Schott AG) sheets are used. The laser processing parameters such as the wavelength, pulse energy, pulse repetition frequency, scan speed and the number of laser passes which are necessary to perform through a cut or to drill a borehole in the TFG substrate are studied in detail. Our results show that the highest effective cutting speeds (220 mm/s for a 50 μm thick TFG substrate and 74 mm/s for a 100 μm thick TFG substrate) are obtained with the 1030 nm wavelength, whereas the 343 nm wavelength provides the best quality cuts. The 515 nm wavelength, meanwhile, can be used to provide relatively good laser cut quality with heat affected zones (HAZ) of <25 μm for 50 μm TFG and <40 μm for 100 μm TFG with cutting speeds of 100 mm/s and 28.5 mm/s, respectively. The 343 nm and 515 nm wavelengths can also be used for drilling micro-holes (with inlet diameters of ⩽75 µm) in the 100 μm TFG substrate with speeds of up to 2 holes per second (using 343 nm) and 8 holes per second (using 515 nm). Optical microscope and SEM images of the cuts and micro-holes are presented.     

Design of a highly nonlinear twin bow-tie polymer photonic quasi-crystal fiber with high birefringence

A twin bow-tie polymer-based photonic quasi-crystal fiber with high birefringence, high nonlinearity and low dispersion as well as maintaining single mode operation is presented in the wavelength range 1.8–2.2 μm. Through optimizing fiber structure parameter using a full-vector finite-element method combined with perfectly matched layers boundary condition, the birefringence is as high as 2.43 × 10⁻³, the nonlinearity is as high as 118 W⁻¹ km⁻¹, and the dispersion is only 25 ps/nm/km at 2 μm with the holes pitch of 3.3 μm. From the point of fabrication, the influences of deviation of each air hole diameter are discussed to verify the robustness of the photonic quasi-crystal fiber designed.     

A novel polarization splitter based on dual-core elliptical-holes hybrid photonic crystal fiber

A novel kind of polarization splitter in dual-core elliptical holes hybrid photonic crystal fiber is proposed. Numerical results show that the splitter can reach small coupling length ratio of 0.5, for wavelength from 1.15 μm to 1.9 μm. At wavelength 1.55 μm, the extinction ratio can achieve ?64 dB and the 1.92-mm-long splitter is suggested to achieve extinction ratio better than ?10 dB, a bandwidth of 150 nm. The fiber has small coupling length ratio, small coupling length and high extinction ratio and it is more suitable for fabricating polarization splitter.     

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.     

Size effects on characteristic magnetic fields of a spin-valve multilayer by computer simulation

The change in characteristic magnetic fields of a spin-valve multilayer is investigated as a function of the size by computer simulation. The spin-valve modeled in this work is IrMn (9 nm)/CoFe (4 nm)/Cu (2.6 nm)/CoFe (2 nm)/NiFe (6 nm). The spin-valve dimensions are varied widely from 20 mm×10 mm to 0.5 μm×0.25 μm, but the aspect ratio defined by the ratio of the length to the width is fixed at 2.0. The magnetostatic interactions begin to affect the magnetic properties substantially at a spin-valve length of 5 μm, and, at a length of 1 μm, they become even more dominant. The main consequences of the magnetostatic interactions are a significant increase of the coercivity and a very large shift of the bias field in both the pinned and free layers. It is shown that these changes can be explained by two separate contributions to the total magnetostatic interactions: the coercivity change by the self-demagnetizing field and the change of the bias field by the interlayer magnetostatic interaction field.     

Design of a compact polarization splitter based on the dual-elliptical-core photonic crystal fiber

We propose a compact polarization splitter based on dual-elliptical-core photonic crystal fiber. Two elliptical cores are introduced to increase the difference of effective index between x-polarized and y-polarized mode and three elliptical modulation air holes are used to control the power transfer between the two cores. By optimizing the structure parameters, the length of the polarization splitter is distinctly shortened. Numerical results demonstrate that the compact splitter has the length of 775 μm and up to 50 dB extinction ratio at the central wavelength of 1.55 μm. The corresponding bandwidth of 32 nm could be achieved from the wavelength of 1.534–1.566 μm with the extinction ratio over 20 dB     

Er/Yb co-doped fiber amplifier with wavelength-tuned Yb-band ring resonator

Erbium-ytterbium co-doped fiber amplifier with wavelength-tuned Yb-band loop resonator is presented. The amplified spontaneous emission (ASE) from Yb ions is utilized to stimulate a laser emission at several wavelengths from the 1 μm band in the 1550 nm amplifier. The wavelength of this lasing is tuned by introducing a fiber Bragg grating (FBG). The results show, that the overall efficiency of the amplifier at nominal 1550 nm wavelength can be increased by introducing a feedback loop with 1040 nm and 1050 nm FBG. This loop also protects the Er/Yb amplifier from parasitic lasing at 1 μm and allows significant output power scaling without risk of self-pulsing.     

Ultrasmall optical logic gates based on silicon periodic dielectric waveguides

An ultrasmall silicon periodic dielectric waveguides-based multimode interference all-optical logic gate has been proposed. The device consists of three 205 nm wide single-mode input waveguides, a 1.1 μm wide and 5.5 μm long multimode interference waveguide, and three 205 nm wide single-mode output waveguides. The total length and width of the device are 13.7 μm and 3.2 μm, respectively. By changing the states of the input optical signals and/or control signals launched into the device, multifunctional logic functions including OR, NAND, NOR, and NOT gates are performed, and each logic function can be realized at a specific output waveguide in accordance with the launched control signals. The ultrasmall multifunctional logic device has potential applications in high density photonic integrated circuits.     

Silicon microspheres for electronic and photonic integration

Silicon microspheres are transparent in the near-infrared telecommunication bands and can be used for electrophotonic integration. We have experimentally observed blue shifts in resonance wavelengths of an electrically driven silicon microsphere of 500 μm in radius, in the near-infrared. We have used a distributed feed back (DFB) laser operating at 1475 nm, and applied electrical potential differences up to 9 V to the silicon microsphere. We have observed blue shifts in the resonance wavelengths up to 0.05 nm, which corresponds to a change in the refractive index of 10⁻⁴.     

Road condition analysis using NIR illumination and compensating for surrounding light

An investigation of a NIR camera system for road surface classification has been conducted for several road conditions. The surfaces were illuminated with three wavelengths, 980 nm, 1310 nm and 1550 nm and a halogen lamp, to simulate a real environment application with surrounding light. A measuring scheme to deal with surrounding light has been implemented enabling road condition classification from NIR images in a real environment. The retrieved camera images have been analyzed and an RGB representation of the different surfaces has been created to classify the different road conditions. The investigation shows that it is possible to distinguish between dry, moist, wet, frosty, icy and snowy road surfaces using a NIR camera system in a disturbed environment.     

Arrayed waveguide grating-based demultiplexer with two central wavelengths

In this paper we have presented an arrayed waveguide grating with two central wavelengths, 1550.12 nm and 1310.12 nm. Introducing a novel architecture for outputs of system, if input light to arrayed waveguide grating consists of wavelengths around 1550.12 nm, proposed system will act as 16 channels demultiplexer with channel spacing of 1.6 nm. On the other hand when input wavelengths are distributed around 1310.12 nm, the same arrayed waveguide grating will divide the input to 27 channels with channel spacing of 0.68 nm.     

β-NaYF4:Er3+(10%) microprisms for the enhancement of a-Si:H solar cell near-infrared responses

β-NaYF₄:Er³⁺(10%) microprisms, synthesized using a hydrothermal method, were applied to the back of a thin film hydrogenated amorphous silicon (a-Si:H) solar cells to investigate response to sub-band gap near-infrared irradiation. Currents of 0.3 μA and 0.01 μA were measured during single-illumination with 60 mW (80 mW/cm²) 980 nm and 1560 nm diode lasers, respectively, due to frequency upconversion (UC). Under co-excitation by 60 mW 980 nm and 100 mW 1560 nm lasers, a current improvement to 0.54 μA was obtained, resulting from enhancements in red emission. The finding indicates that co-excitation with multiple wavelengths accessible to UC materials is very effective in enhancing the efficiency of solar cells.     

Analysis and applications of nanocavity structures used as tunable filters and sensors

This paper reports on a novel design for a tunable filter and plasmonic sensor based on the metal–insulator–metal waveguide with a nanocavity resonator. Simulation results show that as a one-channel filter, the resonance wavelengths show a linear red-shift with an increase in nanocavity length with a slope of 1742 nm/μm and a nonlinear blue-shift with an increase in nanocavity width, respectively. A two-channel filter can be realized using two nanocavities and the arrangement of the two nanocavities with respect to the waveguide and the value of the distance between the nanocavities has only a marginal effect on the filter notch wavelength. Finally, both in-slit and out-slit refractive index plasmonic sensors are investigated with a sensitivity of 710 nm/RIU and 250 nm/RIU, respectively.     

A bridged waveguide intersection in two-dimensional photonic crystal structures

In this work we introduce a symmetric waveguide intersection in photonic crystal structures, which passes the optical power entering from each one of its four ports, directly to the forward port. This junction allows designers to easier and more efficient design of photonic integrated circuits (PICs) and to create bridge-junctions in a 13.3 μm² area with just one linear material in its construction. The minimum pass to stop contrast ratio is 26 db with an 18.1 nm bandwidth for contrast ratio over 15 db. Conformal finite difference time domain (CFDTD) method has been used to analysis the system and numerically demonstrates its working.     

Bio-inspired digital nanoactuators for photon and biomaterial manipulation

This paper presents a bio-inspired digital nanoactuation device (DND) for applications to nano-precision digital manipulation of photons and biomolecules. The structures and principles of DND device have been inspired from those of the biological muscle actuator. The bio-inspired DND, composed of a digital actuator and a nonlinear mechanical modulator, purifies the stroke of the digital actuator in order to generate the high-precision motion required for nano-positioning of photonic or bio-molecule devices. We design and fabricate two kind of DNDs: one with a linear modulator (l-DND) and the other with nonlinear modulator (n-DND), both having an identical input and output strokes of 15.2 μm and 5.4 μm, respectively. In experimental study, n-DND shows the repeatability of 12.3 ± 2.9 nm, superior to that of 27.8 ± 2.9 nm achieved by l-DND. We experimentally verify that the displacement purifying capability of the bio-inspired n-DND achieves nano-precision motion repeatability, applicable to photon and bio-molecule manipulation.