Low-index Layer Research Articles

Wavelength-specific forward scattering of light by Bragg-reflective iridocytes in giant clams.

A surprising recent discovery revealed that the brightly reflective cells ('iridocytes') in the epithelia of giant clams actually send the majority of incident photons 'forward' into the tissue. While the intracellular Bragg reflectors in these cells are responsible for their colourful back reflection, Mie scattering produces the forward scattering, thus illuminating a dense population of endosymbiotic, photosynthetic microalgae. We now present a detailed micro-spectrophotometric characterization of the Bragg stacks in the iridocytes in live tissue to obtain the refractive index of the high-index layers (1.39 to 1.58, average 1.44 ± 0.04), the thicknesses of the high- and low-index layers (50-150 nm), and the numbers of pairs of layers (2-11) that participate in the observed spectral reflection. Based on these measurements, we performed electromagnetic simulations to better understand the optical behaviour of the iridocytes. The results open a deeper understanding of the optical behaviour of these cells, with the counterintuitive discovery that specific combinations of iridocyte diameter and Bragg-lamellar spacing can produce back reflection of the same colour that is also scattered forward, in preference to other wavelengths that are scattered at higher angles. We find for all values of size and wavelength investigated that more than 90% of the incident energy is carried by the photons that are scattered in the forward direction; while this forward scattering from each iridocyte shows very narrow angular dispersion (ca ±6°), the multiplicative scattering from a layer of ca 20 iridocytes broadens this dispersion to a cone of approximately ±90°. This understanding of the complex biophotonic dynamics enhances our comprehension of the physiologically, ecologically and evolutionarily significant light environment inside the giant clam, which is diffuse and nearly white at small tissue depths and downwelling, relatively monochromatic, and can be the same colour as the back-reflected light at greater depths in the tissue. Originally thought to be unique, cells of similar structure and photonic activity are now recognized in other species, where they serve other functions. The behaviour of the iridocytes opens possible new considerations for conservation and management of the valuable giant clam resource and new avenues for biologically inspired photonic applications.

Linearly polarized Q-switched ceramic laser made with anisotropic nanostructured thin films.

A polarizing laser mirror was made of an alternating sequence of low and high refractive index layers of titanium oxide using glancing angle deposition (GLAD). Large refractive index contrast and large birefringence, reaching 0.5 and 0.1, respectively, could be obtained from one single raw material by changing the deposition conditions. The laser mirror could withstand a train of 2.7ns, single-mode pulses at 680Hz, λ=1030 nm, and peak power density of 670 MW/cm2 when used as an output coupler of a passively Q-switched (Yb0.1Y0.9)3Al5O12 ceramic laser. The polarization extinction ratio was found to be better than 30dB both in continuous-wave and pulsed regimes. These results indicate that polarizing laser mirrors made from nanostructured thin films with GLAD, in addition to being simple to fabricate, can withstand high pulse energy density.

Synergetic electrode architecture for efficient graphene-based flexible organic light-emitting diodes.

Graphene-based organic light-emitting diodes (OLEDs) have recently emerged as a key element essential in next-generation displays and lighting, mainly due to their promise for highly flexible light sources. However, their efficiency has been, at best, similar to that of conventional, indium tin oxide-based counterparts. We here propose an ideal electrode structure based on a synergetic interplay of high-index TiO2 layers and low-index hole-injection layers sandwiching graphene electrodes, which results in an ideal situation where enhancement by cavity resonance is maximized yet loss to surface plasmon polariton is mitigated. The proposed approach leads to OLEDs exhibiting ultrahigh external quantum efficiency of 40.8 and 62.1% (64.7 and 103% with a half-ball lens) for single- and multi-junction devices, respectively. The OLEDs made on plastics with those electrodes are repeatedly bendable at a radius of 2.3 mm, partly due to the TiO2 layers withstanding flexural strain up to 4% via crack-deflection toughening.

Periodic transmission peak splitting in one dimensional disordered photonic structures

In the present paper we present ways to modulate the periodic transmission peaks arising in disordered one dimensional photonic structures with hundreds of layers. Disordered structures in which the optical length nd (n is the refractive index and d the layer thickness) is the same for each layer show regular peaks in their transmission spectra. A proper variation of the optical length of the layers leads to a splitting of the transmission peaks. Notably, the variation of the occurrence of high and low refractive index layers, gives a tool to tune also the width of the peaks. These results are of highest interest for optical application, such as light filtering, where the manifold of parameters allows a precise design of the spectral transmission ranges.

Extremely narrow resonances, giant sensitivity and field enhancement in low-loss waveguide sensors

Low-loss waveguides (WGs), which support excitation of waveguide modes (WMs), are based on a dielectric WG separated from an absorptive film by a low-index dielectric spacer layer. We perform numerical and analytical study of the impact of the losses imposed to the WG in a planar sensing structure in the Kretschmann configuration on the resonance properties of the excitation. We demonstrate that the loss degree of the WMs can be controlled by the thickness of the spacer layer for both s and p polarizations. Extremely narrow resonances are discovered in the reflectivity spectra due to excitation of the low-loss WMs, and the maximum of the estimated sensitivity by intensity is found to be of 105-fold higher as compared to the conventional surface plasmon and WG-coupled surface plasmon sensors. We reveal the giant field intensity enhancement of 107-fold on the surface of the sensing structure in aqueous sensing media that can provide stronger fluorescence intensity at lower sample volumes for fluorescent labeling sensing.

Design for an efficient single photon source based on a single quantum dot embedded in a parabolic solid immersion lens.

We have designed a single photon emitter based on a single quantum dot embedded within a single mode parabolic solid immersion lens (pSIL) and a capping low-index pSIL. Numerical simulations predicted that the emitter performance should exhibit a high photon collection efficiency with excellent far-field emission properties, broadband operation, and good tolerance in its geometric (spatial configuration) parameters. Good geometric tolerance in a single-mode pSIL without yielding significant losses in the photon collection efficiency is advantageous for device fabrication. The low-index top pSIL layer provided this structure with a high photon collection efficiency, even in the case of a small numerical aperture (NA). Photon collection efficiencies of 64% and 78% were expected for NA values of 0.41 and 0.5, respectively. In addition to the benefits listed above, our combined pSIL design provided excellent broadband performance in a 100 nm range.

Plasmonic modes in a conductor-corrugated gap-dielectric system at telecommunication wavelength

We propose a conductor-corrugated gap-dielectric structure to realize broadband plasmonic modes at telecommunication wavelength. The structure consists of a corrugated low-index dielectric gap layer sandwiched by high-index dielectric and conductor layers. The electric field of the plasmonic modes is primarily concentrated in the corrugated gap, which not only reduces the metal absorption resulting in long propagation length but also causes good field confinement. It is proved that periodic modulation introduced into the gap can significantly increase the cutoff gap thickness of the structure. The effective medium theory and numerical simulation demonstrate that the cutoff gap thickness can be further increased by decreasing the filling factor of the low-index dielectric in the corrugated gap. Meanwhile, the scattering effects induced by the periodic modulation are also investigated. The results not only provide a new understanding of the surface plasmonic modes but also benefit the designing of compact devices.

Modelling of multilayer dielectric filters based on TiO2/SiO2 and TiO2/MgF2 for flourescence microscopy imaging

We report a design for creating multilayer dielectric optical filters based on TiO2 and SiO2/MgF2 alternating layers. We have selected Titanium dioxide (TiO2) for high refractive index (2.5), Silicon dioxide (SiO2) and Magnesium fluoride (MgF2) as a low refractive index layer (1.45 and 1.37) respectively. Miniaturized visible spectrometers are useful for quick and mobile characterization of biological samples. Such devices can be fabricated by using Fabry-Perot (FP) filters consisting of two highly reflecting mirrors with a central cavity in between. Distributed Bragg Re-flectors (DBRs) consisting of alternating high and low refractive index material pairs are the most commonly used mirrors in FP filters, due to their high reflectivity. However, DBRs have high re-flectivity for a selected range of wavelengths known as the stopband of the DBR. This range is usually much smaller than the sensitivity range of the spectrometer. Therefore, bandpass filters are required to restrict the wavelength outside the stopband of the FP DBRs. The proposed filter shows high quality with an average transmission of 97 % within the passbands and the transmission outside the passband is around 3 %. Special attention has been given to keep the thickness of the filters within the economic limits. It can be suggested that these filters are exceptionally promising for florescence imaging and narrow-band imaging endoscopy.

An extremely low-index photonic crystal layer for enhanced light extraction from organic light-emitting diodes

This paper reports organic light-emitting diodes (OLEDs) with improved light extraction fabricated by embedding an extremely low-index photonic crystal (LIPC) layer. The LIPC layer increases the optical efficiency through the reduced wave-guide mode between the substrate and anode both by increased light resonance and by a strengthened diffraction effect from an extremely low-refractive-index medium, specifically a line structure composed of a vacuum gap. As a result, the current efficiency and power efficiency of the LIPC-OLEDs are 1.51 and 1.93 times higher, respectively, than the reference device at 1000 cd m(-2). Because most of the light extraction is significant, especially in the forward direction, at the specific wavelengths satisfying the Bragg's diffraction equation, it is possible to calculate the anomalous spectrum of the LIPC-OLED through the finite-difference time domain (FDTD) method.

Theoretical Study for the ITO/Si based High Contrast Grating Structure with Focusing Capability and its Fabrication

High contrast grating (HCG) is the structure made up of the sub-wavelength grating of high-index and the surrounding layer of low-index, which reveals high contrast between two materials. Its advantages include high reflectivity over a broad bandwidth, polarization and wavelength selectivity, optical high-Q resonator, and phase modulation. In this work, the HCG structure comprising of indium tin oxide (ITO) and Silicon (Si), for the surrounding layer and the grating layer respectively, was studied. Its theoretical model was established, and transmittance, phase and optical behavior were calculated by rigorous coupled-wave analysis and finite element method. Furthermore, the established structure was fabricated to validate its feasibility. The fabricated structure shows the focusing capability whose length is about 10 μm, and the feasibility of the structure was demonstrated. It is also meaningful that ITO layer can contribute to the fabrication of the HCG structure, leading to enable the structure to be electrical-driven.

Effect of prism index on sensitivity of lossy mode resonance sensors operating in visible region

We present the theoretical results of the optimization of lossy mode resonance sensors at visible wavelengths. Both angular and spectral interrogations are carried out for absorbing indium tin oxide (ITO) films placed on glass prism. The inclusion of a low-index layer between the prism and the lossy (ITO) layer can produce an efficient refractive index sensor for bio/chemical applications. Further increase in sensitivity can be achieved by changing the index of the prism. It is shown that the sensitivity has strong dependence on the index of prism. Sensitivities as high as 4670 nm/RIU for spectral mode and 67 deg/RIU for angular mode with small values of full width at half maximum (FWHM) can be achieved. Dependence of sensitivity and FWHM on refractive index and thickness of low-index matching layer is also investigated.

Charging suppression in focused-ion beam fabrication of visible subwavelength dielectric grating reflector using electron conducting polymer

Nanoscale periodic patterning on insulating materials using focused-ion beam (FIB) is challenging because of charging effect, which causes pattern distortion and resolution degradation. In this paper, the authors used a charging suppression scheme using electron conducting polymer for the implementation of FIB patterned dielectric subwavelength grating (SWG) reflector. Prior to the FIB patterning, the authors numerically designed the optimal structure and the fabrication tolerance for all grating parameters (period, grating thickness, fill-factor, and low refractive index layer thickness) using the rigorous-coupled wave analysis computation. Then, the authors performed the FIB patterning on the dielectric SWG reflector spin-coated with electron conducting polymer for the anticharging purpose. They also performed similar patterning using thin conductive film anticharging scheme (30 nm Cr coating) for comparison. Their results show that the electron conducting polymer anticharging scheme effectively suppressing the charging effect during the FIB patterning of dielectric SWG reflector. The fabricated grating exhibited nanoscale precision, high uniformity and contrast, constant patterning, and complied with fabrication tolerance for all grating parameters across the entire patterned area. Utilization of electron conducting polymer leads to a simpler anticharging scheme with high precision and uniformity for FIB patterning on insulator materials.

The Influence of a Power Law Distribution of Cluster Size on the Light Transmission of Disordered 1-D Photonic Structures

A better understanding of the optical properties of random photonic structures is beneficial for many applications, such as random lasing, optical imaging and photovoltaics. Here we investigated the light transmission properties of disordered photonic structures in which the high refractive index layers are aggregated in clusters. We sorted the size of the clusters from a power law distribution tuning the exponent a of the distribution function. The sorted high refractive layer clusters are randomly distributed within the low refractive index layers. We studied the total light transmission, within the photonic band gap of the corresponding periodic crystal, as a function of the exponent in the distribution. We observed that, for a within the interval [0,3.5], the trend can be fitted with a sigmoidal function.

Theoretical Study of Hybrid Guided Modes in a Multilayer Symmetrical Planar Plasmonic Waveguide

We presented a comprehensive theoretical study of the hybrid guided mode in a multilayer symmetrical planar plasmoinc waveguide, which is constructed with a thin film metal layer symmetrically sandwiched by three dielectric low/high/low-index layers. The seven-layer planar plasmonic structure can support super long-range plasmonic modes with strong subwavelength confinement in the low-index gap layer. We derived the dispersion equations for the guided mode and characterized the hybrid guided mode based on our derived analytical expressions. We explained how the variations in the thickness of the low-index gap and high-index cladding could change the types of the hybrid mode from strong surface plasmon polariton (SPP)-like mode, to SPP-dielectric waveguide (DW)-like mode, and further to strong-DW-like mode. We also found that by tailoring the geometric dimensions of the waveguide, the plasmonic mode of the multilayer structure can be optimized with the strongest mode confinement at the nanoscale gap. The combination of tight light confinement and long-range propagation length makes the seven-layer plasmonic waveguide an excellent candidate for applications in chip-scale plasmonic integrated circuits. The presented theoretical analysis shall be very useful in the design and optimization of active and passive nanoplasmonic devices.

Enhanced sensitivity in injection-molded guided-mode-resonance sensors via low-index cavity layers

We present an investigation on the use of low-index cavity layers to enhance the sensitivity of injection-molded guided-mode resonance (GMR) sensors. By adjusting the sputtering parameters, a low-index cavity layer is created at the interface between the waveguide layer and the substrate. Refractive index measurements show that a sensitivity enhancement of up to 220% is achieved with a cavity layer, in comparison to a reference GMR sensor without a cavity layer. Finite-element-method simulations were performed, and the results indicate that the cavities significantly redistribute the resonance mode profile and thus enhances the sensitivity. The present investigation demonstrates a new method for enhancing the sensitivity of injection-molded GMR sensors for high-sensitivity label-free biosensing.

Control of the average light transmission in one-dimensional photonic structures by tuning the random layer thickness distribution

In this work we have studied the optical properties of disordered photonic structures, in which we have controlled the distribution of the random layer thickness. Such structures are characterized by the usual alternation of high and low refractive index layers, but the layer thickness follows the aforementioned distribution. We have used two types of distributions: a distribution in which each thickness has the same probability to occur and one in which the thickness follows a Gaussian function. We have simulated the average transmission all over the spectrum for photonic structure characterized by a different width of the distribution. We have found that the choice of the distribution of the layer thickness is a control of the average transmission of a random photonic structure. Photonic structures, fabricated following these distributions, can be interesting for the fabrication of bandpass optical filters.

Mode properties of a coaxial multi-layer hybrid surface plasmon waveguide

A novel type of coaxial circular multi-layer hybrid surface plasmonic waveguide is proposed and of which the mode properties are analyzed theoretically at the telecommunication wavelength of 1550 nm. Simulation results show that by tuning the size of the waveguide, the mode propagation properties could be effectively controlled to obtain enhanced mode confinement as well as weaker propagating loss. It is also found that the mode can be strongly localized in the middle low-index dielectric media layer with sub- or deep sub-wavelength scale. Further investigation proves that the mode propagation loss can be well compensated by replacing the high-index dielectric media with gain material to obtain longer propagation length with very tight mode confinement. The proposed hybrid structure can be used in nanophotonic waveguides, high-quality nanolasers, optical trapping, and biosensors.

Enhancement of magneto-optical Kerr effect by surface plasmons in trilayer structure consisting of double-layer dielectrics and ferromagnetic metal.

Giant enhancement of the magneto-optical Kerr effect (MOKE) by surface plasmon polaritons (SPPs) is theoretically shown in a trilayer structure consisting of double-layer dielectrics and a ferromagnetic metal (Al2O3/SiO2/Fe). We calculated the resonant enhancement of the transverse MOKE (TMOKE) and polar MOKE (PMOKE) using the attenuated total reflection (ATR) configuration with the transfer matrix method using a 4 × 4 scattering matrix. At a specific film thickness of the low-index SiO2 layer, where confinement of the SPPs on the Fe surface becomes close to the cutoff condition, the incident light from the Al2O3 couples with the SPPs at the SiO2/Fe boundary most efficiently, resulting in resonant enhancement of the MOKE at an incident angle corresponding to the wave vector of the SPPs. The calculated PMOKE showed orthogonal transformation (90°-rotation) and almost full-orbed deformation (44°-ellipticity) of the polarization, and the TMOKE showed a change in reflectance of about 34 dB upon magnetization reversal.

Dual optical role of low-index injection layers for efficient polarizer-free high contrast-ratio organic light-emitting diodes.

Polarizer-free high contrast-ratio organic light-emitting diodes (OLEDs) are explored with a structure involving a semi-reflective Cr-based bottom electrode and a dielectric-capped thin Ag top electrode. Their efficiency is shown to be improved significantly with little sacrifice in luminous reflectance by adopting low-refractive-index injection layers that can increase the effective reflectance from the bottom electrode and simultaneously reduce the loss owing to surface plasmon polariton modes. OLEDs employing a low-refractive-index injection layer exhibit improved current efficiency by up to ca. 27.4% than those using index-matched injection layers, with luminous reflectance maintained at as low as 4%.

Advantages and challenges of optical coating production with indirect monochromatic monitoring.

In this paper, we present our recent studies on raising the quality of optical coating production with an indirect monochromatic monitoring system. Preproduction error analysis and computational manufacturing are used to estimate potential advantages of application of indirect optical monitoring. It is then demonstrated that a key issue for realization of this advantage is accurate specification of tooling factors for layer thicknesses on test glasses. The tooling factors are precalibrated using single layer depositions and then are corrected using results of reverse engineering for the first production run. It is found that a gradual variation of tooling factors of low index layers is the main error factor in the first deposition run. Finally, we redeposit our coating with a modified monitoring strategy, taking into account this factor. The new experimental results show excellent correspondence with the theoretical spectral performance.