Small Δn Research Articles

Metal-organic framework photonic crystals with bidisperse particles-based brilliant structural colors and high optical transparency for elaborate anti-counterfeiting

Photonic crystals (PCs) have attracted great interest and wide applications in displays, printing, anti-counterfeiting, etc. However, two main challenges significantly hinder their applications: 1) the tradeoff between high optical transparency across the whole visible range and brilliant colors requiring a large refractive index contrast (Δn), and 2) the way of regulating structural colors by altering tens of different sizes. To address these issues, a new type of metal–organic framework (MOF)-based transparent photonic crystal (TPC) has been fabricated through self-assembling MOF particles into three-dimensional ordered structures which were then infiltrated by polydimethylsiloxane (PDMS). Compared to conventional PCs, these TPCs exhibit 1) both brilliant forward iridescent structural colors and high transmittance (>75 %) across the whole visible spectra range, and 2) conveniently adjustable colors based on bidisperse particles. The unique color-generating mechanism of the light diffraction by each plane lattice and the small Δn between MOF particles and PDMS are the keys to TPCs’ characteristics. Moreover, the prepared invisible anti-counterfeit labels can reversibly hide-reveal patterns with elaborate and exchangeable color contrast in a non-destructive way, showing potential applications in anti-counterfeiting, information encryption, and optical devices.

Improved light transmission in nanocrystalline aluminum nitride (AlN)—Enabling a lightweight, thermal shock resistant transparent ceramic

Optically transparent ceramics were traditionally only viable using optically isotropic materials and limited birefringent materials with small Δn such as alumina (Δn≈0.008). Thus heavily birefringent materials such as AlN (Δn≈0.045), polycrystalline ceramics are usually not considered for optical applications due to their high scattering loss, despite its exceptionally appealing thermal conductivity, mechanical stiffness/toughness and relatively low density—attributes that are ideal for light weight optical windows for harsh environments. To further improve the transparency over coarse grain AlN ceramics that have been reported, subwavelength grain size is desired. Here we present significant improvements in visible, NIR and mid-IR transparency in AlN ceramics by reducing the average grain size to 230 nm. A newly developed, non-carbon thermal reduction for producing clean powders at low temperatures is key to the success. The nanocrystalline AlN ceramics reach full density at a densification temperature as low as 1500 °C and showed an in-line transparency of over 55% at 2.5 μm and over 70% in the mid-IR which is the highest transparency in AlN ceramics reported. The successful demonstration of transparent nanocrystalline AlN ceramics makes it the ideal choice for optical windows in quickly fluctuating temperature environments where weight is a consideration such as air and space vehicles.

Influence of linear birefringence in the computation of scattering phase functions

Birefringent media, like biological tissues, are usually assumed to be uniaxial. For biological tissues, the influence of linear birefringence on the scattering phase function is assumed to be neglectable. In order to examine this, a numerical study of the influence of linear birefringence on the scattering phase function and the resulting backscattering Mueller matrices was performed. It is assumed that the media consist of spherical scattering particles embedded in a nonabsorbing medium, which allows us to employ the Lorenz-Mie theory. In the Monte Carlo framework, the influence of linear birefringence on the components of the electric field vector is captured through the Jones N-matrix formalism. The Lorenz-Mie theory indicates that a given linear birefringence value Δn has a bigger impact on the scattering phase function for large particles. This conclusion is further supported by Monte Carlo simulations, where the phase function was calculated based on the refractive index once in the ordinary direction and once in the extraordinary one. For large particles, comparisons of the resulting backscattering Mueller matrices show significant differences even for small Δn values.

Simulation of group-velocity-dependent phase shift induced by refractive-index change in an air-bridge-type AlGaAs two-dimensional photonic crystal slab waveguide

We report on a theoretical study of a refractive-index-change- (Δn-) induced phase shift enhanced by the low group velocity in an air-bridge-type AlGaAs two-dimensional (2-D) photonic crystal (PC) slab waveguide. The calculation was based on a three-dimensional finite-difference time-domain method for the design of a phase-shift arm of the 2-D PC-based symmetric Mach–Zehnder- (SMZ-) type all-optical switch. Δn was assumed to be induced by an optical nonlinearity of InAs quantum dots embedded selectively in the phase-shift arm in the PC SMZ. By changing Δn from 0 to −0.01 and −0.1 for an even guided mode in the triangular-lattice single-line-defect waveguide in the PC SMZ, we calculated a group-velocity-dependent phase shift as well as a band diagram and a transmission spectrum. The result showed that the phase shift is almost inversely proportional to the group velocity. Taking into account Δn of approximately −0.001 predicted for actual InAs quantum dots, we evaluated the length of the phase-shift arm, necessary for the π phase shift in the PC SMZ, to be ~100 μm for a group velocity of 0.031c and a lattice constant of 0.36 μm at a wavelength of ~1.3 μm, where c is the light velocity in the vacuum. The length of the phase-shift arm was significantly reduced because of the low group velocity in spite of the small Δn. As a result, it was found that the phase-shift arm can be designed short enough to achieve a compact ultrafast all-optical switch.

Spin-dependent interactions in hyperon-nucleon scattering

In order to study the ΣN spin-orbit interaction, asymmetries of the polarized Σ + elastic scattering on proton has been measured. The polarized Σ + hyperons were produced through the p( π +,K +) Σ + reaction in a liquid-scintillator active target, and tracks of Σ +p scattering events were recorded with a newly-developed track detector named SCITIC (scintillating track image camera). Analyses of data from an early stage of experiment indicated a large Σ +p spin-orbit interaction in contrast to the case of the small ΔN spin-orbit interaction.

Intrinsic birefringence of poly(trimethylene terephthalate)

AbstractHighly oriented poly(trimethylene terephthalate) (PTT) fiber has a low birefringence that is unexpected for an aromatic polyester with a high refractive index. To explain this observation, the intrinsic birefringence Δn of PTT crystal was calculated from its bond polarizabilities to be 0.029. This Δn is almost an order of magnitude smaller than poly(ethylene terephthalate)'s value at 0.22, although both polymers have nearly identical crystal refractive indices. The small Δn is due to the arrangement of PTT's methylene groups in gauche conformations, causing the chain‐repeating unit to be tilted ∼53° away from the c axis toward the basal plane. Because of the small Δn, the crystalline‐phase orientation made only a small contribution to the overall birefringence despite the fiber's high crystallinity and orientation. To understand the effect of the number of methylene groups on polyester optical anisotropy, the Δn's of a series of poly(m‐alkylene terephthalates) with m = 2–5 were compared and correlated with ψ: an angle made by the normal of the benzene ring with the crystal's axis. As ψ′ decreases, Δn of the polyesters diminishes. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1513–1520, 2002

The relaxation behavior of amorphous PET in global chain orientation but nearly random segmental orientation (GOLR) state

In this report, the relaxation behavior of amorphous poly(ethylene terephthalate) (PET) in global chain orientation but nearly random segmental orientation (GOLR) state is studied by infrared dichroism, optical birefringence and hot shrinkage measurements at temperatures below or up PET's glass transition temperature, Tg. The results indicate that the difference in the relaxing rate between global chains and segments is at least in ∼ 102 times order above PET's Tg (∼ 84°C), with a rough approximation; while the sudden decrease of small Δn of the sample (∼ 10−4) upon being treated at about 85 ∼ 90°C, implies us that the small Δn is closely related with the orientation of global chains.

Design of Imaging Lens Systems that Use Low Dispersive Radial Gradient-Index Rod

Many effective lens design examples using gradient-index (GRIN) materials have been reported so far. However, the GRIN materials that were used in those designs are either very difficult or impossible to produce. We show an effective design and the design techniques of imaging lens systems that utilize realizable materials. The designed lens systems use flat-surfaced radial GRIN lens (Wood lens) with low dispersion, small Δn and small diameter.

Surface freezing in mixtures of molten alkanes and alcohols

Surface freezing is studied in molten binary mixtures of alkanes and alcohols of different lengths using X-ray surface scattering and surface tension measurements. A crystalline monolayer (for alkanes) or bilayer (for alcohols) is formed at the surface a few degrees above the bulk freezing temperatures. The behaviour is found to be dominated by the length difference of the two components, Δn. For small Δn the surface properties and structure vary continuously with concentration between those of the pure components. For large Δn, however, the variation is discontinuous, exhibiting surface segregation. Several new phenomena, not observed in the pure components, are also found: a new surface crystalline structure in alkanes, a suppression of surface freezing for some compositions in both materials, and the inducement of surface freezing in alcohols which do not show the effect when pure. A Flory-Huggins theory based on competition between entropic mixing and a repulsive interaction due to chain length mismatch accounts well for the observed phenomena in alkanes, but requires modifications when applied to alcohols, probably due to the more complex additional headgroup interactions.

A crossed-beam study of the state-resolved integral cross sections for the inelastic scattering of OH(X 2Π) with CO and N2

State-resolved cross sections for the inelastic scattering of OH(X 2Π) with CO and N2 have been measured in a crossed molecular beam apparatus over the energy range from 0.5 to 5.5 kcal mol−1. The magnitude and shape of the excitation functions for the same OH final states were virtually independent of the two targets except near threshold. Both spin–orbit conserving and spin–orbit changing transitions have been investigated. Cross sections for spin–orbit changing transitions of large ΔN were approximately the same magnitude as those for spin–orbit conserving transitions. For small ΔN, however, the spin–orbit conserving transitions became more favorable. The Λ-doublet ratio, Π(A′)/Π(A″), in the Π3/2 (F1) manifold differed from unity only for the N2 target for which the ratio became increasingly larger than unity as N′ increased. The observed fine-structure effects were nearly the same as those from the corresponding (reverse) reactive collisions, and a close connection between the inelastic and reactive events has been conjectured.

(2+1′) rotationally resolved resonance enhanced multiphoton ionization via the E 2∑+(4s,3d) and H 2∑+(3d,4s) Rydberg states of NO

The results of studies of ionic rotational branching ratios and photoelectron angular distributions resulting from (2+1′) resonance enhanced multiphoton ionization of NO via various high J (≊21.5) rotational branches of the E2∑+(4s,3d) and H2∑+(3d,4s) Rydberg states are presented. The rotational branching ratios show the expected ΔN=even rotational propensity rule with very small ΔN=odd signals. The branching ratios for the E2∑+ state are seen to be independent of photoelectron energy with the ΔN=+2 signals strongest and no appreciable higher rotational transfer peaks (‖ΔN‖≥3). The higher rotational transfer signal for ionization of the H2∑+ state are also negligible but the rotational branching ratios are strongly energy dependent due to a Cooper minimum in the l=3 partial wave of the kσ- and kπ-continua at a photoelectron kinetic energy of 2.6 eV and 2.9 eV, respectively. This leads to a strong rotational selectivity that can be exploited to produce ions in a specific rotational level. These consequences of Cooper minima close to threshold are quite general and their influence on rotational distributions should be readily observable in other molecular systems. The photoelectron angular distributions via both states show a strong energy dependence with a rapid change in the angular distributions around the Cooper minimum associated with the H2∑+ state.

Nitrogen-14 nuclear magnetic resonance. Part III. A unified scale for chemical shifts in organic and aqueous solutions

A unified scale for 14N n.m.r. shifts, δN, based on internal standards, is proposed. The primary standards are nitromethane and nitrate ion for organic and aqueous solutions, respectively. The chemical shift between nitromethane and the nitrate ion is shown to be zero by means of internal standards. Tetranitromethane (+48 p.p.m.) and dimethylformamide (+276 p.p.m.) are suggested as secondary reference substances for small δN values.