Sculptured Nematic Thin Film Research Articles

Dyakonov–Tamm waves-based optical sensing using sculptured nematic thin film

Dyakonov–Tamm (DT) waves are highly sensitive to the constitutive properties of the partnering materials near the interface. DT waves are excited at the interface of two dielectric materials of which at least one is anisotropic and periodically nonhomogeneous normal to their interface. Sculptured nematic thin film (SNTF) is a good candidate for the periodically nonhomogeneous dielectric partner for optical sensing of a fluid due to its porosity. The nanoscale parameters of an uninfiltrated SNTF obtained from the inverse Bruggeman homogenization formalism were used in the forward Bruggeman homogenization formalism to determine the constitutive parameters for the infiltrated SNTF. The sensitivity of DT waves to the refractive index was analyzed for two possible sensing modalities and it was found that the sensitivity was comparable to that of the chiral sculptured thin films (STFs) made of the same material as of the SNTF. This implies that the sensing with DT waves is robust, is independent of the morphology of the partnering nonhomogeneous dielectric material and could make the sensing easier since SNTFs are easier to fabricate than the chiral STFs.

Multiple trains of same-color surface-plasmon-polaritons guided by the planar interface of a metal and a sculptured nematic thin film. Part VI: Spin and orbital angular momentums

The spin and the orbital parts of the angular momentum of multiple surface-plasmon-polariton (SPP) waves guided by the planar interface of a metal and a periodically nonhomogeneous sculptured nematic thin film (SNTF) were theoretically investigated. Both the Minkowski and Abraham formulations of angular momentum were considered. Multiple SPP waves guided by the metal/SNTF interface propagating along a direction that does not lie wholly in the morphologically significant plane of the SNTF were found to have different magnitudes and directions of the spin and the orbital parts of the angular momentum. The spin angular momentum is oriented in a variety of directions, but the orbital angular momentum is perpendicular to the direction of propagation and lies in the interface plane. Not only does the periodic nonhomogeneity of the partnering dielectric material engender a multiplicity of SPP waves, but also it can enhance the magnitudes of the spin and orbital parts of their angular momentums.

Prism-coupled excitation of Dyakonov–Tamm waves

The prism-coupled excitation of Dyakonov–Tamm waves guided by the planar interface of a homogeneous, isotropic dielectric material and a periodically nonhomogeneous sculptured nematic thin film (SNTF) was investigated. The excitation of Dyakonov–Tamm waves was identified by correlating a high-absorptance band in the plots of absorptance vs. the incidence angle and the direction of propagation in the interface plane with the solutions of the underlying canonical boundary–value problem. Dyakonov–Tamm waves were found to have a much wider angular existence domain than Dyakonov waves, and broader absorptance peaks than both the Dyakonov and Tamm waves in the plots of absorptance vs. the incidence angle.

Propagation of surface waves and waveguide modes guided by a dielectric slab inserted in a sculptured nematic thin film

Wave propagation guided by a dielectric slab inserted in a sculptured nematic thin film (SNTF) was studied theoretically. Two types of guided waves can be identified: (i) surface (Dyakonov--Tamm) waves guided by one or both of the two planar interfaces of the dielectric slab and the SNTF, and (ii) waveguide modes in the dielectric waveguide formed by the slab with the SNTF as the cladding. As the thickness of the dielectric slab is increased, the number of waveguide modes increases. If the slab thickness is less than twice the e-folding distance into the dielectric slab, the Dyakonov--Tamm waves propagate coupled to both interfaces; the coupling decreases and eventually vanishes as the slab thickness increases, so that Dyakonov--Tamm waves are guided by the individual dielectric-SNTF interfaces independently. The chosen structure supports the propagation of Dyakonov--Tamm waves in all directions, in contrast to the restricted range of propagation supported by a single SNTF-dielectric interface. Propagation of both Dyakonov--Tamm waves and waveguide modes should occur in practice with negligible attenuation, in contrast to that of surface-plasmon-polariton waves that are guided when the dielectric slab is replaced by a metal slab.

Multiple trains of same-color surface plasmon-polaritons guided by the planar interface of a metal and a sculptured nematic thin film. Part V: Grating-coupled excitation

The excitation of multiple surface-plasmon-polariton (SPP) waves by the plane wave illumination of a metallic surface-relief grating coated with a sculptured nematic thin film (SNTF) was studied theoretically. The relevant boundary-value problem was formulated such that the grating plane and the morphologically significant plane of the SNTF can be rotated about a common axis with respect to each other. The rigorous coupled-wave analysis was employed to numerically solve the boundary-value problem. Absorptances were calculated as functions of the angle of incidence of a linearly polarized plane wave with wave vector lying wholly in the grating plane, and the excitation of the SPP waves was inferred from those peaks in the absorptance curves that were independent of the thickness of the SNTF. These peaks were successfully correlated with the solutions of the underlying canonical problem solved in Part IV of this series of papers.

Dyakonov–Tamm waves guided by a phase–twist combination defect in a sculptured nematic thin film

The propagation of Dyakonov–Tamm waves guided by a phase–twist combination defect in a sculptured nematic thin film (SNTF) was studied theoretically by numerical solution of a dispersion equation. The phase defect was fixed at 180 °, whereas the twist defect was kept variable as also the direction of propagation. Multiple Dyakonov–Tamm waves that differ in spatial profile, degree of localization, and phase speed were found to propagate guided by the combination defect, depending on the angle between the morphologically significant planes of the SNTF on either side of the defect as well as on the direction of propagation. The most strongly localized Dyakonov–Tamm waves turned out to be essentially confined within one structural period of the SNTF normal to the combination defect.

Surface plasmon–polariton wave propagation guided by a metal slab in a sculpturednematic thin film

Surface plasmon–polariton (SPP) wave propagation guided by a metal slab in a periodicallynonhomogeneous sculptured nematic thin film (SNTF) was studied theoretically.The morphologically significant planes of the SNTF on both sides of the metalslab could either be aligned or twisted with respect to each other. The canonicalboundary-value problem was formulated, solved for SPP wave propagation, andexamined to determine the effect of slab thickness on the multiplicity and the spatialprofiles of SPP waves. Decrease in slab thickness was found to result in moreintense coupling of two metal/SNTF interfaces. But when the metal slab becomesthicker, the coupling between the two interfaces reduces and eventually SPP waveslocalize to one of the two metal/SNTF interfaces. The greater the coupling betweenthe two metal/SNTF interfaces, the smaller is the phase speed. Both the phasespeed and the attenuation of an SPP wave also depend on the twist between themorphologically significant planes of the SNTF on the two sides of the metal slab.

Multiple surface‐plasmon‐polariton waves localized to a metallic defect layer in a sculptured nematic thin film

AbstractThe propagation of a surface‐plasmon‐polariton (SPP) wave guided by a thin metallic defect layer in a sculptured nematic thin film (SNTF) – a periodically nonhomogeneous, anisotropic, dielectric material – was studied theoretically. If the defect layer is sufficiently thick, SPP waves propagate independently along the two metal/SNTF interfaces formed by the presence of the defect layer in the SNTF. As the thickness of the defect layer is reduced, SPP wave propagation is modified by the coupling of the two interfaces. The number of SPP waves increases and lower attenuation rates along the direction of propagation become possible.magnified image(© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Multiple trains of same-color surface plasmon-polaritons guided by the planar interface of a metal and a sculptured nematic thin film. Part IV: Canonical problem

The canonical problem of the propagation of surface-plasmon-polariton (SPP) waves localized to the planar interface of a metal and a sculptured nematic thin film (SNTF) that is periodically nonhomogeneous along the direction normal to the interface was formulated. Solution of the dispersion equation obtained thereby confirmed the possibility of exciting multiple SPP waves of the same frequency or color. However, these SPP waves differ in phase speed, field structure, and the e-folding distance along the direction of propagation.

Multiple trains of same-color surface plasmon-polaritons guided by the planar interface of a metal and a sculptured nematic thin film. Part III: Experimental evidence

The absorbance, reflectance, and transmittance of a linearly polarized, obliquely incident plane wave were calculated for the planar interface of a metal and a sculptured nematic thin film (SNTF) in the Kretschmann configuration, the wave vector of that plane wave being arbitrarily oriented with respect to the morphologically significant plane of the SNTF. The permittivity profile of the chosen SNTF was supposed to have been sculptured during physical vapor deposition by varying the vapor incidence angle sinusoidally about a mean value. Regardless of the orientation of the wave vector, multiple surface-plasmon-polariton (SPP) trains or waves of the same color but different phase speeds and guided by the metal/SNTF interface can be excited.

Theory of Dyakonov–Tamm waves at the planar interface of a sculptured nematic thin film and an isotropic dielectric material

In order to ascertain conditions for surface-wave propagation guided by the planar interface of an isotropic dielectric material and a sculptured nematic thin film (SNTF) with periodic nonhomogeneity, we formulated a boundary-value problem, obtained a dispersion equation therefrom, and numerically solved it. The surface waves obtained are Dyakonov-Tamm waves. The angular domain formed by the directions of propagation of the Dyakonov--Tamm waves can be very wide (even as wide as to allow propagation in every direction in the interface plane), because of the periodic nonhomogeneity of the SNTF. A search for Dyakonov-Tamm waves is, at the present time, the most promising route to take for experimental verification of surface-wave propagation guided by the interface of two dielectric materials, at least one of which is anisotropic. That would also assist in realizing the potential of such surface waves for optical sensing of various types of analytes infiltrating one or both of the two dielectric materials.

Multiple trains of same-color surface plasmon-polaritons guided by the planar interface of a metal and a sculptured nematic thin film. Part II: Arbitrary incidence

The absorbance, reflectance, and transmittance of a linearly polarized, obliquely incident plane wave were calculated for the planar interface of a metal and a sculptured nematic thin film (SNTF) in the Kretschmann configuration, the wave vector of that plane wave being arbitrarily oriented with respect to the morphologically significant plane of the SNTF. The permittivity profile of the chosen SNTF was supposed to have been sculptured during physical vapor deposition by varying the vapor incidence angle sinusoidally about a mean value. Regardless of the orientation of the wave vector, multiple surface-plasmon-polariton (SPP) trains or waves of the same color but different phase speeds and guided by the metal/SNTF interface can be excited.

Multiple trains of same-color surface plasmon-polaritons guided by the planar interface of a metal and a sculptured nematic thin film

A sculptured nematic thin film (SNTF) is an assembly of parallel nanowires that are shaped in a fixed plane orthogonal to the substrate on which the film is deposited. The absorbance, reflectance, and transmittance of a linearly polarized, obliquely incident plane wave were calculated for a planar metal/SNTF interface in the Kretschmann configuration, the wave vector of the plane wave lying wholly in the morphologically significant plane of the SNTF. The permittivity profile of the chosen SNTF was supposed to have been sculptured during physical vapor deposition by varying the vapor incidence angle sinusoidally about a mean value. Calculations revealed that (i) multiple surface-plasmon-polariton (SPP) trains of the same color can be independently guided by the metal/SNTF interface, (ii) not all SPP trains have to be co-propagating, and (iii) not all SPP trains have to be of the same linear polarization state. As different SPP trains move with different speeds, guided by the interface, exciting prospects emerge for error-free sensing and plasmonics-based communication.

Sculptured nematic thin films with periodically modulated tilt angle as rugate filters

The reflectances and transmittances of linearly polarized light are calculated using the piecewise homogeneity approximation for a sculptured nematic thin film (SNTF) with a sinusoidally varying (local) columnar tilt angle. Empirical data on TiO 2, available in the literature, are used to realistically model the variation of the permittivity dyadic with the tilt angle. Wave propagation is taken to occur in the morphologically significant plane of the SNTF. Several Bragg peaks occur, with the fundamental peak making a tilt-modulated SNTF suitable for exploitation similarly to a rugate filter. Additionally, the appearance of a spectral hole in the fundamental Bragg peak is demonstrated with the inclusion of a central defect layer.

Dielectric sculptured nematic thin films for rugate-like filters

A sculptured nematic thin film (SNTF) slab with sinusoidally nonhomogeneous permittivity dyadic is shown to act as a polarization-discriminatory bandstop filter for normally incident light. A cascade of two matched SNTF slabs is shown to act as a polarization-insensitive rugate-like filter.