DESIGN AND ANALYSIS OF SPECIALITY OPTICAL FIBER AND METAMATERIALS

Abstract

This thesis presents some novel designs related to optical fiber and metamaterials which are useful for the applications in many fields of optics and photonics. Special type of optical fibers like large mode area photonic crystal fiber designed in this thesis can be utilized for high power applications like high power lasers, amplifiers and sensors. The designs of metamaterials can be employed for nanoantenna, optical buffers and sensing applications. The proposed rectangular core large mode area photonic crystal fiber in this work offers extended single mode operation by filtering all higher order modes. Higher order modes have been suppressed to ensure there is no modal dispersion despite large mode area so that non linear effects could be avoided. Good beam quality can be obtained at the output end of the fiber laser using extended single mode operation. Size of air holes and Fluorine doped silica rods in the cladding region have been optimized to get better results. The proposed structure has an effective-mode area of fundamental mode as large as 2147 μm2 at a wavelength of 1.064 μm with very small loss of 1.36 × 10−2 dB/m. However, first higher order mode has very large confinement loss equal to 9.34 dB/m, which confirms effective single mode operation after 2.14 m propagation length. Moreover, this structure offers an extended single mode operation within a broad spectral range from λ = 1 μm to λ = 1.6 μm so that it is applicable in high power applications and communication. Optical metamaterials are the artificially structured materials which are used to manipulate the direction of the flow of the light. In this work, all-dielectric and hybrid metal-dielectric metamaterials have been designed which are useful for the nanoantenna, optical buffers and sensing applications. Electric as well as magnetic dipoles have been optically induced in the nanoparticles. Azimuthally symmetric forward scattering with complete suppression of backward scattering using first Generalized Kerker’s condition has been achieved for the allowed longitudinal mode and transverse modes in the optical region using single ellipsoidal nanoparticle. By changing the direction of the electric field, forward as well as backward scattering can be tuned at different wavelengths. Further, ellipsoidal core (Si) and shell (SiO2) metamaterial has been proposed for highly directional properties. In this case forward scattering has been attributed to the Fano resonance. Wavelengths at which Fano resonance takes place in the ellipsoidal nanoparticle exhibit higher directivity than the Kerker’s type scattering or forward scattering shown by symmetrical structures like sphere and cube. In this VI thesis, cuboidal nanoparticles in the shape of nanodisk and nanorod have been utilized as dielectric nanoantennas in the visible range. The dimensions of the nanoparticles have been tailored to bring the electric and magnetic dipoles together so that both of them spectrally overlap and on-resonance scattering of electric and magnetic dipole moments take place. There is a Fano dip in the backward scattering and therefore there is an enhancement in the forward scattering which leadsto the improvement in directivity and radiation efficiency. These designs have applications in highly directional nanoantennas. In this thesis, electromagnetically induced transparency has been reported in the Terahertz region which is useful for the optical buffers and ultrasensitive refractive index sensors. In the proposed design, electric dipole has been excited in the metal ring which serves as a bright mode and in the dielectric cube resonator, magnetic and electric dipole have been induced which act as quasi-dark mode and bright mode, respectively. Electric dipole of the metal ring interferes with the electric as well as magnetic dipoles of the dielectric cube which creates an EIT window in a broad region from 1.8 THz to 2.2 THz. A steep and constant phase change in transmission results in high group delay of 5 ps in the broad EIT window so that a large delay bandwidth product equal to 2 could be obtained so that it is useful in the application of optical buffers. Also, Metal dielectric structure provides the platform for high quality factor Fano resonance. Two resonance dips have been obtained with high quality factor using the metal dielectric metamaterial Fano resonator. The quality factor of the Fano resonator turns out to be Q = 89.5 at the first dip and Q = 23 for the second Fano dip. This high quality factor leads to high figure of merit of sensor equal to 6 and 4 for first and second resonance dips, respectively which is useful for metamaterial based sensing devices and bio-sensors.