Hamza Kurt

Graded index photonic crystals.

We explore two-dimensional triangular lattice photonic crystals composed of air holes in a dielectric background which are subject to a graded-index distribution along the direction transverse to the propagation. The proper choice of the parameters such as the input beam width, gradient coefficient, and the operating frequency allow the realizations of the focusing (lens) and guiding (waveguide) effects upon which more complex optical devices such as couplers can be designed. Numerical results obtained by the finite-difference time-domain and planewave expansion methods validate the application of Gaussian optics within a range of parameters where close agreement between them are observed.

Annular photonic crystals

A new type of two-dimensional photonic-crystal (PC) structure called annular PC composed of a dielectric-rod and a circular-air-hole array in a square or triangular lattice such that a dielectric rod is centered within each air hole is studied. The dielectric rods within the air holes greatly modify the dispersion diagram of the photonic crystal despite the fact that the percentage of volume occupied by the dielectric rods may be small (<12%). Increasing the radius of the inner-dielectric rod, starting from zero to a critical value, reduces the band gap and closes it completely as expected, because of the addition of more dielectric material inside the unit cell. Continuing to increase the radius of the rod above the critical value surprisingly creates another photonic band gap. Comparison of the dispersion diagrams of the new structure and the original lattice (circular air hole square/triangular array in dielectric background) reveals that the photonic band gap is considerably enhanced in size for both square and triangular lattice with the new structure. This approach preserves the symmetry of the structure and provides a complete photonic band gap away from the close-packed condition and at low normalized frequencies.

Photonic crystals for biochemical sensing in the terahertz region

The propagation of terahertz waves in two-dimensional photonic-crystal waveguides was studied computationally to investigate the effects of introducing small quantities of molecules, such as deoxyribonucleic acid, into selected air holes for sensor applications. Comparison with bulk systems shows that photonic-crystal waveguides are promising for biochemical sensing.

The focusing effect of graded index photonic crystals

We describe an approach to implement graded index (GRIN) structures using two-dimensional photonic crystals (PCs). The lattice spacing along the transverse direction to propagation is altered and we show, both theoretically and experimentally, that such a spatial perturbation is an effective way to obtain GRIN PC. The response of the structure to spatially wide incident beams is investigated and strong focusing behavior is observed. The large spot size conversion ratio can be attainable and is mainly limited by the finite size of the structure. The designed GRIN PC shows promise for use in optical systems that require compact and powerful focusing elements compared to the traditional bulky lenses.

Study of different spectral regions and delay bandwidth relation in slow light photonic crystal waveguides

We investigate slow light propagation in monomode photonic crystal waveguides with different spectral features such as constant group index, high bandwidth and low group velocity dispersion. The form of the waveguide mode alters dramatically and spans three different spectral intervals by tuning the size of the boundary holes. Namely, slope of the band gap guided mode changes sign from negative to positive toward the Brillouin zone edge. In between there is a transition region where modes have nearly zero slopes. Maximum group index occurs at these turning points at the expense of high dispersion and narrow bandwidth. The apparent trade-off relationship between group index and bandwidth is revealed systematically. We show that as the radius of the innermost hole is increased above a certain value, the former one decreases and the latter one increases both exponentially but with a different ratio. The product of average group index and bandwidth is defined as a figure of merit which reaches up to a value of approximately 0.30 after a detailed parametric search. The findings of the frequency domain analysis obtained by plane wave expansion method are confirmed via finite-difference time-domain study.

Coupled-resonator optical waveguides for biochemical sensing of nanoliter volumes of analyte in the terahertz region

We present a detailed study of coupled-resonator optical waveguide (CROW) based sensors for biochemical sensing. The sensitivity dependence on the CROW structure parameters, such as intercavity distance and cavity type, is investigated for the effects in the THz region of the EM spectrum of introducing small quantities of molecules, such as DNA, in the holes. Introducing the absorptive material into the low-index medium greatly affects the shape of the propagating modes of the CROW and the transmitted E-field. The shift of the resonant frequency also depends linearly on the refractive index changes for off-resonant case (dispersive effect).

Asymmetric light propagation in chirped photonic crystal waveguides

We report numerical and experimental investigations of asymmetric light propagation in a newly designed photonic structure that is formed by creating a chirped photonic crystal (PC) waveguide. The use of a non-symmetric distribution of unit cells of PC ensures the obtaining of asymmetric light propagation. Properly designing the spatial modulation of a PC waveguide inherently modifies the band structure. That in turn induces asymmetry for the lights followed path. The investigation of the transmission characteristics of this structure reveals optical diode like transmission behavior. The amount of power collected at the output of the waveguide centerline is different for the forward and backward propagation directions in the designed configuration. The advantageous properties of the proposed approach are the linear optic concept, compact configuration and compatibility with the integrated photonics. These features are expected to hold great potential for implementing practical optical rectifier-type devices.

Ultra slow light achievement in photonic crystals by merging coupled cavities with waveguides

We explore slow light behavior of a specially designed optical waveguide by carrying out structural dispersion using numerical techniques. The structure proposed is composed of square-lattice photonic crystal waveguide integrated with side-coupled cavities. We report three orders of magnitude reduction in group velocity at around υ(g) ≅ 0.0008c with strongly suppressed group velocity dispersion. The analysis is performed by using both plane-wave expansion and finite-difference time-domain methods. For the first time, we succeeded to show such a low group velocity in photonic structures. Slow light pulse propagation accompanied by light tunneling between each cavity is observed. These achievements show the feasibility of photonic devices to generate extremely large group index which in turn will eventually pave the way to new frontiers in nonlinear optics, optical buffers and low threshold lasers.

High efficiency of graded index photonic crystal as an input coupler

A graded index photonic crystal (GRIN PC) configuration was placed at the input side of a photonic crystal waveguide (PCW) in order to efficiently couple the light waves into the waveguide. We compared the transmission efficiencies of light in the absence and presence of the GRIN PC structure. We report a significant improvement in coupling when the GRIN PC is incorporated with the PCW. The intensity profiles were obtained by carrying out the experiments at microwave frequencies. Finite difference time domain based simulations were found to be in good agreement with our experimental results.

Photonic crystals for fluid sensing in the subterahertz range

The authors present experimental results on photonic crystals in the subterahertz range for fluid-sensing applications. Using a standard machining process, a two-dimensional photonic crystal consisting of a triangular array of air holes in the dielectric medium was produced. The insertion of fluids with different refractive indices into the active region affects the transmission profile in the stop band. The experiment agrees well with theoretical calculations.