Photonic crystal waveguides and devices

Abstract

The extra ordinary improvement in the progress of information data processing in the past few decades is associated with the generation of high performance and considerable miniaturization of integrated circuits in semiconductor technology. However it results in increase in resistance and power dissipation in circuits. To overcome these problems, devices using photons rather than electrons as an information carrier were developed. The simple analogy between an electron in a semiconductor and a photon in a periodically varying dielectric medium, known as photonic crystal, can reveal novel electromagnetic phenomenon. These new types of structures (photonic crystals) are able to control electromagnetic waves in three dimensions since they can give rise to bandgaps for photons, analogous to bandgap in semiconductors known as forbidden energy bandgap. Furthermore, devices based on photonic crystals benefit from high speed of optical signals which provide larger bandwidth and reduced cross talks between the channels because of the absence of interaction between the photons. The existence of unique and interesting properties such as -photonic bandgap (PBG) i.e. in the range of frequencies at which optical propagation is completely prohibited in any direction, as well as the existence of defect modes, that may appear within the photonic bandgaps when a defect is introduced into an otherwise perfect photonic crystal, has resulted in fast growth of photonic technology. The possibility of molding the flow of light through these structures has led to the design and development of efficient optoelectronic devices and systems. The high refractive index contrast provided by silicon has led to silicon based micro-photonics. In the present thesis, first the photonic bandgap (PBG) induced wave guiding applications of photonic crystals is exploited to design 2D dual band wavelength demultiplexer (DBWD) for separating the two telecommunication wavelengths, 1.31 m and 1.55 m. Initially, two designs were presented based on air bridge type photonic crystal structures in which both the upper and lower cladding is made up of air. However, these types of structures are mechanically unstable from practical perspective as well as not suitable for large scale integration. On the contrary, structures with solid support are more realistic. To overcome this instability, silicon-on-insulator (SOI) based PhC DBWD is designed. In these structures, mechanical robustness is improved by the supporting dielectric material under the slab. Enhancement in spectral response is further obtained by optimizing the Y junction of de-multiplexer giving rise to high transmission and extinction ratios for the two wavelengths, 1.31 μm and 1.55 μm. Tolerance analysis was also performed to study the effect of the variation of air hole radius, etch depth and refractive index on the transmission characteristics of the proposed design of SOI based photonic crystal DBWD. In addition the strong light matter interaction observed in silicon photonic crystals results in slowing down the group velocity of light within such photonic crystals. By carefully engineering the photonic dispersion relationship, one may obtain unique opportunities for realization of devices that exploit the impact of slow light effects within such photonic crystals. These devices serve as key sources for processing, storing and buffering, required in future all-optical communication networks and information processing systems. PhC channel waveguides can be used as defect-mode slow light structure which enables increased time-delay for optical signals. Therefore, the design of a silicon-on-insulator photonic crystal channel waveguide for slow light propagation, with group velocity in the range of 0.0028c to 0.044c and ultra-flattened group velocity dispersion (GVD), is proposed. The proposed structure is also investigated for its application as an optical buffer with a large value of normalized delay bandwidth product (DBP), equal to 0.778. Furthermore it is also shown that the proposed structure can also be used for time or wavelength-division de-multiplexing. The tunability of PhC lattices can further be extended and controlled by filling their segments with certain types of liquid crystal (LC) material. This combination offers the possibility of shifting the frequency of the defect modes and tuning the dispersion curves, in order to obtain flat slow modes with low group velocity dispersion. Since dynamically tuned devices are essential components in optical systems, PhC waveguide configurations with infiltration of LC material offer a strong potential for realizing integrated micro-photonic devices. An SOI based LC infiltrated slow light PhC channel waveguide having rectangular air holes in silicon core is thus proposed which yields an average group index of 43 over a bandwidth of 1.02 THz and vanishing group velocity dispersion. Propagation losses and their dependence on group velocity are another matter of concern. There is little justification in exploring the slow light regime if any advantage obtained is immediately counteracted by excessive losses. Such issues of concern are tackled directly in the present thesis - in order to obtain a highly efficient slow light PhC waveguide, with a simple design, that is suitable for fabrication. The response of a given material to an incident electromagnetic wave is characterized by the study of induced polarization of the medium. The linear response of the medium is valid only if the incident radiation is weak. However, if the intensity of incident light increases, polarization of the medium is no longer linear and becomes nonlinear. Therefore in such media, light propagation is controlled by the intensity of the incident light. The concept of nonlinear photonic crystals can be employed by taking the advantage of slow group velocity of light achievable in such structures. In this frame of work, we next, report the effect of slow light on two photon absorption (TPA), free carrier absorption (FCA) and self phase-modulation (SPM) processes in silicon-on-insulator (SOI) photonic crystal (PhC) channel waveguides. It is important to mention here that, as optical pulses propagate through photonic crystal waveguide; their evolution in both the time and frequency domains is governed by the interplay of linear dispersion and nonlinearity. It is observed that, in the slow light regime, these nonlinear effects are enhanced and the resulting increase in the induced phase shift can be used to decrease the size and power requirements needed to operate devices such as optical switches, logic gates, etc. However, soliton dynamics will dominate the propagation of femtosecond pulses in PhC waveguides when group velocity dispersion (GVD) is strongly anomalous because of large waveguide dispersion. Keeping above facts in view, I have investigated the propagation of light in a nonlinear slow light medium formed by a channel SOI PhC structure having elliptical holes in silicon core - and it is observed that, while beginning with almost the same spectral width as that of an input pulse, the pulse spectrum broadens as the input power level increases. The rate at which the spectrum broadens with power is larger for slower waveguides. However it is found that the SPM-induced phase shift decreases as the TPA coefficient increases, which in turn depends on the slow down factor, S. The spectral broadening factor calculated for the waveguide shows that a higher input power level of the order of 102 W is required to obtain the same level of spectral broadening in fast waveguides as compared to the power level on the order of 100 W required in the case of slow waveguides. This type of slow light structure has considerable potential for use in photonic device applications such as optical switches, logic gates etc. - as demonstrated in chapter 6 of the present thesis.