Figure 3.6: The vertical confinement factor as a function of the etch depth
Figure 3.5 shows the epitaxial wafer index profile and the mode distribution in the vertical direction. The modal profile is a little bit asymmetric relative to the position of the quantum well region since the upper cladding layer of the SCH structure is only about 500 nm thick. This also implies that the metal contact will introduce a small amount of absorption loss for the laser. As we mentioned before, this is the design compromise we have to take. It is clear that the etch depth and the modal profile will determine how much of the optical mode interacts with the photoniccrystal, as shown in Fig. 3.5. We can also calculate the effective refractive index in the wafer plane n eff = 3 . 26 . Figure 3.6 shows the vertical confinement factor as a function of the etch depth. The vertical confinement factor is very sensitive to the etch depth in the range of 200 nm–500 nm. Thus the etch depth needs to be accurately controlled in this range.
Chapter 6 Conclusion
Controlling the electromagnetic modes in a laser cavity is the key to design- ing high power lasers. Rather than just the total output power from the facet, the more important metric is the total useful power coupled into an optical system. While a large modal volume is necessary for reducing the energy density at the facet, this is contrary to the requirement for single spatial mode operation in a waveguide utilizing total-internal-reﬂection for conﬁnement. We have seen that grating conﬁnement via Brag reﬂection al- lows for modes above the light line, and if these lossy modes can be isolated, the inherent loss mechanism by radiation into the substrate creates a large modal loss discrimination that can favor the defect mode for single spatial mode operation.
Flip-chip bonding method for edge coupling
For the flip-chip bonding method, a diode laser chip is flip-chip bonded on a silicon substrate. Misalignment during the bonding process can result in extra coupling losses. For the horizontal alignment, sub-micron (±0.5 µm) alignment accuracy is usually enough to obtain good coupling efficiency. And this accuracy is feasible with a well- maintained flip-chip bonding system. Since the vertical size of the laser mode is smaller and the thickness of the Si/Si 3 N 4 waveguide is less than 800 nm, the vertical alignment accuracy is more critical for efficient coupling. Multiple steps of etching and film deposition during the fabrication process often present challenges for accurate vertical alignment. In addition, the solder layer thickness could vary during the bonding process, which further increases the difficulty of good vertical alignment. Here, we use dry-etched silicon pedestals on the passive chip to assist the bonding process and precisely control the vertical alignment of the laser diode chip.
Keywords: 1D Photonic Сrystal, Tunable Photonic Сrystal, Multi-component Photonic Сrystal, Photonic Band Gap, Photonic Gap Map
Miniaturization and integration of the optical devices, such as filters, polarisers, splitters and switches in electro-optical microcircuits is the key goals for realisation of all Si based microphotonics 1 . One of the great expectations is associated with the research and development of the optical devices operating on photonic band gap (PBG) effect, namely devices based on Si PhotonicCrystal (PC) structures 2,3 . Owning to the PBG effect, which is the total reflection of all frequencies of light within this gap; Si PCs are used as broadband Bragg mirrors or reflectors 4 . The operational wavelength of these devices possesses a high sensitivity to the refractive indices, n, and thickness, d, of all the components of PC. The Si- based PCs are normally composed of two-components, which are primarily Si with high refractive index n H = n Si and any other material with lower refractive index n L < n Si . Furthermore, by changing the value of n L , tuning and switching ON- and OFF- of the PBGs can be achieved 5 . By variation of the refractive index of one of the PC component, using the external forces, or by introducing any disorder of the components in PC structure the narrow transmission peaks can be formed within the PBG. Thanks to these narrow transmission peaks a variety of Si PC band-pass filters have been proposed nowadays 6-9 . By varying the angle of incidence of light these Si PC band-pass filters, which reflect or transmit the certain polarization, can be designed 10 .
82 hybrid chemistry techniques have been adopted to develop high index refractive polymers which combine an organic polymer matrix with highly refractive inorganic nanoparticles. 97 The factors affecting the refractive index of a high-n nanocomposite include the properties of the individual polymer matrix and nanoparticles, and the chemistry linking inorganic and organic components. 97 In particular one can look at the synthesis of nanocomposite materials based on hybrid networks of organically modified, inorganic oxide clusters dispersed in a polymer host matrix. 98 Organically modified inorganic oxide clusters can be obtained by a reaction of inorganic-based alkoxides with carboxylic acids. 98 Exposing the organic-inorganic hybrid system to UV light results in homogeneous photopatterning and modification of polymer host refractive index properties. 98 Developing hybrid organic-inorganic chemistry amenable for use with TPL DLW would allow for this fabrication methodology to be used in the fabrication of 3D PhC architectures of the sort described in this thesis. Recently, Vyatskikh et. al. described a similar process for fabricating nickel nanolattices via TPL DLW. 99 A ligand exchange reaction was used to synthesize a metal precursor with polymerizable functionalities by reacting nickel alkoxide with a carboxylic acid. Nickel oxide clusters were then mixed with pentaerythritol triacrylate, a common acrylate based monomer, and a small amount of 7-diethylamino-3-thenoylcoumarin photoinitiator. Using this new hybrid photoresin, metallo-organic 3D structures were directly fabricated via TPL DLW. Once these structures were pyrolysed, the result was fully metallic 3D structures with sub-micron features. Based on the synthesis of a nickel oxide cluster-acrylate hybrid resin, one natural extension of hybrid organic-inorganic chemistry techniques would be the development of high index germanium and/or zirconium alkoxide clusters bonded to pentaerythritol tetraacrylate monomers, which will polymerize by two-photon absorption of UV light. The development of such a novel high index Ge-nanocluster resin will allow for the one-step fabrication of high index 3D nanoarchitected structures, more easily enabling the experimental observation of phenomena like negative refraction and full photonic band gaps in complex 3D PhCs.
It has been shown here that, by using a porous core along with the porous cladding of a conventional PCF, the power confinement in the air holes can be significantly increased, which will reduce the effect of material loss by 60% for the solid Teflon. The overall loss value can be further reduced if the material loss can be reduced or the fabrication technology improved to allow a higher d= value than 0.95 (for the outer d =), which is considered in this paper. It has been shown that the leakage loss and the bending losses for such a PCF are very small for practical applications.
in the refractive index profile shows that the quantum well alone can serve to confine light. In such a device however, a difficulty arises in separately engineering the carrier and light confinement. In other words, changing the quantum well thickness changes both the electronic and photonic confinement. By separating the electron and photon confined, an SCH structure provides independent parameters in the form of the waveguide and quantum well, giving us additional design degrees of freedom. The cladding serves the same purpose as that in a fiber optic cable, it is required for optical confinement due to total internal reflection.
x y ,
n n as a consequence of the effective index of the quasi- TM mode is higher than that of the quasi-TE mode as shown in Fig. 12. It can be clearly observed that as the diameter of d 2 is decreased, effective index increases for both the TE and TM polarizations and the modal index difference between the two fundamental quasi-TE and quasi-TM increases, shown by a blue dotted line with stars. Changing of d 2 makes the PCF structure more asymmetric and it is shown here that birefringence value as large as 0.012 can be easily achieved and this structure can also be easily produced by using simpler stack-and-draw approaches. Although higher birefringence value of 0.026 has been reported earlier  but they would require a more complex fabrication process, possibly using the extrusion process.
I also want to thank all members of our group for invaluable help and contribution they made for this work and for the stimulating and inspiring atmosphere which always supported me. Jan-Robert Van Look and Barbara Wild, who worked with me at the beginning of my stay in EPFL, helped me a lot in learning of the basis of photonic crystals and their optical properties. Andrea Dunbar helped me with her deep intuitive understanding of many physical phenomena, which greatly broaden my scientific expe- rience. Very fruitful discussions with Hua Zhang allowed me really to understand many aspects of real-life sample fabrication and processing. Working together with Nicolas Le Thomas and Jana J´ agersk´ a was a real pleasure and their unique experience in op- tical characterization of photoniccrystal structures allowed us to verify the theoretical results presented in this thesis.
Optical polymers such as PDMS depict worse rare-earth ion solubility characteristics than silica and silicate inorganic glass hosts, consequently and it is impossible to design efficient Er 3+ -doped polymer waveguides for engineering lossless splitters, which can then potentially open the opportunity for engineering seamless and complex photon carrier circuits for the backplane of PCs. In an earlier approach , investigations into whether the deposition of a large expansion coefficient glass on a PDMS substrate might be feasible for engineering a suitable gain medium. However it was soon realized that the apparent large mismatch between the coefficients of thermal expansion and elastic constants of an Er 3+ - phosphate modified tellurite glass and PDMS were the two main barriers for film growth. It is for this reason a nano-scale glass-polymer superlattice approach was proposed using excimer PLD [4,6], in which a multilayer structure of these two dissimilar materials were sequentially deposited to grow 100s of nanometer thick layers for waveguide engineering. An exemplar microstructure of PDMS-glass superlattice, grown using sequential deposition of these two materials is shown in Fig. 4, which was characterized for spectroscopic properties, including the lifetime of 1-2 ms in the film and waveguide structures. Note that although the PDMS is possible to process using standard cleanroom techniques, the nano- composite materials are impossible to etch or selectively ion-beam mill. For this reason, the 100fs pulsed laser was used for waveguide inscription and fluorescence characterization . Preliminary data from waveguide engineering is sufficiently encouraging to advance this technique of sequential deposition to the next level for gain characterization in future.
recent theoretical study suggested that the thermal conductivity can be increased at low temperatures via fine-tuning of PnC dimensions and nanopatternings [24,25].
Tuning the phonon dispersion in the PnC structures and in individual nanostructures can affect the optical and electrical properties of the material. Engineering the dispersion in PnCs in such a way that the phonon DOS attains its maximum or minimum within the energy required to trigger the carrier transition between the defect, i.e. trapping state and the conduction or valence band can result in either enhanced or suppressed G-R center recombination . A recent experimental study found that modification of the phonon dispersion in the core-shell GaAs 0.7 Sb 0.3 /InP nano-pillar arrays affects the hot carrier relaxation in such structures . In another example, opening up a band gap in a certain phonon frequency range with an accurate design of the hole PnCs can strongly influence the quasiparticle recombination lifetime in a superconductor .
Based on the above, I presented the main scope of this project, which is the examination of the possibility to employ the aforementioned reflective component as an external resonant mirror in External Cavity (EC) laser configurations that also comprise a III-V-based Reflective SOA (RSOA). External Cavity lasers are in general attractive as they allow the independent design, fabrication and optimization of the passive and the active parts, and make cost-effective use of the III-V materials. The proposed EC laser architecture is especially appealing for Silicon photonic optical interconnects as it features a Silicon photoniccrystal cavity-based reflector, which implies high integration density, and precise and well-controlled lasing wavelength registration. Additionally, the lasing wavelength dependence on the resonance of the photoniccrystal cavity of the reflective chip, combined with the low switching energies demonstrated in photoniccrystal cavities, indicate its potential for energy efficient wavelength tuning/modulation.
In this chapter, a photoniccrystal nanocavity with an ultra-high Q and small mode volume has been achieved even cavity modes do not locate within the gap or even no bandgap at all. The air hole radii have been modulated with a quadratic profile to decouple the cavity mode from possible losses consisting of guiding loss and radiation loss, resulting in strong light confinement in all three directions. By reducing the Fourier amplitude of the dielectric perturbation, governed by the air hole radii profile, at the corresponding vectors from the dominant Fourier components of the cavity modes to the leaky modes in momentum space, leakages into in-plane and vertical directions have been limited, leading to doubly-degenerated modes with very high Q of 120,000 and mode volume V eff of 0.79(λ/n) 3 . The figure of merit Q/V eff in the weak coupling regime is about two times higher than the highest value reported so far for doubly-degenerated modes. Therefore, this cavity is very promising for the realization of entangled photon sources. Finally, the designed cavity has been applied to achieve high Q cavities for material with low index and for quantum cascade lasers, in which a lack of photonic bandgap usually hinders them from applications. These results emphasize a flexibility of the cavity. The results achieved in this chapter
"forbidden" to propagate in the structure. Recently , DBR with a complete TE and TM band gap was achieved . Distributed Bragg reflectors are critical components in vertical cavity surface emitting lasers(VCSEL's) and other types of narrow-line width laser diodes[10,11] such as distributed feedback (DFB) lasers and distributed bragg reflector (DBR) lasers. They are also used to form the cavity resonator (or optical cavity) in fiber lasers and free electron lasers , spontaneous emission , high- reflecting omni-directional mirrors [14,15], and low- loss-waveguiding , all-optical transistors, amplifiers, routers photonic integrated circuits, optical computing .In this paper , reflectance of 1D PC of periodic structure was studied with the aid of Zemax software follows by Teraplot as graphical method . Further, the effect of construction parameters and angle of incident of light also investigate.
3 Centre for Advanced Photonics and Process Analysis, Cork Institute of Technology, Cork, Ireland firstname.lastname@example.org
Energy efficient Wavelength Division Multiplexing (WDM) is the key to satisfying the future bandwidth requirements of datacentres. As the silicon photonics platform is regarded the only technology able to meet the required power and cost efficiency levels, the development of silicon photonics compatible narrow linewidth lasers is now crucial. We discuss the requirements for such laser systems and report the experimental demonstration of a compact uncooled external-cavity mWatt-class laser architecture with a tunable Si PhotonicCrystal resonant reflector, suitable for direct Frequency Modulation.
Quasi-crystal structures do not present a full spatial periodicity but are nevertheless constructed starting from deterministic generation rules. When made of different dielectric materials, they often possess fascinating optical properties, which lie between those of periodic photonic crystals and those of a random arrangement of scatterers. Indeed, they can support extended band-like states with pseudogaps in the energy spectrum, but lacking translational invariance, they also intrinsically feature a pattern of ‘defects’, which can give rise to critically localized modes conﬁned in space, similar to Anderson modes in random structures. If used as laser resonators, photonic quasi-crystals open up design possibilities that are simply not possible in a conventional periodic photoniccrystal. In this letter, we exploit the concept of a 2D photonic quasi crystal in an electrically injected laser; speciﬁcally, we pattern the top surface of a terahertz quantum-cascade laser with a Penrose tiling of pentagonal rotational symmetry, reaching 0.1–0.2% wall-plug efﬁciencies and 65 mW peak output powers with characteristic surface-emitting conical beam proﬁles, result of the rich quasi-crystal Fourier spectrum.
Quasi-crystal structures do not present a full spatial periodicity but are nevertheless constructed starting from deterministic generation rules. When made of different dielectric materials, they often possess fascinating optical properties, which lie between those of periodic photonic crystals and those of a random arrangement of scatterers. Indeed, they can support extended band-like states with pseudogaps in the energy spectrum, but lacking translational invariance, they also intrinsically feature a pattern of ‘defects’, which can give rise to critically localized modes confined in space, similar to Anderson modes in random structures. If used as laser resonators, photonic quasi-crystals open up design possibilities that are simply not possible in a conventional periodic photoniccrystal. In this letter, we exploit the concept of a 2D photonic quasi crystal in an electrically injected laser; specifically, we pattern the top surface of a terahertz quantum-cascade laser with a Penrose tiling of pentagonal rotational symmetry, reaching 0.1–0.2% wall-plug efficiencies and 65 mW peak output powers with characteristic surface-emitting conical beam profiles, result of the rich quasi-crystal Fourier spectrum.
The next characterization tool was the DEKDAK 150 profilometer  whos main uses are for the quantification of the surface roughness of a sample. The process in which the surface roughness is identified constitutes of moving a diamond stylus across the surface for a specified distance with a specificed contact force. As the stylus moves across the sample, small variations in its discplacement is measured which produces data regarding the surface roughness. Depending on the quality of the equipment, a profilometer can measure variations on a sample surface as small as one nanometer. Equally important, however, is the profilometers horizontal resolution. This parameter depends on the radius of the stylus. The smaller the stylus radius, the smaller distance between seperate measurable points. The radius of the DEKTAK 150 profilometer stylus is 12.5 µm, which limits the profilometer resolution considerably when considering extremely small structures. In other words, analyzing the characteristics of constructions in the nanometer-regime is not possible with this profilometer, but it can still give us valuable information regarding surface roughness of various films grown on our samples. More importantly, it is a very useful piece of equipment for measuring step-sizes, an ability which is frequently exploited to measure etching rates of various etching recipes.
The photonic band structure for a silicon slab in air, with relative permittivities as in previous sections, is plotted in figure 2.17. The holes have a relative radius of r/a = 0.25 and the relative thickness of the silicon slab is d/a = 0.5. This structure is made with MPB as the band structures in previous sections. Since MPB requires periodicity, a supercell of one slab unit with surrounding material and a hole is created. In this case first a block of air, then a block of Si with an air hole, and then an air block again. This supercell is repeated infinitely in all directions and a dispersion relation is calculated. The resulting structure will be correct in the plane of the slab, but unrealistic in the z-direction. Thus we need enough vertical space between each sequence of slabs to ensure that the confined modes are not affected noticeably. The non-confined modes will be affected, but this is disregarded as they fall above the light line and are therefore uninteresting to us. This method has been used in several published articles [41, 42, 43, 44, 45].