Study of a confined band-edge photonic crystal laser on a hybrid III-V/silicon platform

  • Md. Tanbir Hasan ((PhD) Student)
  • Roel Baets (Promotor)
  • Günther Roelkens (Co-promotor)
  • Heidi Ottevaere (Jury)
  • Yannick De Koninck (Advisor)

Student thesis: Master's Thesis

Abstract

Confinement of low group velocity band-edge modes in a photonic crystal (PhC) slab on Silicon-on-insulator (SOI) has been studied. This confined-band-edge mode cavity has been utilized to make a CMOS-compatible laser with wavelength around 1.55µm. III-V material stack BCB-bonded to the SOI containing the cavity works as the gain medium. Rigorous, three dimensional, finite-difference time-domain method and block-iterative frequency-domain methods in planewave basis habe been used to compute and design the electromagnetic properties of the modes of the cavity and associated structures. Two sets of structure have been designed: one with the hybrid III-V / silicon and another with only the silico-on-insulator (SOI) to characterize the cavity itself. First set has been fabricated on imec wafer scale CMO-line milti-project wafer (MPW) run and then post processed in our clean room. Second sets are physically realized using a electron-beam lithography.
To get the idea of the existing art, literature citing the recent progress, particularly in band-edge laser are explored. In addition to getting familiarized with the in-house design environment, tools like MEEP (python-wrapper), MPB, Lumerical's FDTD solutions are learned. Techniques involving the estimation of the quality factor of a resonant cavity, calculating DFTF flux spectrum of the field evolution, transmission/reflection characteristic, and numerous other, are acquired.
Band-edge resonance are over a larger are of unperturbed photonic crystal is understood first before attempting to confine it. A comparison between 2D vs 3D simulation was performed. It's found the 3D simulation give more estimation of the quality factor than the effective index approximated 2D simulation. Then the a set design rule are developed to design the the confined band-edge cavity with sufficiently large quality factor, and smaller mode volume. Factors influencing the vertical out-of-plane loss are investigated through k space transformation of the field distribution. Possible way to address them are also discussed. By careful investigation of the cavity resonance spectrum, the origin of different resonance peak are identified. Quality factors are estimated for them. It's well known that fabrication of photonic crystal is very challenging. So tolerance (in terms of quality factor and resonance wavelength) of the designed cavity with respect to the imperfection in the cavity feature size estimated. For a variation of upto 40nm in size the quality factor for the passive component stays within 1.4-2 x 104 and resonance wavelength shifts as much as 1.67% for a variation of 20nm in size. Then optical property of the cavity coupled to line defect waveguide is investigated. The competing behavior of the quality factor and the coupling strength is identified, and a optimum value of the coupling parameter is proposed. Resonance modes originated from the line defect waveguide are also identified. The position of the the mode cut-off frequency and slow light mode of the photonic crystal waveguide made, near the band edges of the mirror layer, made the transmission more complicated. Coupling to the waveguide with different width is also studied. Photonic crystal waveguide's propagation loss are also addressed.
Similar approach as for the passive cavity design is taken with the lybrid cavity. The presence of the III-V material as the cladding layer distort the band diagram of the triangular lattice photonic crystal. In addition to ill defined band edge, the band as densely distributed. However, by detailed investigation, it is found that there are still some critical points on the bands where band-edge and consequently confined-band edge resonance can occur. It is also shown that the energy confinement of the the band edges in the PhC layer, the it's position inside the band separation of the mirror layer are the main factor in designing cavity with high quality factor. At the same time, the hybrid mode maintain over 50% overlap in the gain medium which can be sufficient for achieving lasing threshold. For a particular design with two layer of core with hole size (diameter) of 295nm, two adaptation layer, and mirror layer with hole size 240nm, the confined-band-edge resonance occur at 1.565µm in the hybrid III-V/SOI platform with 100nm bonding layer thickness. For a device with overall dimension as large as 25µ x 22µ , the quality factor has been estimated a value over 1000 and mode volume of about 1.65 (? / n)3. It's also shown that for a successful hybrid cavity implementation, the bonding thickness need to be lower than 200nm. Although not in details, cavity coupled to a PhC waveguide is analyzed. For separation of 5 row between the outermost adaptation layer and the waveguide, the quality factor of the cavity can retain a value of about 352.
Then degeneracy of the cavity resonant modes are investigated with the help of symmetry. It found the confined band edge resonance has two sates. One state is symmetric with respect ot the x=0 plane and anti-symmetric with respect to the y=0 plane and the other state is anti-symmetric in both plane. The field distribution of each state are also calculated. Based on the resonance split up at the degenerate state, in the FDTD simulation, it can be projected that, this two state will split up in the practical measurement. This because that the imperfection in the fabrication, which is more likey, will distort the symmetry of the structure.
Date of Award11 Jun 2012
Original languageEnglish
SupervisorHeidi Ottevaere (Jury), Roel Baets (Promotor), Günther Roelkens (Co-promotor) & Yannick De Koninck (Advisor)

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