Agilent Technologies Optical Interconnects & Networks Department Photonic Crystals in Optical Communications Mihail M. Sigalas Agilent Laboratories, Palo Alto, CA

Agilent Technologies Optical Interconnects & Networks Department Outline -Trends in Optical Communications -Photonic Crystals -Numerical Methods -Photonic Crystal Waveguides -Resonators in Photonic Crystals -Conclusions

Agilent Technologies Optical Interconnects & Networks Department Trends in Optical Communications -Optical interconnects have been replacing electrical interconnects at shorter and shorter distances over time. -Optical interconnects for chip to chip or even within one chip will be needed in the near future. -Very short (microns scale) optical components (waveguides, bends, splitters, resonators) needed to achieve that. -There are two ways to make micron scale optical components: Photonic crystals and high index contrast materials.

Agilent Technologies Optical Interconnects & Networks Department Computer Interconnects Hierarchy

Agilent Technologies Optical Interconnects & Networks Department Finite Difference Time Domain Method 1.Approximate the space and time derivatives in Maxwell’s equations with finite differences. 2.The ``leap-frog’’ scheme for the E and H fields in time: 3.E and H fields in space (Yee grid): EEE HH nn-1/2n+1/2 Time

Agilent Technologies Optical Interconnects & Networks Department Finite Difference Time Domain Method 4. Use absorbing boundary conditions (ABC) to close the space ABC Photonic Crystal

Agilent Technologies Optical Interconnects & Networks Department Requirements for photonic crystals interconnects -Should be easily fabricated (2D slab PC are easier to be made than 3D PC) -Should have low propagation and coupling losses. -Most of the current 2D slab PC waveguides have narrow guiding bands with small group velocities. Small group velocities create higher internal and propagation losses. New structures are needed. -Should be easily integrated with active devices (lasers, LEDs).

Agilent Technologies Optical Interconnects & Networks Department 2D Slab PBG Waveguides Si slab on a SiO 2 substrate Triangular Lattice; Lattice constant: a; R/a=0.29; h/a=0.6 Band Gap for even modes (TE-like): c/a High index contrast confinement perpendicular to the slab. Photonic band gap effect within the slab.

Agilent Technologies Optical Interconnects & Networks Department 2D Slab PBG Waveguides: Circular Air Holes Guiding along a line of circular Air holes with R d =0.45a 3D FDTD Calculations Guided Band is narrow with Small group velocity Aslo see: Loncar, et.al., J. Opt. Soc. Am. B 18, 1362 (2001)

Agilent Technologies Optical Interconnects & Networks Department 2D Slab PBG Waveguides: Elliptical Air Holes Leaky Modes Guiding along a line of elliptical Air holes with Short axis 0.66a and long axis 1.48a. Guided band covers most of the band gap. Plane Wave Expansion Method Johnson, et.al., Opt. Express 8, 173 (2001)

Agilent Technologies Optical Interconnects & Networks Department 2D Slab PBG Waveguides: Elliptical Air Holes Guiding along a line of elliptical Air holes with Ratio of short to long axis 0.45 Short axis: 0.66a, 0.7a, 0.74a Good coupling and low propagation losses

Agilent Technologies Optical Interconnects & Networks Department Fabrication of PC waveguides substrate SiO 2 (0.15um) Si (0.26um) SiO 2 (1um) Alignment MarksDefine PC Waveguide Define Ridge Waveguide Silicon Etch SEM of PBG waveguide

Agilent Technologies Optical Interconnects & Networks Department 2D Slab PBG Waveguide Bends: Circular Air Holes

Agilent Technologies Optical Interconnects & Networks Department Conventional Waveguide Bends Si waveguide on SiO o Bend: ~70% Transmission 60 o Bend: ~90% Transmission Also see: Manolatou et. al., J. Lightwave Techn. 17, 1682 (1999) Good Transmission!

Agilent Technologies Optical Interconnects & Networks Department PBG vs. Conventional Waveguides -Both types of waveguides could give 100% efficiency along tight bends. -There is ONLY one difference between the two types: PBG waveguides can guide light mostly through the air. However, ONLY 3D photonic crystals can do that.

Agilent Technologies Optical Interconnects & Networks Department 3D Photonic Crystals Ho et. al., Solid State Commun. 89, 413 (1994) Layers of Si rods surrounded by air

Agilent Technologies Optical Interconnects & Networks Department 3D PBG Waveguide Bend Photonic Crystal Total Thickness: 20 layers Projection of the 10 th and 11 th layers Straight waveguide (Black) Bend (Red)

Agilent Technologies Optical Interconnects & Networks Department 3D PBG Waveguide Splitter Photonic Crystal Total Thickness: 20 layers Projection of the 10 th and 11 th layers Straight waveguide (Black) Splitter (Red)

Agilent Technologies Optical Interconnects & Networks Department 3D PBG Waveguide Splitter 10 th Layer 11 th Layer Guiding mostly through the air

Agilent Technologies Optical Interconnects & Networks Department Micro-resonators -Micron size resonators needed for sources and detectors. -Micro-resonators also needed for add-drop filters in Wavelength Division Multiplexing. -There are two ways to create micron-size resonators: Photonic crystals and High index contrast materials (micro-disk, micro-ring).

Agilent Technologies Optical Interconnects & Networks Department 2D Slab PBG Resonators R d /a=0.21, 0.17, 0.11 Air Holes in Si slab with SiO 2 substrate Lattice constant: a Air holes Radius: 0.29a Mode Volume: ~a^3 See also: Vuckovic, et.al., Phys. Rev. E 65, (2001)

Agilent Technologies Optical Interconnects & Networks Department Disk Resonators Si on SiO 2 Disk Radius: 2a Disk Thickness: 0.6a Mode Volume: ~  (2a)^2 a=4  a^3

Agilent Technologies Optical Interconnects & Networks Department Conclusions -There is a need for micron scale components (waveguides and resonators) for optical communications. -There are two possible candidate materials for building the optical components: Photonic crystals and high index contrast materials. -For waveguides, both types of materials are expected to perform equally well. -However, 3D photonic crystals can guide light mostly through the air. -Photonic crystal resonators are expected to be 5-10 times smaller in size than micro-disk resonators.

Agilent Technologies Optical Interconnects & Networks Department Future Work - Loss mechanisms - Theoretical models - Coupling to photonic crystal waveguides