Fundamentals In direct bandgap materials - GaAs, InP, for example Lowest energy points of both the conduction & valence bands line up vertically in the wave vector axis In indirect bandgap materials - Si, Ge Free electrons tend to reside X valley of the conduction band, which is not aligned with free holes in the valence band
Fundamentals In indirect bandgap materials Auger recombination - An electron (or hole) is excited to a higher energy level by absorbing the released energy from an electron-hole recombination - Rate increases with injected free-carrier density & inversely proportional to the bandgap Free-carrier absorption (FCA) - The free electrons in the conduction band can jump to higher energy levels by absorbing photons The elctrons pumped to higher energy levels release their energy through phonons
Fundamentals Availability of nanotechnology Breaking the crystal-symmetry or crystalline Si A number of groups have reported enhanced light-emmiting efficiency & optical gain in low dimentional Si at low temperatures - Porous Si, Si nanocrystals, Si-on-insulator(SOI) superlattices, Nanopillars…… Achieving room-temperature continuous-wave lasing remains a challenge!!
Fundamentals Advantages of Si for a good substrate Si wafers are incredibly pure & have low defect density 32 nm CMOS technology is sufficienty advanced to fabricate Si has a high thermal conductivity, which is a very useful characteristic for an active device substrate SiO 2 serves as a protective layer and a naturally good optical waveguide cladding
Silicon Raman lasers Raman Scattering (or Raman effect) Inelastic scattering of a photon by an optical phonon A small fraction of the scattered light(≈1/) Raman gain coefficient in Si is around five orders of magnitude larger than that in amorphous glass fibres Si waveguide loss is also several orders of magnitude higher than in glass fibres Two-photon absorption(TPA) A nonlinear loss mechanism in which two photons combine their energies to boost an electron in the valence band to the conduction band TPA increases with the number of photons in a waveguide A limiting factor when using high optical pump powers
Silicon Raman lasers A high Racetrack ring resonator Cavity A large bend radius helps to minimize waveguide bending losses The directional coupler is designed to utilize the pump power efficiently and achieve a low lasing threshold Overcoming the TPA-induced FCA TPA-induced FCA nonlinear optical loss can also reduced by optimizing the p-i-n reverse-biased diode Silicon Raman lasers nenefit significantly from high spectral purity!!
Epitaxial lasers on silicon Compared with Si, GaAs and InP have lattice mismatches and thermal expansion coefficient mismatches Reducing by special surface treatment (strained superlatiices, low- temperature buffers & growth on patterned substrates) Advanced epitaxial techniques with SiGe & GaSb buffer layers - The realization of GaAs-based CW diode lasers on Si substrates at room temperature Ge-on-Si(or SiGe-on-Si) epitxial growth - Key photonic components from this material system have demonstrated performances comparable or even better than their III-V counterparts in certain aspects
Epitaxial lasers on silicon Germanium has an indirect band structure ! Energy gap from the top of the valence band to the momentum- aligned Γ valley is close to the actual band gap! The tensile strain is able to reduce the energy difference between the Γand L valleys Strain raises the light-hole band, which increases optical gain for high injection These techniques have enabled room-temperature direct- bandgap electroluminescence and CW room temperature optically pumped operation of Ge-on-Si lasers Optically pumped Ge-on-Si laser demonstrating CW operation at room temperature!!
Hybrid silicon lasers It is possible to combine epitaxial films with low threading dislocation densities to the lattice-mismatched Si substrate Advantages over bonding individual III-V lasers to a SOI host substrate The onfinement factor can be dramatically changed by changing the wave guide width
Hybrid silicon lasers Small size, low power consumption and a short cavity design are all critical for optical interconnects a schematic of an electrically pumped microring resonator laser, its cross-section SEM image
Hybrid silicon lasers By lasing inside a compact microdisk III-V cavity and coupling to an external Si waveguide, a good overlap between the optical mode and electrical gain results Schematic of a heterogeneously integrated III-V microdisk laser with a vertically coupled SOI wave guide Results from combining four devices with diameters Increasing thermal impedance causes laser performance to decrease dramatically with smaller diameters A major hurdle in the realization of compact devices
Challenges and opportunities Opportunities Optical interconnects could be a possible solution Achieving smaller interconnect delays, lower crosstalk & better resistance to electromagnetic interference Integration with CMOS circuits can provide low cost, integrated control, signals processing and error correction power consumption must be reduced to 2 pJ bit -1 or lower Silicon Raman lasers are potentially ideal light sources for a variety of wavelength-sensitive regimes Raman lasers will be very competitive in size and cost if a pump source can be integrated