Recent progress in lasers on silicon Recent progress in lasers on silicon Hyun-Yong Jung High-Speed Circuits and Systems Laboratory

Outline  Fundamentals  Silicon Raman lasers  Epitaxial lasers on silicon  Hybrid silicon lasers  Challenges and opportunities

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