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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

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Recent progress

in lasers on silicon

Hyun-Yong Jung

High-Speed Circuits and Systems Laboratory



  • Fundamentals
  • Silicon Raman lasers
  • Epitaxial lasers on silicon
  • Hybrid silicon lasers
  • Challenges and opportunities


  • 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


  • 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


  • 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!!


  • 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
  • SiO2 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(≈1/𝟏𝟎𝟕)
  • 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

  • Overcoming the TPA-induced FCA
  • 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
  • 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