Recent progress in lasers on silicon
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Recent progress in lasers on silicon
Hyun-Yong Jung
High-Speed Circuits and Systems Labo-ratory
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Outline
FundamentalsSilicon Raman lasersEpitaxial lasers on siliconHybrid silicon lasersChallenges and opportunities
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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 val-ley of the conduction band, which is not aligned with free holes in the valence band
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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
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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 di-mentional Si at low temperatures
- Porous Si, Si nanocrystals, Si-on-insulator(SOI) superlattices, Nanopillars……
Achieving room-temperature continuous-wave lasing remains a challenge!!
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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
SiO2 serves as a protective layer and a naturally good optical waveguide cladding
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Silicon Raman lasersRaman 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 magni-
tude 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
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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 re-duced by optimizing the p-i-n reverse-biased diode
Silicon Raman lasers nenefit significantly from high spectral purity!!
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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 demon-strated performances comparable or even better than their III-V coun-terparts in certain aspects
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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 be-tween 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 opti-cally pumped operation of Ge-on-Si lasers
Optically pumped Ge-on-Si laser demonstrating CW operation at room temperature!!
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Hybrid silicon lasers It is possible to combine epitaxial films with low threading disloca-
tion densities to the lattice-mismatched Si substrate Advantages over bonding individual III-V lasers to a SOI host sub-strate
The onfinement factor can be dramat-ically changed by changing the wave guide width
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Hybrid silicon lasers Small size, low power consumption and a short cavity de-
sign are all critical for optical interconnects
a schematic of an electrically pumped microring resonator laser, its cross-section SEM image
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Hybrid silicon lasers By lasing inside a compact microdisk III-V cavity and cou-
pling to an external Si waveguide, a good overlap between the optical mode and electrical gain results
Schematic of a heterogeneously integrated III-Vmicrodisk laser with a vertically coupled SOI wave guide
Results from combining four devices with diam-eters
Increasing thermal impedance causes laser per-formance to decrease dramatically with smaller diameters A major hurdle in the realization of compact devices
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Challenges and opportunities
Opportunities
Optical interconnects could be a possible solution Achieving smaller interconnect delays, lower crosstalk & better re-sistance to electromagnetic interference
Integration with CMOS circuits can provide low cost, integrated con-trol, 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