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
In indirect bandgap materials
- GaAs, InP, for example
•
Lowest energy points of both the
conduction & valence bands line
up vertically in the wave vector
axis
- 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
 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, lowtemperature 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 momentumaligned 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 directbandgap 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