Carrier density dynamics in optically pumped VECSELs

1
JRA5 Multi-GHz pulse sources based on
mode-locked VECSELs
Coordinator Ursula Keller (keller_at_phys.ethz.ch),
Deputy Coordinator Heiko Unold
(unold_at_phys.ethz.ch)
Partners ETHZ-ULP, COM, CAM, UoS-Phys, UoS-ORC,
HHI, 35Ph
Modeling and Design
Design of electrically pumped VECSELs

• Carrier density dynamics in optically pumped
  VECSELs
• The carrier dynamics of an optically pumped gain
  structure are analyzed. The gain structure is CW
  pumped and amplifies a pulse train S. By changing
  the pump spot size wp , we can maximize the
  average gain. This optimum is important because
  when we use the gain structure in a laser, we
  achieve the highest output powers, reduce
  temperature increase and have good suppression of
  higher order modes.
• The Model
• The 2-D rate equation is given by

• The electrically pumped VECSELs are designed in
  the same upside-down geometry as the existing
  optically pumped devices. After growth, the
  epitaxial structure is patterned into mesas,
  passivated, and metallized. The devices are then
  soldered upside-down to a patterned heat sink,
  the substrate is selectively removed and a
  patterned backside contact is deposited.
• The first-most layers after the etch-stop layers
  are current-spreading layers, which serve the
  purpose of ensuring a uniform carrier-profile
  across the aperture of the emitter. The
  following graph shows the horizontal component of
  the current in and around such a
  current-spreading layer.

Carrier density

• Carrier injection
• Ipump pump intensity
• ? optical frequency
• d QW thickness
• The optical pump intensity I W/m2 is constant
  in time, The radial dependence is assumed to be
  gaussian.

• Stimulated recombination
• vg group velocity
• g(N) gain
• S(r,t) photon density

• Diffusion
• D diffusion coefficient

Gain The gain is assumed to be

• Recombination
• t carrier lifetime