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Ultrafast Light Coupling to NonequilibriumCarriers in Extended Semiconductor Media
J.V Moloney, S.W. Koch, J. Jones
M. Scheller, A. Laurain, I. Kilen, C. Baker, K. Gebele
Arizona Center for Mathematical Sciences
College of Optical Sciences
University of Arizona, 85721
Support: AFOSR BRI Grant # FA9550-14-1-0062
Talk Outline
Background to the BRI Project and Team Members
Introduction and Key Past Results
UA Experimental Project
UA Microscopic Simulations Hartree Fock Resonant Periodic Gain Structure Multiple QW Nonequilibrium Femtosecnd Disk Laser
UA Microscopic Theory and Simulation Second Born
Conclusion & Summary
BRI Project: Development and Verification of Effective First Principles Modeling (Maxwell-Bloch Equations) of Semiconductor Lasers under Non Equilibrium Operating Conditions.
Project Goal: Use nonequilibrium microscopic physics modeling to develop new generations of sub 100 fs semiconductor laser mode-locked sources.
Lead Institution: University of Arizona
1. Nonequilibrium Theory and Simulation2. Fundamental experiments to support theory/simulation
Electrical Transport: M. Grupen AFRL/RYDD, S. Badescu AFRL/RYDD
Semiconductor Materials Growth: University of Marburg W. Stolz
Mode-locking support experiments: University College Cork J. McInerney
Mode-locking edge emitters: Bochum University M. Hoffmann
Talk Outline
Background to the BRI Project and Team Members
Introduction and Key Past Results
UA Experimental Project
UA Microscopic Simulations Hartree Fock Resonant Periodic Gain Structure Multiple QW Nonequilibrium Femtosecnd Disk Laser
UA Microscopic Theory and Simulation Second Born
Conclusion & Summary
Semiconductor Disk Laser (OPSL)
Wavelength versatile, high brightness and power source
Key Properties: Low single pass gain
Gain clamping involvescomplex interplay of highcarrier densities and carrier temperatures
Flexible gain/index controlfor gain/dispersion compensation
Gain bandwidth engineering
Barriers and wells Barrier pumping
Stronger absorption (thicker) but stronger heating (larger quantum defect).
Thermal management issues.
Wells vs. Barrier Pumping
The structure can be designed such that pump absorption takes place in:
Wells only Well pumping
- Weaker absorption but smaller quantum defect.
- Swaps thermal issues for absorption issues.
- Resonant Pumping with Fabry-Perot sub-cavity: QWs placed at pump and cavity field antinodes.
Barrier vs Quantum Well Pumping
Transport
Nonequilibrium Effects in Carrier Transport
M. Grupen, S. Badescu AFRL
0 20 40 60 80 100 120 1400
4
8
12
16
Ou
tpu
t P
ow
er
(W)
Incident Pump Power (W)
T=15C
Pout > 15W
Single frequency
Circular TEM00
Low divergence
M < 1.4
l = 1016nm
Stabilized single frequency VECSELStabilized single frequency VECSEL
Mode-locking of our VECSELs with SESAMs
5.1 W average output power at 25 W net pump 682 fs pulse duration 1.7 GHz repetition rate Peak power above 3 kW Nearly transform limited TEM00-mode
Autocorrelation RF-signal
Passive Mode-Locking of VECSEL
Maik Scheller et al. Electronics Letters, 48 (2012)
Talk Outline
Background to the BRI Project and Team Members
Introduction and Key Past Results
UA Experimental Project
UA Microscopic Simulations Hartree Fock Resonant Periodic Gain Structure Multiple QW Nonequilibrium Femtosecnd Disk Laser
UA Microscopic Theory and Simulation Second Born
Conclusion & Summary
Experimental Characterizations
VECSEL SESAM Cavity
Gain depletion Kinetic hole
burning and filling Gain recovery
Saturation fluence Recovery time Nonsaturable
losses
Dispersion Thermal and
Kerr lensing
High sensitivity is required because of the small changes (tens of percent) High temporal resolution (10 tens of femtosecond) Long time window to track the dynamics during a full cavity roundtrip
Dual comb spectroscopy
Time frame determined by repetition rate of the laser Scan rate by the difference between the repetition rates Sampling over a time frame of several ns with scan rates of hundreds of Hertz No mechanical delay line and potential misalignments
The use of two frequency combs with slightly detuned repetition rateallows for a temporal sampling as the time spacing between theemitted pulses is slightly different for the two lasers
FC1
FC2
Phase lock
SamplePulse train
Laser system DURIP
Phase locking of two frequency combs to perform pump and probe measurements Locking of one comb to the VECSEL cavity to perform time resolved in-situ spectroscopy
Spectral coverage : 980nm to 1400High time resolution: pulse down to 10fs
AFOSR FA2386-14-1-3001
First step: A custom laser systembased on Yb doped fiber technology
1060nm center wavelength 80fs pulse duration 2W output power 80MHz repetition rate
PZT/mirror
Ytterbium gain fiber 980nm Pump diode
Coupler(50/50) Collimator
Polarization controller
HI 1060
WDM
Isolator
Collimator
Collimator
QWP
PBS
QWP
Isolator
QWP
Grating pairs
WDM
980nm Pump diode
Collimator
Isolator
Compressor
Laser output
Nufern Yb-doped fiber 5m
The VECESL cavity is locked close to a harmonic of the fiber lasers repetition rate
Basic setup for in-situ probing
Preliminary data on the gain depletion and recovery of a mode-locked VESCEL
Two time constant due to the different lengths of the cavity armsDepletion on the time scale of the VECSEL pulses duration (30ps)
1 2
Talk Outline
Background to the BRI Project and Team Members
Introduction and Key Past Results
UA Experimental Project
UA Microscopic Simulations Hartree Fock Resonant Periodic Gain Structure Multiple QW Nonequilibrium Femtosecnd Disk Laser
UA Microscopic Theory and Simulation Second Born
Conclusion & Summary
Semiconductor Bloch Equations (2-Band)
all contributions beyond singlets
nonlinearities: phase-space filling, gap reduction, Coulomb enhancement
HF field renormalization
HF energy renormalization
Talk Outline
Background to the BRI Project and Team Members
Introduction and Key Past Results
UA Experimental Project
UA Microscopic Simulations Hartree Fock Resonant Periodic Gain Structure Multiple QW Nonequilibrium Femtosecnd Disk Laser
UA Microscopic Theory and Simulation Second Born
Conclusion & Summary
Simulation Set-up
Linear Cavity with RPG and SESAM
DBR Mirror Output
Simulation Parameters: QW relaxation time: 30 ps, SESAM relaxation time: 0.5ps, Cavity roundtrip time: 20ps, Cavity length 3.2 cm, SESAM carrier Density: 14 25.0 10 m
I. Kilen et al., Optica, 4, No. 1, 192 (2014)
Low Gain Limit Net Gain Picture Holds
Net gain is difference between the blue (full gain) and black (SESAM absorption) in left upper picture.
Inversion
Pulse and Phase
Pulse Spectrum
Gain and Loss
Intermediate Pump Levels High gain
Unused carriers
Interference of two time-shifted longer pulses
Inversion near pulse center wavelength bleaches out leaving unused carriers for further amplification time shifted pulses
A. H. Quarterman et al. Nat. Photonics 3, 729731 (2009).
Pulse Molecule
Higher Pump Levels Pulse Separation
Time separated pulses not harmonic mode-locking!
Spectral Peaks separated- Each peak associated with separate pulse
Consistent with: S. Husaini and R. Bedford, Applied Physics Letts. 104, 161107 (2014)
Slow SESAM Absorber Allows unused carriers after bleaching to amplify new pules waveforms
SESAM relaxation time : 5 ps
Conclusion on RPG Structures
QW arrangement in an RPG structure cannot avail of all of the active carriers
- Unused carriers promote instability and multiple pulses
Solution
Talk Outline
Background to the BRI Project and Team Members
Introduction and Key Past Results
UA Experimental Project
UA Microscopic Simulations Hartree Fock Resonant Periodic Gain Structure Multiple QW Nonequilibrium Femtosecnd Disk Laser
UA Microscopic Theory and Simulation Second Born
Conclusion & Summary
Nonequilibrium Femtosecond Semiconductor Disk Laser
Simulation Parameters: QW relaxation time: 30 ps, SESAM relaxation time: 0.5ps, Cavity roundtrip time: 20ps, Cavity length 3.2 cm, SESAM carrier Density: 14 25.0 10 m
Nonlinear MQW Femtosecond Disk Laser Sub-wavelength densely packed QWs cooperatively emit to create a giant sub 100fs mode-locked pulse Mode-locking intra-cavity elements needed to start and sustain pulse circulation
Provisional patent filed.
MQW Mode-locking Sustained with Large Net Absorption
Zero Net Gain
MQW Cooperative Emission
Net Gain Picture Meaningless
Nonequilibrium Inversion Extracted More Efficiently in MQW
Talk Outline
Background to the BRI Project and Team Members
Introduction and Key Past Results
UA Experimental Project
UA Microscopic Simulations Hartree Fock Resonant Periodic Gain Structure Multiple QW Nonequilibrium Femtosecnd Disk Laser
UA Microscopic Theory and Simulation Second Born
Conclusion & Summary
Nonequilibrium Simulations
Semiconductor Bloch Equations
currently implemented
Relaxation of Kinetic Holes
relaxation without mixing of different k-states
Carrier Scattering
Quantum Boltzmann Equation
scattering rates:
Kinetic Hole Burning
short pulse injected into VECSEL cavity
full microscopic calculation with Boltzmann scattering
5 band model
Many-Body Relaxation of Kinetic Holes
replenishing rate corresponds to 100s of fs
+
Full Nonequilibrium VECSEL & SESAM Simulations
single pulse propagating through inverted QWs and SESAM extraction of effective roundtrip gain semiconductor Bloch equations in screened Hartree-Fock approximation 10 quantum wells SESAM = single QW with rapid relaxation time (500 fs) field 10 x focussed on SESAM
1% outcoupling
we cant do full mode-locking simulations at this level, but .
Full Nonequilibrium VECSEL & SESAM Simulations
single pulse of chosen duration propagating through gain medium and SESAM
extraction of effective roundtrip gain
two regimes of efficient pulse amplification
fs regime: broad pulse spectrum
efficient use of resonant part of inversion (overlapping spectrum)
ps regime: narrow pulse spectrum
small percentage of resonant inversion kinetic-hole filling during pulse = replenishing of resonant inversion
Many-Body Relaxation of Kinetic Holes
replenishing rate corresponds to 100s of fs
most efficient for longer pulses
+
Dependence on SESAM Relaxation Time
0.5 ps
if they can form, longer pulses receive more amplification viahole filling during the pulse
fast absorption recovery in SESAM suppresses longer pulses
1 ps
2 ps 3 ps
Extended Relaxation Rate Model
Parameters
relaxation rates are obtained by fits to microscopic calculations
is calculated considering energy and particle conservation
is calculated considering particle conservation
carrier-carrier scattering is an order of magnitude faster than carrier-phonon scattering and relaxation due to pumping