Moloney 10022014

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







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







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

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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







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







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







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

Moloney 10022014

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