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$: Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 1 Nanostructured Quantum Cascade Lasers for Longitudinal Single Mode$
Technische Physik, Universität Würzburg (jpr\powerpoint\2004\2004_ESLW\QCL_talk) foil 1

Nanostructured Quantum Cascade Lasers for Longitudinal Single Mode Control

Nanostructured Quantum Cascade Lasers for Longitudinal Single Mode Control

• Motivation and structure of quantum cascade (QC) lasers

• Ultra-Short QC Microlaser

• Two segment distributed feedback (DFB) lasers

J.P. Reithmaier1,3, S. Höfling1, J. Seufert2, M. Fischer2, J. Koeth2, A. Forchel1

1 Technische Physik, Universität Würzburg, Germany

2 nanoplus, Nanosystems and Technology GmbH, Germany

3 present address: Technische Physik, Universität Kassel, Germany


MotivationMotivation• Many important gases have

their fundamental absorptionin the mid-infrared spectralregion (e.g NH3, O3, CO2)

• Quantum cascade lasers (QCLs) are reliable mid-infrared laserscapable of room temperature operation

Single mode emission is requestedfor gas sensing applications

• Detection of NH3 demonstrated with single mode distributed feedback lasers in cooperation with:


Active Region DesignsActive Region Designs

321

Page et al., Appl. Phys. Lett. 78(22) (2001)

Three quantum

well design

• Resonant tunneling between lowest injector state and upper laser level 3

• Fast depopulation of lower laser level 2 by interminibandscattering processes

Pflügl et al., Appl. Phys. Lett. 83(23) (2003)

bound-to-continuum design

• Resonant tunneling between lowest injector state and upper laser level 3

• Fast depopulation of lower laserlevel 2 by LO-phonon resonancewith ground state 1


Advantages of micro-lasers: • Increased device density compared to conventional ridge waveguide lasers by approximately a factor 10 is possible• Low threshold currents• Short cavity devices can exhibit single mode emission due to limited gain bandwidth and large mode spacing

[Höfling et al, Electr. Lett. 40, 120 (2004)]

Wavelength tuning should be possible by controling the cavity length

Use of highly reflective deeply etched semiconductor-air Bragg mirrors

allows the fabrication of ultra-short ridge waveguide micro-lasers:

Why Micro-LasersWhy Micro-Lasers

LngFP 2

1~/1

Ln

m

gFP 2

~


Fabrication ProcessFabrication Process

Monolithically integrated: ridge waveguide and Bragg-mirror fabrication

(1) RWG definition(optical lithography + lift-off)

(2) Bragg mirror definition(e-beam lithography + lift-off)

(3) Pattern transfer(dry etching by ECR-RIE)


0.0 0.5 1.0 1.5 2.0 2.50

2

4

6

8

Pow

er (

a.u

.)

Current (A)

Ultra-Short MicrolasersUltra-Short MicrolasersMicrolasers with ridge lengths down to 30 µm (< 10 x wavelength) realized

• High-quality Bragg mirrors

• Optically smooth surfaces

15 µm


Room Temperature Operation of MicrolasersRoom Temperature Operation of Microlasers

Ridge length ~150 µm

Devices based onBound-to-continuum active region design

- 85 mW , 80 K

- 3.4 mW , 293 K (20 °C)

900 910 920

0.02

0.04

0.060.08

0.1

0.2

180 K2A9 mW

Inte

nsity

(a.

u.)

Wavenumber (cm-1)

> 10 dB


Wavelength Tuning with Cavity LengthWavelength Tuning with Cavity Length

m=33

m=33

m=34

ng = 3.41

• changes in cavity length: 0.2 µm

• tuning over 38 cm-1 (420 nm) centered around 955 cm-1 (10.5 µm)

Results based on bound-to-continuum design: 50 µm device


Single Mode Emission StabilitySingle Mode Emission StabilityLasers with of ~50 µm ridge length based on bound-to-continuum design

• single mode operation

up to 1.5 x Ith

• mode jump due to blue shift by increased voltage

• mode spacing about 30 cm-1 (340 nm)


Wavelength tuning observed with:

• Heat sink temperature -0.062 cm-1/K

• Drive current -1.0 cm-1/A

Tuning with Temperature/CurrentTuning with Temperature/CurrentResults based on threequantum well design

Ridge length ~100 µm

~/


Mode Switching with TemperatureMode Switching with Temperature

940 950 960 970

0

1

2

3

4

5

6

7

8

10.6 10.5 10.4 10.3

200 K

180 K

160 K

140 K

120 K

100 K

Inte

nsity

(a.

u.)

Wavenumber (cm-1)

80 K

• Discontinuous tuning by temperature and according drive current variation

• Spacing between modes 16 cm-1


Two Segment Distributed Feedback LasersTwo Segment Distributed Feedback Lasers

Two segment distributed feedbacklLaser with different grating periods

1

front segment rear segment

2

ii n

2

1~


Reversible Mode SwitchingReversible Mode Switching

If= current injected in front segmentIr= current injected in rear segment

926 927 928 929 930 931 932 933 934

0

2

4

6

8

10

10.78 10.76 10.74 10.72

2.0 : 1

0.9 : 1

1.7 : 1

1.5 : 1

1.3 : 1

1.1 : 1

1 : 1

0.8 : 1

0.7 : 1

0.6 : 1

If + I

r = 3.5 AI

f : I

r= 0.5 : 1

120 K

Inte

nsity

(a.

u.)

Wavenumber (cm-1)

Wavelength (µm)

0.0 0.5 1.0 1.5 2.00

50

100

150

200

Pow

er (

mW

)

Current (A)

120 K

If


Evolution of DFB Modes with TemperatureEvolution of DFB Modes with Temperature

100 120 140 160 180 200 220 240 260920

922

924

926

928

930

932

10.86

10.84

10.82

10.80

10.78

10.76

10.74

Mode2

Wav

enum

ber

(cm

-1)

Temperature (K)

2.5 cm-1

Mode1

Wav

elen

gth

(µm

)


Quasi-Continuous TuningQuasi-Continuous Tuning

920 922 924 926 928 930 932

0.0

0.2

0.4

0.6

0.8

1.0

10.86 10.84 10.82 10.80 10.78 10.76 10.74

Inte

nsity

(a.

u.)

Wavenumber (cm-1)

9 cm-1

Wavelength (µm)

900 920 940 960

0.050.1

0.51

510

1:1

120 K

inte

nsi

ty,

a.u

.

wavenumber, cm-1

23 dB

• Tuning with temperature and segment drive current control

• Single mode emission over > 9 cm-1

• Side mode suppresion ratio (SMSR) up to 23 dB


SummarySummary

• QC Microlasers with monolithically integrated Bragg mirrors

• Single mode emission achieved due to large mode spacing and limited gain bandwidth

• Wavelength tuning demonstrated with:

- Temperature

- Drive current

- Cavity length

• Room temperature operation achieved (>3 mW @ 20 °C, > 10 dB SMRS @ 180 K)

• Two segment QC distributed feedback lasers

• Mode switching over 1.5 and 2.5 cm-1

• Quasi-continuous tuning over 9 cm-1 (105 nm); SMRS up to 23 dB

Acknowledgement:

A. Wolf, M. Emmerling, S. Kuhn, C. König, J. Goertz, B. Rösener

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