The quantum cascade laser: a high power semiconductor...

The quantum cascade laser: The quantum cascade laser: a high power semiconductor laser for a high power semiconductor laser for midmid--infrared sensing applicationsinfrared sensing applications

Oana Malis

Collaborators:

Deborah L. Sivco, Jianxin Chen, Liming Zhang, A. Michael Sergent, Loren Pfeiffer, Kenneth West, Bell Laboratories, Lucent Technologies

Claire Gmachl, Dept. of Electrical Engineering and PRISM, Princeton Univ.

Alexey Belyanin, Department of Physics, Texas A&M University

Introduction to quantum cascade lasers

material

CB

VB

diodelaser:

layer thickness

CB

QC-laser:

Conventional semiconductor laser

Quantum cascade laser: unipolar semiconductor laser using intersubband transitions

Quantum cascade lasers: mid-infrared light sources

mid-infrared light source wavelength agile: InPrange 5 – 20 µm high powerhigh-speed

12

312

Ibott

Itop

Itop

activeregion

injector

injector

activeregion

e

e

3

Ibott

InGaAs/InAlAs lattice-matched to InP

QCL: compact, rugged light source

Grown by MBE

InGaAs/InAlAs lattice matched to InP

What makes the QC-laser special?

Wavelength agility: layer thicknesses determine emission wavelengthHigh optical power: cascading re-uses electronsFabry-Perot, single mode (DFB), or multi-wavelength (dual-wavelength, ultra-broadband) Temperature tunableUltra-fast carrier dynamics: no relaxation oscillationsActive research field in semiconductor physics

QCL operating modes

Fabry-Perot mode

8.0 8.2Wavelength (µm )

Single mode DFB

4.96 5.00 5.04

4.92 4.96 5.00 5.04 7.36 7.40 7.44 7.48

no grating

Inte

nsity

(arb

. uni

ts)

Wavelength (µm)

Dual-wavelength

a

0.1

1

10

Pow

er (

arb.

uni

ts, l

og. s

cale

)

Wavelength (µm)

5 6 7 8 9

2, 3, 4 A5 ... 13 A

Ultra-broadband

8.6 8.8 9.0 9.2 9.4 9.60

50

100

150

200

Inte

nsity

(a.

u.)

pump wavelength (µ m)

4.3 4.4 4.5 4.6 4.7 4.80

50

100

Inte

nsity

(a.u

.)

second-harmonic (µm)

Nonlinear light generation:second-harmonic

laser SH

What makes the QC-laser special?

Wavelength agility: layer thicknesses determine emission wavelengthHigh optical power: cascading re-uses electronsFabry-Perot, single mode (DFB), or multi-wavelength (dual-wavelength, ultra-broadband) Temperature tunableUltra-fast carrier dynamics: no relaxation oscillationsActive research field in semiconductor physics

Free-space optical telecommunications

Applications

In-situ trace gas sensing: NO, CO, NH3, CH4, H2O (isotopes), and more complex molecules – ppmto ppb levels ⇒Chemical and biological sensing (air quality, chemical and biological weapons, breath monitoring) Remote sensing: LIDAR

QC-laser

DC - Source

MCT-det.

Spectrum Analyzer

Satellite Set-Top Box

DC - Voltmeter

Physical Sciences, Inc.

200 m

Ongoing QCL research

Goal: to extend the functionality and Goal: to extend the functionality and performance of midperformance of mid--infrared emittersinfrared emitters

New materials and fabrication techniques to optimize InP QCL performance

New light generation processes:

Ø Nonlinear light generation in QCLs

Ø Hole quantum cascade laser

Optimization of InP-based laser properties

Design of high-gain active region

Minimization of waveguide losses using InP top and side-claddings

Growth of high-purity materials

Thermal management

2

3

4

IB

IB

e

active

injector

injector

1e

n InP, 1-2×1017 cm-3, substrate

n InGaAs, 3-5 × 1016 cm -3

Waveguide core:Active regions and injectors

30-50 stages

n InGaAs, 3-5 × 1016 cm -3

n InP, 1017 cm -3

n InP, 8 × 1018 cm -3

Ti/Au top contact

InP substrate electroplated Au

In solder waveguide core

Advanced fabrication and processing

MBE and MOCVD overgrowth: Liming Zhang, Jianxin Chen

InP substrate

MBE MOCVDMOCVD

Laser core

Plated gold

Improvement of cw max. temperature by 50K (with HR coating)

Metal electroplating

Recent highlight: room-temperature, continuous-wave operation at 8 µm

1.0 1.5 2.0 2.5 3.0 3.5 4.00

10

20

30

40

300 K

320 K

280 K260 K

240 K220 K

200 K

current density (kA/cm2)

cw o

utpu

t pow

er (m

W)

cw mode

0 2 4 6 80

2

4

6

8

10

12

14

current density (kA/cm2)

Vol

tage

(V)

0

50

100

150

200

250

300

350

400

450

300 K

320 K300 K280 K260 K

240 K

220 K

Pea

k ou

tput

pow

er (m

W)

pulsed mode

Nonlinear light generation in QCLs: Outline

Nonlinear light generation in intersubband transitions

Sum-frequency and second-harmonic generation in QCLs

Enhancement of second-harmonic response in InP QCLs

Phase-matching for second-harmonic generation

Summary and discussion of second-harmonic QCLs

Future projects: parametric down-conversion

Introduction to nonlinear light generation in QCLs

Motivation:Extend operation of InP-based QCLs outside the limits imposed by material system (i.e. below 5 µm)Light sources with new functionalityApplications: high-resolution chemical and biological sensing to quantum cryptography

Goal: develop monolithically integrated Goal: develop monolithically integrated nonlinear QC lasersnonlinear QC lasers

Nonlinear light generation using resonant intersubband transitions

P = ε0 (χE + χ(2)E2 + χ(3)E3 + …)

ω 2ω

( ) ( )13131212

132312

0

3)2(

222 γωγωεχ

⋅−−⋅⋅−−∝

iEiEzzz

Ne

e hh

M.K. Gurnick and T.A De Temple, IEEE JQE 19, 791 (1983).F. Capasso, C. Sirtori, and A.Y. Cho, IEEE JQE 30, 1313 (1994).

ωω 1

23

2ω

Monolithically integrated nonlinear QCL

N. Owschimikow et al., Phys. Rev. Lett. 90, 043902 (2003).

Sum frequency and second-harmonic generation

Conditions for efficient SHG:Efficient pumping ⇒ monolithic integrationPhase-matching

2ω1

ω2

active region

ω1+ ω2

2ω2

ω1

ω1

ω2

SL

First demonstration of nonlinear QCL

• two active regions (7.1 µm and 9.5 µm) and mixing superlattice section • 7.1 µm active region includes resonant IS cascades for SFG and SHG

superlattice 7.1 µm active region

60 and 80 mW of laser power ⇒ 30 nW SFG and 15 nW SHG

5

4

3

23

11

injector

injectoractiveregion

Optimized nonlinear QC laser active region

InGaAs/ InAlAs QCL Two nonlinear cascades:2 – 3 – 4, and 3 – 4 – 5

χ(2) = 4.7 × 10-5 esu= × 3 highest measured value in any material system

( ) ( )

Γ−

+Γ−

Γ+

Γ−

+Γ−

Γ=

43

43

54

54

53

354534

32

23

43

43

42

24342323 E

2nnnnzzznnnnzzzNe

P xe

hω

ω

Nonlinear QC laser general characteristics

8.6 8.8 9.0 9.2 9.4 9.60

50

100

150

200

Inte

nsity

(a.u

.)

pump wavelength (µm)

4.3 4.4 4.5 4.6 4.7 4.80

50

100

Inte

nsity

(a.

u.)

second-harmonic (µm)

Fundamental Second-harmonic

• InGaAs/InAlAs QCL grown by MBE on n-type InP• deep-etched ridge waveguide devices• 1.5 – 2.25 mm long, 4 – 15 µm wide

10 KMCT detector

10 KInSb detector

D2912

Linear and nonlinear L-I for D2912

00

.05

0.1 600

400

200

054321

Current (A)

Non

linea

r po

wer

PN

L (n

W)

Line

ar p

ower

PL

(W)

0 0.010.0050

0.6

0.3

(PL (W))2P

NL

(µW

)68 µW/W2

65 µW/W2

49 µW/W2

0.1

1

10

100

D2616D2882

Pow

er c

onve

rsio

n e

ffici

ency

(µW

/W2 )

D2886 D2912

firstsample

optimizeddesign

2)()(2 W W ωω η=

Second-harmonic generation of QC laser

∆k = k2ω – 2kω = 2ω (µ2 – µ1)/c = the phase mismatchµ1,2 = effective refractive indices of modes α2 = total losses of a given cavity mode at λ2 = λ1/2L = the cavity length, R1,2 = reflection factors of a cavity Σ = nonlinear overlap factor of the two interacting modes

( )[ ]( )( )( )2

122

2222

21

225

11cos21128

~22

RkcRkLee LL

−+∆−∆−+Σ −−

αλµµπ

ηαα

Waveguide design for second-harmonic generation in QCLs

10 nm InGaAs 1e20 cm-3 Au contact

InGaAs, 6.5e18 cm-3, 850 nmInAlAs, 1e17 cm-3, 1300 nm

InGaAs, 1e17 cm-3, 1600 nm

active region 50 stages 2475 nm

InGaAs, 1e17 cm-3, 1500 nm

InP 1-5e17 cm-3 substrate10

8

6

4

2

0

0 2 4 D

ista

nce

(µm

)

SH refractive index profi le

Modal phase-matching for second-harmonic generation in QCL

IR refractive indices for InGaAs, InAlAs, and InP:Indices for undoped alloys were interpolated linearly from the published values for the end compounds for each wavelengthDrude formula to calculate the complex refractive indices of the doped alloysOne-dimensional solution of the wave equation assuming infinitely wide ridges ⇒ effective refractive indices, mode profiles

Problem: IR refractive indices are not known accurately

flexibility in waveguide design ⇒ modal phase-matchingno need for birefringence or quasi phase-matching

0 2 4 6 8 10 12

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

n=3.1861

n=3.2868

n=3.3094

n=3.2127

pump TM00

SH TM00

SH TM01

SH TM02

Mag

netic

fiel

d H

pro

file

(a.u

.)

Thickness (µm)

Mode selection for SHG phase-matching

Phase-matching of pump TM00 mode with SH TM02mode

Spatial distribution of modes determines the overlap with each other and with the active region

Exact phase-matching using ridge-width dependence

w

2 4 6 8 10 12 14 16 18 20 223.04

3.06

3.08

3.10

3.12

3.14

3.16

3.18

3.20

3.22

phase-match

second-harmonic

fundamentalre

frac

tive

inde

x

width (µm)

InAlAs-based waveguide development for phase-matching

0.1

1

10

100

1000

10000

0

50

100

150

200

250

Non

linea

r ef

ficie

ncy

(µW

/W2 )

Non

linea

r po

wer

(µW

)

4 cm-197135α = 84367 cm-1711715837∆k = 58129572944293529272912

D2944: η = 35 mW/W2

D2957: PNL = 240 µW

0

20

40

60

80

0 0.5 1.0 1.5 2.0 2.50

40

80

120

160

Current (A)

Lin

ear

pow

er P

L (

mW

)

Non

-lin

ear

PN

L (µW

)

0

40

80

120

160

0 0.002 0.004 0.0060

40

80

120

160

35 mW/W2

PL2(W2)

PN

L(µ

W)

0

100

200

300

0 1 2 30

100

200

300

Current (A)

Lin

ear

pow

er P

L (m

W)

Non

-lin

ear

PN

L (

µW)

0

50

100

150

200

0 0.02 0.04 0.06 0.08 0.100

50

100

150

200

2.4 mW/W2

PL2(W2)

PN

L(µ

W)

O. Malis et al., Appl. Phys. Lett. 84, 2721 (2004).

Effect of phase-matching for SHG with InAlAs waveguides

Ridge-width dependence

Agreement with calculation on the position of the maximum

Recent result: 2 mW second-harmonic generation

What made it possible:InP top cladding regrowthby Jianxin ChenHR coating of back facet

Power >1 mW interesting for spectroscopy

O. Malis et al., Electron. Lett. 40, 1586 (2004).

0.00 0.04 0.08 0.120.0

0.5

1.0

1.5

2.0

17 mW/W2

PL

2(W2)

PN

L (mW

)

0.0

0.5

1.0

1.5

2.0

2.5

Non

linea

r po

wer

PN

L(m

W)

0 1 2 3 40

50

100

150

200

250

300

350

400

450

500

Lin

ear

po

wer

PL(

mW

)

Current (A)

D3014

Far-field pattern of second-harmonic mode

Far-field pattern consistent with TM02 modeSharp, high feature in the far-field

cryostat

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0o rotation +13o rotation -13o rotation

SH

Inte

nsity

(arb

. uni

ts)

angle ( o )

Top view Side view

2” ZnSewindow

Discussion of experimental results

Max theoretical:η = 2 W/W2

Experimental limitationsRidge width within 0.5 µm from wet etchingHigher non-resonant mid-infrared losses due to higher dopingHigher resonant losses due to accidental band alignment

Max experimental:η = 35 mW/W2

Future work

• Continue to improve nonlinear conversion efficiency and power

• AR coating on front facet

• CW, room temperature, and single mode operation

• Lower wavelength (< 4.5 µm) in the spectral region that is difficult to reach with InGaAs/InAlAs QC lasers

Summary of second-harmonic project

Developed monolithically integrated nonlinear QCLsDeveloped technique for phase-matching of nonlinear QC lasersIncrease by over two orders of magnitude in the second-harmonic power generation and nonlinear efficiencyMilliwatt second-harmonic generation promising for applicationsPhase-matching technique can be applied for other parametric processes

Collaborators: Loren Pfeiffer, Ken West

MotivationNew type of quantum cascade laserNew functionality of QCLs: Ø surface emitting QCLs and VCSELsØ Device that extends the operating range of present GaAs

QCLsØ Advantages similar to GaAs electron QCLs: better

temperature behavior, lower losses Ø Alternative to InP-based devices using the mature GaAs

MBE technology and GaAs substrates New physics of intersubband transitions in the valence band and hole relaxation processes

GaAs-based hole quantum cascade lasers

Background

Previous work in hole QC structuresSi/SiGe intersubband absorption, electroluminescence and photocurrent

Challenges: strained heterostructuresMaterial issuesTheoretical complexity

Advantages of GaAs/AlGaAs material systemStrain-free material Mature material systemExtensive experience from electron GaAs QCLsUnique materials opportunities in-house

Intersubband absorption

HH1LH1SO1

HH2LH2

conduction band

valence band

�k

p

s

ifps

W

pQW

psW

fcnm

eNndEL

NnLL

LTT

θθ

ε

ρπα

θ

α

cossin

2

cos

/)/ln(

2

00int

int

int

h=

=

=

∫

oscillator strength

multipass waveguide

Mid-infrared bound-to-bound hole intersubbandabsorption in GaAs/AlGaAs quantum wells

Structures: MBE grown on GaAs(001)

1% thickness control, confirmed by x-ray measurements

25 Å – 45 Å GaAs quantum wells

57% AlGaAs digital alloy barriers: 8.5 Å GaAs/ 11.3 Å AlAssuperlattice

P-type modulation doping with Carbon from novel solid source

1-2×1012/cm2 p-type doping

Mobility 8000 cm2/Vs at 5 K

Mid-infrared absorption measurements

dipole matrix element:z = 6 Å

31 Å QW

25 - 45 A QWs ⇒ 126 - 206 meVFWHMInGaAs = 20 meV < FWHMGaAs < FWHMSiGe = 30 meV

Comparison of experimental and simulation results

25 30 35 40 45100

150

200

250

HH

hol

e tr

ansi

tion

ener

gy (m

eV)

QW width (Å)

experiment calculation 57% analog alloy calculation digital alloy

6-band k·p calculations with nextnano3 package

digital alloy effectively increases the band offset

10 20 30

-0.4

-0.2

0.0

Ene

rgy

(eV

)

Position (nm)

31 Å QW

Parameters:Band offset: 0.51 eVDoping: 1.6·1012/cm2

GaAs: γ1=8.64, γ2=2.44, γ3=3.27AlAs: γ1=5.03, γ2=0.8, γ3=1.55

O. Malis et al., Appl. Phys. Lett.. 87, 091116 (2005).

Intersubband hole electroluminescence

GaAs/AlAs/Al0.3Ga0.7As2-level systemAl0.3Ga0.7As injector

20 40

1.0

1.2

Ene

rgy

(eV

)

distance (nm)

hh1

E = 57kV/cm

h

h

200 300 400 500

0

4

8

p-polarized

Pho

tocu

rren

t (a.

u.)

Energy (meV)

160 180 200 220 240

p-pol/s-pol

Abs

orpt

ion

(a.u

.)

Energy (meV)

Absorption and photocurrent measurements on luminescence structures

zero bias

expected ∆Ehh1 = 210 meV

120 140 160 180 200

100 200 300 400

0

30

60

J = 1.34 kA/cm2

J = 1.08 kA/cm2

J = 0.54 kA/cm2

Inte

nsity

(a.u

.)

Energy (meV)

Abs

orpt

ion

(a.u

.)

Energy (meV)

Electroluminescence results

3 peaks: hh1, hh2, thermalexpected ∆Ehh1 = 190 meVmeasured ∆Ehh1 = 162 meV

20 40

1.0

1.2

En

erg

y (e

V)

distance (nm)

hh1hh2

hh2

hh1thermal

Effect of active QW thickness

200 400

0

30

thicker QWs

thinner QWs

J = 1.08 kA/cm2

Inte

nsity

(a.

u.)

Energy (meV)

Using growth non-uniformity:10% thickness difference (1ML) ⇒⇒ 15 meV energy difference

Broadening of the hh emission peak: FWHM hh1 20 - 45 meV

Electroluminescence L-I-V

Upper-level lifetime of approx. 0.4 psLifetime consistent with estimate based on m*

GaAs=0.266, m*

AlAs=0.2915 from Luttinger parameters

0.0 0.4 0.8 1.2

10

20

30

J (kA/cm2)

Vol

tage

(V)

0

1

2

3

Pow

er (nW)

5

4

103.2

104

/

−

−

⋅=

⋅=

=

QC

coll

QCcoll

N

ehNIP

η

η

υηη

assuming z = 5 Å

O. Malis et al., Appl. Phys. Lett.. 88, 081117 (2006).

Summary of hole intersubband absorption and electroluminescence

Mid-infrared bound-to-bound hole intersubband absorption range of 126 – 206 meV for 25 – 45 Å C-doped GaAs/AlAs QWsAgreement between experimental results for hh-transitions in wide wells and calculations considering the full band structureHeavy-to-light transitions and hh-transitions in narrow QWs still challengingHole intersubband electroluminescence and photocurrent measurementsEmission wavelength slightly lower in energy than expectedAdditional emission peaks possibly due to other hh-transitionsBroadening of the hh emission peakUpper-level lifetime of approx. 0.4 ps

The quantum cascade laserThe quantum cascade laser

Collaborators:Deborah L. Sivco, Jianxin Chen, Liming Zhang, Loren Pfeiffer,

Kenneth West, A. Michael Sergent, Claire Gmachl, AlexeyBelyanin

Unipolar, intersubband laser operating in the mid-infrared rangeApplications in trace-gas sensing and free-space communications Active field of research into new materials and new light emission processesØ New materials and fabrication techniques to

optimize InP QCL performance Ø Nonlinear light generation in QCLsØ Hole quantum cascade laser

The quantum cascade laser: a high power semiconductor...

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