Les lasers à cascade quantique :une technologie des lasers

semiconducteurs pour le MIR et le FIR

Université Paris Diderot, Laboratoire Matériaux et Phénomènes QuantiquesCNRS, UMR7162, Paris, France.

Carlo Sirtori

Collaborators

D. Dolfi

X. Marcadet

M. Carras

H. BeereD. Ritchie

R. Colombelli

Y. Chassagneux

I. Sagne

U. Gennser

S. Barbieri W Maineault

A. Vasanelli Y. Todorov

P. Jouy A. Calvare

S. Laurent P. Gellie

P. Filloux D. Barate

C. Manquest A. DetailS. Dhillon

N. Jukam

J. TignonLPA - ENS

Semiconductor lasers

Edge emitting semiconducteur laser

Au wires

Copper block

Laser

6 m

m

Laser

In solder

Alumina

i

Single laser mounted on copper block

18 Au wires (17µm)In solder

Alumina

6 m

m

LaserCopper block

Laser

FABRICATION STEPS FOR A SEMICONDUCTOR LASER

1- SUBSTRATE 2- EPITAXIE 3- LASER PROCESSING

4- FACETS CLEAVING 5- SINGLE CHIPPREPARATION 6- MOUNTING, BONDING

Diode lasers

00 5 10 15 20

00.5 1.0 1.5

Wavelength (µm)

Wavelength (µm)

GaAs/GaInAs

GaAs/AlGaInPGaN/AlInGaN InP/GaInAsP

? II-VI compounds ?

GaSb/AlGaInSbAs

QC lasers

0 2.0

GaAs/AlGaAsGaInAs/AlInAs/InP

Diode lasers

THz andmm wave

PbTe/PbSe IV-VI

Le jeu des matériaux

Diode lasers

Quantum Cascade lasers

Growth direction

The quantum well: the building block

AlSb AlSbInAs

2.5nm

E12

E1

E2

The quantum well is the elementary constituent of our system

Light-matter interaction with photon energies lower than the band gap

Light-matter interaction with photon energies lower than the band gap

Energy vs. position in a quantum well structure

E1

E2

AlAs

GaAs

AlAs

V(z)

Position z

Ene

rgy

Conduction bandquantum well

Valence bandquantum well

Electrons are free particles in the plane

2

1

The confinement potential is onlyin the direction of growth ( z)

1

2

Injection barrier

Minigap

Electron reservoir

QC laser design

1

2 Minigap

1

2 Minigap

Cascade Action:1) Np photons per electron traversing the structure (Np ~100 @THz)

2) The total population inversion is distributed over all the period

1

2 Minigap

Activeregion

Injector

hω = 5 – 400 meV

"MINIBAND"

"MINIGAP"

INJECTOR EMITTER

E

x 25

45 n

m

3

21

Band diagramTEM Micrograph

Quantum cascade design and material

Courtesy of C. GmachlBell Labs, Lucent Tech.

hν1

Same material systemDifferent quantum well widths

Diffeferent transition energieshν1 = 4 hν2

hν2

Intersubband transitions depend primarilyon width of the quantum well and not on the materials constituent the heterostructure

Intersubband transition energies

Tmax of QC lasers vs. wavelengthPulsed operation

Data (some are unpublished):� ETH-Z� THALES� Walter Schottky (Munich)� Northwestern University� Humboldt University Berlin� TU Vienna� MIT� University of Sheffield� University of Montpellier

Range of operation:

2.7 - 400µm or 100 – 0.8THz

0

100

200

300

400

2 5 10 20

Tem

pera

ture

(K

)

III-V compoundsphonon bands

LN2

Peltier

Atmospheric windows

500

Wavelength (µm)50 100

GaAs based lasers

InP based lasers

200

Sb based lasers

Uni. Montp. 2.6µm!

Quantum Cascade laserperformances

A. Lyakh, et al., APL 92, 111110 (2008) Y. Bai, et al., APL 92, 101105 (2008)

CW operation of QC lasers (State-of-the-art)

Division of Engineering and Applied SciencesHarvard University

Y. Bai, et al., APL 95, 221104 (2009)

Very High power broad area QCLsWide strip lasers are used to increase the volume of the active material

High power QCLs with no Buried Het� Au plated, HR coating and epi down mounting

ICP Etching

Courtesy of X. Marcadet

0.0 0.5 1.0 1.5 2.0

0

2

4

6

8

10

12

Vol

tage

(V

)Current (A)

0

200

400

600

800

1000

HR coatingEpi downL=4 mmCWRT on TEC

14 µm 12 µm 10 µm 8 µm 6 µm

Out

put p

ower

(m

W)

THz lasers

1 THz = 300cm-1 = 4.1meV = 300µmFrequency Energy (cm-1) Energy (meV) Wavelength

0,0 0,5 1,0 1,5 2,0

0

50

100

150

0 100 200 3000

1

2

3

95K

77K

70K

60K

40K

20K

4K

Pea

k po

wer

(m

W)

Vol

tage

(V

)

Current (A)

90K

80K

Current density(A/cm 2)

Performances of a 3THz QC laser

Pulsed

0 20 40 60 80 100

100

150

200

250

300

J th (

A/c

m2 )

Temperature (K)

� Tmax = 96K(pulsed); 70K(cw)

� 2% peak wall-plug efficiency at 4K

� 0.4% wall-plug efficiency in CW

3 x 0.2mm2 device

S. Barbieri et al., APL (2004)

75

50

25

Applications

� QC lasers are a mid infrared laser technology based onIII-V semiconductor compounds such as GaAs and InP

Spectroscopic applications (Gas, molecular detectio n)

Optical countermeasure (High power devices)

� Medical (breath analyses, )� Environmental� Security (Explosive detection)

� 3-4µm for missile out-steering� 8-10µm for night vision blinding

Output power = 10mW, CW operation, control on the l inewidth

Output power > 1W, CW operation non strictly necess ary

1)2)3)4)5)6)7)8)9)

10)11)12)13)14)15)16)17)

List of companies providing QCL wafers, sources and sensor systems

Company name

R.F. Curl et al. Chem. Phys. Lett. 487 (2010) 1–18

Conclusions

� QC lasers are a technology based on III-V semiconductor compounds

� QC cascade lasers ar

� Wavelength can be adjusted by changing the quantum design of the active region rather than its material composition

�QC lasers are unipolar optoelectronic devices (electron only!)

� High performance devices are fabricated :� QC laser operating CW at 300K with Watt level power (λ = 5µm)� QC lasers with tens mW of power in the THz region @ 77K

� QC lasers are the only semiconductor solution in the THz region

�Electrical control of the complex refractive index for frequency and amplitude modulation

� Lasers stabilisation (QC THz stabilised on a frequency comb , NP July)

� Merging QC lasers with other important technologies as: µ-wave, telecom, TDS (femto second).

� Plasmonics and metal guiding

� Very high frequency modulation > 50 GHz

Future

Far Infrared Optics in Si-Inversion layers

EF

K

E

ELO

Intersubband lifetime

Photon emission (10 - 100 ns)

Electrons leave excited subbands by spontaneous emission ofoptical phonons (bleu arrow)This time can be estimated using bulk phonons

Radiative efficiency = 10-4 - 10-5

Phonon emission (1 ps)

Elastic scattering (2 -5 ps)

Applied bias on a QC laser (λ = 4.3µm)

0 200 400 600 800

0

100

200

300

400

500

600

700

800

900

1000

1100

Ene

rgy

(meV

)

Z (Å)

V = 0

0 200 400 600 800

0

100

200

300

400

500

600

700

800

900

1000

1100

Ene

rgy

(meV

)

Z (Å)

V = Vth

Saw tooth

Stair case

0

FWHM = 4.6 meV0.10

0.20

100 120 140 160

Photon energy (meV)

Abs

orba

nce

(Log

10T

)

11 10 9 813Wavelength (µm)

T = 10 K Lorenzianline-shape

Absorption measurements

1

2

+++++++++

δ doping

EF

Modulation doping allows to observe narrow lines ≅ 5%∆EE12

Surfa

ce w

ith Sh

otky c

ontac

t

< Φ1| z |Φ1 > = <z12> == dipole matrix element

Selection rules:only the electric field parallel to z is absorbed

No absorption for light normal to the surface!!

Population inversion between subbands

� Injection and extraction of electrons from quantum wells by tunnelling

1

3

2

E

k||

� Population inversion condition τ32 > τ2

τout

τphonon

k||k||

Population inversion at High temperature

The number of states below the lower states of the laser transition helps its depletion

The rate of depletion is proportional to the number of final states accessible

Closely spaced levels increase the elastic scattering and favor the occupation of the subbands at high k// (essential for THz laser)

Two phonon resonanceBound to continuum

∆E >> KT

Increase the energy separation between thelower state of the laser transition and the ground state of the structure

"MINIBAND"

"MINIGAP"

INJECTOR EMITTER

E

x 25

45 n

m

3

21

Band diagramTEM Micrograph

Quantum cascade design and material

Courtesy of C. GmachlBell Labs, Lucent Tech.

Material systems for QC laser

GaInAs

= 0.043 m0

AlInAs/GaInAs//InP

AlGaAsx=45%

me* = 0.067 m 0

GaAs/AlGaAs//GaAsAlSb

AlInAs

520 meV

~ 2000 meV

GaAs

InAs

AlSb/InAs//InAs or GaSb

= 0.024 m 0

GaAs/AlAs~ 1000meV

AlInAs/GaInAs/InPstrain compensated~ 600/700 meV

~~

390 meV

Indirect discontinuitylimits to 400meV

Even in strain compensatedmaterials no evidence oflateral valleys

Exact position of the lateralminima is unknown

me* me

*

700 meV

InAs is the upper limitat 400meV

~~

400meV

InAs Band gap

GaN?

0

1.5

3.0

4.5

6.0

7.5

9.0

10.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0

50

100

150

200

250

300

350

400380K

360K

340K

320K

300K

280K

260K

240K

380K

240K

Current [A]

Vol

tage

[V]

Pow

er [m

W]

Threshold current density [kA/cm2]

L. Dhiel et al. APL (2006)

Burried heterostructuresfor optimum power dissipation

Burried heterostructures foroptiumum power dissipation

InP cladding

re-grown Fe:InP

InP cladding

n+ InP substrate

re-grown Fe:InP

Plated Au n+ InGaAs layer

2 µm

Active core

Division of Engineering and Applied SciencesHarvard University

Wall

plug

effic

iency

(%)

Y. Bai, et al., APL 93, 21103 (2008)

High power QCLs with no Buried HetDouble tranches, epilayer down bonded to a diamond heat sink

Gain dynamics in QCL

M1 M2

High speed IR

detector

Probed Fabry Perot QCL

Source:chirped DFB QCL

It

TM or TE polarization

K = contrast is the

L

Io

L

K

K α−=+

−Re

1

1

-0,5 0,0 0,5 1,0 1,50,0

0,5

1,0

Am

plitu

de

(u

.a.)

Time (µs)

K

mirror or grating

Gain measurement setupE. Benveniste et al. APL 94 (2009)

Oscilloscope

F-P Fringes TE

1270 1269 1268 1267 1266 1265

0,4

0,6

0,8

1,0

Tra

ns

mis

sio

n (

a.u

.)

Wave number (cm-1)

• Contrast independent from the current

αmeas-TE= αWG-TE +αmirror-TE

1270 1269 1268 1267 1266 12650,0

0,2

0,4

0,6

0,8

1,0

Tra

nsm

issio

n (

u.a

.)

Wavenumber (cm -1)

J

αmeas-TM=αmirror-TM+αWG-TM- Gain

F-P fringes in TE F-P fringes in TM

Gain measurements

- 8

- 6

- 4

- 2

0

2

0 0.5 1.0 1.5 0

0.4

0.1

0.2

0.3TE losses

Threshold

Current (A)

Optic

al po

wer (W

)

Gain

(cm-1

)

Gain measurement: intersubband gain and losses

Mirror losses

λ = 5µm

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0-20

-15

-10

-5

0

5

10

15

dG/dJ= 4.1 cm/kA

-αWG

Jth

αM

G-α

ISB

-αW

G (

cm

-1)

J (kA/cm2)

dG/dJ= 9.2 cm/kA

Gain

(cm-1

)

Current density (kA/cm2)

Mirror losses

Gain measurement: intersubband gain and losses

Evidence of a strong transparency current

λ = 8µm

0

5

10

15

0 50 100 150 200

Gain measurement: intersubband gain and lossesλ = 150µm

Measurements performed using a technique first demonstrated in J. Kroll et al Nature (2008)

Gain

(cm-1

)

Current density (kA/cm2)N. Jukam et al. Appl. Phys. Lett (2008)

JthOnset of the gain

1Injection

Extraction2

Injection

Extraction

3

GAIN!

Transparency currentAbsorption

Evolution of the gain

0 1 2 3 4 5 6 7 8

1

Sig

nal (

u.a.

)

time (µs)

0 1 2 3 4 5 6

300

320

340

360

380

Tem

pera

ture

(K

)

time (µs)

7

8

9

10

Mod

al g

ain

(cm

-1)

320 340 360 380

7,0

7,5

8,0

8,5

9,0

9,5

Mod

al g

ain

(cm

-1)

Temperature (K)

TnLT

∂∂=∆

2

λTemperature increment between 2 peaks :

Gain and temperature

DFB QCL in CW mode

Quantum Cascade laserperformances @ THz frequencies

0,0 0,5 1,0 1,5 2,0

0

50

100

150

0 100 200 3000

1

2

3

95K

77K

70K

60K

40K

20K

4K

Pea

k po

wer

(m

W)

Vol

tage

(V

)

Current (A)

90K

80K

Current density(A/cm 2)

Performances of a 3THz QC laser

Pulsed

0 20 40 60 80 100

100

150

200

250

300

J th (

A/c

m2 )

Temperature (K)

� Tmax = 96K(pulsed); 70K(cw)

� 2% peak wall-plug efficiency at 4K

� 0.4% wall-plug efficiency in CW

3 x 0.2mm2 device

S. Barbieri et al., APL (2004)

75

50

25

THz QCL main challenge: Tmax Operation Balkin et al. APL 2008Tmax = 178K

Terahertz QCLs performance vs. wavelength

Data from:Nest-Pisa

ETHZMIT Teraview-CavendishParis 7-Thales

Harvard/Leeds

1 THz3 THz

Courtesy of G. Scalari, ETHZ

Unpilar devices summary and perspectives Quantum cascade lasers are devices based on mature III-V compound semiconductors that have been developed and used for other technologies

Unipolar optoelectronic have imported material, processing and device architecture from the well know III-V platform (diode lasers and detectors). However fundamental physical differences have not yet been fullyexploited:

• Gate contacts and three terminal devices• Parallel transport (acceleration of electron in the quantum well) • Phonon engineering• Extremely short upper state life-time (Ultrafast modulation)• Optical guiding and photonic structures based on metal patterning• Extremely short upper state life-time (Ultrafast modulation)• Optical guiding and photonic structures based on metal patterning

Ultrafast unipolar optoelectronics

τ3 ~ 1nspττ sτ3 >> >�

1E8 1E9 1E10 1E11

1E-15

1E-14

1E-13

1E-12

1E-11

1E-10

3dB

25GHz

10GHz

3GHz

45GHz

17GHz

5GHz

τp = 2ps; τ

3 = 3ns

10Lo

g(S

ac/J

ac)

(ar

b. u

nits

)

Frequency (Hz)

Pout = 1W --> τs = 12.5ps

Pout = 100mW --> τs = 125ps

Pout = 10mW --> τs = 1.25ns

Frequency response function of a diode laser

Frequency response function of QC lasers

1E8 1E9 1E10 1E111E-14

1E-13

1E-123dB

Frequency (Hz)

10Lo

g(P

ac/I ac

) (

arb.

uni

ts)

140GHz67GHz14GHz

Pout

= 10mW (Pintra

= 100mW) --> τs = 0.5 ps

Pout

= 1mW (Pintra

= 10mW) --> τs = 5 ps

Pout

= 100µW (Pintra

= 1mW) --> τs = 50 ps

THz QC laser; τp = 2ps; τ

3 = 5ps

50µm x 12µm 2-metal ridge

� Short non-radiative lifetime ~ 1ps� no relaxation oscillations!

� Short stimulated lifetime ~ 1ps � very wide band of modulation

The system acts as an over-dumped oscillator: the equilibrium of carrier population is restored within a round photon round trip

The cascade enhance the number of photon in the cavity

Results on QCLs RF modulationDouble metal QCL; f=2.9THz3mm x 50µm x 12 µm ridge

S. Barbieri et al., Appl. Phys. Lett. 91, 143510 (2007)

R. Paiella et al., Appl. Phys. Lett. 77, 169 (2000)

8.1µm QCL3.75 mm x 4.5 µm ridge with chalcogenide glass insulation layer to reduce devicecapacitance

Double-metal waveguide allow for a low parasitic capacitance without any additional processing �High f modulation limited by wire-bond inductance

Modulation at 25GHz

� Narrow-band impedance adaptation

1 10

0.01

0.1

1

Measured Computed (with cables attenuation)

Nor

mal

ised

Pow

er T

rans

mis

sion

Frequency (GHz)

4mm x 70µm ridge 19.7GHz

2.20 2.25 2.30 2.35

Frequency (THz)

21.5GHz

20.5GHz20.0GHz

19.5GHz

19.0GHz

18.5GHz

18.0GHz

PRF = 13dBm

Spectra measured with up-conversion technique (res. 100MHz) S. Barbieri et al., Appl. Phys. Lett. (2007)

W. Maineult et al., Appl. Phys. Lett. (2010)

25 GHz

2.1 2.2 2.3 2.4 2.5Frequency (THz)

We have already demonstrated 35GHz

En,k- Em,k=∆Enm Vk

Intersubband absorption

The absorption of a photon is related only to the energy difference between the subbands et does not depends on k//

Absorption peaks like in 3D confined potentials : atoms

Three terminal devices

Three terminal QCL lasers with new functionalities

Vlaser

VControl

Active region

Control Contr

ol cu

rrent

(mA)

Control Voltage (V)

0

0.2

0.4

0.6

-300 -200 -100 0 1000

0.2

0.4

0.6

-300 -200 -100 0 100

V1 =0 V1 ∼2V

8µm 10µm

0 200.00

0.02

0.04

0.06

0.08

0.10

Γ = 100%α = 13 cm-1

met

al la

yer

met

al la

yer

activ

e re

gion

Mod

e in

tens

ity (

arb.

uni

ts)

Distance ( µm)

Double-metal waveguide

Advantage: Overlap factor 100%, independent from l and doping; strong lateral confinement

Drawback:Low out-coupling (R > 0.9); More difficult fabrication

AR

Top metal contact

Bottom metal contact

B. S. Williams et al., Appl. Phys. Lett, vol. 83, 5143 (2003)

n+ host-substrate

Far field of a metal-metal QC THz laser

y

x

12mm

52mm

mode intensityprofile

The strong interaction between the guided mode and the metal excite the top contact which act as an antenna

Metal-metal devices

z

y

x

fqsubstrate

-20 0 20 40 60 80

-20

0

-40

20

40

(b)�

Lase

r T

op

Data

Horn antennas for THz QCL

W. Maineult et al., APL (2008)

Exploiting the extreme optical confinement

h

Double metal

A.R.

Metal AR air

Mismatch between the DM mode and Metal-AR-air modeIndex contrast

A. Bousseksou et al., Opt. Express 2009

First step towards the la generation electrical injected surface plasmons Experimental data

Au

Electric Field

Evanescent wave devices

R. Colombelli