g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no...

19
g = Γ . G - (α i + α m ) Active region Waveguide Cladding Contact Waveguide Cladding Contact To achieve maximum material gain G at given l To provide excellent injection properties with minimum optical loss α and heating To optimize optical confinement Γ Semiconductor laser design goals

Transcript of g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no...

Page 1: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

g = Γ.G - (αi + αm)

Active region

Waveguide

Cladding

Contact

Waveguide

Cladding

ContactTo achieve maximum

material gain G at given l

To provide excellent injection properties with minimum optical loss α

and heating

To optimize optical

confinement Γ

Semiconductor laser design goals

Page 2: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Semiconductor Lasers: 2D vs. 3D confinement

electrons E

)(3 EDρ

holes

double-heterostructure laser

Eg

quantum well (QW) laserelectrons

holes

Ec1-h1

E

)(2 EDρ

c

v

3D joint DOS

νh

Eg

gr Ehmh −⎟⎠⎞

⎜⎝⎛= ν

πνρ

23

222

21)(

h

c1

h1

c2

h2

2D joint DOS

νh

2)(hπ

νρ rmh =

11111 −−−−− +=−= hcvcr mmmmm

Ec1h1 Ec2h2

c1

h1

c2

h2

hνhν

hνhν

Page 3: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Advantages of intersubband scheme:

Lasing wavelength is no longer defined by Eg and can be tuned by the QW width

Hole transport is eliminated

δ-like joint DOS provides for higher gain and better temperature stability

Monopolar transport offers electron recycling

electrons

electrons

holes

Eg >> hν

electrons

holes

Eg,eff ≈ hνhν

Wavelength is restricted by

material bandgap

Wavelength is uncoupledto the material bandgap

Interband and Intersubband lasing

Page 4: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Interband and Intersubband Lasing

interband lasing scheme intersubband lasing scheme

Bipolar laser: electron AND hole

injection

monopolar transportoffers electron recycling

electrons

holes

electrons

electrons

holeshole transport iseliminated

lasing wavelength is nolonger defined by Eg

lasing wavelength can betuned by the QW width

Eg,eff ≈ hν

Eg >> hν

δ-like joint DOS providesfor higher gain and bettertemperature stability

fast electron relaxationallows HF modulation

Advantages:

electrical injection

optical pumping

Ωh hνhν

Page 5: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

• In intersubband lasers δ-like joint density of states provides for higher optical gain

• Transparency current is negligible in intersubband lasers due to small electron population in the lower lasing states

(2)

(1)

K

Monopolar laser)1()2(

ee ff >

Bipolar laser

(c)

(v)

)()( ve

ce ff >

hνhν

[ ])()()()( 12 KfKfhhG −∝ νρν

δ-like joint DOS provides for higher gain

Page 6: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Optical Gain

interband lasing intersubband lasing

Bipolar laser

(c)

(v)

(2)

(1)

K

Monopolar laser)1()2(

ee ff >)()( ve

ce ff >

hνhνhν

[ ])()()()( 12 KfKfhhG −∝ νρν

)()()()(12 KVKV

KE−

=ρρ 2

1

122

112

2111)()()(

hh πππρ rD

mmmdK

KdEdKKdEKE =−=−=

−−

Page 7: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Intersubband Kinetics

(2)

(1)nrR

21τ

(0)

K

(2)

(1)

rR 11τ

2110 ττ <<

Ec

(2)

(1)(0)

3-level scheme

LOhE ν≈10

LOhE ν>21

K2 K1

K0

LOhvE ≈10

hνLO

hνLO

hνLO

hνLO

21τ

Optical phononresonance !

Tunneling

Optical phononemission

101

201

112

212 −

−−

− −∝<<−∝ KKMKKM

LOhvE >>21

( )Jnn==

1

1

2

2ττ

1221 nn >⇒<ττ

Tunneling ?

Tunneling

Page 8: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Quantum Cascade Laser

quantum cascade laser

E ~ 50 kV/cm(2)

(1) (2)

(1) (2)

(1)

Ener

gy

Distance

(2)

(1)

E = 0

quantum well laser

νhνh

νh

νh

νh

νh

Advantages of the cascaded scheme:

• quantum efficiency in excess of 1 allows high-power RT operation;

• electric field tunability due to Stark effect;• multy-wavelength operation;• electrically uniform active region;• large confinement factor.

(1)

(1)

(1)

(1)

Page 9: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Laser design elements: Active Region

2QW, direct(intrawell)

Optical phononresonance

3QW, indirect(interwell)

Optical phononresonance

4QW, directtransition

Double optical phononresonance

Bound-to-continuuminjectorless

QW

SL

Continuum-to-continuum(superlattice QCL)

SL1

SL2

Tunneling

Tunneling

Tunneling

Tunneling

Page 10: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

(2)

(1)

minibandsminigap

(2)

(1)

energylevels

tunneling

miniband transport

(2)

(1)

Laser design elements: Superlattice Injector

Page 11: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Laser design elements: Superlattice Injector

GaInAs/AlInAs superlattice

Lw/Lb = 6.0/1.8 nm

infinite superlattice

finite superlattice with 9 periods

(2)

(1)

E = 0

(2)

(1)

i = 1

i = 9

i = 1

i = 9

i = 1

i = 9

i = 1

i = 9

zkz k||π/d

Ene

rgy

d=Lw+Lb

minigap minibands

(2)

(1)

i = 1

(2)

(1)

i = 2

. . .+

isolated quantum wells

Page 12: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Advantages of the cascaded scheme:

• quantum efficiency in excess of 100%•electrically uniform active region• large confinement factor• multy-wavelength operation

Ec

(2)

(1)(0)

3-level scheme

LOhE ν≈10

LOhE ν>21

E ~ 50 kV/cm(2)

(1) (2)

(1) (2)

(1)

Ener

gy

Distance

(2)

(1)

E = 0

νh

νh

νh

Monopolar transport offers electron recycling

Page 13: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

60 nm52

0 m

eV

3

2

activeregion

injector

injector

e

activeregion

Doped injectors separate

active regions electrically

Courtesy of Claire Gmachl - Princeton University

Intersubband-based QC-laser λ ~ 7.5 µm

Page 14: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

F. Capasso et al. Physics Today 55, 34 (May 2002)

Real laser design: three level QCL

(x 25 stages)

8 µm

ban

d o

ffse

t 725 m

eV

QW1 QW2 QW3ACTIVE QWs - layer sequence (nm): 4.4/0.9/0.9/4.8/1.7/4.4/2.8

AQWs

INJECTOR

INJECTOR

minigap

miniband

miniband

minigap

Ga0.38In0.62As/Al0.48In0.52As

E32 = 280 meVE21 = hνLO = 30 meV

τ21 = 0.3 psτ32 = 2.6 psτ31 = 3.0 psNdop = 2.4×1011 cm-2

R. Kohler et al. APL 76, 1092 (Feb. 2000)

n-InP substrate

λ = 4.5 µm, pulsed

Voltag

e (V

)

Peak

outp

ut

pow

er (

mW

)

0 1 2 3 4

10

8

6

4

2

0

400

300

200

100

0

Current (A)

cladding regionactive regioninsulating layertop metal contact

Page 15: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Related problem: Active Region Heating

Double-phonon depopulationscheme

M. Beck et al. Science 295, 301 (2002)

E3 - E2 = E2 - E1 = hνLO~30 meVE4 - E3 = 135 meV (λ = 9.1 µm)

CW-RT Operation:• buried stripe geometry• epilayer-down mounting

(x 35 stages)

TAR = THS + U×Jth/G ; ∆T = TAR-THS

Jth = J0exp(TAR/T0)

J0= 560 A/cm2 ; T0= 170KJth~ 4.3 kA/cm2 (RT, CW)Jth~ 3.1 kA/cm2 (RT, pulsed)

hνLO

AR

HS

Page 16: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Pea

k w

avel

ength

(µm

)

Pea

k en

ergy

(meV

)

250

225

200

175

150

8

7

6

5

8 10 20 30

Ultra-broadband QCL

C. Gmachl et al. Nature 415, 883 (Feb. 2002)

Active region index

Many-Wavelength Operation:• all stages are designed for different wavelengths(heterogeneous cascade);

• optical gain compensates opticalloss (Ith= const) in the wholewavelength spectrum.

Wavelength (µm)

Wav

eguid

e lo

ss

Modal

gai

n

5 6 7 8

Wavelength (µm)

Optica

l pow

er (

arb.u

n.)

5 6 7 8 9

10

1

0.1

2 A3 A4 A

5 A9 A

Page 17: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Wav

elen

gth

(µm

)

Grating period (µm)

neff= 3.18

C. Gmachl et al. IEEE Journal QE 38, 569 ( 2002)

0.5 1.0 1.5 2.0

20

15

10

5

Wavelength (µm)

Pow

er (

log)

Pow

er (

linea

r)

Wavelength (µm)8.6 8.7 8.8 8.5 8.6 8.7

DFB and Tunable QCL

M.Kisin and S Luryi. Appl. Phys. Lett. 82, 847 ( 2002)

[110]

λac

[001]z

λ0 = 2 λac

Piezo-Acoustic Wave

( )αλπβ −Γ−= Gineff 2

2

DFB mechanisms:

• refractive index modulation;• optical gain modulation.

Tem

per

ature

(K)

Wavelength (µm)

T (

%)CO2 CO2H2O H2O

4 5 6 7 8 10 12 14 18

4.59 4.65 5.35 5.4 8.5 8.6 9.5 9.6 9.9510.05

16.2 16.22

100

200

300

0100

0

Page 18: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Applications Example: Environmental Monitoring

Mid IR spectrum is called molecular fingerprint region.

Two atmospheric transparency windows3-5 µm and 8-13 µm lack water-vaporabsorption and are particularly importantfor chemical-sensing applications.

Advantages of laser-based optical methodsin trace-gas analysis include:

• noninvasive character,• high sensitivity and selectivity,• real-time detection.

Other exemplary applications:

• combustion diagnostics in the power and automobile industries, medical diagnostics, • detection of explosives and drugs, chemical and biological weapons of mass destruction,• military countermeasures as blinding the IR sensor of a heat-seeking missile,• optical wireless communications in the eye-safe atmospheric transmission windows.

TIME

Carbon monoxide concentrations in ambient air(monitored at Rice University on March 2001, A. Kosterov et al.)

CARBO

N M

ON

OXID

E C

ON

CEN

TRATIO

N(p

arts

per

bill

ion in v

olu

me)

500

400

300

200

Mid

nig

ht

8 A

M

F. Capasso et al. Physics Today 55, 34 (May 2002)

Page 19: g = ΓG - (αoe/ESE519/lecture7qcl.pdfAdvantages of intersubband scheme: Lasing wavelength is no longer defined by E g and can be tuned by the QW width Hole transport is eliminated

Recommended Literature

• J. Faist et al. Science (Apr. 1994), v.264, p.553.• J. Faist et al. Nature (June 1997), v.387, p.777.• C. Gmachl et al. Nature (Feb. 2002), v.415, p.883.• M. Beck et al. Science (Jan. 2002), v.295, p.301.• F. Capasso et al. IEEE Journal on Selected Topics in Quantum Electronics (Nov. 2000),

v.6, p.931.• J. Faist et al. IEEE Journal on Quantum Electronics (June 2002), v.38, p.533. • F. Capasso et al. Physics Today (May 2002), v.55, p.34.