ece5343_2015sp_07a (1)

RuiQ.Yang,ECE,Univ.ofOklahoma 2/26/2015

1

References: F. Capasso, et al. JQE, 38, 511 (2002), J. Faist, et al, Chap. 1, Semicond. and Semimetals, 66, Academic (2000). Yang, Chap. 2, Long Wavelength Infrared Emitters based on Quantum Wells and Superlattices, Ed. M. Helm, 2000

Chapter7IntersubbandQuantumCascadeLasersandInterbandCascadeLasers

RuiQ.YangSpring,2015

University of OklahomaSchool of Electrical & Computer Engineering

ECE 5343 Quantum Structures and Devices

ObjectivesandOutlineObjectives: Understand the basic principle and operation of quantum cascade (QC) and interband cascade (IC) lasers Gain knowledge of QC & IC lasers and their applicationsOutline Short review of historic development of lasers Basic semiconductor laser physics and types Intraband quantum cascade (QC) lasers

Original idea and subsequent development Design considerations and device performance

Interband cascade lasers Basic concept and structure Single-mode DFB lasers and Applications Other aspects and recent progress


2

Outline Short review of historic development of lasers Basic semiconductor laser physics and types Intraband quantum cascade (QC) lasers



1917 - Albert Einstein developed the theoretical concept of light traveling in waves of particles (photons) and of Stimulated Emission which later become known as Light Amplification by the Stimulated Emission of Radiation (LASER)

Stimulated Emission

E2

E1hvhvhv

in out

A. Einstein, Phys. Z. 18, 121 (1917)


3

Charles Townes and his colleagues were the first to build a maser in 1954, which operated in the microwave frequency range. It was the precursor of the laser. In 1958 Townes and Arthur Schawlow of Bell Labs proposed a system that would work at infrared and optical wavelengths. Townes shared the 1964 Nobel Prize in physics for his work on masers and lasers

microwave amplification by stimulated emission of radiation (MASER)

= 24 GHz

First Maser (1954)

Townes published 25 papers on spectroscopy before this work

Theodore (Ted) H. Maimanthen at Hughes Aircraft facilities

First Laser (1960)

Rejected by PRL. accepted by Nature

=694.3 nm

pulsed

optical pumping

Cr3+ ions in Al2O3


4

First cw Laser (1960)

Ali Javan, William Bennett, and Donald Herriott (Bell Labs) adjust the helium-neon laser, the first laser to generate a continuous beam of light at 1.15 microns and the first of many electrical discharge pumped gas lasers.

The Laser - A Possibility in the 1930sIn the scientific world, they always say that when the time comes for an invention or a discovery to be made, if you don't do it, someone else will. To a large extent, that's true. But it's not always the case. People can miss a good idea. When it comes to the laser-my kind of laser, the Gas Laser-I'm convinced it could have been invented in the 1930s, not thirty years later in 1960 when I managed to do it.

Semiconductor Lasers (1962)

Robert N. Hall 1962 First GaAsP laser diode (Holonyak & Bevaqua)

Todays diode lasers


5

Izuo Hayashi and Morton Panish (Bell Labs) design the first semiconductor laser (GaAs-AlGaAs) that operates continuously at room temperature.

First CW RT Semiconductor Lasers (1970)

1917 - Concept of Stimulated Emission - Albert Einstein1954 First Maser - Charles Townes et al. (Columbia University)1958 - Laser Theory - Charles Townes & Arthur Shawlow (G. U. & Bell Labs)1960 - First Laser (Ruby laser) - Theodore Maiman (Hughes Aircraft)1960 - He-Ne Laser - Ali Javan (Bell Labs)1962 - Semiconductor Laser (pulsed, 77 K)- Robert Hall (GE Research Lab)1964 - CO2 Laser - Kumar Patel (Bell Labs)1964 - PbTe diode laser (pulsed, 6.5 m at 12 K) J. F. Butler, et al. (Lincoln Lab)1964 - Nd:YAG Laser - Guseic, Markos and Van Uiteit (Bell Labs)1964 - Argon Laser - William Bridges (Hughes Aircraft)1966 - Dye Laser - Sorokin and Lankard (IBM Watson Research Center)1970 - Excimer Laser - Nikolai Basov's Group (Lebedev Labs, Moscow)1972 - RT cw semiconductor laser - Izuo Hayashi & Morton Panish (Bell Labs)1976 - GaInAsP/InP DH laser diode at 1.2 m (cw, 300 K) (Lincoln Lab)1977 - Free Electron Laser - John M J Madey's Group (Stanford University)1980 - X-ray lasing action - Geoffrey Pert's Group (Hull University, UK)1994 - Quantum Cascade Laser (pulsed, 4.3 m 125K) J. Faist, et al. (Bell Labs)1994 - Concept of Interband Cascade (IC) Lasers - Rui Q. Yang (Univ. of Toronto)1997 - First demonstration of IC lasers (U. of Houston & Sandia National Labs)2002 - Quantum Cascade Laser (cw, RT) - Beck, Faist, et al. (University of Neuchtel)2008 - IC lasers (cw, 319K) (NRL), plamson-waveguide IC lasers near 6 m (OU)

Events in Laser Development


6




Lasers

At thermal equilibrium (Boltzmann statistics), the relative population

1//)(12 1212 kThkTEE ee

nn

LASER: Light Amplification by Stimulated Emission of Radiation

Two-level system

Spontaneous emission: occur randomlyStimulated emission: generated by a photon field. The emitted photon is in phase with the radiation field, in addition to hv12=E2-E1

Monochromatic precisely equal energyCoherent in phase, directional

for the two levels containing an equal number of available states


7

Einstein Coefficients and Their Relationship

Absorption = Spontaneous emission + Stimulated emission

Einstein coefficients

For steady state

B12 n1 (v12) = A21 n2 + B21 n2 (v12)

Photon energy densityNo photon density is required for a transition from an upper state to a lower state

Population Inversion

Ratio between stimulated emission rate and absorption rate

n2 > n1

At equilibrium, the stimulated emission rate st is very small

)()( 122121

22112221

221

AB

nAnB

nAst

To enhance st is to have a large photon filed density (v12), which can be realized with an optical resonant cavity

12

1221

1211212221

)()(

nn

BB

nBnB

abs

st

If st is dominant over absorption of photons, we must have more electrons in the upper level than in the lower level, which is unnatural. The conditionPopulation Inversion


8

Bernard-Duraffourg inversion condition

For a semiconductor at non-equilibrium with quasi Fermi-levels EFn and EFpPopulation Inversion in a Semiconductor

1/)(2 ]1[)( 2 kTEEc FneEf

1/)(1 ]1[)( 1 kTEEv FpeEf

)()()](1[ 1212 EfEfP vcabs )()](1)[( 1212 EfEPf vcst

0)()]()([ 1212 EfEfP vcabsstfc(E2 ) - fv(E1 ) > 0

p g(EFn EFp ) >(E2 E1) =hv12>Eg

Threshold ConditionAfter a round trip, light intensity is

If lasing, the light intensity does not change at thresholdgth = i + m

0 L zR2R1

Mirror 1 Mirror 2zgikz iee )( Lg ieRRII )(2210

Phase change is e2ikLk=2/, =0/n, n is refractive index, g is gain, i is the internal lossR1, R2 - the reflection coefficient

1)(221 Lg ieRR Threshold condition)1ln(2

121RRL

m -- Mirror lossIn phase condition cos(2kL)=1, sin(2kL)=0

2kL=m(2), m=1, 2, 3,

Total loss: = i + m


9

Modes in a laser cavityk =2n/0, L=m0/(2n)m =2nL/0, m=1, 2, 3,

resonant modes in a laser cavity 00200

22 d

dnLLnddm

Drawing from Andersons book

mddn

nnL

1

00

20

0 12

nL220

0

Adjacent mode space :

Example: =3 m, L=1 mm, n=3.4, # of modes m=2x3.4x103/3~2266~32/(2x3.4x103)~0.0013 m=1.3 nm . some modes are in gain curves

Basic Diode Laser Physicsg (cm-1)

I (A)Itr

Ith

g (cm-1) = a(I Itr )a: differential gain (cm-1/A)Itr: transparency current where population inversion occur

At threshold: gth (cm-1) = = i + ma (Ith Itr ) = i + m

)(1 imtrth aII threshold current increases with mirror loss, internal loss


10

Diode laser performance characteristicsP (W)

I (A)

Slope efficiency s=dP/dI (W/A)External quantum efficiency e: # of received photons per electron

e= s (W/A)/(hv/q)

im

mie

i: internal Q.E. which is # of emitted photons inside of the cavity per electron

Output power : P=s (I Ith)=e(I Ith)hv/qpower conversion efficiency (wall-plug efficiency):

im

mthip I

IIqVh

IVP

when I Can p increases with I forever?

Increases with I

Power (wall-plug) efficiency of diode lasers

M. Razeghi, IEEE J. ST QE, 15, 941, 2009

Operating voltage V = Vth + (I Ith)R, Vth: threshold voltage, R: differential resistance

eoVip IVP voltage efficiency V =Nhv/(eVth), N: no. of stages

the optical efficiency o=m/(m + w)

electrical efficiency ththth

ele VIIRII

1

1 It does not monolithically increases with Iele peaks at

RIVII thththpeak 2max 1 1 ththele VIR

If i, o, R are keep constant for any high current, i.e., without power saturation, the electrical efficiency, hence p, would peak at I = Ipeak.

ele/I=0


11

Temperature Dependence

2910 2920 2930 2940 2950 2960

Wavelength (nm)

150K 38mA

36mA

J430W1A20mx1.5mm

cw

3030 3040 3050 3060 3070 3080 3090

208K161mA

210K

163mA

example

P (W)

I (A)

T 1

Ith3Ith2Ith1

T 2 T 3

T 1


12

Maximum cw Temperature an example

80 100 120 140 160 180 200 220 240 260 280 300 320 340 36010

100

1000

4

5

6

7

8

9

10Th

resho

ld cu

rrent

dens

ity (A

/cm2 )

Temperature (K)

T0~51K

solid: cwopen: pulsed

Thres

hold

volta

ge (V

)

Rsth~5.4 Kcm2/kWNear Tmaxcw, Jthchanges more rapidly

Threshold currents and voltages for an IC laser at various temperatures

Maximum cw PowerOutput power : P=e( I Ith )hv/q

]})1(exp[{0

0 TVIRT

IIhv

qP pthhe

0]})1(exp[)1(1{00

0 TVIRT

TVRI

qhv

dIdP pthhpthe

)1(}])1({ln[)1(0max,

0000

pth

hcwhh

pthpthpeak VR

TTTTT

VRIT

VRTI

e is the external QE in the case of negligible heating and is nearly constant within a certain temperature range. By neglecting current dependence of p, one has

Neglect differences of V, p, at Th,maxcw

)1(}1])1({ln[)1(max,

0000

maxpth

hcwheh

pthpth

e

VRTT

qhv

TT

VRIT

VqRhvTP


13

Maximum cw Power an example

0 50 100 150 200 250 300 350 400 450 5000123456789

10

110K

80-86K

cw150mx1mm mesa stripe

J165X3E

Volta

ge (V

)

Current (mA)0

20

40

60

80

100

120

140

140K

Powe

r (mW/

facet)

3800 3820 3840

190mA150K

Wavelength (nm)

Current-voltage-light (I-V-L) characteristics of a 150-m-wide and 1-mm-long mesa-stripe laser in cw mode at several temperatures and its lasing spectrum (inset) at 150 K.

Thermal rollover

Various types of semiconductor lasers Homojunction laser (p-n junction)- developed first early 1960s Heterojunction lasers Single heterojunction Double-heterojunction

Carrier confinement

Optical waveguideSeparate confinement heterostructure QW lasers

Visible lasers (400-700nm) Storage (DVD) Laser pointers

Infrared lasers (>700nm) Optical link for fiber communication Pumping sources


14

Stepsformakingsemiconductorlasers

Vertical cavity surface-emitting lasers (VCSELs)

The output beam of an edge-emitting laser is elliptical and divergent

VCSEL

The output beam of an VCSEL is more circular

(Distributed Bragg Reflector stack of quarter wavelength alternating layers)


15




Light Emission based on Intersubband Transitions

1971: R. Kazarinov and R. Suris proposed using intersubband transitions in a biased superlattice for light amplification

R. F. Kazarinov, R.A. Suris, Sov. Phys. Semicond. 5, 707 (1971)


16

TwoApproaches

interwell photon-assisted tunneling transition (the optical transition is between quantum states in adjacent QWs

intrawell transition (optical transition is between states in the same QW) with resonant tunneling in superlattices

F. Capasso et al, JQE (1986)

PopulationInversionandThresholdUsing the rate equation and considering the steady state of two level system E2 and E1, the population inversion is

)( 121212

212

eJnn requiring 21>1 for inversion

J e n nth thin

( )

( )2 1

21 1

Threshold current density (by re-arranging the above Eq.)in=2/(2+21) is electron injection efficiency

21 is the relaxation time from level E2 to the lower level E1

g LT n n LT n nth s thR

sth

R ( ) ( )2 1 2 121 R=21/R is the radiative efficiency

J eLT

gth

s

th

PI R in PI =(21 - 1)/21 is population inversion efficiency

Jth does not explicitly depend on the individual time scales, but is inversely proportional to the efficiencies of three critical processes, in, R, & PI, which are ratios of these time scales.

L has the unit of length and is related to the emission wavelength, and Ts has the unit of time and depends on the inverse line-width of the spontaneous emission


17

DesirableconditionsTo minimizing the threshold current, it is desirable to have these time scales satisfy

2 >> 21 >> 1and R 21, such that in PI R 1. 21 is ~ps or shorter with LO phonons.

For interwell & intrawell intersubband transitions, radiative efficiency R is about the same, but in & PI can differ significantly in the two approaches. interwell transitions, 21 is longer, benefits the population inversion, higher PI, may lower injection efficiency inintrawell transitions, 21 is shorter, raises the injection efficiency in but lower the population inversionPI, it is preferable to make the lower energy state lifetime 1 much smaller than the relaxation time 21 and utilize the intrawell transition to achieve a low threshold current for intersubband lasing. making 1 small is challenging!

Variousearlystructuresviaresonanttunneling

P.-F. Yuh and K. L. Wang, Appl. Phys. Lett. 51, 1404 (1987); H. C. Liu, J. Appl. Phys. 63, 2856 (1988)S. I. Borenstain & J. Katz, Appl. Phys. Lett. 55, 654 (1989)A. Kastalsky, V. J. Goldman, J. H. Abeles, APL. 59, 2636 (1991)Q. Hu and S. Feng, Appl. Phys. Lett. 59, 2923 (1991).


18

Firstintersubbandluminescence

Resonant tunneling in a periodic superlatticeEmission observed in the Far-Infrared

M. Helm et al, Phys. Rev. Lett. 63, 74 (1989)

A. A. Andronov, Semicond. Sci. Technol. 7, B629 (1992)Phononmediatedpopulationinversion

Yang, Superlatt. & Mircrostruc. 17, 77 (1995);Chapter 2 in Helms book (2000)

Scattering is fast with LO phononsAcoustic phonon scattering is slower


19

FirstDemonstrationofQuantumCascadeLasers

1994: Bell LabsTmax= 125K (pulsed), Pmax = 10mW, = 4.26 m, Jth> 4 kA/cm2.

J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A.L. Hutchinson, A. Y. Cho, Science 264, 553 (1994)

25 cascade stages, ~500 layers on InP sub.

SomeFeatures Wavelength tailoring determined by layer thickness instead of band-gap Relatively insensitive to temperature: high T0 (>100K) due to parallel subbands and optical phonon limited lifetime Ultrafast: high speed modulation High quantum efficiency (>100%) due to cascade process Narrow linewidth (


20

NoteonCascadeConfigurations

An early work on cascade lasers (on GaAs) by Van der Ziel & Tsang [APL 41, 499 (1982)]

Explicitly pointed out that the quantum efficiency (QE) may be greater than unity

cascaded active regions are in different waveguides

e hvhv

hvhv

hv hvhv

Cascading

For both intersubband QC and interband cascade lasers, every cascade stage (active region) is in the same waveguide

Not only QE>1, but also (dg/dI) Nc

p

p

p

n

n

n

p+n+

p+n+

active layer

active layer

active layer

Quantum efficiency exceeding 100% is not new

Faists VG


21

Faists VG

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


22

Faists VG

Computation of Energy States


23

Faists VG

Faists VG


24

One can also use two-band and one-band models

RateEquationsandParameters


25

Faists VG

RateEquations

All the population is lumped together in k=0 state!

Gain cross section


26


27

Lifetime reduced with the transition energy close to the LO phonon energy

Population inversion exist only if 2


28

Faists VG


29

Justification for photon-assisted tunneling transition


30

Faists VG


31

Faists VG

Active region optimization


32

Faists VG

Faists VG


33

Faists VG

Faists VG


34

Faists VG

Schematic potential prole and design of the active region of a quantum cascade laser based on three quantum wells. a)Two quantum well before the application of the electric eld. b)The electric eld bring the two ground states in resonance and yields a splitting equal to the optical phonon energy. A third thinner well is added up stream. If the state of this well is resonant with the excited state of the coupled well, the resulting transition is diagonal (c), if the well is thinner and the resonance is above, the transition is vertical (d).


35

Faists VG

Faists VG


36

Faists VG

Faists VG


37

Faists VG

Faists VG


38

Faists VG

Faists VG


39

A. Lyakhet al., Pranalytica, Harvard, Adtech, Applied Physics Letters 92, 111110 (2008)

Fully Packaged CW/RT 4.6 m High Power QCLs

Pranalyticas PoyntIR laser system, consisting of a packaged uncooled QCL & driver electronics and it emits up to 1 W of average power at 4.6 m with an overall system efficiency of ~10%

Handheld IR illuminator that runs on batteries for over 2 hours at 1 W output power level at 4.6 m

Recent QCL Systems

ece5343_2015sp_07a (1)

Documents