The quantum cascade laser: a unifying concept for generating electromagnetic radiation from 3 to 300µm wavelength

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1 The quantum cascade laser: a unifying concept for generating electromagnetic radiation from 3 to 300µm wavelength C. Sirtori Matériaux et phénomènes quantiques, Université Paris 7 - Denis Diderot ALCATEL-THALES III-V lab, Palaiseau FRANCE

2 Collaborators S. Barbieri W. Maineault A. Vasanelli L. Sapienza S. Dhillon C. Ciuti S. Laurent P. Filloux C. Manquest D. Dolfi A. De Rossi M. Calligaro X. Marcadet I. Sagne, U. Gennser R. Colombelli, Y.Chassagneux J. Faist G. Scalari M Giovannini J. Alton H. Beere D. Ritchie

3 Outline Introduction on QC lasers Performances and applications in the 3 15 µm wavelength region THz lasers THz side band generation Phased match interaction between telecom frequencies and THz radiation High frequency modulation of a 3THz laser Conclusions

4 What does a QC laser look like?

5 Single Laser mounted on copper block Au wires In solder Laser 6 mm Copper block Laser Alumina µm

6 Quantum Cascade Laser, what dimensions? 1m 1mm 100µm 10µm 1µm 100nm 10nm 1nm 1Å 1 atome 3 Å 1 cheveu 50µm = m Semiconductor layers deposited by MBE

7 Photo TEM 8.3nm Courtesy of I. Sagnes, LPN

8 The materials

10 GaAs transistors and high power laser diode InP telecom devices

11 The quantum well: the building block 25 Å AlSb InAs AlSb E 2 E 12 The quantum well is the elementatary constituent of our system E 1 Croissance

12 Energy versus position in a quantum well structure Conduction band quantum well V(z) 2 Energy AlAs E 2 E 1 AlAs 1 GaAs Valence band quantum well Position z The confinement potential is only in the direction of growth (z) Electrons are free particles in the plane

13 Intersubband transition energies hν 1 Same material system Different quantum well widths Diffeferent transition energies hν 1 = 4 hν 2 hν 2 Intersubband transitions depend primarily on width of the quantum well and not on the materials constituent the heterostructure

14 T max of QC lasers vs. wavelength Pulsed operation Atmospheric windows III-V compounds phonon bands Temperature (K) InP based lasers Peltier GaAs based lasers LN 2 Sb based lasers Data (some are unpublished): Bell Labs Neuchatel THALES Walter Schottky (Munich) Northwestern University AOI (Houston) TU Vienna MIT University of Montpellier Wavelength (µm) Range of operation: µm or THz

15 Quantum design

16 Coupled-quantum-wells quasi molecules Quantum engineering of intersubband transitions is enabled by coupling of quantum wells through thin tunneling barriers: Symmetric coupled-quantum-wells Asymmetric coupled-quantum-wells A 1S 2 1 Formation of a symmetric and antisymmetric doublet Hybridisation of orbitals with different quantum numbers molecule of hydrogen

17 QC laser design Injection barrier 2 Minigap 1 Electron reservoir Injector 2 Minigap 2 Minigap 1 1 Active region 2 Minigap 1 Cascade Action: 1) N p photons per electron traversing the structure (N p 2) The total population inversion is distributed over all the period

18 Quantum cascade design and material TEM Micrograph Band diagram INJECTOR EMITTER x 25 "MINIGAP" "MINIBAND" 3 45 nm 2 1 E Courtesy of C. Gmachl Bell Labs, Lucent Tech.

19 Cascade Cascade: répétition d une période -> 1 électron peut générer plusieurs photons

20 Three well active region (λ = 4.3µm) Energy (mev) Z (Å) V = V th V = 0 Energy (mev) Z (Å)

21 Today s best performances in the mid-infrared (3-10µm)

CW operation of QC lasers (State-of-the-art) Threshold current density [ka/cm 2 ] 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 10.5 380K 400 Voltage [V] 9.0 7.5 6.0 4.

22 CW operation of QC lasers (State-of-the-art) Threshold current density [ka/cm 2 ] K 400 Voltage [V] K 340K 320K 300K 280K 260K 240K 240K Power [mw] re-grown Fe:InP Plated Au InP cladding Active core InP cladding n + InGaAs layer re-grown Fe:InP K Current [A] n + InP substrate 2 µm Burried heterostructures for optiumum power dissipation L. Dhiel et al. APL (2006) Division of Engineering and Applied Sciences Harvard University

Single mode QCLs for spectroscopy applications Wavelength (µm) 5,47

) 100 300 K 10 1 0,1 QCL DFB 616-12-6 0,01 1828 1830 1832 1834 1836

wavelength via the top grating design (advanced modelling available)

23 Single mode QCLs for spectroscopy applications Wavelength (µm) 5,47 5,46 5,45 5,44 5,43 5,42 Intensity (a.u.) K ,1 QCL DFB , Photon Energy (cm -1 ) Control of the wavelength via the top grating design (advanced modelling available) Control of the chirp vs time via the AR design and injector doping level Full 2" processing for the realisation of DFB QCLs

24 DFB QCL product characteristics Intensity (a.u.) 1 0,1 0,01 1E-3 1E K Wavelength (µm) 7,9 7,8 1E Photon Energy (cm -1 ) Wavelength range covered : from 4 to 10 µm Control of the emission wavelength : +/- 1 cm -1 SMSR higher than to 30 db Peak power higher than 200 mw Average power higher than 10 mw Voltage (V) ns, 500kHz 300K Intensity (a.u.) Current density (ka/cm 2 ) 1 DFB QCLs E-3 Wavelength (µm) Optical peak power (mw) xavier.marcadet@3-5lab.fr 1E Photon Energy (cm -1 )

71 ka/cm 2 ) / @30C: I th =130mA (j th ~ 1.

25 Single-mode CW-RT operation up to >50C at 5.23µm I th =75mA (j th ~ 0.71 ka/cm 2 ) I th =130mA (j th ~ 1.24 ka/cm 2 ) P el, < 1W -> very low consumption

26 Applications QC lasers are a mid infrared laser technology based on III-V semiconductor compounds such as GaAs and InP Spectroscopic applications (Gas, molecular detection) Output power = 10mW, CW operation, control on the linewidth Medical Environmental Security (Explosive detection) Optical countermeasure (High power devices) Output power > 1W, CW operation non strictly necessary 3-4µm for missile out-steering 8-10µm for night vision blinding

27 THz generation (70-350µm)

28 Active region design: 2THz QCL Injection well Injection barrier Al 0.1 Ga.0.9 As/GaAs Miniband Barbieri et al., APL, vol. 85, 1674 (2004) Miniband 16 mev Miniband Miniband 8meV 14 mev Length of a period (1276Å) Al concentration 10% (all previous structure 15%) 1 2 Reduce the roughness scattering Increase the barrier thickness

29 Device Processing

30 Performances of a 3 THz QC laser T max = 96K(pulsed), 70K(cw) Voltage (V) 0,0 0,5 1,0 1,5 2, Pulsed 3 x 0.2mm 2 device Current (A) Current density(a/cm 2 ) 4K 20K 40K 60K 70K 77K 80K 90K 95K Peak power (mw) % peak wall-plug efficiency at 4K 0.4% wall-plug efficiency in CW

20 25 30 35 40 45 50 55 3.0 2.5 Voltage (V) 2.0 1.5 1.0 0.

31 THz QC µ-disk lasers I. Microcavites THz seuils ultra faibles (5mA) 40µm Current (ma) Voltage (V) K 26K 42K 57K 70K Output power (a.u.) Y. Chassagneux et al., APL (2007) Current Density (A/cm²)

33 THz transfer on an optical carrier

34 THz side band generation at telecom frequencies Top metal contact ω 1 - Ω 3 ω 1 ω 1 + Ω 3 ω 1 Ω 3 THz QCL active region NIR guiding layer Bottom doped layer/ metal contact THz mode Top contact layer NIR guiding layer NIR mode Bottom contact layer

THz transfer on an optical carrier ω sideband = ω telecom - Ω THz ω telecom THz QCL Ω THz THz inscribed onto NIR Sideband leaving QCL NIR beam entering QCL A telecom beam (ω telecom ) is coupled into

35 THz transfer on an optical carrier ω sideband = ω telecom - Ω THz ω telecom THz QCL Ω THz THz inscribed onto NIR Sideband leaving QCL NIR beam entering QCL A telecom beam (ω telecom ) is coupled into a THz QCL emitting at Ω THz (2.8THz). The interaction of these frequencies with the laser material generates THz sidebands (ω sideband ) with the phase and amplitude of the THz beam inscribed onto this NIR carrier.

36 THz side band 1.3µm and 1.5µm Power (W) 1E-7 1E-8 1E-9 1E-10 Frequency (THz) ω 1 2.8THz 2.8THz ω 1 + Ω 3 ω 1 - Ω 3 Power (W) 1E-4 1E-5 1E-6 1E-7 1E-8 Frequency (THz) ω 1 2.8THz 2.8THz ω 1 + Ω 3 ω 1 - Ω 3 1E-11 1E-9 1E Wavelength (nm) 1E-10 Typical NIR input power 100µW 1mW Wavelength (nm) By injecting 100mW of power ~ 1µW on the sidebands S. Dhillon et al. Nature Photonics (2007)

37 High frequency modulation

38 THz + GHz sidebands on a telecom carrier Experimental setup dc bias RF Optical Spectrum Analyzer GHz sidebands at 9GHz Diode laser 38 THz QCL From centimetres to micrometers RF generator l > 1.5cm 0 < f < 20GHz THz QCL λ = 103mm W = 2.9THz Diode laser λ = 1571nm f = THz Power (µw) E-3 Frequency (THz) (b) 2.9THz E Intensity (arb. u.) GHz Wavelength (nm) Frequency (THz)

39 Continuous sideband tuning up to ~13GHz P RF = 20dBm 12.8GHz Intensity (arb. units) Intensity (arb. units) f RF = 5GHz Frequency (THz) f RF = 12.5GHz Intensity (arb. units) 12.6GHz 12.5GHz 12.3GHz 10GHz 9GHz 8GHz 7GHz 6GHz 5GHz 4GHz 2GHz Modulation off Frequency (THz) Frequency (THz)

40 Resonance effect at the round-trip frequency Number of sidebands increases at f roudtrip and f roudtrip /2 Signature of mode-locking f RT /2 = 6.05GHz f RT = 12.3GHz Round trip measured Independently on 3mm multi mode device. f Rtrip = GHz # of sidebands Frequency (GHz)

41 Conclusions Direct THz generation Quantum cascade lasers have been demonstrated down to 0.85THz Tens of mw of THz power are produced in cw up to 77K Using microcavity current thresholds of few ma have been demonstrated Nonlinear THz generation Frequency mixing is a phase-matched process in III-V compounds THz on fibre High frequency modulation of a THz laser THz applications (2 4THz) Imaging (medical and security) Local oscillators (Astronomy)

42 THz and NIR guided modes Doped Layers Al 0.02 Ga 0.98 As GaAs Al 0.02 Ga 0.98 As Semi-insulating GaAs substrate Phase matching between Optical intensity (80µm) FIR QC active region NIR guide Semi-insulating GaAs substrate NIR mode; λ = 1.3µm; α = 0.46cm -1 FIR mode; λ = 80µm; α = 7.15cm Optical intentsity (1.3µm) fundamental modes! Very high overlap factor Distance (µm)

43 Refractive index of GaAs Refractive Index Bulk GaAs Reststrahlenband Index 3.6 Phase matching is possible between two near-infrared beams and a far-infrared beam We propose to obtain the phase matching by using waveguide for THz quantum cascade lasers Wavelength (µm)

44 Polaritonic phase matching 3.9 g Vienna data GaAs bulk Refractive index Reststrahlen band Wavelngth (µm) V. Berger, C. Sirtori; Semicond. Sci. Technol. 19, 964 (2004)

45 Parametric down conversion using QC n 1 dn( ω) dω + 1 ω 1 ( ω Ω ) 3.9 g Vienna data GaAs bulk Refractive index Reststrahlen band Wavelngth (µm) V. Berger, C. Sirtori; Semicond. Sci. Technol. 19, 964 (2004)

46 Towards Phase 1.55µm Engineering of the modal refractive index by increasing the overlap of the mode with surrounding air 3.9 GaAs bulk Refractive index Reststrahlen band 50µm Wavelngth (µm) Use of erbium amplifiers!!

47 2.0 THz QCL: performance back coated 3.15 x 0.25 mm 2 device: T max = 77K(pulsed); 47K(cw) Current (A) 0,0 0,5 1,0 1,5 Current (A) 0,0 0,4 0,8 1,2 Voltage (V) 1,8 0,9 4K 20K 30K 40K 50K 60K 70K 77K 0, Current density (A/cm 2 ) Peak Power (mw) Power (mw) Intensity (arb. units) A T = 4K 1.35 A 0.5 Frequency (THz) Frequency (cm -1 ) 4K 20K 30K 5 35K 40K 45K 47K Current density (A/cm 2 ) C. Worrall, et al. Optics Express (2006)

48 Double-metal waveguide B. S. Williams et al., Appl. Phys. Lett, vol. 83, 5143 (2003) 0.10 Mode intensity (arb. units) metal layer active region metal layer Γ = 100% α = 13 cm -1 AR n + host-substrate 0.00 Top metal contact 0 20 Distance (µm) Bottom metal contact Advantage: Overlap factor 100%, independent from λ and doping; strong lateral confinement Drawback: Low out-coupling (R > 0.9); More difficult fabrication

49 Waveguide engineering to µm The overlap with the air surrounding the ridge decreases the n modal Efficiency (a.u.) Single Double metal Plasmon 170µm µm Phase matched Wavelength (µm) Pump Wavelength Wavelength (nm) (µm) Ridge Size Width (µm) (µm) Phase matched point with NIR shifts to longer wavelengths 1.5µm

50 Tuning the modal phase matching between 1.3 to 1.5µm Phase matching curve for 4 different device widths Efficiency (10-5 ) µm 42µm 52µm 150µm Wavelength (nm)

51 Nonlinearity from bulk GaAs χ (2) is provided by the GaAs xyz P x (ω 1 +Ω 3 ) = χ (2) E y (Ω 3 )E z (ω 1 ) xyz Angle between polarisations ( ) ω 1 TE polarised Ω 3 TM polarised ω 1 TE polarised Ω 3 TE polarised ω 1 + Ω 3 TE polarised No generation

52 Fabry-Perot fringes Efficiency (x 10-5 ) 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 Efficiency (x 10-5 ) 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0, Wavelength (nm) 0, Wavelength (nm)

53 0,0 0,2 0,4 0,6 0,8 1,0 Wavelength (µm) 1315,0 1315,1 1315,2 Fabry Perot Fringes 1,0 0,8 0,6 0,4 0,2 0,0 (x Efficiency ) 10-5 Efficiency (x 10-5 ) 1590,0 1590,3 1590,6 Wavelength (nm)

54 Efficiency (x 10-5 ) Group refractive index and losses Group refractive index 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 Wavelength (nm) ,0 0,8 0,6 0,4 0,2 0,0 1315,0 1315,1 1315,2 Wavelength (µm) Efficiency (x 10-5 ) n=3.54 n= ,0 0,8 0,6 0,4 0,2 Wavelength (nm) 0,0 1590,0 1590,3 1590,6 Wavelength (nm) At 1.3µm α ~ 1cm -1 At 1.55µm α = 3.5 cm -1

55 Telecom applications Fibre transmission of signal from a THz laser Modulated THz will be ascribe on the NIR beam Optical fibre Ω (t) 1550 nm filter Detector

56 THz up conversion Wavelength (µm) Intensity (pw) ω 1 - Ω meV ω meV ω 1 + Ω 3 1E-7 1E-8 1E-9 1E-10 1E-11 Power (W) Wavelength (nm) 1E Energy (mev) Better resolution than common FTIR

57 Non-linear efficiency η = 2 x 10-5 η 1 x 10-4 Experiment Theory η = P( ω1 + Ω P( ω ) 1 3 ) = 2 P( Ω 3 ) Z 3 0 ( ω1 n 1 Ω n n ) 2 2 d L A eff 2 L = interaction length This parameter can be easily increased of a factor hundred by changing the reflectivity of the facets (R ~ 0.9) BUT THE OPTICAL LOSSES limit us to 1/α, thus < 3mm

58 ω 3 = ω 2 +Ω 1 Quantum cascade OPO QC laser just below threshold ω 2 Ω 1 What is for? If the Idler (Ω 1 ) is contained in the gain curve of the QC laser then its losses can be vanishing small The threshold can be reached by only overcoming the mirror losses Interplay between linear and non-linear gain Tunability Phase locking of between µ-wave and telecom

59 Conclusions THz side band generation Transfer of a THz wave on an optical carrier The process is phase matched The phase matched from 1.3 to 1.6µm can be tuned by waveguide engineering The process can be used to generate THz (Frequency mixing) OPO?

60 Conclusions Direct THz generation Quantum cascade lasers have been demonstrated down to 1.9THz Tens of mw of THz power are produced in cw up to 77K Using double metal structures ma current threshold are demonstrated Nonlinear THz generation Difference frequency generation is phase-matched in III-V THz waves on fibre Injecting THz pulses into a QC THz laser Amplification of THz pulses Rep-rate multiplier

High power and single frequency quantum. cascade lasers for gas sensing. Stéphane Blaser

High power and single frequency quantum cascade lasers for gas sensing Stéphane Blaser Alpes Lasers: Yargo Bonetti Lubos Hvozdara Antoine Muller University of Neuchâtel: Marcella Giovannini Nicolas Hoyler