Advanced semiconductor lasers

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1 Advanced semiconductor lasers Quantum cascade lasers Single mode lasers DFBs, VCSELs, etc.

2 Quantum cascade laser

3 Reminder: Semiconductor laser diodes Conventional semiconductor laser CB diode laser: material VB InSb

4 Intersubband transitions CB intersubband transitions E g VB AlGaAs GaAs AlGaAs Device applications: quantum cascade laser (QCL) and quantum well infrared photodetector (QWIP)

5 Quantum Cascade Laser CB V= Four-level laser E n n * 2 2m L L = layer thickness layer thickness unipolar semiconductor laser using intersubband transitions

6 Schematic of charge transport in QCLs CB CB ph active region injector CB ph active region injector ph active region

7 Quantum cascade lasers: mid infrared InGaAs/InAlAs lattice matched to InP light sources 3 I top 3 e I bott 2 1 e I top 1 2 injector active region injector I bott active region Band engineering wavelength agility: InP range 5 20 m

8 QCL: compact, rugged light source Grown by Molecular Beam Epitaxy InGaAs/InAlAs lattice matched to InP

9 Semiconductor growth: Molecular Beam Epitaxy Prof. Manfra s GaN and GaAs MBE machines at Purdue Device fabrication at the Birck Nanotechnology Center

10 What makes the QC laser special? Wavelength agility: layer thicknesses determine emission wavelength High optical power: cascading re uses electrons Fabry Perot, single mode (DFB), or multi wavelength (dual wavelength, ultrabroadband) Temperature tunable Ultra fast carrier dynamics: no relaxation oscillations Active research field in semiconductor physics

11 What makes the QC laser special? Wavelength agility: layer thicknesses determine emission wavelength High optical power: cascading re uses electrons Fabry Perot, single mode (DFB), or multi wavelength (dual wavelength, ultrabroadband) Temperature tunable Ultra fast carrier dynamics: no relaxation oscillations Active research field in semiconductor physics

12 What makes the QC laser special? Wavelength agility: layer thicknesses determine emission wavelength High optical power: cascading re uses electrons Fabry Perot, single mode (DFB), or multi wavelength (dual wavelength, ultrabroadband) Temperature tunable Ultra fast carrier dynamics: no relaxation oscillations Active research field in semiconductor physics

13 QCL operating modes Fabry Perot mode Single mode DFB Dual wavelength Wavelength ( m) Nonlinear light generation: second harmonic Intensity (arb. units) no grating Wavelength ( m) Power (arb. units, log. scale) Ultra broadband a 2, 3, 4 A A Intensity (a.u.) laser pump wavelength ( m) Intensity (a.u.) SH second-harmonic ( m) Wavelength ( m)

14 What makes the QC laser special? Wavelength agility: layer thicknesses determine emission wavelength High optical power: cascading re uses electrons Fabry Perot, single mode (DFB), or multiwavelength (dual wavelength, ultrabroadband) Temperature tunable Ultra fast carrier dynamics: no relaxation oscillations Active research field in semiconductor physics

15 Single mode and tunable QC DFB lasers CO 2 H 2 O CO H 2 O T (%) CO NO CH 4 N 2 O CO Wavelength ( m) NH Temperature (K)

QCLs are ideal for sensing applications In situ trace gas sensing: NO, CO, NH 3, CH 4, H 2 O (isotopes), and more complex molecules ppm to ppb levels Chemical and biological sensing (air quality,

16 QCLs are ideal for sensing applications In situ trace gas sensing: NO, CO, NH 3, CH 4, H 2 O (isotopes), and more complex molecules ppm to ppb levels Chemical and biological sensing (air quality, chemical and biological weapons, breath monitoring) Remote sensing: LIDAR Non invasive medical diagnosis Free space optical telecommunications Pranalytica s optical nose technologies/gas sensors.php

17 What makes the QC laser special? Wavelength agility: layer thicknesses determine emission wavelength High optical power: cascading re uses electrons Fabry Perot, single mode (DFB), or multiwavelength (dual wavelength, ultrabroadband) Temperature tunable Ultra fast carrier dynamics: no relaxation oscillations Active research field in semiconductor physics

18 Lasers Pump source Gain medium Optical resonator Four-level laser Reminder: lasers Population inversion Stimulated emission

19 Gain medium 3 I top 3 e I bott 2 1 e I top I bott injector active region 1 2 injector J th w g g active region m ez32 0n ef L p

QCL research directions Design of high gain active region IB Understanding mid infrared waveguide losses e 3 4 IB injector 2 1 e Growth of high purity materials active injector Heat extraction from

20 QCL research directions Design of high gain active region IB Understanding mid infrared waveguide losses e 3 4 IB injector 2 1 e Growth of high purity materials active injector Heat extraction from active region Ti/Au top contact ninp, cm -3 ninp, cm -3 InP substrate electroplated Au n InGaAs, cm -3 Waveguide core: Active regions and injectors stages n InGaAs, cm -3 In solder waveguide core ninp, cm -3, substrate

mode 220 K 240 K 260 K 280 K 300 K 320 K 450 400 350 300 250 200 150 100 50 Peak output power (mw) 0 1.0 1.5 2.0 2.5 3.

21 Room temperature, continuouswave operation MBE or MOCVD InP overgrowth Metal electroplating Plated gold Laser core cw output power (mw) K 220 K 240 K 260 K cw mode 280 K 300 K 320 K Voltage (V) K pulsed mode 220 K 240 K 260 K 280 K 300 K 320 K Peak output power (mw) current density (ka/cm 2 ) current density (ka/cm 2 ) J. Chen, et.al., J. Vac. Sci. Tech. 25 (2007), 913.

22 Highlights of recent results Room temperature high power cw operation (M. Razeghi et al.)

23 Highlights of recent results Terahertz QCLs Highest operating temperature ~ 175 K in pulsed regime Narrow tunability Q. Hu (MIT), F. Capasso (Harvard), J. Faist (ETH), A. Tredicucci (Pisa)

24 Other fun stuff: Monolithic integration of QCLs with resonant optical nonlinearities energy 5 I g z 2 1 active region 5 I. II. (2) ~ 10 5 pm/v Difference frequency generation in QCLs cladding ω q 3 Laser1 section Side contact layer Laser 2 section ω p 2 P (2) * ( p q EpEq substrate 1 M. Belkin, F. Capasso, A. Belyanin et al. Nature photonics 1, 288 (2007). M. Belkin, F. Xie et al., 2008

25 Single mode lasers

0 Light intensity [a.u.] 2.5 2.0 1.5 1.0 0.

26 Laser modes: longitudinal and transverse 4.0 QCL 2743 DR1-3 ridge B, 77K cw Longitudinal modes Light intensity [a.u.] ma 300 ma 200mA 150 ma Transverse modes Wavenumber [cm -1 ]

27 Single Mode Laser Single mode laser is mostly based on the index guided structure that supports only the fundamental transverse mode and the fundamental longitudinal mode. In order to make single mode laser we have four options: 1 Reducing the length of the cavity to the point where the frequency separation of the adjacent modes is larger than the laser transition line width. This is hard to handle for fabrication and results in low output power. 2 Vertical Cavity Surface Emitting laser (VCSEL) 3 Structures with built in frequency selective grating

28 Single Frequency Semiconductor Lasers: Distributed Bragg reflector (DBR) laser Frequency selective dielectric mirrors a cleaved surfaces. Only allow a single mode to exist Periodic corrugated structure that interfere constructively when the wavelength corresponds to twice the corrugation periodicity (Bragg wavelengths) Distributed Bragg reflector A B q( B /2n) = (a) Active layer Corrugated dielectric structure (b) (a) Distributed Bragg reflection (DBR) laser principle. (b) Partially reflected waves at the corrugations can only constitute a reflected wave when the wavelength satisfies the Bragg condition. Reflected waves A and B interfere constructive when q( B /2n) = S.O. Kasap, Optoelectronics (Prentice Hall) q=integer, B = Bragg wavelength of the mirror output

29 Single Frequency Semiconductor Lasers: Distributed Feedback (DFB) laser The corrugated layer, called the guiding layer, is now next to the active layer In the DFB structure traveling wave are reflected partially and periodically as they propagate. 2 2 B m 1/ 2 B B Corrugated grating n q Ideal lasing emission Optical power 2nL Guiding layer Active layer 0.1 nm (a) (b) B (c) (nm) (a) Distributed feedback (DFB) laser structure. (b) Ideal lasing emission output. (c) Typical output spectrum from a DFB laser S.O. Kasap, Optoelectronics (Prentice Hall)

30 DFB (Distributed Feed Back) Lasers In DFB lasers, the optical resonator structure is due to the incorporation of Bragg grating or periodic variations of the refractive index into multilayer structure along the length of the diode.

32 Thermal Properties of DFB Lasers Light output and slope efficiency decrease at high temperature Agrawal & Dutta 1986 Wavelength shifts with temperature The good: Lasers can be temperature tuned for WDM systems The bad: lasers must be temperature controlled, a problem for integration

33 Vertical cavity surface emitting lasers (VCSELs)

34 VCSEL

35 Edge emitting vs. surface emitting laser Ridge waveguide Laser Vertical Cavity Surface-Emitting Laser

36 /4n 2 /4n 1 Surface emission Contact Dielectric mirror Active layer Dielectric mirror Substrate Contact A simplified schematic illustration of a vertical cavity surface emitting laser (VCSEL) S.O. Kasap, Optoelectronics (Prentice Hall) Optical cavity axis along the direction of current flow rather than perpendicular to current flow Radiation emerges from the surface of the cavity rather than from its edge Reflectors at the edges of the cavity are dielectric mirrors layers for mirror, MQW active region

37 Edge-emitting laser VCSEL Large distance between cavity modes: single-mode laser Circular beam shape Low threshold and power consumption 2D laser arrays Wafer-scale testing Ultrafast modulation

too resistive as a result Current spreading, many

38 For long wavelength laser based on InGaAsP/InP: index contrast is too low, need too many layers, the device is too resistive as a result Current spreading, many transverse modes -> need confinement for current and for the EM field

40 Oxidized aperture VCSEL

41 Other advanced optical cavities Photonic crystal lasers Microlasers: microdisk, micro pillar, etc. semiconductor heterostructures (n=3.3) 5 µm (McCall et al., 1992) Phys. Rev. Lett. 98, (2007)

42 Light confinement in optical microresonators n=1.47 Maxwell s Eq. n=1 J. Wiersig, PRA 2003 MH and K. Richter, PRE 2002

, 1998) resonator geometry mode characteristics active (lasing) material

43 QCL Microlaser Bow-tie (Gmachl et al.,1998) Quantum Cascade Laser (Faist, Capasso et al., 1994; Sirtori et al., 1998) resonator geometry mode characteristics active (lasing) material amplification small amplification devices defined direction changes upon coupling lasing varnishes (thresholdless)

for antennas micro resonators and waveguides Active devices low threshold nonlinear devices microlasers and

44 Photonic Crystals: Opportunities Photonic Crystals complex dielectric environment that controls the flow of radiation designer vacuum for the emission and absorption of radiation Passive devices dielectric mirrors for antennas micro resonators and waveguides Active devices low threshold nonlinear devices microlasers and amplifiers efficient thermal sources of light Integrated optics controlled miniaturisation pulse sculpturing

45 Defect-Mode Photonic Crystal Microlaser Photonic Crystal Cavity formed by a point defect O. Painter et. al., Science (1999)

46 Photonic Crystal Applications:PBG Laser The smallest defect mode laser is shown ( Axel Scherer, California Institute of Technology). Periodic air holes in high index material forms a 2D photonic crystal. The center air hole is removed and forms a resonant cavity. Light is confined in the cavity. Spontaneous emission in the band gap is prohibitted, but for the defect mode is enhanced. This produces a microlaser with very low threshold. PBG Defect Laser

Introduction Fundamentals of laser Types of lasers Semiconductor lasers

ECE 5368 Introduction Fundamentals of laser Types of lasers Semiconductor lasers Introduction Fundamentals of laser Types of lasers Semiconductor lasers How many types of lasers? Many many depending on