Advanced semiconductor lasers Quantum cascade lasers Single mode lasers DFBs, VCSELs, etc.
Quantum cascade laser
Reminder: Semiconductor laser diodes Conventional semiconductor laser CB diode laser: material VB InSb
Intersubband transitions CB intersubband transitions E g VB AlGaAs GaAs AlGaAs Device applications: quantum cascade laser (QCL) and quantum well infrared photodetector (QWIP)
Quantum Cascade Laser CB V= Four-level laser E n n 2 2 2 * 2 2m L L = layer thickness layer thickness unipolar semiconductor laser using intersubband transitions
Schematic of charge transport in QCLs CB CB ph active region injector CB ph active region injector ph active region
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
QCL: compact, rugged light source Grown by Molecular Beam Epitaxy InGaAs/InAlAs lattice matched to InP
Semiconductor growth: Molecular Beam Epitaxy Prof. Manfra s GaN and GaAs MBE machines at Purdue Device fabrication at the Birck Nanotechnology Center
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
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
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
QCL operating modes Fabry Perot mode Single mode DFB Dual wavelength 8.0 8.2 Wavelength ( m) 4.96 5.00 5.04 Nonlinear light generation: second harmonic 200 100 Intensity (arb. units) no grating 4.92 4.96 5.00 5.04 7.36 7.40 7.44 7.48 Wavelength ( m) Power (arb. units, log. scale) 10 1 0.1 Ultra broadband a 2, 3, 4 A 5... 13 A Intensity (a.u.) laser 150 100 50 0 8.6 8.8 9.0 9.2 9.4 9.6 pump wavelength ( m) Intensity (a.u.) SH 50 0 4.3 4.4 4.5 4.6 4.7 4.8 second-harmonic ( m) 5 6 7 8 9 Wavelength ( m)
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
Single mode and tunable QC DFB lasers CO 2 H 2 O CO 2 4 5 6 7 8 10 12 14 18 H 2 O 0 100 T (%) CO NO CH 4 N 2 O CO 2 4.59 4.65 5.35 5.4 8.5 8.6 9.5 9.6 9.95 10.05 16.2 16.22 Wavelength ( m) NH 3 0 100 200 300 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, chemical and biological weapons, breath monitoring) Remote sensing: LIDAR Non invasive medical diagnosis Free space optical telecommunications Pranalytica s optical nose http://www.pranalytica.com/core technologies/gas sensors.php
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
Lasers Pump source Gain medium Optical resonator Four-level laser Reminder: lasers Population inversion Stimulated emission
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 2 3 1 32 2 4 ez32 0n ef L p 1 2 32
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, 8 10 18 cm -3 ninp, 10 17 cm -3 InP substrate electroplated Au n InGaAs, 3-5 10 16 cm -3 Waveguide core: Active regions and injectors 30-50 stages n InGaAs, 3-5 10 16 cm -3 In solder waveguide core ninp, 1-2 10 17 cm -3, substrate
Room temperature, continuouswave operation MBE or MOCVD InP overgrowth Metal electroplating Plated gold Laser core cw output power (mw) 40 30 20 10 200 K 220 K 240 K 260 K cw mode 280 K 300 K 320 K Voltage (V) 14 12 10 8 6 4 2 300 K pulsed 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 3.0 3.5 4.0 0 0 0 2 4 6 8 current density (ka/cm 2 ) current density (ka/cm 2 ) J. Chen, et.al., J. Vac. Sci. Tech. 25 (2007), 913.
Highlights of recent results Room temperature high power cw operation (M. Razeghi et al.)
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)
Other fun stuff: Monolithic integration of QCLs with resonant optical nonlinearities energy 5 I 4 4 3 g 3 2 1 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
Single mode lasers
Laser modes: longitudinal and transverse 4.0 QCL 2743 DR1-3 ridge B, 77K cw Longitudinal modes 3.5 3.0 Light intensity [a.u.] 2.5 2.0 1.5 1.0 0.5 400 ma 300 ma 200mA 150 ma 0 1230 1240 1250 1260 1270 1280 Transverse modes Wavenumber [cm -1 ]
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
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) =. 1999 S.O. Kasap, Optoelectronics (Prentice Hall) q=integer, B = Bragg wavelength of the mirror output
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. 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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.
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
Vertical cavity surface emitting lasers (VCSELs)
VCSEL
Edge emitting vs. surface emitting laser Ridge waveguide Laser Vertical Cavity Surface-Emitting Laser
/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). 1999 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 20-30 layers for mirror, MQW active region
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
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
Oxidized aperture VCSEL
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, 043906 (2007)
Light confinement in optical microresonators n=1.47 Maxwell s Eq. n=1 J. Wiersig, PRA 2003 MH and K. Richter, PRE 2002
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)
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
Defect-Mode Photonic Crystal Microlaser Photonic Crystal Cavity formed by a point defect O. Painter et. al., Science (1999)
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