Semiconductor lasers: What's next?

1. Semiconductor lasers: What's next? Grigorii Sokolovskii Ioffe Institute, St Petersburg, Russia gs@mail.ioffe.ru

2. Outline Applications of semiconductor lasers. ‘Revolution’ of light. Basic principles of laser operation (only to remind). Absorption and gain in semiconductors, inversion of population and conditions for it's achievement. Rate equations. Lasing threshold. Laser efficiency. Modulation of the laser signal. Gain clamping. Fiber-optical applications. DFB and DBR lasers. VCSELs and VECSELS Waveguide in LD structure. Modes of the waveguide. Beam-propagation (‘beam-quality’) parameter M2 and how to measure it. Achieving maximum power density with LDs. Interference focusing of LD beams. What’s next? New applications and problems to solve.

3. Applications of LDs $7B → $1T • Telecoms • Data reading/recording • Laser printing • Pumping of the solid-state and fiber lasers • ‘Direct’ applications: cutting/drilling/welding • Biology and medicine

4. Applications of LDs • Telecoms • Data reading/recording • Laser printing • Pumping of the solid state and fiber lasers • ‘Direct’ applications: drilling/cutting etc • Biomedicine

5. Types of LDs 1. Surface and edge-emitting (e.g. VCSELs, VECSELs, GCSELs, etc and ‘striped’ LDs) 2. Broad and narrow area Broad: high power, poor beam quality;Narrow: low power, better beam quality

6. LDs for Mid-IR (1600-5000 nm) In Mid Infrared spectral range 1600-5000 nm lies strong absorption bands of such important gases and liquids as: CH4 , H2O, CO2, CO, C2H2, C2H4, C2H6, CH3Cl, OCS, HCl, HOCl, HBr, H2S, HCN, NH3 , NO2 , SO2 , glucose and many others. 80 mW/nm f=500 Hz LED20 70 LED22 60 LED18 50 Spectral density 40 30 20 10 0 1300 1500 1700 1900 2100 2300 2500 2700 Wavelength, nm

7. Optical tweezers Waveguide: light in matter ‘Inverse’ waveguide:matter in light – optical tweezers

8. Laser projectors Microvision SHOWWX Laser Pico Projector Samsung H03 Pico Pocket Sized LED Projector Aiptek PocketCinema T30 MacWorld 2010 Best of Show award

9. Basic principles of laser operation Gain + Feedback

10. Basic principles of laser operation Lasing threshold

11. E2 E2 E1 E1 Basic principles of laser operation Spontaneous and stimulated emission Spontaneous Absorption Stimulated

12. n E2 T E1 Basic principles of laser operation Gain: inversion of population ‘Negative’ temperature

13. E3 E3 E2 E2 E1 E1 E4 3- & 4-levelsystems

14. E3 Ec E2 Eg E1 Ev E4 Laser diodes: Some basics Vibronic laser Diode laser Ec – conduction band Ev – valence band Eg – energy gap

15. Laser diodes: Some basics QWs and QDs are the ‘artificial atoms’:)

16. Light generation and absorption in semiconductors ‘Golden’ rule of Quantum mechanics: Absorption probability: Radiation probability: The total radiation probability:

17. Light generation and absorption in semiconductors What is the condition for stimulated emission? Inversion of population:

18. Inversion of population Forward-biased p-n-junction is both the source for the inversion of population and for the name of the “laser diodes” http://britneyspears.ac/physics/fplasers/fplasers.htm

19. Stimulated and spontaneous emission rate: 3D T↑ r ρ(E) fce fvh Eg rsp ћω=E rst -ρ(E)

20. Density of states: Low-Dimensional vs 3D Bulk semiconductor Quantum Well Quantum Wire Quantum Dot

21. Evolution of the threshold current density: from 3D to Low-Dimensional

22. Density of states: Low-Dimensional vs 3D Bulk semiconductor~1 kA/cm2 Quantum Well~100 A/cm2 Quantum Wire??? Quantum Dot~10 A/cm2

23. QDs vs 3D rst QD Lower threshold 3D But: at low QD density threshold may NOT be reached E Lower temperature dependence of the laser parameters Narrow spectrum for IDENTICAL QDs But: broad spectrum for inhomogeneously broadened QDs

24. φc nc kx k0nf kxf nf f kz kxf ns φs Laser diodes: The waveguide Eg ↓ ↔ n↑

25. x x k0ns k0ns k0nc k0nc k0nf k0nf z z Modes of the waveguide

26. Composing Rate Equations The simplest model J is the average pump current density n is the average carrier concentration in the active region S is the average photon concentration (average intensity) Carriers, steady-state: Pump rate = Recombination rate Carriers, time-dependent:

27. Composing Rate Equations Photons, steady-state: Total radiation probability = Recombination rate Photons, time-dependent: Г - confinement factor β - spontaneous emission factor nt - transparency concentration A - linear gain coefficient τs - spontaneous recomb. time τp - photon lifetime

28. Lifetimes Spontaneous: Photon lifetime: τs ~ 1 ns if n ~ nt, then: L ↑ →τp ↑ αin ↑ → τp ↓ R ↑ → τp ↑ τp ~ 1 ps

29. τWE ES τE τWG GS τG Rate Equations for QD LD …where f is the filling factor

30. 0 Steady-state solution of the Rate Eqs

31. n nth nt β↑ Jth S β↑ J Jth Steady-state solution of the Rate Eqs J

32. Lasing Threshold R↑ R↑ R↑

33. Lasing Threshold

34. Threshold vs Temperature

35. Ec Ec Ec Ev Ev Ev Evv Laser efficiency Pumping efficiency Internal quantum efficiency free-carriers Auger

36. P R↑ ηd I Ith Laser efficiency differential efficiency:

37. 1/ηd L P U P η(1/2) rserial↑ rserial↑ I Ith I Ith Laser efficiency Measuring IQE: Lasing efficiency:

38. J Jth t S S t t Δt Turn-on delay ?

39. J Jth t S t Δt J S Jth t t Turn-on delay RZ modulation If J = 2Jth, τs= 1 ns: Δt = 0.7 ns Non-return-to-zero (NRZ) modulation

40. Δt Cτp J/Jth Turn-on delay of QD LDs Non-QD laser diode: QD LD:

41. Small-signal moduation

42. Small-signal moduation

43. Amplitude-freq. response of photons

44. Amplitude-freq. response of photons

45. Phase-frequency response of photons

46. Amplitude-freq. response of carriers

47. Amplitude-freq. response of carriers

48. Phase-frequency response of carriers

49. π/2 Phase-frequency response π/2 shift in the phase-frequency responses means the energy flow between the photons and carriers (similar to the kinetic and potential energy in pendulum) which is called ‘relaxation oscillations’.

50. n(t) S(t) Relaxation oscillations NB: no relaxation oscillations in QD lasers! Typical explanations: 1) QD LDs are too fast; 2) QD LDs are too slow