Quantum Dots in Photonic Structures

1. Lecture 14: Implemenatations, perspeectives Quantum Dots in PhotonicStructures Wednesdays, 17.00, SDT Jan Suffczyński Projekt Fizyka Plus nr POKL.04.01.02-00-034/11 współfinansowany przez Unię Europejską ze środków Europejskiego Funduszu Społecznego w ramach Programu Operacyjnego Kapitał Ludzki

2. Plan for today Reminder 2. QD lasers 3. Other…

3. The source of polarizationentangledphotons H Linearpolarizer V H V

4. Biexciton Exciton Emptydot Enangledphotons from a QD The method: biexciton – excitoncascade Obstacle: anisotropy The energycarries the information on the polarization of the photon

5. Biexciton Exciton Emptydot Entangledphotons from a QD The method: biexciton – excitoncascade Anobstacle: anisotropy The energycarries the information on the polarization of the photon (in circularpolarizationbasis:)

6. Fine structure of neutral exciton ( + )/ X δ1~0.1meV ( – )/ X Anisotropic exchange δ0~1meV Isotropic exchange ( + )/ δ2 ≈0 Xdark ( – )/

7. START STOP X XX time 0 Entanglementtest STOP (H) START (H) • XX-X cascade

8. Influenceof the in-plane electricfieldon the photoluminescence of individualQDs Kowalik et al., APL’2005 InAs/GaAs Quantum Dots

9. Evolutionof the anisotropy exchange splittingwith theapplied voltage Kowalik et al., APL’2005

10. experiment B=0 m -PL 1.8904 1.891 Energy [eV] 0.18 0.14 AES [meV] 0.10 0 2 4 6 Magnetic field [T] [meV] 0.45 p AngleJ-2f [rad] 2 0 0 0 2 4 6 8 10 Magnetic Field [T] Influence of the in-plane magneticfieldon the photoluminescence of individualQDs • model Increaseordecrease of the anisotropysplitting, depending on the magnetic field direction K. Kowalik et al., PRB 2007

12. QD in a pillarmolecule: anultrabrightsource of entangledphotons

13. QD as an entangled photons source The idea: obtain polarization entangled photon pairs from biexciton-exciton cascade Main obstacle: anisostropy of the QD  exciton level splitting Hindrance: low collection efficiency (a few %) Energy XX X Ground state • The solution: coupling of the X and XX to the modes of the photonic molecule • When exciton level homogeneous linewidth larger than exciton anisotropy splitting: polarization entangled photons emitted in XX-X cascade •  Increased extraction efficiency due to photon funneling into cavity mode

14. Pillar molecules R Distance PL Intensity (arb. units) 1,315 1,320 1,325 1,330 1,335 Energy (eV) Photon Energy (meV) Electronic lithography Radius Distance

15. Experimental realization • Purcell effectevidenced on X and XX transitions •  The proof of entanglement: • polarizationresolvedsecond order XX-X crosscorrelations A. Dousse, at al. Nature 2010

16. Characterization of the source - entanglement Density matrix of the two-photon state  67 % degree of entanglement  Entanglement criteria fullfilled

17. Quantum Dot Lasers

18. A laser – basic characteristics mirror mirror cavity

19. A laser – basic characteristics Active material mirror mirror cavity

20. A laser – basic characteristics pumping Active material emission mirror mirror cavity

21. A laser – basic characteristics Components of a laser • An energy pump source • An active medium to create population inversion by pumping mechanism: • - photons at some site stimulate emission at other sites while traveling • Two reflectors: • to reflect the light in phase • multipass amplification

23. Potential Advantages for Quantum Dot Semiconductor Lasers • Wavelength of light determined by the energy levels not by bandgap energy: • improved performance & increased flexibility to adjust the wavelength

24. Potential Advantages for Quantum Dot Semiconductor Lasers • Wavelength of light determined by the energy levels not by bandgap energy: • improved performance & increased flexibility to adjust the wavelength

25. Potential Advantages for Quantum Dot Semiconductor Lasers • Wavelength of light determined by the energy levels not by bandgap energy: • improved performance & increased flexibility to adjust the wavelength • Small volume: • low power high frequency operation • large modulation bandwidth • small dynamic chirp • small linewidth enhancement factor • Superior temperature stability of I threshold I threshold (T) = I threshold (T ref).exp ((T-(T ref))/ (T 0)) • High T 0 decoupling electron-phonon interaction by increasing the intersubband separation. • Undiminished room-temperature performance without external thermal stabilization

26. QDs as anactive medium in lasers:the firsttheoreticalpredictions Extremelylowcurrenttreshhold Increasedgain M. Asadaet al., IEEE J. Quantum Electron. 22, 1915 (1986). Y. Arakawaet al., Appl. Phys. Lett. 40, 939 (1982).

28. Increased maximummaterial gain

29. Potential Advantages for Quantum Dot Semiconductor Lasers • Lower Threshold • HigherModulationSpeed • SmallerLinewidth • Less TemperatureSensitivity • ReducedAugerRecombination → Mid-Infrared Semiconductor Lasers

30. Q. Dot Laser vs. Q. Well Laser In order for QD lasers compete with QW lasers: • A large array of QDs since their active volume is small • An array with a narrow size distribution has to be produced to reduce inhomogeneous broadening • Array has to be without defects • may degrade the optical emission by providing alternate nonradiative defect channels • The phonon bottleneck created by confinement limits the number of states that are efficiently coupled by phonons due to energy conservation • Limits the relaxation of excited carriers into lasing states • Causes degradation of stimulated emission • Other mechanisms can be used to suppress that bottleneck effect (e.g. Auger interactions)

31. QDL – Application Requirements • Same energy level • Size, shape and alloy composition of QDs close to identical • Inhomogeneous broadening eliminated  real concentration of energy states obtained • High density of interacting QDs • Macroscopic physical parameter  light output • Reduction of non-radiative centers • Nanostructures made by high-energy beam patterning cannot be used since damage is incurred • Electrical control • Electric field applied can change physical properties of QDs • Carriers can be injected to create light emission

32. ElectricallypumpedQuantumDot Laser Fujitsu Temperature Independent QD laser (2004)

33. Temperature Independent QD laser Fujitsu (2004)

35. QD laser already on the market

36. Stable operation up to 60C without a cooler • Modulation rates up to 500MHz • 2VDC operation • 532nm output (100-200mW power level, with frequency doubling) • Tiny TO-56 package (5.6mm diameter)

37. Lasing in a QD-microdisc system InAs/GaAsQDs cavityQ exceeds 15 000

38. Lasing in a QD-microdisc system „In most of oursamples lasing persists when the sample is tuned from 6 to55 K (a QD tuning range of 1.5 nm). This indicates thelasing is not based exclusively on observable QD statesresonantly coupled to the mode.”

39. Lasing in a QD-microdisc system „However, the relativespectral tuning of observed QDs emission states and cavitymodes does influence the L-I curve.” Z. G. Xie et al., PRL’2007

40. A recipy for a good QD laser „To achieve single statelasing the processes associated with the loss must be suppressedand more efficient lasing via the single-emitterstate (i.e., higher effective oscillator strength and higherQ), must be implemented.” Z. G. Xie et al., PRL’2007 + … a good QD-cavitymodespatialmatching

41. The investigations clearly visualize a smooth transition fromspontaneous to predominantly stimulated emission which becomes harder to determine for high beta. S. M. Ulrich et al., PRL’2007

42.  = t2 – t1 Od źródła fotonów Karta do pomiaru korelacji Dioda „STOP” Liczba skorelowanych zliczeń n() Dioda „START” t1 = 0 wejście STOP t2 = 20 wejście START

43. Increased g(2) (t) atlasingtreshold S. M. Ulrich et al., PRL’2007

44. bof a mode = the ratio of SE into thatmode divided by the total SE into all modes S. M. Ulrich et al., PRL’2007

45. Measured second-order photon correlation function at zero delay time (top) and output intensity versus input pump power, Pexc (bottom), for three different microcavity lasers. Q = 1850 20 QDs Q = 9000 30 QDs Q = 19000 15 QDs Wiersig et al.,Nature’2009

46. k-spaceimaging Fourier plane

47. k-spaceimaging Fourier planeimaging

48. Real-spaceimaging

49. Angleresolvedemission from QDsin planarcavity GaAs/InGaAs planarcavity PhotonEnergy

50. Angleresolvedemission from QDs in planarcavity