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PhD Thesis Defense Daniel Navarro Urrios

Obtención, caracterización y aplicaciones de dispositivos ópticos basados en nanoestructuras de silicio. Realisation, characterisation and applications of optical devices based on silicon nanostructures. PhD Thesis Defense Daniel Navarro Urrios. Dpto. de Física Básica, Universidad de La Laguna.

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PhD Thesis Defense Daniel Navarro Urrios

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  1. Obtención, caracterización y aplicaciones de dispositivos ópticos basados en nanoestructuras de silicio. Realisation, characterisation and applications of optical devices based on silicon nanostructures. PhD Thesis Defense Daniel Navarro Urrios Dpto. de Física Básica, Universidad de La Laguna

  2. Outline • General Introduction • Amplification studies in planar waveguides based on oxidised porous silicon • Form birefringence in Si-Nc planar waveguides • Er coupled to Si-Nc optical amplifiers • General conclusions Daniel Navarro Urrios

  3. Outline • General Introduction • Amplification studies in planar waveguides based on oxidised porous silicon • Form birefringence in Si-Nc planar waveguides • Er coupled to Si-Nc optical amplifiers • General conclusions Daniel Navarro Urrios

  4. General introduction • Silicon is the leading material for microelectronics. Huge processing technology (CMOS) infrastructure, process learning and capacity. • In photonics it can do also well (waveguides, fast modulators, power splitters and combiners, tuneable optical filters, detectors…) • It is the ideal plattform for integrating optical and electrical devices. Daniel Navarro Urrios

  5. General introduction • However Silicon is NOT an efficient light emitter, internal quantum efficiencies int~10-6 Need for an optical amplifier and a signal generator (laser)based on Silicon Indirect band-gap Daniel Navarro Urrios

  6. Different strategies to amplify light based on Silicon materials • Dye-doped planar waveguides based on oxidised porous silicon • Si-Nc in SiO2 planar waveguides • Er coupled to Si-Nc planar amplifiers Daniel Navarro Urrios

  7. Outline • General Introduction • Amplification studies in planar waveguides based on oxidised porous silicon • Form birefringence in Si-Nc planar waveguides • Er coupled to Si-Nc optical amplifiers • General conclusions Daniel Navarro Urrios

  8. HF I Si I time Porous silicon formation Pore dimension 2 ... 50 nm It is luminiscent (quantum confinement) But hard to realise population inversion Effective refractive index (air + silicon) High absorption in the visible Daniel Navarro Urrios

  9. Silicon Porous silica SiO2 Bulk silica Partial oxidation T > 400ºC Complete oxidation T > 800ºC Densification T > 1000ºC (many hours) Natural oxidation Effect of thermal oxidation • Consequences: • It becomes transparent for the visible • It becomes a passive material • Porous structure maintained  good for impregnation • We have still control of the refractive index (1.15<n<1.40) • Low refractive indices (n1.15), close to air  good cladding material for building waveguides Daniel Navarro Urrios

  10. Core (Porous silica, less porous, n1.3) Core (Porous silicon, less porous, n1.8) Cladding (Porous silicon, n1.35) Cladding (Porous silica, more porous, n1.15) Cladding (Porous silica, n1.15) Cladding (Porous silicon, more porous, n1.35) Silicon (n=3.5) Silicon (n=3.5) Silicon (n=3.5) Silicon (n=3.5) core Cut this way: cladding Si substrate Planar waveguides Two types • Polymeric waveguides with oxidized porous silicon cladding • Oxidised porous silicon waveguides PSW PMW Core (PMMA-polymethylmethacrylate-,n1.49) After oxidation (900ºC, 3h) After oxidation (900ºC, 3h) Daniel Navarro Urrios

  11. m-line characterisation Detector l=633nm We can excite each of the modes supported by the planar waveguide Knowledge of the effective refractive indices of the supported modes Daniel Navarro Urrios

  12. Modelling the waveguides parameters Nice agreement between experiments and simulations Daniel Navarro Urrios

  13. 1) PSW Core (500nm) Core (400nm) TE 50% Cladding (5-10mm) Cladding (5-10mm) Silicon substrate Silicon substrate 2) PMW TE 70% Single-mode waveguides (l=633nm) Daniel Navarro Urrios

  14. Dye impregnation (Nile Blue-LC 6900) Pulsed pump at 532nm Chosen because it is quite robust when dried. Impregnation: PSW: 5 min inmersion of the waveguides in ethanol+dye solution PLW: Precursor PMMA solution mixed with the dye Daniel Navarro Urrios

  15. 10 cm Spot size: ~3cm  300m Experimental setup Guided PL setup Sample Cylindrical lens lens To monochromator and PMT if there is gain, the PL shape should narrow and grow superlinearly with power Doubled Nd:YAG (532 nm, 5ns pulses) Daniel Navarro Urrios

  16. 2) PMW Guided PL vs Pump Power 1) PSW Daniel Navarro Urrios

  17. L Amplified Spontaneous Emission (ASE) Variable Stripe Length (VSL) g depends on power! And passes from negative to positive values by increasing the pump flux Daniel Navarro Urrios

  18. 1) PSW 2) PMW Guided PL vs Pump Power Net optical gain has been observed in both cases Daniel Navarro Urrios

  19. Optical Fiber Further studies on the impregnation of PSW N.A.<0.025 This signal is not travelling through the waveguide because putting a screen it disappears. Interferences Observation of narrow and linearly polarised spectral peaks Daniel Navarro Urrios

  20. 0 z1 1 z2 2 Collecting the light at 90º Incoherent emitters. Each emitter point can interfere only with itself.

  21. z Silicon The first 200-300 nanometers are emitting much stronger Daniel Navarro Urrios

  22. Possible explanations NO Could it be that the pump is being strongly attenuated through the structure? The contrast of the interferences is independent of the pumping wavelength. Also losses would be more than 105cm-1 Decreasing concentration The concentration of dye in the first hundreds of nanometers is orders of magnitude higher than in the rest of the sample No dramatic reduction of the contrast Daniel Navarro Urrios

  23. In this kind of samples we have observed net gain in the guided configuration. We believe that the main contribution to the gain is due to these first hundreds of nanometers, because we have built micro and macro-cavities and no amplification behaviors with pump power were observed when detecting normal to the sample. Daniel Navarro Urrios

  24. Outline • General Introduction • Amplification studies in planar waveguides based on oxidised porous silicon • Form birefringence in Si-Nc planar waveguides • Er coupled to Si-Nc optical amplifiers • General conclusions Daniel Navarro Urrios

  25. 2 Introduction Si nanocrystals usual fabrication techniques Courtesy of J. Linnros Daniel Navarro Urrios

  26. Introduction • Usual growing techniques : • Large size dispersion: • Inhomogeneous broadening of the emission band • Reduction of the stimulated emission efficiency (not all nanocrystals show optical gain). Courtesy of B. Garrido Daniel Navarro Urrios

  27. Optical gain under pulsed pumping has been demonstrated in these samples M. Cazzanelli, D. Navarro-Urrios, et al. Journal of Applied Physics, 96, 3164 (2004). The studied samples SiO2 substrate, alternating layers of SiO and SiO2 are grown by evaporation. After the annealing in N2 at 1100ºC for 1h monodispersed Si-nanocrystals are formed in a waveguide configuration. Investigation of the waveguiding properties of these samples Daniel Navarro Urrios

  28. Prism Detector Sample 543nm 633nm M-line measurements (543nm-633nm) We are able to know the effective index of each mode for each wavelength measured Daniel Navarro Urrios

  29. Information extracted from m-line, TEM images and mode solver simulations -Thicknesses and number of periods of the SL (TEM images) -Dimension of each period (TEM images) -Effective modal indices (m-line)  Material refractive indices (simulations) Daniel Navarro Urrios

  30. Material birefringence= Air SiOx Isotropic SL SiO2 Information extracted from m-line, TEM images and mode solver simulations It is impossible to fit the extracted effective indices unless we assume a negative birrefringent structure. no>ne The origin of the observed birrefringence is found on the particular structure of the core, i.e., a multilayer periodic structure made of two different kind of layers of different refractive indexes. “Form Birefringence” Daniel Navarro Urrios

  31. Air . . . no n1,d1 ne SL n2,d2 . . . SiOx TM TE SiO2 dSiO2+dNS=SL period no and ne:ordinary and extraordinary refractive indices of the equivalent layer Nw:number of Si-NC layers in the SL dSiO2 (nSiO2) and dNS (nNS):thicknesses (refractive indexes) of the SiO2 and nanocrystal single layers of the SL dt: total thickness of the SL ? dNS, nNS Modellization of the structure Theoretical model for a superlattice Daniel Navarro Urrios

  32. Unique solution!! 0.63 Results Daniel Navarro Urrios

  33. Results nNS(633nm)= 1.705 nNS(543nm)=1.735 Independent calculations …but the same dNS (independent of l) Daniel Navarro Urrios

  34. Similar samples growed by sputtering technique showed similar results N. Daldosso et al., Journal of Luminescence, 121, 2, 2006 Ion implanted samples with random distributed nanocrystals gave isotropic behavior no=ne 50nm Other studied samples Daniel Navarro Urrios

  35. z ne no y x Other studied samples Another type of material birefringence have been also studied Combination of m-line + transmission (linearly polarised light) measurements Reactive Si deposition method + annealing 1100ºC for 1h Vertical structures b>0 The Si-NC shape would be similar to the ellipsoid of indices Daniel Navarro Urrios

  36. Transmission measurements z z M-line measurements ne nz   ny no y nx y x x Other studied samples Reactive Si deposition method + annealing 1100ºC for 1h Non-perpendicular geometry between the Si beam axis and the substrate We need TEM images for confirmation Daniel Navarro Urrios

  37. Outline • General Introduction • Amplification studies in planar waveguides based on oxidised porous silicon • Form birefringence in Si-Nc planar waveguides • Er coupled to Si-Nc optical amplifiers • General conclusions Daniel Navarro Urrios

  38. EDWA (Erbium doped Waveguide Amplifier) Usual EDFAs (Erbium doped Fiber Amplifier) Erbium (Er3+) abs10-21 cm2 Expensive pumping source (resonant, intense and coupled) buffer SiO2 x x x x x x x x x x x x x x x x x x x x x x x x Si substrate x x x x x x x x x x x x x x x x x x By taking advantage of the coupling between Si-Nc and Er3+ ions Introduction by using We want to improve Daniel Navarro Urrios

  39. Why Si-Nc? Broad band absorption (UV-VIS) Increment of excitation for Er3+ : sexc from ~10-21 (in SiO2) to 10-16-10-18 cm2 (with Si-Nc) Fast (~ 1ms) and efficient (~55%) energy transfer from Si-nc to Er3+ Possibility of electrical pumping Higher index contrast for light confinement CMOS compatibility Daniel Navarro Urrios

  40. Transfer to Er3+ Intrinsic recombination Exciton generation and strong Auger 4I11/2, 4I9/2 4I13/2 4I15/2 Er3+ Nexc Introduction Excitons: Steady state: Nexc : density of excitons NNC: total density of Si-Nc NC: absorption cross section : intrinsic lifetime of the exciton kt: average coupling rate Cind: percentage of Er3+ coupled to Si-Nc Daniel Navarro Urrios

  41. Absorption and stimulated emission term Important for pump and probe measurements Excitation term De-excitation mechanisms N2 N1 Er3+ Nexc Introduction ? sabs, sem, sexc, td, Cup Daniel Navarro Urrios

  42. Dep. conditions (2÷6 mm) SiO2 800 nm Si-nc doped Er3+(1mm) SiO2 (15mm) Si-substrate Annealing T n increases with annealing time The samples Er:Si-nc produced by Reactive Magnetron co-Sputtering and successive annealing to get phase separation and reduction of non radiative defects Optical litography and Reactive ion etching F. Gourbilleau et al., JAP, 94, 3869 (2003) JAP 95, 3717 (2004). Annealing time Daniel Navarro Urrios

  43. Determination of sabs andsem Mc Cumber relation: From transmission measurements sabs and sem similar to that of Er3+ in SiO2 sabs and sem Daniel Navarro Urrios

  44. Medium field effects prevail Local field effects prevail Radiative lifetime determination Also, from Mc Cumber analysis: Daniel Navarro Urrios

  45. Quantitative measurements of the photon flux emitted from the samples. It is so possible to correlate the number of emitted photons with N2 Total lifetime and cooperative up-conversion Daniel Navarro Urrios

  46. Total lifetime and cooperative up-conversion td and Cup Daniel Navarro Urrios

  47. Excitation cross section at low pump power sexc exc is orders of magnitude higher than that of Er3+ in pure silica (~10-21 cm2), for samples B and C, resonant (488 nm) and non-resonant (476 nm) result in the same exc …but seems to be flux dependent, the slope is changing with increasing pump flux  Daniel Navarro Urrios

  48. Simulation Experimental Excited Er3+ vs pump flux …but Photon flux (ph/cm2s) Daniel Navarro Urrios

  49. Modelling Er3+ ions near the Si-NC are efficiently coupled to them, whereas Er3+ ions far away behave more and more as Er3+ in SiO2 that can be excited only directly. Model for sexc We consider that the first Er to be excited and therefore the strongest coupled would be the closest to the Si-Nc The coupling diminishes with the distance We have divided discretely the region around the Si-Nc into shells of different probability. The rate equation: is then solved for each R Thus, by integrating over all the shells, we get the temporal dependence of the total excited state population. Daniel Navarro Urrios

  50. Short range interaction Simulations td=3.8 ms, Cup=2x10-17cm3s-1, so=3x10-16cm2, sd=5x10-21cm2, Rnc=4nm , Ro=0.5nm, NNC=1x1017cm-3. Doing this for each flux we obtain…. And this means that only 2-3% of the whole erbium population can be excited trough transfer from Si-Nc. In any case it is about 10-100 excitable Er3+ per Si-Nc Daniel Navarro Urrios

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