1. KTH ROYAL INSTITUTE OF TECHNOLOGY Rare earth doped waveguide lasers and amplifiers Dimitri Geskus Department of Materials and Nano Physics KTH - Royal Institute of Technology, Kista, Sweden

2. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Contents Basics of amplification of light Rare earth doped waveguide amplifiers Spectroscopy Basic principles of lasers Rare earth doped waveguide laser Few special lasers

3. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se The rare earth ions Have unique electron configurations: incompletely filled subshells, and a completely filled outer shell (giving the chemical properties). Transitions of electrons in the subshells, change the energy potential of the ions. Interaction with light. Ytterbium, 1025 nm Thulium, 2000 nm Erbium, 1500 nm Neodymium, 1064 nm

4. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Energy diagram of an ion, and some transitions Ion in excited state Ion in ground state Absorption Stimulated emission Duplicated photon Original photon Incident pump photon Incident signal photon Upper laser level Ground level Upper pump level Lower laser level Fast non-radiative decay

5. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se wave amplifier: metastable Buoy in water A buoy with stored energy, will generate a wave in itself when tipped over. The tumbling can be triggered by an incident wave, which gets stronger after tipping over the buoy. A sea filled with “excited” buoys has the potential to generate a tsunami.

6. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se 4 level transition and 3 level transitions 2 things to keep in mind: - We need millions of ions - Absorption and emission processes have the same probability Absorption Stimulated emission Stimulated emission Absorption

7. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se 4 level transition and 3 level transitions Population inversion: N upper laser level / N lower laser level > 1  Lets take 100 ions: Can I get population inversion by absorbing a single photon? Can I get inversion in a 2 level system? Absorption Stimulated emission Stimulated emission Absorption

8. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Popular rare earth ions and their energy levels Pr 3+ Nd 3+ Yb 3+ Tm 3+ Er 3+ Ho 3+ Dy 3+

9. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se The Ytterbium ion The two principal energy levels are Stark split, making it a quasi tree level ion. The sublevels are thermally populated according to the Boltzmann distribution Few transitions and their Wavelength are indicated.  Verify by “ein gedankenexperiment” that you need a larger fraction of ions excited (larger population inversion) to get gain at 980 nm, than at 1025 nm. Energy (cm -1 ) 10695 10476 10187 568 407 169 0 2 F 5/2 2 F 7/2 Absorption cross-section Emission cross-section (3) (2) (1) (4) (3) (2) (1)

10. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se The Erbium ion

11. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Compact, high-gain amplifier What do we need? To achieve high overall gain we need ….. To have a compact device we need…. Fundamental law of everything: “Preservation of challenges” If you solve one challenge, you create another: so we can’t have it all! The high concentrations, are a limitation, especially in amorphous materials.

12. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Amorphous vs. crystalline host materials Ions bond to specific sites in the crystal. Therefore the ions are kept at “safe” distance from each other. In amorphous materials the ions can cluster, hereby they start to interact with each other. This results in: Pump absorption, but no emission Reabsorption of the signal Energy transfer processes Pump

13. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se The Erbium ion and energy transfer up-conversion

14. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Life with Erbium Erbium doped fiber amplifier (glass, meters length) Erbium doped Al 2 O 3, microchip amplifier (poly crystalline 13-25 cm length) ~5 cm ~3 mm

15. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Er Al Rare-earth-doped Al 2 O 3 planar waveguides Deposition by reactive co-sputtering on thermally oxidized Si K. Wörhoff et al., IEEE J-QE 45, 454 (2009)  integration with Si-technology Microstructuring by chlorine-based reactive ion etching J.D.B. Bradley et al., Appl. Phys. B 89, 311 (2007) propagation losses @ 1550 nm ~ 0.21 dB/cm

16. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Al 2 O 3 :Er 3+ integration with Si photonics L. Agazzi et al., Opt. Express 28, 27703 (2010) Inverted taper Al 2 O 3 :Er 3+ waveguide SOI waveguide SiO 2 buffer layer (2  m) Si substrate 2 μm 270 nm 1 μm 450 nm 220 nm 100 nm 400 μm Fabrication of Si-on-insulator waveguides, deposition of Al 2 O 3 :Er 3+ layer on top, etching of rib waveguides by RIE

17. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Amplifier performance Without ETU, the Er 3+ amplifier would be a marvellous tool, producing tens of dB/cm gain. With ETU we would choose an Er 3+ conc. of ~8  10 20 cm -3 to achieve ~7.5 dB/cm gain. With quenched ions the gain is limited to ~2 dB/cm. L. Agazzi et al., J. Phys. Chem. C 117, 6759 (2013)

18. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se When transmitting pump light through the channel waveguide, there is a large amount of nonsaturable absorption. A fraction of Er 3+ ions cannot be bleached. These ions absorb pump power, but “immediately” decay back to the ground state. These ions do not emit a photon! NSA Non-saturable absorption

19. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Al 2 O 3 :Er 3+ spiral amplifiers Spiral amplifier (small foot-print) S.A. Vázquez-Córdova et al., (2014) Total small-signal gain ~20 dB for optimized length Internal net gain

20. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se In Al 2 O 3 :Er 3+ the fraction of quenched ions increases to 32% for 3.66  10 20 cm -3 Er 3+ ions, while in Al 2 O 3 :Yb 3+ only 11% of ions are quenched at 6.67  10 20 cm -3. Likely there are two quenching mechanisms: 1. Transfer to impurities 2. ETU among Er 3+ pairs Fraction of ions with fast quenching

21. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Ytterbium in a crystalline host Combining the best of all worlds: Yb doped KY(WO 4 ) 2 (KYW) Large emission cross-sections, which is even enhanced by strong crystal fields. Single crystalline host, providing large interionic distances hence, reducing energy transfer processes Only 2 principal energy levels, so no upconversion. Fabrication a bit challenging, especially integration on a silicon platform. Ytterbium doped KYW waveguide (crystal)

22. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se LATTICE MATCHING: FINDING THE BALANCE 22 KYbW KGdW KYWKYWKYWKYW KLuW KYWKYWKYWKYW KYWKYWKYWKYW F. Gardillou et al., Opt. Lett. 32, 488 (2007) No Matching  instable layers! Matched Composition KYW: 49.1%Gd 3+ 48.4% Lu 3+ 2.5% Yb 3+ Gd 3+ KYW:Gd 3+,Lu 3+,Yb 3+ KYWKYW Matched  Crack free! Design of: - Crystal Lattice - Yb Concentration - Refractive index

23. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se MAKING CHANNELS 23 Coat with Photoresist Litho and development Argon ion beam etching Removal of photoresist Pure KYW overgrowth

24. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se PUMP PROBE EXPERIMENT

25. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se EXPERIMENTAL RESULTS D. Geskus, et al. Adv. Mater., 24 (10) (2012)

26. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Concentration limit of Yb:KYW F. Auzel et al., “Radiation trapping and self-quenching analysis in Yb 3+, Er 3+, and Ho 3+ doped Y 2 O 3,” Opt. Mater., (2003)

27. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Channel structures and bulk devices The confinement of light in Waveguides possibly allows higher concentrations without a severe impact on performance

28. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se OVERVIEW OF GAIN MATERIALS D. Geskus, et al. Adv. Mater., 24 (10) (2012)

29. Rare-Earth Lasers and Integrated Devices Group KTH  Royal Institute of Technology Dept. of Materials and Nano Physics Dimitri Geskus geskus@kth.se Size of amplifiers based on glass and crystalline host material Erbium doped fiber amplifier (glass) Ytterbium doped KYW waveguide (crystal)