Quasi-Phase-Matched Faraday Rotation in Semiconductor Waveguides with a Magneto-Optic Cladding for Monolithically Integrated Optical Isolators

Transcription

1 Quasi-Phase-Matched Faraday Rotation in Semiconductor Waveguides with a Magneto-Optic Cladding for Monolithically Integrated Optical Isolators Prof. David C. Hutchings, Barry M. Holmes and Cui Zhang,

2 Acknowledgements Oxide deposition kindly provided by: Beth J. H. Stadler, Sang-Yeob Sung and Andrew Block Electrical & Computer Engineering, University of Minnesota This work is supported as a Materials World Network Complex Oxides for Heterogeneous Optoelectronic Integration and a CSC scholarship

3 Motivation Semiconductor Photonic/Optoelectronic Integrated Circuits (PICs/OEICs) beginning to impact... Si photonics III-Vs for sources, detectors, modulators additional functionality, improved yields, reduced costs...but optical isolator/circulator solution not yet deployed 2 approaches to non-reciprocity Non-reciprocal phase shift in asymmetric waveguide with Transverse Magnetic Field Faraday Rotation (polarisation mode conversion) with Longitudinal Magnetic Field this talk

4 Bulk, Faraday-Effect Optical Isolator Incident Reflected Faraday Rotator θ Analyser Mirror θ = VHd Polariser Applied Magnetic Field Bulk components need assembly and alignment therefore relatively costly to produce Missing-link in integrated optoelectronics related technologies for circulators and polarisation agility

5 Integration of III-V with high MO material Only the upper cladding is magneto-optic Core guiding layer remains as III-V semiconductor MO effect only through evanescent tail Device length is extended commensurately but MO effect (Verdet constant) in garnets can be HUGE! M-O layer Core (III-V) Substrate (III-V) n=2.2 n=3.6 n=3.3

6 Stokes Parameters and Poincaré Sphere * 3 * ) 2Im( ) 2 Re( S S S S H H S H H S H H S H H S x y x y x y x y = + + = = = + = Field components are, in general, complex Instead consider Stokes parameters (real valued)

7 Guided Mode Poincaré Sphere approximation use averaged Stokes parameters across mode profile then each mode is a point on the modified Poincare Sphere coupled-mode approximation assumes just 2 guided modes mode-beating trajectory is a rotation about the axis through these 2 modal points

8 Magneto-Optic Waveguide for λ=1.55 µm 100nm Garnet (n=2.2, ε xy =0.05i) S1 (mode 1) S2 (mode 1) S3 (mode 1) GaAs Al 0.6 Ga 0.4 As S 1 S 2 S 3 S1 (mode 2) S2 (mode 2) S3 (mode 2) half-beat length = 150 µm Averaged Stokes Parameters (0.9979,0, ) and ( ,0, )

9 Magneto-Optic Waveguide for λ=1.55 µm 100nm Garnet (n=2.2, ε xy =0.10i) 1.6 S1 (mode 1) 1.6 S2 (mode 1) 1.6 S3 (mode 1) SiO Si SiO S 1 S 2 S S1 (mode 2) 1.6 S2 (mode 2) 1.6 S3 (mode 2) half-beat length = 9.95 µm Averaged Stokes Parameters (0.9915,0,0.0082) and ( ,0, )

10 Coherent mode conversion Problem: waveguide introduces shape birefringence Coherent mode conversion includes: - 2 nd & 3 rd order optical frequency conversion - Faraday rotation One mode acts as a source for another The relative phase of the source and the generated mode matters Require phase-matching to maintain continual growth 2 Irradiance k=0 DR DD k=/ 0 0 L c 2L c 3L c 4L c 5L c distance

11 Our QPM solution for MO in waveguides Heterogeneous upper cladding patterned to be MO / non-mo (or +MO/-MO) Uniform magnetic field Look to match refractive indices to minimise scattering Non M-O M-O Core (III-V) Substrate (III-V) n=2.2 n=3.6 n=3.3

12 First-generation Periodic MO waveguides Si 3 N 4 Coated sputtered from Ce-YIG 120µm Period (2 x L beat ) 2.7µm

13 First-generation QPM MO Results Length=8mm, multi-moded and using remanent magnetisation as-deposited MO film from sputtered Ce:YIG target

14 Anneal of MO film As Deposited Annealed at 800 C for 10 mins Looked like the surface had exploded!! due to large mis-match in thermal expansion one solution is to pattern garnet into islands

15 garnet lift-off process development: Attempt 1 collaboration with Univ. Minnesota lift-off process (PMMA) allows fabrication of garnet islands anneal in O2 garnet/semiconductor interface compromised mask realignment better than 10nm (can be ~1nm)

16 avoid side-wall deposition with overhanging bilayer resist ~1200nm of 15% PMMA diluted capped with ~200nm 4% PMMA diluted e-beam dose ~600µC.cm -2 on GaAs ~85nm garnet thickness for 120nm deposited lift-off with PMMA bilayer on GaAs

17 Anneal of YIG on GaAs RTA in 150mB O 2 for 120s (Glasgow) 700ºC, no buffer layer 700ºC, ~5nm sputtered MgO buffer layer 800ºC, ~5nm sputtered MgO buffer layer

18 avoid side-wall deposition with overhanging bilayer resist ~1200nm of 15% PMMA diluted capped with ~200nm 4% PMMA diluted e-beam dose ~976µC.cm -2 on SOI ~85nm garnet thickness for 120nm deposited lift-off with PMMA bilayer on SOI

19 Anneal of YIG on Si-on-insulator RTA in 150mB O 2 for 120s (Glasgow) 800ºC, no buffer layer 800ºC, ~5nm sputtered MgO buffer layer 900ºC, ~5nm sputtered MgO buffer layer

20 Anneal of YIG on Si-on-insulator RTA in 10slpm O 2 for 120s (UMN) 850ºC, no buffer layer 850ºC, ~5nm sputtered MgO buffer layer

21 Effect of composite garnet cladding 100nm total thickness garnet upper cladding Ce:YIG (-4500º/cm) on top of YIG (200º/cm) average S 3 parameter shown for TE-like mode GaAs waveguide example SOI waveguide example no buffer layer 5nm MgO buffer layer 70 no buffer layer 5nm MgO buffer layer average S 3 / average S 3 / YIG layer thickness (nm) YIG layer thickness (nm)

22 Proposed integrated waveguide isolator Non-reciprocal (Magneto-Optic) polarisation mode converter (NR-PMC) Reciprocal polarisation mode converter for halfwave-plate functionality (R-PMC) Integration with Laser Diode D.C. Hutchings, J. Appl. Phys. D 36, 2222 (2003)

23 Mode-Beating Toolset Components Reciprocal Polarisation Mode Converter (R-PMC) plane-wave equivalent: ½- or ¼-waveplate Non-Reciprocal Polarisation Mode Converter (NR-PMC) plane wave equivalent: Faraday rotation Fixed (and Variable) Differential Phase Shifter (DPS) Polarisation Mode Selector or Splitter (PMS) plane wave equivalent: Polarising Beam Splitter

24 RIE-lag R-PMC waveguide fabrication RIE Lag Taper (10µm long) Rectangular input/output guide

25 RIE-lag 3 trench R-PMC optical results

26 R-PMC design considerations for isolator and modulator designs, require 50% reciprocal mode conversion (3dB coupler) could use ¼-beat length converter with modal Stokes points [i.e. S 2 axis] S = ( 0,1,0 ), ( 0, 1,0 ) but this design solution can only be approached asymptotically with increasingly stringent tolerances we propose instead a ½-beat converter with modal Stokes points S =, ±,0,, m,

27 Al-Q 50% R-PMC Design for λ=1.55 µm Single trench half-beat length = 254 µm Averaged Stokes Parameters (0.702,0.707,0) and (-0.699,-0.710,0)

28 Al-Q 50% R-PMC for λ=1.55 µm Single Trench Waveguide Core Transparency conditions for DPS

29 Poincare Sphere Representation

30 Mode-Beating Toolset Components Most Polarisation Functional Waveguide Devices can be constructed from the components: Reciprocal Polarisation Mode Converter (R-PMC) use tapered transitions universal 3dB coupler design Non-Reciprocal Polarisation Mode Converter (NR-PMC) quasi-phase-matched evanescent Faraday rotation new garnet lift-off process under development to permit annealing Fixed and Variable Differential Phase Shifter (DPS) Polarisation Mode Selector or Splitter (PMS)

31 Integrated Waveguide Devices Polarisation Modulator/Controller TE (TM) to ANY, or ANY to TE (TM) Polarisation inverter TE (TM) to TM (TE) with fixed DPS ANY to ANY Non-Reciprocal Devices Integrated Optical Isolator Directional MUX/DEMUX

32 Polarisation Modulator/Controller Topologically equivalent to Mach-Zehnder Interferometers 2 3dB couplers 2 polarisation modes correspond to the 2 interferometer arms

33 Progress towards integrated isolator Realise Faraday effect with a semiconductor core, using QPM MO upper cladding Lift-off process allows annealing of segmented garnet cladding MgO buffer layer provides protection for semiconductor during anneal GaAs/AlGaAs waveguides silicon-on-insulator waveguides Reciprocal polarisation conversion with asymmetric waveguides single slot reproducibly defined with etch-stop in III-V identified universal 50% R-PMC design complete integration platform is evolving with tapers, intermixing, etc.