Low Loss Waveguide Technologies & Purcell-Enhanced Emission

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1 MIT Microphotonics Center Fall Meeting, Boston, October 11, 2011 Low Loss Waveguide Technologies & Purcell-Enhanced Emission R. Tummidi, R. Pafchek, J. Li, G. Sukumaran, K. Kim, T. L. Koch Lehigh University Bethlehem, Pennsylvania, USA

2 Low Loss Waveguide Technologies & Purcell-Enhanced Emission T. L. Koch The MIT Si-Laser MURI Program Gernot Pomrenke, AFOSR Extrinsic light emitter design & test structure concept: Novel materials & fabrication research advances: Ultra-smooth etchless waveguide definition for low waveguide & resonator losses (w/maximal vertical confinement) Oxidation/implantation/deposition sequence for nm-scale Er 3+ -doped SiO 2 slot waveguides Record high-q TM slot resonators Strategy: Shallow ridge guide to enable maximal vertical confinement, lateral electrical access and low loss, high Q rings for laser resonator Horizontal slot to avoid etched interfaces nm scale slot to enable tunneling or dipole transfer based excitation (rather than hot electron injection) Novel electromagnetic & quantum electrodynamic research advances: Deep understanding of remarkably strong (> 10X) broadband Purcell enhanced spontaneous emission without resonators Deep understanding of designs to almost completely eliminate fundamental TM TE radiation conversion losses in waveguides, ring & disk resonators Investigations into potential structural QED enhancements into non-radiative energy transfer 11/7/11 2

3 SOI Waveguide Geometries Strip Silicon wire Quasi-Planar Ridge Large-Area Rib 3~5µm ~400nm ~200nm ~1µm ~200nm 3~5µm Si (n=3.48) SiO 2 (n=1.44) Si (n=3.48) Small mode-area Excellent bending performance (~µm range) Higher propagation loss (surface roughness scattering) Tightest vertical field concentrations for overlap with gain Provides means for electrical access for engineering active devices Lower propagation loss Scalable bending performance by ridge depth (down to wire if needed) Efficient coupling to SM fiber without taper Low propagation loss (~0.1 db/cm) Poor bending radius

4 Thermal Oxidation ( Etchless ) Waveguide Definition* in contrast to prior results with wet-etched guides PECVD SiO 2 and Si 3 N 4 Mask Thermal Oxidation AFM trace 0.2 nm surface roughness *Lehigh patented process: US 7,563,628 Fabrication of optical waveguide devices

5 Waveguide Loss Measurements Fabry-Perot Fringe Approach Wet etch est. 0.7dB/cm Ring Resonator Approach (higher precision) - use intentionally under-coupled ring so output coupling doesn t load waveguide Wet etch ring Q technique Thermal oxidation Q technique Waveguides using Wet-Etching Waveguides using Thermal-Oxidation Fundamental Mode 2 nd (odd) Mode

6 Ray Picture Describing Leaky TM Mode Phenomenon Wave impinging on ridge boundary causes mode conversion, i.e., generation of components of other polarization also must determine whether those components evanescent or propagating! W t 1 t 2 TE case TM case TM Evanescent TE Propagating! TE TM Destructive interference when Note: This is NOT the usual round trip phase condition, which would have 2k x W (and Goos-Hanchen shift if evanescent reflection)

7 Interferometric Radiation Cancellation for TM Modes at Magic Widths Experiments confirm dramatic effect:

8 Design at magic width for low-loss TM Designing for Low TE AND TM Loss Waveguides formed using ultra-smooth thermal oxidation process lowest waveguide transmission losses reported for high vertical confinement waveguides 400 µm Ring, 1.43 µm Magic Width Ridge Waveguide Drop port response (arb.) Q TM =6.81x10 5 Δν FWHM,TM = MHz α TM =1.0 db/cm Q TE =1.57x10 6 Δν FWHM,TE = MHz α TE =0.42 db/cm Wavelength (nm) Net actual waveguide loss 0.36 db/cm for TE and 0.94 db/cm for TM Note that TM mode loss is ~0.6dB/cm higher than TE mode loss even at magic width More sensitive to surface scattering & new curvature loss effects

9 Does Curvature Have Some Impact on Effectiveness of Magic Width? Straight waveguide Curved waveguide R. S. Tummidi, T. G. Nguyen, A. Mitchell and T. L. Koch, paper WN3, Proceedings of the IEEE LEOS 2008 Annual Meeting, Newport Beach, 9-13 November 2008.

10 TM TE W T TE2 Bent Waveguide R TE2 R TE1 R T TE1 + Increased loss at magic width for curved waveguides 1.43 µm Waveguide Increase in loss is not just due to increased bending loss Significant contribution to the loss due to imperfect cancellation of radiated wave fronts even at the magic widths

11 Even More Complex Possibilities In Rings & Disks T. G. Nguyen, R. S. Tummidi, T. L. Koch and A. Mitchell, Lateral leakage of TM-like mode in thin-ridge Silicon-on- Insulator bent waveguides and ring resonators, Opt. Express, 18, (7), pp , March 2010

12 TM Horizontal nm-scale Slot Waveguides What s the appeal of TM mode? For small changes in material index (inc. imaginary component or gain) The modal effective change for a slab waveguide where the weighting function is given by Here is the rigorous position-dependent effective thickness of the optical mode and then 23.5% of the local gain in the 8.3 nm slot seen by the TM mode!!n For example: With a peak gain of 13.7 db/cm in appropriate composition nm glass slot with Er Waveguide Position (µm) Note: Thanks to Jacob Robinson (Cornell) for reminding us (i.e., boxing us around the ears) to use proper expressions from literature! 8 nm SiO 2 Slot TE TM 100 nm a-si 150 nm c-si Weighting Function (µm -1 ) ( or inverse effective mode thickness) 14X higher gain enhancement from slot for TM mode compared to TE mode TM modal gain would be 3.2 db/cm (>1.83 db/cm TM modal loss) 1. F. D. Patel, S. DiCarolis, P. Lum, S. Venkatesh, and J. N. Miller, A Compact High-Performance Optical Waveguide Amplifier, IEEE Photon. Tech. Pomrenke Lett. 16, MURI Review (2004). 10/11/11

13 Low-Loss 8-nm Slot Waveguides 8 nm SiO 2 For low loss: W=1.51 µm (magic width) BOX 100 nm a-si 143 nm c-si Use thermal oxidation waveguide definition for ultra-smooth shallow ridge waveguide Design at Magic Width to mitigate lateral TM to TE radiation loss Input Couplers Ring Resonator loss measurement Through Drop R. M. Pafchek, J. Li, R. S. Tummidi, T, L, Koch, Low Loss Si-SiO2-Si 8nm Slot Waveguides, IEEE Phot. Tech. Let., 21, pp , (2009).

14 Next Step: Erbium Doped Horizontal Slot Waveguide 1. PECVD SiO 2 and Si 3 N 4 Mask Nitride Oxide 2. Thermal Oxidation 3. HF Strip 7.5 nm Ridge 205 nm c-si 205 nm 4. Blanket Thermal Oxidation 5. HF Strip 150 nm 6. Thermal oxide slot 8 nm Silica Slot 45º Erbium Implantation 2 KeV at 3x10 20 (6x10 20 peak) H 2 pre-treatment and α-silicon deposition 7.5 nm Ridge 8 nm Er/SiO 2 W=1.51 µm (magic width) BOX 100 nm a-si 143 nm c-si Activation anneal 700 C, N 2 ambient, 15 min Si Substrate 14

15 Experimental Results 1480 nm Band pass Er doped Slot waveguides 1550 nm Band pass Pump 1480 nm Polarization controller Cascaded Filters Cascaded Filters OSA PSD (dbm) (~ 40mW) (~ 8mW) RBW=0.5 nm Luminescence decay Signal Fast PL decay: Non-radiative recombination? Purcell enhancement? 152 µsec Wavelength (nm) Time (X100) µsec Measured Er room temperature PL by pumping straight slot waveguides with 1480nm TM pump laser Emission power saturates, suggesting all optically active Er are inverted as much as possible with 1480 nm pump Measured PL power too high for standard 10 msec Er lifetime (Purcell Enhancement?) Measured PL lifetime too fast to be purely Purcell Enhanced

16 Quantum Electrodynamics of Spontaneous Emission (homogeneous dispersive medium or free space ) just a plane wave with polarization vector Do the easy integrals and get the classic result: Aside: material n g does not enter in rate! 1. P. W. Milonni, Field quantization and radiative processes in dispersive dielectric media, Journal of Modern Optics, Vol. 42, No. 10, pp. Pomrenke MURI (1995). Review 10/11/11

17 For partial spontaneous emission rate into just one waveguide mode same as before Spontaneous Emission Into a Waveguide Mode but now where A n (x) are the modes of system and satisfy the orthogonality condition 1 Example: slab modes Use slab density of states: vector mode amplitude due to polarization and get L L Now position-dependent due to mode amplitude old result 1 B. J. Dalton, E. S. Guerra, 1 and P. L. Knight, Field quantization in dielectric media and the generalized multipolar Hamiltonian, Pomrenke Phys. MURI Rev. Review A, Vol. 54, 10/11/11 No. 3, pp (1996).

18 It is easy to re-write this as Spontaneous Emission Just Into Slab Waveguide Mode Here correction factor (Purcell Enhancement Factor F P-1D ) i.e., the mode thickness is just the inverse of the weighting function and the average of this inverse mode thickness is simply the correctly defined confinement factor Exceed homogeneous medium rate even into just one mode if But for our narrow slot waveguide, just looking in vertical direction, we have (or inverse modal thickness) 1/30 th a wavelength in the material!!! Get F P-1D = 11.1

19 Full Numerical Calculation of Enhanced SER in 8.3nm Slot Waveguides R. Tummidi Radiation Modes Guided Modes 100 nm Si W nm SiO nm Si Position, z(nm) BOX 8.3 nm Slot Г enhancement in slot TM TE Total Radiation TM guided efficiency F(z)/F free Slot Width W(nm) 1. Pomrenke H.P. Urbach MURI and Review G.L.J.A 10/11/11 Rikken Spontaneous emission from a dielectric slab Physical Review A, Vol 57, 5, May 1998

20 More Physics of Spontaneous Emission This is a large broad band PURCELL EFFECT but without a resonator. We can also easily illustrate how this relates to the usual Purcell enhancement for a resonator: First extend the calculation in a straightforward way to guiding in two dimensions for spontaneous emission into a channel guide mode; we obtain Purcell Enhancement Factor F P-2D with the position-dependent effective mode area defined by This averages over an active volume to the expected expression and we thus get a spontaneous emission rate into just the one channel waveguide mode that exceeds the total rate in a homogeneous medium if we can achieve

21 Resonator Purcell Factor But Purcell enhancement factor F generally associated with resonators If we start with the waveguide enhancement we have just derived, we can insert mirrors with reflectivity a distance apart in a arbitrarily long waveguide E out If it is easy to show that there is R no change in density of states (#states/unit energy) in the large waveguide volume no change in the normalized mode amplitudes outside the resonator in the presence of the cavity L c E in L Previous waveguide results still applicable but the mode amplitude inside the resonator is just increased by a factor R E out

22 Resonator Purcell Factor For a Fabry-Perot resonator is related to resonator As dipole transition rates inside the resonator scale as from Fermi s Golden rule, the increased SER inside the resonator is given by which gives the Purcell factor (with group velocity correction) where we have substituted

23 Investigate Potential of Remarkable TM Slot Mode Field Amplitude to Impact Nonradiative Decay i.e., dipole interactions can be analyzed in terms of exchange of virtual photons in 2 nd order QED perturbation theory Should we see impact of slot? A simplified Model Calculate energy transfer rate from a dipole in the slot to another dipole in the adjacent silicon layer Two ways to calculate the energy transfer rate QED second order perturbation approach accounting for retardation effects Instantaneous electrostatic coupling Hamiltonian Förster approach For homogeneous media, it is straightforward to show that both techniques give equivalent results in the near zone ( radiation less energy transfer) regime where retardation effects can be neglected 1 1. D. P. Craig, T. Thirunamachandran, Molecular Quantum electrodynamics An Introduction to radiation molecular interactions Academic Pomrenke Press, London, MURI Review /11/11

24 [Enorm(Slot)/Enorm(Non-Slot)] nm nm Distance into Si layer from SiO2 interface in nm Electrostatic simulations indicate <20% enhancement in the Förster interaction for a normal-oriented dipole in a slot relative to a non-slot configuration Since this has to give same result as QED analysis in short distances where retardation effects are not important, this means that resonant photon energies do NOT play a special role in Forster transfer Supports conclusion of Polman group

25 1. Developed complete suite of design, materials, and fabrication techniques enabling maximal vertical confinement, nm-scale slot waveguides with record low losses & high-q cavities 2. Experimentally observed & co-developed good basic understanding of broadband Purcell-Enhanced spontaneous emission in slot waveguide structures Conclusions & Accomplishments - This advance may present opportunities for future highly efficient single spatial mode waveguide emitter i.e., many of the attributes of laser, but spectrally broadband (patent application filed) 3. Developed excellent collaborative work across MURI team in extrinsic emitter research (patent application filed). Outstanding spirit of shared ideas in design, analysis, and applications. - Heartfelt thanks to Kim & the entire MURI team! - Special thanks to: Ryan Briggs & Harry Atwater (Caltech) Young Chul Jun & Mark Brongersma (Stanford) Jacob Robinson & Michal Lipson (Cornell) & Gernot Pomrenke, AFOSR

26 MIT Microphotonics Center Fall Meeting, Boston, October 11, 2011 Low Loss Waveguide Technologies & Purcell-Enhanced Emission T. L. Koch University of Arizona Lehigh College University of Optical Sciences Bethlehem, Tucson, Pennsylvania, Arizona USA

27 So to recap whenever Spontaneous Emission Just Into Slab Waveguide Mode for an emitter at position x, we will have a spontaneous emission rate into just the one slab waveguide mode that exceeds the total homogeneous medium spontaneous emission rate! Alternatively, for an average over the active emitter volume, this will be true when where we have the usual (properly defined) confinement factor

Lateral leakage of TM-like mode in thin-ridge Silicon-on-Insulator bent waveguides and ring resonators

Lateral leakage of TM-like mode in thin-ridge Silicon-on-Insulator bent waveguides and ring resonators Thach G. Nguyen *, Ravi S. Tummidi 2, Thomas L. Koch 2, and Arnan Mitchell School of Electrical and