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High Quality Factor Trapezoidal Subwavelength Grating Waveguide Micro-ring Resonator Zheng Wang1, 2, †, Xiaochuan Xu3, †, D.L. Fan1, 4, Yaguo. Wang1, 4 and Ray T. Chen 1,2,3 1Materials

Science and Engineering Program, Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, USA 2Department of Electrical and Computer Engineering, The University of Texas at Austin, 10100 Burnet Rd., MER 160, Austin, Texas 78758, USA 3Omega Optics, Inc., 8500 Shoal Creek Blvd., Bldg. 4, Suite 200, Austin, TX 78757, USA 4Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, USA †These authors equally contributed to this work

The authors acknowledge the Department of for sponsoring this research under SBIR grant # DE-SC0013178. 1

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1. Background

OUTLINE 2. Challenge and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion 2

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1. Background

OUTLINE 2. Challenge and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion 3

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Subwavelength grating(SWG) waveguide Λ W h

Features: •

Waveguide built with subwavelength grating: a periodic arrangement of core and cladding materials with a pitch less than one wavelength1.

•

Theoretical lossless: Bloch mode can be supported.

•

Engineering friendly: the ratio of core and cladding material can be engineered microscopically for desired waveguide properties macroscopically.

•

Large mode overlap with cladding materials: a promising platform for lightmatter interaction research2.

l

Basic parameters: •

Width of waveguide core: w

•

Height of waveguide core: h

•

Length of waveguide core: l

•

Grating period: Λ

•

Duty cycle: δ=l/Λ

[1] Bock, Przemek J., et al. "Subwavelength grating periodic structures in silicon-on-insulator: a new type of microphotonic waveguide." Optics express 18.19 (2010): 20251-20262. [2] Wangüemert-Pérez, J. Gonzalo, et al. "Evanescent field waveguide sensing with subwavelength grating structures in silicon-on-insulator." Optics letters 39.15 (2014): 4442-4445.

4

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SWG waveguide based micro-ring resonator (SWGMR)

1. The micro-ring and bus waveguide have been replaced with SWG waveguide1. 2. The sensitivity of a SWGMR can be as high as 400nm/RIU, which is 1.7 times of strip waveguide based micro-ring resonator1. [1] Schmidt, Shon, et al. "Improving the performance of silicon photonic rings, disks, and Bragg gratings for use in label-free biosensing." SPIE NanoScience+ Engineering. International Society for Optics and Photonics, 2014.

5

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1. Background

OUTLINE 2. Challenge and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion 6

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The challenge of SWGMRs The micro-ring radius (r) of a SWGMR is large while its quality factor (Q) is unsatisfied Reference

Top cladding

r (μm)

Q

Wang, Junjia, Ivan Glesk, and Lawrence R. Chen. "Subwavelength grating filtering devices." Optics express 22.13 (2014): 15335-15345.

SU8 (n~1.58)

10

8,800

Donzella, Valentina, et al. "Design and fabrication of SOI microring resonators based on sub-wavelength grating waveguides." Optics express23.4 (2015): 4791-4803.

Air (n~1)

30

6,000

Schmidt, Shon, et al. "Improving the performance of silicon photonic rings, disks, and Bragg gratings for use in label-free biosensing." SPIE NanoScience+ Engineering. International Society for Optics and Photonics, 2014.

Water (n~1.33)

30

3,900

An approach for high Q SWGMRs with small r is highly desired. 7

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Our solution: trapezoidal silicon pillar

1.For a small SWGMR, the bend loss from its micro-ring dominates its quality factor.

2.We have demonstrated that using trapezoidal silicon pillar could significant reduce the bend loss of curved SWG waveguide1. 3.For a same micro-ring radius, a SWGMR built with trapezoidal silicon pillars(TSWGMR) would have a higher quality factor then a SWGMR built with rectangular silicon pillars (R-SWGMR). [1] Wang, Zheng & Xu, Xiaochuan, et al. "Geometrical tuning art for entirely subwavelength grating waveguide based integrated photonics circuits." Submitted to Scientific Report.

8

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1. Background

OUTLINE 2. Challenge and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion 9

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Design of a T-SWGMR Optimize the shape of trapezoidal silicon pillars for achieving the highest unload quality factor.

r

g

Figure out the gap size (g) for triggering critical coupling. 10

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Optimization of the shape

Simulation parameters: •

w=500 nm

•

h=250 nm

•

Λ=300 nm

•

Duty cycle: δ=0.5 (l= 150 nm)

•

Micro-ring radius: r=5 μm

Target:

•

Find out the LT and LB for the lowest bend loss

optimized trapezoidal silicon pillar: LT= 140 nm and LB=210 nm.

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FDTD simulation for critical coupling

12

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1. Background

OUTLINE 2. Challenge and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion 13

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Fabrication of T-SWGMR and R-SWGMR T-SWGMR

R-SWGMR

14

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Transmission spectra

Q~1,600

Q~2,800

Q~11,500

The Q of the transmission peak is 4 times larger. 15

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Transmission spectra

Q~4,5000 Q~1,3000

Q~1,5000

Q~1,5000

The Q of the transmission peak is 3 times larger. 16

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1. Background

OUTLINE 2. Challenge and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion 17

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Conclusion 1.

For the first time, we demonstrated the smallest SWGMR (5 μm radius) with a quality factor as high as 11,500.

2.

For the same micro-ring radius (r), the quality factor (Q) of a TSWGMR is higher than the quality factor of a R-SWGMR. For r=5 μm, the Q of our T-SWGMR is our times of a R-SWGMR For r=10 μm, the Q of our T-SWGMR is our times of a R-SWGMR For r=10 μm, the Q of our T-SWGMR is 5 time of other group’s result1.

3.

Strong dispersion of SWGMRs has been observed. Rigorous and deep analysis will be performed later.

4.

This approach can be readily applied to other cladding materials like water (bio-sensing) and air (gas-sensing).

[1] Wang, Junjia, Ivan Glesk, and Lawrence R. Chen. "Subwavelength grating filtering devices." Optics express 22.13 (2014): 15335-15345.

18

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Thank you for your attention! 19

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Low-loss Curved Subwavelength Grating Waveguide Based on Index Engineering Zheng Wang1, 2, †, Xiaochuan Xu3, †, D.L. Fan1, 4, Yaguo. Wang1, 4 and Ray T. Chen 1,2,3 1Materials

Science and Engineering Program, Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, USA 2Department of Electrical and Computer Engineering, The University of Texas at Austin, 10100 Burnet Rd., MER 160, Austin, Texas 78758, USA 3Omega Optics, Inc., 8500 Shoal Creek Blvd., Bldg. 4, Suite 200, Austin, TX 78757, USA 4Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, USA †These authors equally contributed to this work

The authors acknowledge the Department of for sponsoring this research under SBIR grant # DE-SC0013178.

THE UNIVERSITY OF

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1. Background

OUTLINE 2. Challenges and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion

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1. Background

OUTLINE 2. Challenges and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion

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Subwavelength grating(SWG) waveguide Feathers: •

Waveguide built with subwavelength grating: a periodic arrangement of silicon and cladding materials with a pitch less than one wavelength.

•

Theoretical lossless: Bloch mode can be supported.

•

Engineering friendly: the ratio of silicon and cladding material can be engineered microscopically for desired waveguide properties macroscopically.

•

Large mode overlap with cladding materials: a promising platform for lightmatter interaction research.

Λ W h l

Bock, Przemek J., et al. "Subwavelength grating periodic structures in silicon-on-insulator: a new type of microphotonic waveguide." Optics express 18.19 (2010): 20251-20262. Wangüemert-Pérez, J. Gonzalo, et al. "Evanescent field waveguide sensing with subwavelength grating structures in siliconon-insulator." Optics letters 39.15 (2014): 4442-4445.

23

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1. Background

OUTLINE 2. Challenges and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion

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Challenges for curved SWG waveguides

1. Large bend radius (r): 10 μm 2. High duty cycle (δ): 0.7 and 0.8 3. High loss: for δ=0.7 and r= 10 μm case, loss per 90° bend is about 1.5±0.4 dB. Donzella, Valentina, et al. "Sub-wavelength grating components for integrated optics applications on SOI chips." Optics express 22.17 (2014): 21037-21050.

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Solution: trapezoidal silicon pillars Physical intuition: To make photons “walk” at the same pace

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1. Background

OUTLINE 2. Challenges and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion

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Optimization via 3D FDTD Smaller bend radius: r=5 μm Lower duty cycle: δ=0.5

For a 5 μm radius SWG waveguide (w=500nm,h=250nm and Λ=300nm) bend and quasi-TE polarization , the optimized trapezoidal silicon pillar has 140 nm top base and 210 nm bottom base.

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Optical filed profile Conventional rectangular silicon pillar

Our trapezoidal silicon pillar (optimized)

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1. Background

OUTLINE 2. Challenges and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion

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Fabrication of trapezoidal silicon pillars None-tuned

Under-tuned

Pattern Transfer Fidelity Factor (PTFF)

PTFF 

Optimally tuned

 A

d

A f dS

 A dS  A dS 2 d

Type

Desired shape

Non-tuned Under-tuned Optimally tuned Over-tuned

150 nm top base and 150 nm bottom base, 500nm height 120 nm top base and 190 nm bottom base, 500nm height 140 nm top base and 210 nm bottom base, 500nm height 70 nm top base and 210 nm bottom base, 500nm height

2 f

Over tuned

silicon 1 Ai   0 non-silicon PTFF dS is the resolution of SEM images 0.98 0.97 0.95 0.98

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Transmission spectra

We cascade 4 bends in one device

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Statistical loss at 1550nm

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1. Background

OUTLINE 2. Challenges and Solution

3. Design and Optimization

4. Experimental Demonstration

5. Conclusion

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Conclusion 1.

2.

3.

We developed a novel approach to achieve low-loss curved SWG waveguide. All aforementioned goals have been successfully achieved Smaller bend radius: r=5 μm Lower duty cycle: δ=0.5 Lower loss: -1.10 dB per 90° bend This approach could be readily applied to curved SWG waveguide based integrated photonics devices such as micro-ring resonators.

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Thank you for your attention! 36