High Quality Factor Microdisk Resonators for Chip-scale Visible Sensing

overview

2

Introduction and motivation High Q SiN microcavities on substrateCritical Coupling to SiN waveguides Experimental demonstrationConclusions

SiN for Visible Sensing

3

Multi-modal sensing in visible Low water absorption [2]Fluorescence sensingRaman sensing :nanoparticles’ plasmon

Silicon Nitride High index, low loss, low auto-fluorescence background Ease of fabrication (Planar/multilayer) (LPCVD)Source/detector integration (CMOS)

[2] lsbu.ac.uk/water/

Fabrication of SiN Microdisks

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Stoichiometric SiN on thermal oxide Electron beam lithography on ZEP reflow of ZEP ICP etching with CF4 gas (85deg, 5nm

roughness)

1 mm

200 nm

Critical Coupling

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Conventional straight WG Short coupling length-> narrow gaps

Pedestal[1]Controlled etching time Increasing field overlap

Pulley CouplingWaveguide looping around the diskIncreasing coupling length

R=20mm

Straight waveguide

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Semi-phase-matching Short coupling length-> narrow gaps

-5 0 5 10 1510

6

107

coupling Q vs limit

Coupling vs. Waveguide Width

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Pulley Scheme

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PulleySignificant increase in coupling lengthLess coupling induced lossPhase matching -> mode selectiveSensitive to waveguide width Large gap -> ZEP reflow for smooth sidewalls

Pulley Scheme’s Phase Matching

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Long coupling lengthStrict phase matching requirements Sensitive to waveguide width for phase matching nwg=nd [R/(R+g+w/2)]

order

neff FSR (rad/c

m)

TE 1 1.704 490

TE 2 1.626 498

TM 1 1.577 486

TE 3 1.559 504

TM 2 1.500 498

TE 4 1.503 509

140 145 150 155 160 165 1708.5

9

9.5

10

Ko (

rad/

um)

Azimuthal mode number m

Resonant modes of a 10 um radius disk

r=10 mm

Disk-Waveguide Phase Matching

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Phase matching optimized by choosing the waveguide width

100 200 300 400 500 600

1.5

1.55

1.6

1.65

1.7

1.75

Effe

ctiv

e in

dex

of W

G

Waveguide width (nm)

Waveguide 100nm away from the disk

100 200 300 400 500 600

1.5

1.55

1.6

1.65

1.7

1.75

Effe

ctiv

e in

dex

of W

G

Waveguide width (nm)

Waveguide place farther at 400nm coupling gap

TETE

TM

TM

g= 100 nm

g= 400 nm

TE1

TE2

TE2

TE1

TM1

r=10 mm

Pedestal and Pulley Coupling to R=100 mm disk

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Phase matchinglarger gaps, single mode operation

653 654 655 656 657 658 659 660-20

-15

-10

-5

0

653 654 655 656 657 658 659 660-12

-10

-8

-6

-4

-2

0

Pulley CouplingPedestal=40 nm

Wavelength (nm) Wavelength (nm)

Nor

mal

ized

tra

snm

issi

on

(dB

) Gap=400 nm

Effect of Phase Matching

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-30 -20 -10 0 10 20 30-200

-100

0

100

200phase difference

-30 -20 -10 0 10 20 3010

4

106

108

1010

1012 coupling Q vs limit

-30 -20 -10 0 10 20 30-200

-100

0

100

200phase difference

-30 -20 -10 0 10 20 3010

0

105

1010

1015 coupling Q vs limit

m and W for phase matching

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Splitting

143 3.1 3.2 3.3 3.4

x 10-7

8.3

8.4

8.5

8.6

8.7

x 106

250 260 270 280 290

7.6

7.8

8

8.2

8.4

8.6

x 106

1.68 1.7 1.72

x 10-7

8.105

8.11

8.115

8.12

x 106

Conclusions

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SiN is an excellent material for visible and NIR photonics applications.

By optimizing the fabrication process, microdisks with Qs as high as 8M can be achieved.

Critical coupling to adjacent waveguides is achieved by using pedestal and pulley coupling schemes.

Pulley coupling also enables critical coupling to selected mode(s) of the cavity without sacrificing Q.

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