http://photonics.intec.ugent.bePhotonics Research Group

Silicon optomechanics

13th IEEE Photonics Society 13th IEEE Photonics Society Benelux Annual WorkshopBenelux Annual Workshop ::

11/09/200911/09/2009

Joris Roels, Dries Van ThourhoutJoris Roels, Dries Van Thourhout

*Ghent University*Ghent University--IMEC, Dept. of information technology, Gent, BelgiumIMEC, Dept. of information technology, Gent, Belgium

Optomechanics: brief history 1871: theoretical prediction of ‘radiation

pressure’ (James Clerck Maxwell)

1899: first experimental proof of tiny ‘optical force’ (Lebedev)

1986: optical trapping of polarizable microparticle (ªdipole), particle moves along field gradient towards region with highest intensity (beam waist)

Exploitation of optical forces on a chip

Today: nanophotonic structures with shrinking device dimensions fi enables relatively large forces

Optomechanics today Possible applications:*optical cooling of micromechanical resonators (see further)*sensitive read-out of small displacement/vibration*light as ‘novel’ actuation force, from MEMS (Micro-Electro Mechanical Systems) to NOMS (Nano-Opto Mechanical Systems)

…but first we need to understand the very basics

*broad class of integrated optically tunable components possible

Felec

Outlinewaveguide optomechanicscavity optomechanicsoptical cooling

How to move a waveguide?

Povinelli et. al, Optics Express, (2005)

Mo Li et al. , Harnessing optical forces, Nature (2008)

Experimental demonstration of attractive force between waveguide and substrate

Waveguide-waveguide interaction (repulsive force?)

Fopt

Physics similar to optical trapping: dipoles in field gradientwaveguide mode tries to increase its effective index

TheoryTwo single mode Si nanophotonic waveguides in close proximity fi bimodal: symmetric/anti-symmetric eigenmodeLight couples back-and-forth between waveguides

symmetric anti-symmetric

g445nm

Si Si220nm

Fopt

symm vs. anti-symm mode: different (force) sign

repulsive forceattractive force

Ln

dgd

cPF group

opt ωω−

=

power

eigenmode frequency

group index

waveguide length

k

Force simulation

controlling symm/anti-symm mode excitation = controlling force

@gap = 220nm

Fsymm,att ª -0.23pN/µm/mW

Fantisymm,rep ª 0.1pN/µm/mW

Superposition of 2 modes:Beating force up to 20% of the total force magnitude

Beat period ª 120µm

gap445nm

Si SiFopt

220nm

symmetric

anti-symmetric

attractive

repulsive

Device

pump

sweeping pump λ fi fields arrive with different phase at waveguide coupler

entrance

in phase fields favor symmetric (attractive) mode

symmetric anti-symmetric

counter phase fields favor anti-symmetric (repulsive) mode

sweeping wavelength enables tuning: attractive ↔ repulsive

3-dB optical power splitter (MMI) + two waveguides (delay length ∆Lª113µm) + freestanding waveguide coupler (length Lª25µm)

Transduction scheme

probe

full device = unbalanced Mach-Zehnder Interferometer (delay length ∆L) with one

MMI coupler and one waveguide coupler

1e+0

1e-1

1e-4

1e-3

1e-5

Tran

smis

sion

(mW

)

1e-2

Tran

sduc

tion

(nm

-1)

1e+0

1e-1

1e-2

1e-3

1e-4

1e-5

Wavelength (nm)1546 1548 1550 1552 1554

waveguide displacement ∆g

↓

coupling ∆κ

↓Transmission ∆T

3-dB couplerwaveguide coupler

Probe laser @1548nm

Calibration

main damping factor: air fi vacuum conditions: Qmech↑

vacuum fiber feedtroughs

vacuum chamber

pressure sensors

pump group

both optical and thermal forces are very small (ªfN-pN): mechanical resonator fi vibration amplitude x Qmech for given AC-force

Calibration: thermal forces (can be calculated with known params e.g. T, oscillator mass)

IN OUT

Mechanical Q

vacuum conditions boost up Qmech x100

-56

-54

-52

-50

-48

-46

-44

5.6 5.8 6 6.2 6.4 6.6

f [MHz]

mod

ulat

ion

dept

h [d

B]

p_atm

10-4mBar

Qmechº60

Qmechº6000

100 fm/√Hz

10 fm/√Hz

max expected optically induced displacement (Hooke’s law): Qmech x F/k ª nm

Calibration (2)

probe

Transduction (without pump) is measured/calibrated by recording thermal vibration noise (peak spectral density PSD)

5.5 5.6 5.7 5.8Frequency (MHz)

PSD

(pW

/Hz)

0

100

200

300

400

500

Two peaks = two suspended waveguides

Pump probe set-up

probe laser

pump laser+EO modulator

detector+ESA

filter

vacuum chamber

pump laser light is modulated with electro-optical modulator and injected simultaneously (circulator)

probe laser light is detected and analyzed with electrical spectrum analyzer

sweeping modulation frequency enables to record driven displacement spectra for different pump λ

λ=1548nm

tunable λ

Displacement spectra

red circles for λ=1551.4 (attractive), blue circles for λ=1554nm(repulsive)

curves are phase-shifted 180°

@gap = 220nm

Fsymm,att ª -0.23pN/µm/mW

Fantisymm,rep ª 0.1pN/µm/mW

5.785.77

-10

-14

-18

-22

-26

-30

f (MHz)5.7755.765

P mod

/ Pcw

(dB

) pump λ=1551.4nm

pump λ=1553.5nm

Optical force

Excellent agreement theory vs. experiment

Fsymm,att ª -0.2pN/µm/mW

Fantisymm,rep ª 0.1pN/µm/mW

Experimental demonstration: attractive vs. repulsive force

Error bars originate from uncertainty on exact power level in device

J. Roels et al., Tunable optical forces between nanophotonic waveguides, Nat. Nanotechnology (2009)

repulsive

attractive

1553 1554155215511550-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

1555

pump λ=1551.4nm

pump λ=1553.5nm

wavelength (nm)

Opt

ical

forc

e (p

N/µ

m/m

W)

1549

Outlinewaveguide optomechanicscavity optomechanicsoptical cooling

Cavity optomechanics

power enhancement

larger force per photon Fopt ~ dωcav/dx

optomechanically tunable filters

Mo Li et al., Optomechanical coupling in photonic crystal supported nanomechanical waveguides, Opt. Express (2009)

Coupled resonators

2 vertically stacked ring resonators (spider web)

filter tuning over 4nm range

Rosenberg et al., Static and dyamic wavelength routing via the optical force, Nature Photonics, (2009)

Mechanical resonator optical cooling vibrating atoms cause force Fbrown ~ √T fithermal motion at resonance frequency ωmech

optical cooling = reducing thermal vibration

photon life time cavity ª mechanical period (high optical Q!) fi optical force Fgrad creates drag force

m

kΓ

Fbrown

ωωmech ωmech

displacement noise [m²/Hz]displacement

noise [m²/Hz]

m

k

Fbrown Fgradω

~Tinit Γ

~Teff <Tinit

Cavity cooling (1)

microtoroids on tiny pillarradial mechanical modesQoptª108 , finesse > 105

Qmechª105

Schliesser, Kippenberg et. al, Nature Physics, 4, 415-419 (2008)

having both high mechanical and optical Q is very challenging!

Cavity cooling (2)

Implications:-fundamental (quantum mechanics in microscale objects!)-ultrasensitive displacement sensing-quantum computing/cryptography-on chip photonic clocks

Conclusions & perspectives

Recent advances have enabled exploitation of optical forces on chip

Immature research field, much more work needs to be done towards practical applications.

Optically tunable components/ optical cooling