Slightly fishy? Combining NSOM with

SEA TADPOLE

Johanna Trägårdh, H. Gersen

University of Bristol

Project aim

Measure the full electric field of light propagating in a photonic structure.

• Spectral amplitude, spectral phase and polarizationStructure properties, n(w,x), dispersion

• Locally / on a sub-wavelength scale

• In a single point

• In the near-field

Description of a laser pulse

)()()(

www ieSE

ccetIttti

.)()())((

21

w

t w

Fourier transform

(w)

S(w)

Absolute phase

...)()()( 2

2

021

100 wwwww

Equivalent to a time delay

2nd order dispersionLinear chirp in time

Crossed beam spectral interferometry

t=0

))()(sin2cos, ww refsigsigrefsigref θky()ω()Sω(S)ω(S)ω(S)yω(S

...)()()( 2

2

021

100 wwwww

Signal

Reference

NSOM on a waveguide

Optical amplitude

Amplitude

Si

SiO2

SiON 850nm

Sample

Near-field scanning optical microscope

Tuning fork

Fiber probe

10 nm

Spatially Encoded Arrangement for Temporal Analysis by Dispersing a Pair of Light E-fields

Signal

(NSOM probe)

Reference

P. Bowlan, et al., Opt. Express 14, 11892(2006).

NSOM on a waveguide

Optical amplitude

Amplitude

40mm x 4.8mm

Topography

Si

SiO2

SiON 850nm

Optical Amplitude

NSOM and SEA TADPOLE

This measures:• Spectral amplitude and phase• In the near-field• In a single point• In 70 ms

Frequency w

y

))()(sin2cos, ww refsigsigrefsigref θky()ω()Sω(S)ω(S)ω(S)yω(S

Measuring the dispersion

The fringes in the interferogram will shift as the phase of the detected light changes when the probe is moved

Measuring the dispersion

Spectral Phaset=0

c

Lnw

Refractive index

nmodel= 1.445nMZ = 1.46

MZ

Measuring the group velocity

x=0 x=22 mm

The change in fringe tilt shows that the signal pulse has moved in time.

...)()()( 2

2

021

100 wwwww

Measuring the group velocity

A linear term in the spectral phase is equivalent to a time delay

t=0 ...)()()( 2

2

021

100 wwwww

w

w

d

dg )(Group delay:

Measuring the group velocity

Retrieved group velocity 1.98108

Group index ng=1.51

Group index from model ng=1.47

Fourier transform )()()()(

tEeSEi

www

Probe position

Conclusion

NSOM + SEA TADPOLE measures the full electric field of light propagating in a photonic structure.

• Spectral amplitude, spectral phase (and polarization)

• In the near-field

• In a single point

• Retrieve group velocity and n(w,x)

SEA TADPOLE

Spectral interferometry on bulk optics

t=0

Angular frequency (THz)

Ph

ase

(rad

)

(w)

S(w)

820 785 753Wavelength (nm)

GDD=1163 fs2

GDDcalc=1142 fs2

...)()()( 2

2

021

100 wwwww

Frequency w

25mm BK7

No sample

Another way to track an ultrafast laser pulse

in a photonic structure

AOM f=80.0 MHz

AOM f=80.07 MHz

Heterodyne detection in a Mach-Zehnder interferometer

)cos()()(2det ttItII SR w

Another way to track an ultrafast laser pulse

in a photonic structure

We want to measure the phase and amplitude of the electric field of the light pulse, and also where it is in time.

AOM

AOM

Another way to track an ultrafast laser pulse

in a photonic structure

We want to measure the phase and amplitude of the electric field of the light pulse, and also where it is in time.

AOM

AOM

Measuring the group velocity

t=0

Amplitude

t=67 fs40mm x 4.8mm

Measuring the effective refractive index

t=0

Position (mm)

Fourier transform

n=1.46ncalc=1.445

Frequency (1/nm)

t=0

A cos

Comparing the methods

Heterodyne detection in Mach-Zehnder interferometer:

Measures effective refractive index (for center frequency).

Measures group index (with an accurate delay)

Requires scanning a large area – slow and requires periodic or invariant structures

t=0

Spectral interferometry:

Measures n(w)

Measures group index

Only requires measurements in two or a few points.

In this configuration it requires a camera.