Date post: | 25-Jul-2015 |
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Science |
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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.