3P077
Ultrashort laser shaping for scanning near-field optical microscopy(Institute for Molecular Science,a The Graduate University for Advanced Studies,b WasedaUniversity c) ٤Yuqiang Jiang,a Huijun Wu,a Tetsuya Narushima,a,b Kohei Imura,c Hiromi
Okamoto,a,b[Introduction] Scanning near-field optical microscopy with ultrashort laser pulses would bea highly potential method to detect ultrafast dynamics with ultrahigh spatial resolution. However, the laser pulses after passing through a near-field probe made of optical fiber is seriously broadened due to the group velocity dispersion in the fiber. In this work, by using theFourier synthesis techniques with a programmable phase modulator, we tried to recover femtosecond laser pulses which were broadened by the optical fiber used for the near-field probe.
[Experimental]
The most
successful techniques for ultrashort laser pulse shaping involve Fourier synthesis, where
the optical field is modified bymodulation of its spatially dispersed frequency components.
In our work, a programmablephase modulator (PPM, X8267,Hamamatsu) was used as thespatial modulator. The schematic diagram of the experimental
setup is shown in Figure 1. In
this setup, we have installed aFigure 1. The schematic diagram of experimental setup.
simple single-mode optical fiber to examine the system performance, but the fiber will be
replaced by a near-field probe
in the future for practical near-field experiments. The femtosecond laser pulses were dispersed by a grating. The dispersed frequency components were collimated by a concave cylindrical mirror (CCM), and introduced to the PPM that is located on the focal plane of theCCM. The frequency components were modulated by PPM, converged again by the CCM tothe grating, and then collinearly combined by the grating as a single beam. The beam was focused on a BBO crystal after passing through an optical fiber. The SHG radiation emitted
from the BBO crystal was detected by a photomultiplier tube (PMT), and its intensity wasused as the feedback signal to control the PPM.
One prominent merit of PPM is its high spatial resolution of modulation. It consists of590,000 pixels within a 20×20 mm2 area; each of those can be individually controlled. The
phase shift of each pixel is controlled by the brightness signal output from a PC. For incident
light of 800-nm wavelength, the maximum of phase shift is more than 4p rad. As the laser
beam was dispersed horizontally, the modulation of PPM to the laser beam can be controlled
by a series of vertical gray bars. A simulated annealing algorithm (SAA) was employed toIntensity

phase shift (p rad)Intensity

produceintensity
800
terns of the bars and to800
750
seek for the best
700
700
650
tion (a set of bars),
600
600
Dt=3.516 ps
that generates the550
Dt=377 fs
500
500
highest SHG signal. A
400
450
Ti:sapphire femtose400
300
350
cond laser (MaiTai,200-202468
10 12 14 16-202468
10 12 14 16
Spectra Physics) wastime (ps)time (ps)used as the source of(a)(b)
femtosecond pulses.
Its pulsewidth was
4.0
about 90 fs (autocorre3.5
3.0
lation width ~140 fs),
2.5
and the wavelength
2.0
1.5
was continuously tun1.0
able from 780 nm to0.5
0.0920 nm.
-0.5
A typical result
050
100
150
200
250
X Axis (bar sequence)is shown in Figure 2.
(d)(c)The 90-fs laser pulses
were broadened by theFigure 2. Compression of laser pulses broadened by an optical fiber
fiber to a few picose(1 meter in length). (a) The laser pulses broadened to 3.516 ps byconds because of
the fiber; (b) The laser pulses compressed to 377 fs by the PPM sysgroup velocity dispertem; (c) The brightness pattern applied to PPM produced by SAA
sion while propagatingwhen laser pulses were compressed; (d) The phase shift generated by
the PPM corresponding to the pattern (c).
in the fiber (Fig. 2a).
They were compressedby the PPM system toa few hundreds femtoseconds (Fig. 2b). Figures 2c and 2d show the pattern of the optimized
brightness signal applied to PPM and the relative phase shift respectively, after compressionof the laser pulses.
[Discussion] When the optical fiber was absent, the dispersed laser pulses can be recovered
to 100-fs level. We also tested the compression with a 20-fs laser source. The setup was also
working well. With an optical fiber in the path, however, the results were not good as expected. This should derive from the following reason. The modulation capability of the PPM
is limited. As mentioned above, the maxmium phase shift generated by the pixels of PPM isabout 4p. On the other hand, the dispersion produced by the long (1-meter length) optical fiber is so large that it is out of the compensation range that the PPM can provide. Also somenonlinear effects when high power laser passing thought the fiber might increase the difficulties of the compression.