Three - University of Arkansas Physics Department

CLEO '02. Technical Digest. pp. 243-244 vol.l,2002
TUESDAY AFTERNOON / CLEO 2002 / 243
low. To overcome these difficulties, we propose
using the optical near-field as a carrier for signal
transmission, since it does not have to follow the
wavevector conservation law. Consequently, an
increase in the PL quantum efficiency is expected.
To evaluate the near-field components of the
PL, we compared the PL spectrum using the conventional
diffraction-limited optical method
with that obtained using an illumination-collection
mode near-field optical microscope (Fig. I)
at 10 K. For the near-field measurement, we used
a fiber probe with an aperture diameter of 140
nm. Free-standing Si nanocrystals in the range of
5-10 nm were fabricated by wet etching (10 ml
HF + 30 ml HNO, + 50 ml CH,COOH).
As shown in Fig. 2, the emission peak energy
for the far-fieldmeasurement was close to 1.53 eV
(h = 800 nm), which corresponds to the luminescencc
from the luminescence center. Furthermore,
the emission peak energy for the near-field
measurement was close to 2.1 eV (h = 530 nm),
which corresponds to the luminescence from the
quantum-confinement state of Si nanocrystals
with a diameter of 2.5 nm."
Since visible PL in the near-field measurement
was observed only when the fiber probe was in
close proximity to the sample surface (-10 nm),
the spectral shift is believed due to the near-Aeld
coupling of the probe and Si nanocrystals. Figure
3 illustrates the energy diagram of Si nanocrystals.'
The PL from the quantum-confinement
state of Si nanocrystals is subjected to both the
charge transfer (denoted a1 and recombination
(denoted B) mechanisms. In the case of freestanding
Si nanocrystals, the charge transfer rate
of a is larger than the decay rate of B.~ Consequently,
wc detect the PL from the luminescence
center (denoted A) in the far-field measurement.
However, the spectral shift of the PL in the nearfield
measurement implies that the charge transfer
rate of p increases in comparison with that of
a due to the near-field coupling of the Si
nanocrystals and the probe.
Wavelength [nm]
CTuK67 Fig. 2. PL spectrum of Si nanocrystals
at 10 K.
CTuK67 Fig. 3. Three-level-state model for
free-standing Si nanocrystals.' QC-Y: quantumconfinement
state of Si nanocrystals.
Furthermore, considering the throughput of
the fiber probe (- 10.~) and the respective peak
PL intensities, the PI. intensity for near-field
measurement was 5 times larger than that for farfield
measurement. This implies that the increase
in the quantum efficiency is due to its wavevector
non-conservation.
References
I. M. Ohtsu, "Near-field nano-optics toward
nanoiatorn deposition:' Proc. of SPIE, 3749,
47%479 (1999).
2. T.Yatsui, M. Kourogi, and M. Ohtsu,"A plasmon
waveguide for optical farinear-field
conversion:' Appl. Phys. Lett. (in press).
3. H. Takagi, H. Ogawa, Y.Yamazaki,A. Ishizaki,
and l: Nakagiri, "Quantum size effew on
photoluminescence in ultrafine Si particles:'
Appl. Phys. Lett. 56,2379-2381 [1990).
4. C. Delerie, G. Allan, and M. Lannoo, 'Theoretical
aspects of the luminescence of porous
silicon:' Phys. Rev. B, 48, 31024-11036
(1993).
5. T. Matsumoto, T. Futagi, H. Mimura, and Y.
Kanemitsu. "Ultrafast decay dynamics of luminescence
in porous silicon," Phys. Rev. B
47,13876-13879 (1993).
6. Y. Kanemitsu, H. Uto, and Y Masumoto,
"Microstructure and optical properties of
free-standing porous silicon films: Size dependence
oiabsorption spectrum in Si nanometer-sized
crystallites:' Phys. Rev. B 48,
2827-2830 (1993).
CTuL
Room: IOTA
Solltons and Waveguides
Sermei Ste~anov, INAOE. Mexico. Preside
CTUU
2:30 pm
The Photoretractlve Sollton as an
Electrrboptlc Modulator
Aqiang Guo, Yongan Tang, Baolai Liang and
Gregory 1. Salamo, Department of Physics,
University of Arkansas, Fayttteville, Arkansas
72701, Ernall: salamo@uarkedu
Mordechai Segev, Technion-Israel Ir~stitute of
Technology, Haifa 320W, Israel, Email:
msegev@~x.~echnion.acil
In integrated optics, most of the developmental
efforts have centered on the use of LNbO, and
LiTaO, crystals primarily because of their ready
commercial availability and well established techniques
for fabricating optical waveguides. Significant
interest however, lies in producing optical
waveguide devices in materials with a higher electro-optic
coefficient which could be used for
making compact low-voltage electro-optic modulators
and switches. A suitable choice for this is
strontium-barium niobate crystal (such as
SBN:75),The crystal exhibits a very large electrooptic
coefficient. which is more than one order
higher than that of LiNbO,. In this paper we discuss
how to fabricate an electro-optic modulator
in SBN using a permanent two-dimensional single-mode
waveguide impressed into a crystalline
lattice by a real-time photorefractive soliton.
Photorefractive solitons have been observed at
low light powers and exhibit robust trapping in
both transverse dimensions. Sol~ton self-induced
waveguides can he engineered by use of the soliton
existence curve. Such soliton-induced waveguides
can be used in various waveguide applications
and in multipleconfigurations.'" Although
the self-induced and easily erascd nature of photorefractive
soliton-induced waveguides is anractive
for dynamic applications, for many applications
it is advantageous to impress waveguides
into the crystalline structure permanently, that is,
to have the induced wa~guide last indefinitely
without an a plied field. Recently, Klotz et
DelRe et 01.'
and A. Guo el al." demonstrated
how to transform a "real-time" screening soliton
into one or multiple permanent waveguides by
means of ferroelectric domain reversal. This report
shows, by using a similar procedure as,9."
the fixed waveguide can be used as a modulator.
We use the standard setup for formingand fixing
screening so~itons.'.""~ The crystal is a I-cm
cube of SBN:75 doped with 0.02% cerium by
weight. An argon laser beam oscillating at 514.5
nm provides the input sol~ton beam (about 12
pm FWHM diameter ordinary-polarized beam)
into the crystal. Normally, the incident beams,
which propagate along an a-axis, diffract to about
100 pm at the exit face. When an electric field is
applied along the c-axis, the beams self-trap.
Once the beam self-focuses to its initial width
(soliton formed) at 3 kV/cm, we switch off the
laser light and the applied electric field sequentially.
At this stage, a space-charge field (which is
responsible for screening the external applied
field before switch off) within the soliton region
has formed. When the space-charge field is
greater than the crystal'coercive field (in our experiment
case), the domains will flip in the soliton
&gain. After the domains flip, the soliton and
background beams are switched back on and
space charge redistributes back to the equilibriunl
state. Finally a permanent waveguide left.
After furing the waveguide, we investigate the
effect of the induced waveguide on the soliton
polarization by using an 840-nm probe laser
beam. An ordinary-polarized probe beam is
launched to the SBN crystal. At the output side a
polarizer at the crossing position is set so that the
output beam is almost completely blocked without
applying an external voltage. This configuration
serves as the basis of the electro-optic amplitude
modulation of light. After the output
polarizer, a CCD camera or a photo-detector is
used to record to beam shape, intensity distributions
and intensity response with respect to the
applied electric field. Figure I shows the output
intensity [(a)] and photographs [(b)] of the output
probe beam as a function of the applied external
DC field. Both (a) and (b) are periodic
functions with DC voltage. The half-wave voltage
is l I0 V. Figure 2 shows the result for an AC applied
voltage as a function of time [(a)] and the
corresponding transmitted intensity rrsponse
[(bl and (c)] of the output probe beam with respect
to the applied AC voltage. In Fig. 2 (c) a low
frequency chopper was put between output polarize~
and photo-detector so that one can calculate
the modulation depth. The modulation
drpth l~rrr is about 65% and the modulation frequency
is about 1.4 KHz. Consider the confinement
of the experiment conditions (the size of
the crystal and the high voltage amplifier), there
are a lot of rooms to optimize above results.