Nonlinear optical tuning of a two-dimensional silicon photonic crystal
H. W. Tan and H. M. van Driel
Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario M5S1A7, Canada
S. L. Schweizer and R. B. Wehrspohn
Nanophotonics Materials Group, Department of Physics, University of Paderborn, 33095 Paderborn, Germany
U. Gösele
Max-Planck-Institute for Microstructure Physics, Weinberg 2, D-06120 Halle, Germany
(Received 30 October 2003; revised manuscript received 1 June 2004; published 8 November 2004)
We use the real (Kerr) and imaginary (two photon absorption) parts of a third order optical nonlinearity to
tune the long 1.6 m and short wavelength 1.3 m band edges of a stop gap in a two-dimensional silicon
photonic crystal. From pump-probe reflectivity experiments using 130 fs pulses, we observe that a 2 m pulse
induces optical tuning of the 1.3 m edge via the Kerr effect whereas a 1.76 m pulse induces tuning of the
1.6 m band edge via both Kerr and Drude effects with the latter related to two-photon induced generation of
free carriers with a lifetime of 900 ps.
DOI: 10.1103/PhysRevB.70.205110 PACS number(s): 42.70.Qs, 42.65.Hw, 42.65.Re
I. INTRODUCTION
Photonic crystals (PC) have unusual dispersion properties
that strongly influence the propagation characteristics of light
beams. However, while linear optical properties of PCs have
received considerable attention, nonlinear properties are not
nearly as well understood, including the extent to which non-
linear effects can be used to tune PC optical properties
quickly.
1–8
In earlier work within our group,
6
linear optics, in
particular single photon absorption by 800 nm, 150 fs pulses,
was used to inject high carrier densities which tuned the
mode frequencies of a two-dimensional (2D) photonic crys-
tal through Drude-induced changes to the linear optical sus-
ceptibility; time-resolved frequency shifts of a Bragg gap up
to 29 nm were observed. Although pulse-width limited
turn-on times were observed, the recovery time was limited
by carrier lifetime 100 ps. In general, overall pulse-width
limited response can only be achieved using nonresonant,
nonlinear induced changes to material optical properties such
as the optical Kerr effect (a third order nonlinearity) or the
optical Stark effect which has been employed by Shimizu et
al.
9
in a multicomposite 1D system. There has previously
been a suggestion of Kerr-induced reflectivity changes
4,5
for
PCs, but the limited data is insufficient to confirm this, or
permit detailed analysis.
Here, using 130 fs pump pulses, we clearly demonstrate
tuning of a 2D silicon PC using a Kerr nonlinearity. Because
silicon has an indirect band gap of 1.1 eV (equivalent to
= 1.1 m) at 295 K and a direct gap at a relatively high
energy of 3.5 eV =355 nm, relatively weak phonon as-
sisted linear or two-photon absorption occurs across the vis-
ible and near infrared; there may then be an advantage to
considering nonlinear effects in PCs made from this semi-
conductor. In particular we show that the short wavelength
edge 1.3 m of a photonic band gap can be redshifted via
the Kerr effect with a pump beam at 2.0 m. For a pump
wavelength of 1.76 m and probe wavelength of 1.6 m
(the long wavelength edge of the same gap) a redshift also
occurs via a Kerr effect. However at high pump intensities,
generation of free carriers from two-photon absorption (2PA)
becomes apparent, leading to a blueshift of the photonic
band edge via a Drude contribution to the linear dielectric
constant.
II. EXPERIMENTAL DETAILS
The experimental results were obtained with a parametric
generator pumped by a 250 kHz repetition rate Ti-sapphire
oscillator/regenerative amplifier which produces 130 fs
pulses at 800 nm at an average power of 1.1 W.
10
The signal
pulse from the parametric generator is tunable from 1.2
to 1.6 m and the idler pulse is tunable from 2.1 to 1.6 m.
The 2D silicon PC sample has a triangular lattice arrange-
ment of 560 nm diameter, 96 m deep air holes with a pitch,
a, equal to 700 nm. Figure 1(a) shows a real space view of
the sample while Fig. 1(b) illustrates the photonic band
structure for the - M direction, which is normal to a face of
the PC. Of particular interest is the third stop gap for
E-polarized (E-field parallel to the pore axis) light. Lying
between 1.3 and 1.6 m, this gap falls between two dielec-
tric bands that are sensitive to changes in the silicon refrac-
tive index. Our purpose is to optically induce changes to the
two edges with idler pulses from the parametric generator
and probe these changes via time-resolved reflectivity of the
signal pulses. Note that, because of the link between the
signal and idler wavelengths, different pump wavelengths
(2.0 m for a 1.3 m probe; 1.76 m for a 1.6 m probe)
must be used when the probe wavelength is changed. How-
ever, as will be shown in what follows, small changes in the
pump wavelength can lead to significant changes in the in-
duced optical processes.
For the time-resolved reflectivity geometry, as indicated
in Fig. 1(a), pump pulses polarized along the - M direction
are focused to a spot size of 25 m on the top surface of the
PC near a crystal face and propagate along the direction z
of the pore axis. An E-polarized probe beam focused to a
PHYSICAL REVIEW B 70, 205110 (2004)
1098-0121/2004/70(20)/205110(5)/$22.50 ©2004 The American Physical Society 70 205110-1