Short Pulse Generation by Adiabatic
Tuning of Light
Takasumi Tanabe, Masaya Notomi, Hideaki Taniyama and Eiichi Kuramochi
We can change the pitch of an acoustic resonator such as a guitar
even after plucking the string by varying
the tension of the string before it stops
vibrating. When we translate this phenomenon to the world of photons, we
should be able to change the wavelength
of the light trapped in a high-Q cavity
by modulating its resonance in a time
much shorter than the photon lifetime. 1
We call this adiabatic wavelength shifting (AWS). Ultrahigh-Q cavities have
now been fabricated on-chip, 2 which
enables us to demonstrate AWS. 3-5
;e photonic crystal nanocavity
that we used in our experiment is a
width-modulated line defect cavity that
is inline connected to input/output
waveguides. Because line defects with
di;erent widths have various cut-o;
wavelengths, light can be satisfactorily
confined in the cavity region if we create
a mode-gap by shifting the air holes
slightly towards the outside of a narrow
(barrier) line defect. Wider line defects
are used as I/O waveguides.
First, we charge the cavity with continuous laser light at a wavelength that
is in resonance with the cavity. ;en we
turn the input laser o; at −100 ps and
record the output spectrogram. Since the
input has been turned o;, the output after −100 ps is the light that was trapped
in the cavity. ;e output exhibits a
smooth exponential decay, which gives a
photon lifetime of 0.32 ns (Q= 3. 7 105).
Naturally the wavelength of the output
light is identical to the input.
Next, we inject a short pump pulse
from the top of the slab to generate carriers at a timing of 0 ps. As a result of
the carrier-plasma dispersion e;ect, the
cavity resonance shifts toward a shorter
wavelength in a time much shorter than
the photon’s lifetime. ;en we observe
a modulated output spectrogram, where
a short wavelength component appears
(a) Nanocavity i/o waveguide i/o waveguide Barrier line defects
W avelength [n m]
(a) Scanning electron microscope image of a fabricated width-modulated line-defect silicon
photonic crystal (PhC) nanocavity. The air holes are shifted slightly outside the line defect.
The 3-D plots in (b) and (c) are spectrograms of the output light at the waveguide after they
have been trapped in a PhC nanocavity that have a mode volume of 0.13 (µ)m3. The graphs
behind the plots are spectrally convoluted temporal waveforms. (b) Output without modulation. (c) Output when the cavity is modulated at 0 ps. AWS shifts the wavelength of the
trapped light toward a shorter wavelength, and the light is immediately output into the waveguides and generates a short pulse.
immediately after the modulation, even
though we have not entered light at this
wavelength. ;is is direct evidence of
AWS. Unlike wavelength conversion
based on optical nonlinearity, AWS is a
completely classical process, which enables us to convert the light wavelength of
a single photon. ;erefore, we believe this
may o;er the possibility of developing
quantum information processing on chip.
Another aspect of AWS is that it allows us to achieve dynamic control of the
Q of the cavity, which is required to demonstrate photonic memory. A 0.06-ns
wide pulse, whose width is much shorter
than the original photon lifetime, is extracted from the high-Q cavity. When we
change the resonance of the cavity and
activate the AWS process, the wavelength
of the trapped light shifts close to the
mode-gap ends of the barrier line defects.
;is strengthens the coupling between
the cavity and I/O waveguides, resulting
in a low-Q mode.
Takasumi Tanabe ( firstname.lastname@example.org), Masaya Notomi ( email@example.com), Hideaki Taniyama and Eiichi
Kuramochi are with NTT Basic Research Laboratories, NT T Corporation, in Kanagawa, Japan.
1. M. Notomi and S. Mitsugi. Phys. Rev. A 73, 051803(R)
2. T. Tanabe et al. Nature Photon. 1, 49-52 (2007).
3. T. Tanabe et al. Phys. Rev. Lett. 102, 043907 (2009).
4. S. Preble et al. Nature Photon. 1, 293-6 (2007).
5. M. McCutcheon et al. Opt. Express 15, 11472-80 (2007).