R&D Information
Success in Trapping and Delaying Light for over 1
ns in Photonic Crystals—Slowing Down the Speedof Light to 1/50,000

NTT has succeeded in trapping light for longer than
1 ns within a wavelength-sized microcavity using an
artificial periodic structure called a photonic crystal.
Using the same device, it has also reduced the speedof light in the structure to 1/50,000 of that in air,which delayed the light for over 1 ns.
It has long been believed that it is fundamentallydifficult to trap and delay light in any easy and convenient manner. Previous achievements required
bulky apparatus and/or ultralow-temperature cooling.
NTT has resolved this problem with a novel designfor ultrasmall optical resonators using photonic crystals and has constructed actual devices based on thisdesign using state-of-the-art nanofabrication technology. This opens up various possibilities for photonics
such as a large-scale photonic information processing
chip with extremely small power consumption and an
optical quantum information processor operatingwith single photons.
Experiments and results
The device is based on a photonic crystal withhexagonally arranged air holes 200 nm in diameter
fabricated by very fine electron-beam lithography
and high-resolution dry etching on a silicon chip. Ourdesign for ultrasmall optical cavities consists of a single missing line of air-holes in a photonic insulator(namely, the periodic part of the lattice). This linedefect part can transmit the light, and its loss is theoretically zero. The key point of our design is that several holes surrounding this line defect are intentionally shifted outwards by only 3–9 nm. This tiny shifteffectively modulates the width of this line defect partand produces strong confinement. Numerical calculations have shown that an optical confined mode withextremely small leakage loss is formed in the widthmodulated part.
Cavities of this type have been fabricated and integrated with input and output waveguides on a silicon
chip. The transmission spectrum showed an ultrasharp resonance peak with a width of 1.3 pm. This66

width corresponds to a cavity quality factor (Q-value)of 1.2 million, which is the highest value ever reported for any wavelength-sized optical cavity. Next, thetime-resolved output signal was measured after theinput was abruptly switched off. The results show thatlight was trapped in the cavity for over 1 ns. Subsequent pulse input measurements showed that the cavity delayed the transmission of an optical pulse by
1.45 ns. This delay corresponds to a reduction in thepropagation speed of the light to 1/50,000 of that inair. The device is only 8 µm long, but light travels
approximately half a meter in air in the same time.
The resultant trapping and delaying times are thelongest reported for any wavelength-sized dielectric
medium, and the reduction of the speed of light by
50,000 is over two orders of magnitude larger thanthat reported for any artificial dielectric slow-light
medium.
Future developments
NTT laboratories will try to apply these ultrasmall
optical cavities to ultrasmall optical processing
devices, such as all-optical switches, having extremely low power consumption. In addition, NTT will
investigate the possibilities for application to opticalbuffers with much longer delay times and to photonic memories that can store photon quanta directly.
Ultimately, the combination of these technologies
could lead to a photonic information processing chipwith extremely small power consumption and toquantum information processors that can be operated
by single photons.
For further information, contact:
E. Tamechika
Planning department
NTT Science and Core Technology Laboratory
Group
Tel: +81-46-240-5152
http://www.ntt.co.jp/sclab/contact_e/

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