Quantum-mechanical switch is fast

Oct. 1, 2000
IBARAKI—A group at the Femtosecond Technology Research Association (FESTA) has successfully created an ultrafast optical switch using submicron-sized semiconductor nonlinear optical materials. The thickness of the semiconductor was designed based on the unique hypothesis of the group.

IBARAKI—A group at the Femtosecond Technology Research Association (FESTA) has successfully created an ultrafast optical switch using submicron-sized semiconductor nonlinear optical materials. The thickness of the semiconductor was designed based on the unique hypothesis of the group.

The conventional theory of nonlinear optical response assumes that the electromagnetic wave passing through the material is uniform. Semiconductors are designed with this assumption in mind. The theory proposed by the FESTA group is different. Their "energy/size double-resonance" theory takes into account the material and its precise optical response. More specifically, they include nonlocal effects due to the spatial distribution of the electromagnetic wave, which leads to new nonlinear effects. At a specific semiconductor thickness, light that resonates with a particular electron level is strongly trapped; thus, the nonlinear process is further enhanced.

The group worked with materials widely used in ultrafast electronic and optical devices. The structure involved sandwiching gallium arsenide between layers of aluminum gallium arsenide. The film thickness was 110 nm. The researchers used molecular-beam-epitaxial crystal-growth methods for the atomic-level manipulations.

A pulsed beam with a 0.82-µm wavelength and pulse half-width of 1.5 ps was used to probe the material. The polarization perpendicular to the input signal polarization was chosen as the output signal to be detected. When the switching signal polarization was positioned at 45°, polarization rotation of the output signal occurred due to nonlinear optical effects (see figure).

Multilayer semiconductor structure serves as an ultrafast optical switch (left). Nonlinear optical effects induced by a 1.5-ps pulse striking the 110-nm-thick multilayer film cause polarization rotation (right).
Click here to enlarge image

The experiments confirmed high switching efficiency and ultrafast response properties. The output signal relaxation time is 1.5 ps, which is about three orders of magnitude faster than seen in nonlinear response using carrier absorption. In addition, with a switching signal of 100 aJ/µm2 (1 aJ = 10-18 J), the reflected signal intensity for the same polarization direction was only 20% compared to the case with no switching signal. This corresponds to a 0.3pi phase shift due to nonlinear effects. This nonlinear sensitivity is two orders of magnitude greater than reported for bulk gallium arsenide and shows progress toward the ideal pi-shifting for switches.

This experiment was a proof-of-principle experiment for pure optical switches; the chosen wavelength of light was not in the range used for current optical communications. In addition, the temperature of the materials during measurement was 5 K.

Courtesy O plus E magazine, Tokyo

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