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Two photons reveal crystal defects

17 Jun 2002

Japanese scientists make a 3D image of the defects in a zinc selenide crystal.

Using two-photon excitation, Japanese scientists have, for the first time, observed the internal defects of semiconductor crystals in three-dimensions. Having studied defects to a depth of 200 µm in zinc selenide (ZnSe), the researchers now plan to do the same with photonic crystal structures (Optics Letters 27 297).

Yoshimasa Kawata and colleagues from Shizuoka University use a laser-scanning microscope to excite blue photoluminescence from within ZnSe crystals. Kawata told Optics.org: "By scanning the focused spot, we can get a three-dimensional photoluminescence distribution." In areas where no photoluminescence is seen, there is a defect at that point.

The laser-scanning microscope comprises a Ti:Sapphire laser to excite the photoluminescence and a photomultiplier tube to detect the emission. The laser operates at 790 nm (doubled to 395 nm), emitting 80 fs pulses that are focused to a selected depth between the surface and 200 µms into the crystal.

By imaging the crystal in 1 µm sections, this allowed the researchers to generate a 3D map of the defects. The resolution of the system is approximately 500 nm.

Whilst Kawata admits that many researchers use two-photon microscopy, he points out that this is mostly to study biological samples. He said: "As far as we know, we are the first to use two-photon-induced photoluminescence for observing three-dimensional internal defects in semiconductor crystals."

The researchers have also carried out unpublished work on GaN crystals. Using the same technique, Kawata says that his team now plans to examine the internal structures of photonic crystals. "We also hope to study semiconductor devices with quantum-well structures," he added.

Blue- and violet-emitting laser diodes are based on semiconductor crystals, such as ZnSe and GaN. Defects affect the efficiency of both the laser diodes and quantum-well structures.

As well as examining defects, Kawata will attempt to fabricate three-dimensional structures inside the crystals. He told Optics.Org: "We are trying to fabricate photonic crystals in semiconductor materials by inducing the optical damage with a higher intensity laser spot."

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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