This report introduces the "Micro-origami" technique, which can make three-dimensional microstructures by using a self-assembling mechanism based on the deformation of strained epitaxial layers. We also discuss its application to optical semiconductor devices.

1. Introduction
　Currently, a large amount of information is exchanged throughout the world, and thus research on optical telecommunication and optical recording techniques that make use of the superior properties of light will become more and more important. In this situation, semiconductor lasers, detectors, mirrors and lenses play very important roles in optical telecommunications for transmission, exchange, division, detection, and storage. To attain the functionality of an integrated system, a three-dimensional assembling technique to arrange these elements on the designed position is required. Normally, this is done by an assembling machine or by hand. In order to improve the accuracy of assembling micron sized elements, which is not easy by the conventional method, a new type of technique for self-assembling these elements is required.
　In this report, we would like to introduce a technique to fabricate three-dimensional microstructures, which we call "Micro-origami". This report also describes some examples of optical semiconductor devices made by this technique.

2. What is the "Micro-origami" technique ?
　A Micro-origami structure is fabricated by using an epitaxial growth technique, the method of depositing single-crystal films in an ultra-high vacuum. In this technique, multilayered film is designed with folding hinges etched into its surface, in the manner of the creased folds of paper. When the film is removed from its substrate by etching, the micro-structure takes its designed form due to the released stress in the film. In this way, our fabrication method reflects the design principles of "Origami", the traditional Japanese art of sculpting objects from folded paper.
　Because it is possible to fabricate these three-dimensional structures on a semiconductor substrate, we can integrate them with lasers and photodetectors.1)

3. Fabrication of three-dimensional microstructure
　We fabricated a mirror and a retro reflector, which consists of three mutually perpendicular mirrors, by using the "Micro-origami" technique on a single-crystal semiconductor GaAs substrate. When illuminated, the retro reflector reflects light back in the direction of the source, and it can transmit a modulated signal back to the interrogating source by misaligning one of its mirrors. These structures were fabricated using a self-assembling mechanism by releasing the stress. The angle between each plate can be strictly controlled by adjusting the thickness and the length of each hinge that connects the plates to each other (Fig. 1).

　　　
Fig. 1. Self-assembling three-dimensional microstructures fabricated by
"Micro-origami" technique.

　As shown in the above figure, these "Micro-origami" structures only use one type of hinge, which is the valley fold in origami. By changing the layered structure of the multilayered film, it is also possible to fabricate more complex structures by using both the "valley fold" and "mountain fold".4)
Figure 2 shows a self-assembling micro-stage that keeps the plate parallel to the substrate.

Fig. 2. Self-asssembling micro-stage using the combination of
"mountain fold" and "valley fold".

4. Optical semiconductor devices using "Micro-origami" technique
　We are also conducting research to realize functional elements and a device.

4-1 Mirror-array with moving mechanism
　Sometimes an electrostatic or other type of actuation mechanism must be introduced into the system to control the reflected light beam. By introducing an electrode, we have already verified that we can control the direction of the reflected beam (Fig. 3).

　　　
Fig. 3. Moving mirror-array by electrostatic actuation.

4-2 Directional sensing photodetector
　By exploiting the advantages of "Micro-origami", it is possible to introduce functional devices by monolithically integrating photodetectors and other possible elements such as mirrors. Figure 4 shows an example of the design and fabrication of a direction sensing photodetector that can detect the direction of the light beam coming into this element by using the shadowing effects on a light beam caused by "Micro-origami" wall.s

Fig. 4. Direction sensing photodetector.