Professor, Quantum Nanoelectronics Research Center, Tokyo Institute of Technology
Room-temperature continuous-wave lifetime exceeding 10,000 hours
—A quantum-wire laser realized by a top-down fabrication method—
In 1978, the possibility reducing transmission loss in optical fibers at a wavelength of 1.5 to 1.6 microns was only theoretical prediction. Prof. Arai, who was then a graduate student, developed a laser with a wavelength of 1.5 microns under the supervision of Professor Emeritus Yasuharu Suematsu (the former president of Tokyo Institute of Technology). Around the time, a Japanese company developed low transmission loss optical fibers with a wavelength of 1.5 microns. The progress in communication devices led to the enhancement of long distance optical communication. Since then, Prof. Arai has devoted his life to the development of optical communication devices.
In 2003, Prof. Arai developed a quantum-wire laser with a wire width of 23 nm and a period of 80 nm. GaInAsP active layers and InP cladding layers are stacked alternatively up to five layers thick using organo-metallic vapor-phase-epitaxy, and the layers are patterned by using electron beam lithography. Then, vertical gratings are fabricated by reactive ion etching, and the gratings are filled with InP. This is how quantum wires are fabricated. This laser with a quantum-wire structure showed no noticeable performance degradation even after more than 10,000 hours (22,600 hours as of 27th Oct. 2005) of operation at room temperature. He says, “I have been challenging myself to see how narrow I can draw patterns using a top-down method. So far, I have been able to draw a pattern with a period of 80 nm.
Prof. Arai’s 5-layered quantum-wire lasers with a wire width of 43 nm perform better than commercialized quantum-film lasers. He says “However, my team’s desire is to fabricate quantum-wire lasers with narrower wires and make use of quantum effects at room temperature. For wires wider than 30 nm, which cannot be expected to show clear quantum effects at room temperature, we do not use the word, ’quantum’ effect.” Currently, the threshold current density of a quantum-wire laser with a wire width of 23 nm is 1.6 times higher than that of a quantum-film laser. This is partly because the threshold gain for laser oscillation increases due to a decrease in the volume of an active layer based on narrow wires and the increase of the optical loss. Prof. Arai thinks that the volume ratio of quantum-wires to other parts of the laser of 1 to 1 is the best solution to the problem. He says, “The performance of a quantum-wire laser won’t be maximized unless patterns are drawn very densely. It is important to develop technology to fabricate fine patterns in desired positions and with desired densities.” In a quantum-wire laser with a wire width of 23 nm, the wire width fluctuation is currently about 8%. “The width of its emission spectrum is about the same as that of a normal quantum-film laser. I would like to make it half the current emission spectrum width. Thus, I have to obtain wire width fluctuations lower than 5%,” says Prof. Arai.
However, increasing the precision of patterning and etching is not a satisfactory solution to all of the problems. Currently, strained layer epitaxy with a strain of about 1% is used in laser film fabrication in order to decrease the threshold current. Prof. Arai says, “It seems that the strain in the first layer is different from that in the second layer when strained layer epitaxy is used. Theoretically, it is possible to fabricate a quantum-wire laser with half of the current emission spectral width when a size fluctuation is lower than 5%; however, unexpected things such as a broader spectral width happen with fabricated lasers.” He, then, fabricated a quantum-wire laser with only one active layer to eliminate the influence of strain on the properties of the laser. It requires strong reflectors because the optical gain is not high enough in a laser with one active layer. Then, his team thought of increasing the refractive index difference between the active layer and the cladding layers to 50% by using cladding layers made of SiO2 glass or benzocyclobutene, which has a low refractive index of 1.5. He says, “Placing electrodes on both sides of the layered film makes the device as thin as 0.1 microns. We call it a membrane laser.” It took three years for his team to make the laser operating under a room temeperature continuous wave (RT-CW) condition by photoexcitation. He says, “However, we would like to fabricate a high performance laser that is operated by current injection and would like to know how much we can increase its performance by both increasing optical confinement in an active layer and reducing the size of an active layer.” The optical gain in an ideal quantum-wire laser is two times larger than that of a quantum-film and four times larger when the refractive index difference between the active layer and cladding layers increases from 5% to 50%.
In optical device development, Prof. Arai insists on using a top-down method to fabricate ultrafine structures. He says, “In 1994, when a German and Russian team collaborated together to fabricate a quantum-dot laser using a bottom-up method, those who had crystal growth equipment rushed into developing devices using the method. However, even if you were able to fabricate a desired device using a bottom-up method, you are just one of the followers. If you fabricate a device using a top-down method, you have your own new field to explore.” When he fabricated the laser with a wavelength of 1.5 microns while he was in school, he felt that he played a role in the era of rapidly developing technology because his laser was commercialized within a short time. Now he advises young researchers as a professor that, although they may not be able to get good results, they should pursue their research interests in order to establish their own new fields. Prof. Arai says, “If you give up, you set your own limit there. You may not be able to get what you want or may find phenomena that you cannot understand, but that is only natural because you are doing something nobody has ever done before.”
