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20 July 2009

 

Sumitomo Electric claims first pure-green semiconductor laser

Sumitomo Electric Industries Ltd’s Semiconductor Technologies R&D Laboratories in Itami, Japan have developed what is claimed to be the first semiconductor laser diode emitting in the pure-green region of the spectrum - at a wavelength of up to 531nm under pulsed operation at room temperature (Enya et al. Appl. Phys. Express 2 (2009) 082101, 17 July).

Light sources using lasers for display applications, such as laser TVs and pocket laser projectors, are expected to have superior properties in terms of size, weight and power consumption. However, up to now, only red and blue semiconductor laser diodes have been commercially available. Commercial green lasers (e.g. for use in laser pointers) work by down-converting the frequency of light emitted from a 1064nm infrared laser and re-emitting it at a green wavelength of 532nm. R&D targeting commercialization of green-emitting semiconductor laser diodes has therefore expanded rapidly in the past several years.

Gallium nitride (GaN), used for blue light-emitting devices, is expected to also be key for green light-emitting devices. However, up to now GaN has been plagued by the luminance efficiency declining sharply as the wavelength increases (e.g. from blue towards green). This is a combined result of the internal field effects as well as the deterioration of the crystal quality of the active layer. The main reason for the low luminous efficiency of GaN-based green light-emitting devices is the polarization of charge carriers due to the piezoelectric field caused by the large distortion in the GaN crystal structure (which has a larger effect on green lasers than on blue lasers). Several organizations are trying to alleviate this problem by using various crystal orientations.

Previously, in late February Osram Opto Semiconductors GmbH of Regensburg, Germany demonstrated the first gallium nitride (GaN)-based laser with an emission wavelength of 500nm (blue-green), and at the end of May Japan’s Nichia Corp reported blue-green lasing at 510-515nm (both using InGaN quantum wells grown on a c-plane GaN substrate).

Sumitomo Electric now says that it has inhibited the drop in efficiency by developing GaN crystal grown on the semipolar {2021} plane of a free-standing GaN substrate. This has weakened the internal field effects and yields improved-quality homogeneous InGaN quantum wells (QWs) in the laser’s light-emitting active layer, even at the high indium composition required for green wavelengths. Also, the spectral width of spontaneous emission from the laser is narrower than that on other planes, the researchers claim.

The firm also says that, by designing the active layer appropriately, it has succeeded in covering the entire range of the green lasing spectrum. While the lasing wavelengths of conventional frequency-converted lasers are locked at a specific wavelength, Sumitomo Electric says that its new laser can be tuned to any wavelength in the green region. For the average laser emission wavelength of 520nm, the typical threshold current was 491mA and the threshold current density was 8.2kA/cm2. For the longest lasing wavelength achieved (531nm), the threshold current was 924mA.

Furthermore, the lasing frequency remains almost unchanged even in the high-current range, so the firm believes that its device has advantages in high-power applications. Another advantage is that the dependence of the wavelength on ambient temperature is minimal, it is claimed.

The development of a green laser diode could enable red-green-blue (RGB) laser light sources and lead to new applications. Sumitomo Electric says that it has applied for over 60 patents related to the technology.

See related items:

Nichia pushes InGaN lasers nearer to green via 515nm emission

Osram Opto claims first 500nm InGaN laser

Non-polar InGaN lasers near green light

Search: Sumitomo Electric Green laser diode GaN

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