Light sources using lasers for display applications, such as laser TV’s and pocket laser projectors, are expected to have superior properties in terms of size, weight and power consumption. For this reason, R&D activities aimed at commercialization of these devices have expanded rapidly over the past several years. Up to now, only red and blue laser diodes were commercially available, while green lasers (*1) were obtained by frequency conversion of infrared lasers. Gallium nitride (GaN) semiconductors, commercially available for blue LEDs, are also expected to be the key material for light-emitting devices in the green region. However, the material has been plagued with a phenomenon where the luminance efficiency shows a rapid decline with increasing wavelength.
At Sumitomo Electric, they have overcome this problem by developing a GaN crystal which inhibits the efficiency drop, resulting in room temperature pulse operation of a laser diode emitting in the pure-green region at 531nm. It is the first green laser diode in the world.
Features of their green laser diodes are as follows:
1. High quality crystal emitting in the green region
The luminous efficiency of GaN light-emitting devices is known to show a sharp decline with increasing wavelength (*2). This is a combined result of the internal field effects as well as the deterioration of the crystal quality of the active layer. Several organizations are studying to alleviate this problem by changing the crystal orientation.
At Sumitomo Electric, they have developed a crystal which weakens the internal field effects and also improves the quality of the active layer. This has lead to the successful development of the world’s first green laser diode.
2. Tunability of the wavelength in the green region
With a proper design of the active layer, Sumitomo Electric have succeeded in covering the entire range of the lasing spectrum in the green region. While the lasing wavelengths of the conventional frequency-converted lasers are locked at a specific wavelength, their device can be tuned to any wavelength in the green region. Furthermore, the lasing spectrum remains virtually unshifted even in the high current range, and therefore, we believe our device has advantages in high power applications.
Another advantage of their green laser diode is that the dependence of wavelength at ambient temperature is minimal.
They have applied for over 60 patents related to this technology. Details of the device will be presented in the July 17th edition of the Applied Physics Express.
The successful development of the green laser diode leads to the red-green-blue (RGB) laser light sources and should lead to new applications.
A laser operates in the green region of the optical spectrum. Until now, green lasers based on semiconductors materials, such as blue or red lasers, did not exist. Commercial green lasers, such as those used in laser pointers, emit green light at the wavelength of 532nm by down converting a 1064nm infrared laser light. The needs for large number of mechanical parts resulting in high costs, along with their inherent low efficiencies, have prevented widespread commercialization of these devices.
Sharp efficiency decline with increasing wavelength
The main reason for the low luminous efficiency of green light-emitting devices based on GaN materials is an electric field, commonly known as “the piezoelectric field.” Piezoelectric polarization caused by a large distortion in the crystal structure is the origin of this field. The piezoelectric field, while it has less effect on blue lasers, has a serious influence upon green lasers.
Source: Sumitomo Electric Industries