Electric lamp and discharge devices – With luminescent solid or liquid material – Solid-state type
Reexamination Certificate
2001-08-03
2004-05-18
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Solid-state type
C313S498000, C428S917000
Reexamination Certificate
active
06737802
ABSTRACT:
Japanese Patent Application No. 2000-244747, filed on Aug. 11, 2000, is hereby incorporated by reference in its entirety.
TECHNICAL FIELD
The present invention relates to a light-emitting device using electroluminescence (EL).
BACKGROUND ART
A semiconductor laser is used as a light source for optical communications systems. The semiconductor laser excels in wavelength selectivity and is capable of emitting light in a single mode. However, it is difficult to fabricate a semiconductor laser because many stages of crystal growth are needed. Moreover, since types of light-emitting materials used for the semiconductor laser are limited, the semiconductor laser cannot emit light with various wavelengths.
Conventional EL light-emitting devices emit light with a broad spectral width and are used in some applications such as for displays. However, conventional EL light-emitting devices are unsuitable for optical communications and the like in which light with a narrow spectral width is required.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a light-emitting device which can emit light with a remarkably narrow spectral width in comparison with conventional EL light-emitting devices and with directivity, and is applicable not only to displays but also to optical communications and the like.
First Light-emitting Device
A first light-emitting device according to a first aspect of the present invention comprises:
a light-emitting layer which emits light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer; and
an optical element for causing light generated in the light-emitting layer to be transmitted in a predetermined direction,
wherein the optical element forms an incomplete photonic band which inhibits spontaneous emission of light in one dimension or two dimensions; and
wherein light generated in the light-emitting layer is emitted by inhibiting spontaneous emission in two dimensions.
The incomplete photonic band used herein refers to a band formed in the case where a complete photonic band gap is not formed. For example, in the case where the optical element is in a shape of grating in which a first medium layer and a second medium layer are arranged alternately, a complete photonic band gap may not be formed when the difference in the refractive indices between the first medium layer and the second medium layer is small.
According to the first light-emitting device, electrons and holes are injected into the light-emitting layer respectively from the pair of electrode layers, specifically, a cathode and an anode. Light is emitted when the molecules return to the ground state from the excited state by allowing the electrons and holes to recombine in the light-emitting layer. Spontaneous emission of this light is inhibited in two dimensions, whereby the light has a very narrow spectral width and high efficiency.
Specifically, in the first light-emitting device, a photonic band is formed by the optical element. In this band, a high density of states is obtained at energy at a specific band edge. If the optical element is formed so that the energy level of the light spectrum emitted in the light-emitting layer includes the energy level at this band edge, emission of light in the light-emitting layer tends to occur at the energy level at this band edge. Therefore, the first light-emitting device is capable of emitting light with a wavelength corresponding to the energy level at a predetermined band edge and with a narrow spectral width, thereby exhibiting high emission efficiency.
Second Light-emitting Device
A second light-emitting device according to a second aspect of the present invention comprises a substrate and a light-emitting section,
wherein the light-emitting section includes:
a light-emitting layer which emits light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer;
an optical element for causing light generated in the light-emitting layer to be transmitted in a predetermined direction; and
an insulating layer which is disposed between the pair of electrode layers, partially has an opening through which current is supplied to the light-emitting layer, and functions as a current blocking layer which determines a region in which current flows,
wherein the optical element forms an incomplete photonic band which inhibits spontaneous emission of light in one dimension or two dimensions; and
wherein light generated in the light-emitting layer is emitted by inhibiting spontaneous emission in two dimensions.
According to the second light-emitting device, since the insulating layer in the light-emitting section functions as a current blocking layer in addition to the effects of the first light-emitting device, the region for current supplied to the light-emitting layer can be specified. Therefore, current intensity and current distribution can be controlled in the region from which it is desired to emit light, whereby light can be emitted with high emission efficiency.
In the case where the insulating layer functions as cladding, in the case of a waveguide including a light-emitting layer as a core and an insulating layer as cladding, the waveguide mode of light transmitted to the waveguide section through a light-transmitting section can be controlled by specifying the opening in the insulating layer. Specifically, the waveguide mode of light transmitted through the light-emitting layer (core) can be set at a predetermined value by specifying the width of the region in which light is confined (the width perpendicular to the direction in which light is transmitted) by the insulating layer (cladding). The waveguide mode and the waveguide generally have a relation represented by the following equation.
N
max+1
≧K
0
·a
·(
n
1
2
−n
2
2
)
½
/(&pgr;/2)
K
0
: 2&pgr;/&lgr;
a: half width of waveguide core
n
1
: refractive index of waveguide core
n
2
: refractive index of waveguide cladding
Nmax: maximum value of possible waveguide mode
Therefore, in the case where the parameters of the above equation such as the refractive indices of the core and cladding are specified, the width of the light-emitting layer (core) specified by the width of the opening in the current blocking layer is selected depending on the desired waveguide mode. Specifically, the width (2
a
) of the light-emitting layer corresponding to the core in a desired waveguide mode can be calculated from the above equation by substituting the refractive indices of the light-emitting layer provided in the current blocking layer and the insulating layer as a current blocking layer for the refractive indices of the core and cladding of the waveguide, respectively. It is preferable to determine the width of the core layer in the waveguide section to which light is supplied from the light-emitting section while taking into consideration the width of the light-emitting layer determined as described above, the calculated value obtained from the above equation based on the desired waveguide mode, and the like. Light in a desired mode is transmitted from the light-emitting section to the waveguide section with high connective efficiency by setting the width of the light-emitting layer, the width of the core layer, and the like to optimum values. In the light-emitting section, there may be a case where the light-emitting layer in the current blocking layer formed by the insulating layer does not uniformly emit light. Therefore, it is preferable that the designed values for each section such as the light-emitting layer, light-transmitting section, and waveguide section be suitably adjusted based on the width (2
a
) of the core (light-emitting layer) calculated using the above equation so that each section exhibits high connective efficiency.
The waveguide mode of the light-emitting device is preferably 0 to 1000. In particular, the waveguide mode is preferably about 0 to 10 in communication applications. Light with a predetermined waveguide mode can be efficiently obtained by s
Kaneko Takeo
Koyama Tomoko
Oliff & Berridg,e PLC
Patel Ashok
Seiko Epson Corporation
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