Light-emitting device

Details Light-emitting device Light-emitting device

2000-08-22

2004-03-09

Eckert, George (Department: 2815)

Coherent light generators

Particular active media

Semiconductor

C372S096000, C372S102000, C257S043000, C313S503000, C313S504000

Reexamination Certificate

active

06704335

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a light-emitting device utilizing electroluminescence (EL).
BACKGROUND OF ART
Semiconductor lasers have been used as a light source for an optical communication system. Semiconductor lasers excel in wavelength selectivity and are capable of emitting light with a single mode. However, semiconductor lasers require many stages of crystal growth and are difficult to manufacture. Another problem with semiconductor lasers is the limitation to the types of light-emitting materials which can be used. This restricts the wavelength of light which can be emitted by semiconductor lasers.
Conventional EL light-emitting devices can emit light with a wavelength having a broad spectral width and have been applied to displays and the like. However, such EL light-emitting devices are unfit for application to optical communications and the like which require light with a narrow spectral width.
DISCLOSURE OF INVENTION
An object of the present invention is to provide a light-emitting device which can emit light with a wavelength having a remarkably narrow spectral width in comparison with conventional EL light-emitting devices, exhibits directivity, and can be applied to not only displays but also optical communications and the like.
A light-emitting device according to the present invention comprises:
a light-emitting layer being capable of emitting light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer; and
an optical waveguide for transmitting light emitted from the light-emitting layer,
wherein a grating is formed in the optical waveguide.
According to this light-emitting device, electrons and holes are introduced into the light-emitting layer respectively from the pair of electrode layers (cathode and anode). Light is emitted when the molecules return to the ground state from the excited state by recombination of the electrons and holes in the light-emitting layer. The light emitted from the light-emitting layer is provided with wavelength selectivity and directivity by the grating formed in the optical waveguide, specifically, a grating with alternating two medium layers each of which has a different refractive index arranged periodically.
It is preferable that the grating be a distributed feedback type or distributed Bragg reflection-type grating. Such a distributed feedback type or distributed Bragg reflection-type grating causes the light emitted from the light-emitting layer to resonate. As a result, light having wavelength selectivity, a narrow emission spectral width, and excellent directivity can be obtained. The pitch and depth of the grating are designed depending on the wavelength of the light to be emitted.
Light can be emitted with a single mode by providing the distributed feedback type grating with a &lgr;/4 phase shifted structure or a gain-coupled structure. “&lgr;” used herein indicates the wavelength of the light inside the optical waveguide.
A grating of a distributed feedback type having a &lgr;/4 phase shifted structure or a gain-coupled structure is a preferable configuration common to the light-emitting devices of the present invention. It is sufficient for the grating to achieve the above functions. The grating may be formed in any layers constituting the optical waveguide.
It is preferable that the light-emitting layer comprises organic light-emitting materials. Use of organic light-emitting materials expands selection of the materials and enables emission of light having various wavelengths in comparison with the case using a semiconductor or inorganic materials.
The aspects described below from (a) to (d) can be given as examples of such a light-emitting device.
(a) In a first aspect of the light-emitting device, the optical waveguide comprises a core layer mainly transmitting light, and a cladding layer having a refractive index lower than the refractive index of the core layer; and
the core layer comprises a layer which is different from the light-emitting layer.
A feature of this light-emitting device is that the core layer, which the light transmits mainly, is formed from a layer different from the light-emitting layer. The core layer is preferably made from materials having a refractive index higher than that of the light-emitting layer. This refractive index relationship ensures efficient introduction of the light emitted from the light-emitting layer into the core layer. The grating may be formed in the core layer. The grating may be formed in the boundary area between the cladding layer and a layer in contact with the cladding layer such as the core layer.
In the case where the light-emitting layer is an organic light-emitting layer comprising organic materials, the core layer may serve not only as a light transmitting layer, but also as at least one of a hole transport layer, electron transport layer, transparent electrode layer, and the like. The cladding layer is designed to have a refractive index lower than that of the core layer. The cladding layer may serve not only as a layer for light confinement, but also as an electrode layer, substrate, hole transport layer, electron transport layer, and the like.
(b) In a second aspect of the light-emitting device, the optical waveguide comprises a core layer mainly transmitting light, and a cladding layer having a refractive index lower than the refractive index of the core layer; and
the core layer comprises a layer including the light-emitting layer, and
the grating is formed in the optical waveguide.
A feature of this light-emitting device is that the light-emitting layer is included in the core layer which is the main light transmitting layer. The grating may be formed in the core layer. The grating may also be formed in a boundary area between the cladding layer and a layer in contact with the cladding layer, such as the core layer. In this light-emitting device, the light-emitting layer may be formed continuously or discontinuously one after another.
In the case where the light-emitting layer is an organic light-emitting layer formed by organic light-emitting materials, the core layer may further comprise at least one of a hole transport layer, an electron transport layer, a transparent electrode layer, and the like. The cladding layer may be designed to have a refractive index lower than that of the core layer. The cladding layer may serve not only as a layer for light confinement, but also as an electrode layer, a substrate, an hole transport layer, an electron transport layer, and the like.
In the light-emitting device according to this aspect, the grating may be formed by the light-emitting layer and a layer in contact with the light-emitting layer. According to the device having such a configuration, light emitted from the light-emitting layer resonates directly by the grating in the region including the light-emitting layer. As a result, light is emitted with a selected wavelength and excellent directivity.
(c) The light-emitting device as a third aspect of the present invention comprises an optical fiber section formed in one body,
wherein the optical fiber section comprises a core layer and a cladding layer, and
wherein the optical waveguide is formed continuously with at least one of the core layer or the cladding layer of the optical fiber section.
In this light-emitting device, because at least either the core layer or the cladding layer of the optical fiber section is formed in one body waveguide, light having excellent wavelength selectivity and directivity can be emitted from the light-emitting layer in the optical waveguide and supplied to the transmission system with high efficiency.
In this light-emitting device, the light-emitting layer may be included in the optical waveguide. The optical waveguide may be either continuous with the core layer of the optical fiber section or formed separately while being optically connected. Furthermore, it is preferable that the optical waveguide comprises a core-layer-continuing portion which continues from the core layer of the optical fiber

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