Electroluminescent laser

A solid-state laser in which an inverse population is formed at the levels of dope ions of the elements with unfilled electron shells by means of impact excitation of said ions with hot carriers, made on the basis of an electroluminescent capacitor in which the layer of electroluminescent substance is optically homogeneous, doped with said elements, and serves as the active element of the laser. The electrodes of the capacitor perform the functions of the Fabry-Perot cavity mirrors and those of the electrodes in order to produce the electrical field required to excite the active element. One of these electrodes is partially transparent to the laser emission.

BACKGROUND OF THE INVENTION 
The invention relates to lasers and in particular to solid-state lasers for 
use in quantum electronics, computer devices, optoelectronics, holography 
and display devices. 
A solid-state injection laser employing a p-n junction monocrystal as an 
active element is known in the art. The facets of the crystal which are 
normal to the junction are polished to serve as mirrors of a Fabry-Perot 
cavity. The monocrystal is located between two excitation electrodes which 
is energized by a required voltage in order to excite the active element. 
This laser suffers from a number of drawbacks, vis. the small size of the 
emission surface (about 1 sq.mm); also high current densities in the 
generation mode (the threshold current is about 100 A/cm.sup.2 at 
4.2.degree. K and 10.sup.5 A/cm.sup.2 at the room temperature); no 
generation at room temperature under the conditions of continuous 
excitation. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a solid-state laser 
having a large emission surface and low current densities in the 
generation mode which is able to operate at the room temperature under the 
conditions of continuous excitation and which assures a high efficiency of 
converting electric energy into light energy. 
This object is achieved by providing a solid-state laser which comprises a 
Fabry-Perot cavity having an active element excited by an electric field 
that is produced by a voltage being applied to the excitation electrodes 
having the active element located between them and which, according to the 
invention, is arranged around a so called electroluminescent capacitor 
with the layer of the electroluminescent material of said capacitor being 
optically homogeneous and being doped with elements having unfilled 
electron shells and which serves as the active element of the laser while 
the electrodes of the capacitor perform the functions of Fabry-Perot 
cavity mirrors and those of the active element excitation, with one of 
them being partially transparent to the laser emission. 
It is preferable that the active element be made in the form of a film. 
The preferred material for said film is ZnS:Mn. 
The active element may be made as a crystal having plane-parallel facets. 
It is also feasible that the active element could use a powder in an 
immersion medium which serves as a dielectric material transparent to the 
laser emission whose refractive index is equal to that of said powder. 
In the preferred embodiment of the invention, it is at least expedient to 
design the partially transparent electrode mirrors as a structure 
consisting of a number of interlaced layers having high and low refractive 
indices, which would have a high refraction coefficient for the laser 
emission wavelength, and a layer of a conductive material which would be 
transparent to the laser emission. 
It is feasible that the multi-layer structure should be made of interlaced 
ZnS, and MgF.sub.2 ; and the layer of the conductive material be made of a 
SnO.sub.2 film. 
The proposed solid-state laser possesses a number of advantages, the major 
of them being: a simple design, a small size, low-cost for the materials, 
the efficiency of the conversion of electric energy to light energy said 
conversion being almost equal to unity with a low current threshold, which 
are attributed to the method of obtaining the inverse population through 
the impact excitation of the impurity centres under the conditions of the 
electrical fields direct excitation. Besides, the laser has a wide 
emission surface which can have an area of scores of square centimeters. 
It also makes it possible to obtain generation at the room temperature.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The solid-state laser shown schematically in FIG. 1 comprises a substrate 1 
and an electroluminescent capacitor placed onto the substrate which is 
formed by a lower electrode 2 applied to the substrate 1 and an upper 
electrode 3 both made of a highly reflective material and serving also as 
the Fabry-Perot cavity mirrors. Between the electrodes a layer of 
electroluminescent substance is located which comprises an active element 
4 of the laser, a protection layer 5 and an insulator layer 6. 
The substrate 1 should have a mirror-like surface and should be able to 
withstand the temperatures encountered in the process of depositing the 
electroluminescent material. If the laser emission is brought out through 
the substrate 1, the latter shall be made of transparent materials, e.g. 
glass, quartz crystal, etc. 
In case the emission is brought out through the upper mirror-electrode 3, 
the functions of the substrate 1 can be performed by a plate of a highly 
reflective metal having a mirror-like surface which simultaneously serves 
as the lower mirror of the cavity. 
The active element 4 of the laser should be made of an optically 
homogeneous electroluminescent substance. Suitable for this purpose are 
substances doped with elements having unfilled electron shells, e.g., the 
transition or rare-earth elements in which it is possible to produce 
inverse populations in the impurity levels through out the electric field 
excitation. 
It is preferable that wide-zone high-resistance semiconductors, such as 
ZnS:Mn, should be used for this purpose. 
The active element 4 can be made by applying a film or a powder onto the 
mirror substrate, with the powder being immersed into a medium of a 
dielectric material which is transparent to the laser emission whose 
refractive index is equal to that of the powder. 
The active element 4 can also be designed as a crystal having 
plane-parallel polished facets. In this case the substrate 1 is excluded 
from the design. 
The electrode mirrors 2 and 3 are made as layers of a conductive and a 
highly reflective material. 
In the laser embodiment under discussion, the emission is brought out 
through the electrode mirrors 5 which is partially transparent to this 
emission. The electrode mirror 2 and 3 are designed either as continuous 
members or as intermittent pieces consisting of areas which are 
electrically isolated from one another and have preset sizes and shapes, 
e.g. rectangles, circles, segments, certain symbols or systems of parallel 
stripes. Such arrangements allows for multiple sources of stimulated 
emission of a required geometry on a single substrate to be obtained as 
well as flat matrix or mosaic screens. The electrodes 2 and 3 are made of 
metal films, such as Al, or Au. 
The electrode mirros 2 may also be made as a polished plate of a highly 
reflective conductive material which can simultaneously perform the 
functions of the substrate 1. 
The protection layer 5 serves to prevent the mutual diffusion between the 
material of the electrode mirrors 3 and the electroluminescent substance 
referred to thereafter as a phosphor in the course of deposition of the 
latter. Materials that could be used to make the protection layer should 
meet a number of requirements imposed by the temperatures involved in the 
process of manufacturing the laser. These are: a low diffusion coefficient 
for the material of the electrode mirrors 3, chemical resistance to the 
substances used for the layers adjacent to the protection layer 5, a low 
rate of diffusion of its components into the layer of the phosphor. 
Suitable substances for the protection layer are, for example, SiO, 
Tn.sub.2 O.sub.5, CaF.sub.2. In case the active element 4 is made of 
ZnS:Mn, but it is feasible that the protection layer should be made of a 
recrystallized film of an undoped basic phosphor substance, e.g. a ZnS 
film. The protection layer 5 should be made as thin as possible in order 
to reduce the voltage drop that occurs across it. 
The insulator layer 6 performs the same function as in ordinary 
electroluminescent films, i.e., it prevents the short circuiting of the 
phosphor layer in the process of deposition of the upper mirrors 3, thus 
raising the break down level of the electric field strength in the laser. 
This layer is made as an insulator film (for instance, SiO) whose 
thickness is much smaller than that of the active element 4. 
FIG. 2 shows another embodiment of a solid-state laser made as an 
electroluminescent capacitor which is applied onto a substrate 1. In 
contrast with the embodiment described above and in order to increase the 
Q-factor of the cavity, the electrode mirrors 2 and 3 are arranged as 
multi-layer structures consisting of interlaced layers 7,8,9 and 10,11,12 
having high and low refractive indices and of layers 13 and 14 (one for 
each structure) made of a conductive material which is transparent to the 
laser emission. An active element 4 is placed between the structures. The 
multi-layer structures should have a high reflection coefficient at the 
laser emission wavelength. The layers 7,8,9 and 10,11,12 can be made of 
ZnS and MgF.sub.2 or of ZnS and Na.sub.3 AlF.sub.6. They should be 
arranged interdigitally. The layers 13 and 14 can be made as transparent 
films of a conductive substance selected from the SnO.sub.2, In.sub.2 
O.sub.3, CdO, ZnO group. The number of layers in the above mentioned 
structures can exceed the number of three as is shown in FIG. 2. The lower 
and upper electrode mirrors 2 and 3 respectively can have various numbers 
of interlaced layers havng high and low refractive indices, the actual 
number depends upon which is partially transparent and used to transmit 
the laser emission. 
There can be different ways of arranging the conductive layers 13 and 14 
with respect to the interlaced layers 7,8,9 and 10,11,12. The layers 13 
and 14 can be arranged so that the layers 7,8,9 and 10,11,12 as well as 
the active element 4 are located between them or so that placed between 
them are either the layers 7,8,9 (or 10,11,12) and the active element 4, 
or only the active element 4. The two latter ways of arranging the layers 
13 and 14 are preferable from the points of view of being able to lower 
the value of the laser operating voltage. 
The laser can be of a simpler design with a sufficiently high Q-factor for 
the cavity, where the non-transparent electrode is made as an opaque metal 
layer having a high reflection coefficient (for instance, an A1 film), 
while the partially transparent electrode mirrors is made as a multi-layer 
structure. 
The proposed solid-state laser operates as follows. A voltage from a power 
suply not shown in the drawing is applied to the electrode mirrors 2 and 3 
(FIG. 1) or to the layers 13 and 14 (FIG. 2) which creates an intensive 
electric field (about 10.sup.6 V/cm) in the active element 4, which is 
sufficient to produce impact excitation of the electroluminescence. When 
the excitation reaches its threshold level, the inverse population is 
created at the levels of the impurity centres (transistion or rare-earth 
elements) and the laser starts generating. The function of the Fabry-Perot 
cavity in the proposed laser embodiment is similar to that in the known 
devices. The wavelength of the laser emission depends on the type of 
phosphor and its doped impurity, as well as on the thickness of the active 
element 4 and on the angle at which the emission leaves the laser cavity. 
A fuller understanding of the essence of the present invention may be had 
from the following description of an example of its embodiment, its 
operation and manufacturing procedure. 
The proposed laser is arranged around thin (about 1 micron) ZnS:Mn 
electroluminescent films. A partially transparent gold layer is applied 
onto the glass plane-parallel substrate 1 having a reflection coefficient 
of R = 65 to 75% which simultaneously serves as the electrode 2 and a 
Fabry-Perot cavity mirror. It carries the protection layer 5 made as an 
undoped ZnS film 150 to 200 nanometers thick. Placed above the layer 5 is 
an electroluminescent ZnS film doped with Mn having a concentration of 
Mn-C.sub.Mn = 1-2 weight percent and being within 0.6 to 1.0 micron thick 
(the active element). A Si0-insulator layer 6 being 20-25 nanometers thick 
is then placed and an opaque A1 film which serves as the second electrode 
3 of the electroluminescent capacitor and at the same time as the upper 
Fabry-Perot cavity mirror. It has a reflection coefficient of R=90%. In 
operation, the electrodes 2 and 3 are fed with an audio frequency 
effective voltage of 65 to 75 V. The laser emission is brought out through 
the lower electrode 2 and the substrate 1. At a voltage below 60 V the 
system being discussed would produce a spontaneous emission the spectrum 
of which is shown in FIG. 3 (curve 15) where the X-axis gives the 
wavelength in nanometers and Y-axis indicates the intensity of emissions 
related to the intensity at the maximum of a emission band. After the 
threshold of 60 V has been reached, the emission band would get 
considerably compressed and in the generation mode it half-width would be 
6 to 8 nanometers (curve 16), which is one order of magnitude below the 
natural emission band ZnS:Mn (curve 17). The divergence angle of the laser 
emission in this case does not exceed 10.degree.. The compression of the 
emission band is accompanied by a sharp increase of its power as 
illustrated by curve 18 (FIG. 4) where the X-axis shows the intensity of 
the electrical field in volts per centimeter and the Y-axis presents the 
specific power emission in milliwatts per square centimeter of the laser 
emission surface. 
The laser being described has been manufactured in the following way. 
A glass plane-parallel substrate 1 cleaned beforehand is used as a basis 
onto which a partially transparent gold layer is deposited by evaporation 
in a vacuum of (1+2).times.10.sup.-5 torr. The reflection coefficient of 
the gold layer is 65 to 75%. It serves as an electrode 2 and at the same 
time as a Fabry-Perot cavity mirror. On undoped film of ZnS 150-200 
nanometers thick is deposited onto it by evaporation in a vacuum of 
(1+2).times.10.sup.-5 torr which serves as a protection layer 5. This film 
is crystallized by means of annealing same in a vacuum of 10.sup.-5 torr 
at 550 to 600.degree. C for 5 to 10 minutes. Placed above this layer is an 
electroluminescent film (the active element 4) of the ZnS:Mn active 
substance which is about 600 nanometers thick. The process consists of two 
steps. At first a ZnS film is applied by means of evaporation in a vacuum 
of 10.sup.-5 torr. Simultaneously an Mn-activator is vaporized into this 
film. Then, the total layer is annealed in a vacuum of about 10.sup.-5 
torr at 650 to 670.degree. C for 5 to 10 minutes. 
During the procedure, the film is crystallized and simultaneously the Mn 
activator becomes uniformly implanted into the ZnS lattice throughout the 
thickness of the film. The concentration of the implanted manganese is 
from 1 to 2 weight percent. The vacuum evaporation process is then used to 
apply a SiO insulator layer 6 being 20 to 35 nanometers thick and an 
opaque A1 film having a reflection coefficient of R=90%, which serves as 
the second electrode 3 of the electroluminescent capacitor and as the 
upper mirror of the Fabry-Perot cavity.