Organic lasers

Description of equivalent WO 9850989 A2

ORGANIC LASERS

Field Of The Invention

The present invention relates to the field of light

emitting devices, in particular, to laser light emitting devices

containing organic materials.

Background Information

Several recent publications have reported phenomena

such as superluminescence and amplified spontaneous emission in

polymeric organic light emitters such as conjugated polymers. (N.

Tessler et al . , Nature 382, 695 (1996); F. Hide et al . , Science

273, 1833 (1996), both of which are incorporated herein by

reference) . The materials used in those emitters are spin-coated

from a solution of the polymer or its chemical precursors.

Optically pumped, stimulated emission from organic laser dyes,

introduced into inert, spin-coated polymers or gels has been described in the literature. (R. E. Hermes, et al . , Appl . Phys .

Lett. 63, 877 (1993); M.N. Weiss et al . , Appl . Phys . Lett . 69,

3653 (1996); H. Kogelnik et al . , Appl . Phys . Lett . 18, 152

(1971); M. Canva et al . , Appl . Opt .. 34, 428 (1995), each of

which is incorporated herein by reference) .

When compared with other electrolumninescent materials,

however, spun-on polymeric materials do not exhibit particularly

good thickness uniformity, ability to achieve extremely high

materials purity, operating lifetimes, and ease of integration

with other conventional semiconductor fabrication processes. In

the field of organic light emitting devices (OLEDs) for flat

panel display applications, for example, small molecule OLEDs

currently offer better operating lifetimes by an order of

magnitude over their spin-coated, polymeric analogs. (L.J.

Rothberg et al . „ "Status of and Prospects for Organic

Electroluminescence", J. Mater. Res. 1996, 11:3174; N.C. Greenham

et al . , "Semiconductor Physics of Conjugated Polymers", Solid

State Physics 1995, 49:1, both of which are incorporated herein

by reference) .

There is much recent interest in lasing action and

stimulated emission in thin films of small molecular weight

organic semiconductors and polymers as organic semiconductor

lasers ("OSLs") . The low cost of organic materials and ability to grow them as quasi- and non-epitaxial thin films facilitates

integration of OSLs with other optoelectronic devices, making

them attractive for a number of applications. The particular

optical and electronic properties of organic semiconductors

result in OSL performance that is significantly more temperature

stable than conventional inorganic laser diodes, a potential

advantage in optical communications and sensor applications. For

example, the lasing action in optically-pumped slab waveguide

structures of vacuum-deposited thin films of small molecular

weight organic semiconductors has been recently demonstrated.

(See V.G. Kozlov et al . , Conf on Lasers and Electro-optics CLEO

^■97, CPD-18, Opt. Soc. Am., Baltimore, MD, May 1997,

incorporated herein by reference) . The output power,

differential quantum efficiency, and emission wavelength of these

organic semiconductor lasers (OSLs) are significantly more stable

to changes in temperature than conventional inorganic laser

diodes. This benefit of organic laser structures combined with

the inherent advantages of organic semiconductors such as low

cost, quasi- and non-epitaxial growth (S.R. Forrest et al . , Phys.

Rev. B 49, 11309 (1994) , incorporated herein by reference) , and

ease of integration with other optoelectronic devices, provides

strong motivation for further research. Presently, there is an

interest in the development of OSL structures that result in desired OSL properties such as narrow bandwidth emission, the

minimal use of active organic materials, and the facilitation of

wavelength tuning and electrical pumping.

Summary of the Invention

The present invention is directed to laser devices that

include organic materials that lase when pumped to thereby

produce laser light. The invention includes both optically and

electrically pumped embodiments of unique structure and organic

laser materials. In contrast to spun-on polymeric materials, the

organic materials used in the present invention offer the

advantages of excellent thickness uniformity, extremely high

materials purity, and ease of integration with conventional

fabrication processes. Moreover, the laser devices of the

present invention provide a clear threshold in output power,

well-defined laser beams, cavity modes, and spectral line

narrowing .

The present invention includes a laser comprising a

mirror layer and a layer of active organic material over the

mirror layer. The active organic material lases when pumped to

thereby produce laser light. The mirror layer reflects at least

a portion of the light produced by the first active organic

material . In one aspect, the present invention relates to

optically-pumped lasers which include a source of optical pump

energy for pumping the active organic material.

In another aspect, the present invention relates to

electrically-pumped lasers in which the active organic material

is electroluminescent and is disposed between a pair of

electrodes. The active organic material is pumped when an

electric current is passed between the electrodes.

Brief Description of the Drawings

Fig. 1 shows an arrangement including an embodiment of

a laser device according to the present invention.

Fig. 2 shows an arrangement including an embodiment of

a laser device according to the present invention.

Fig. 3 is a graphical representation of the

relationship between peak output power and pump energy density

for an embodiment of the present invention.

Fig. 4 (inset to Fig. 3) shows the polarization of the

emission of an embodiment of the present invention, as a function

of the angle between a plane orthogonal to the film surface and

the plane of a polarizer.

Figs. 5A and 5A show electrically-pumped laser device

embodiments of the present invention. Fig. 6 shows a cross-section of a laser cavity

structure in accordance with an embodiment of the present

invention.

Fig. 7 shows a cross-section of an optically-pumped

laser, in accordance with an embodiment of the present invention.

Figs. 8A-8H show the chemical formulae for DCM, Alq₃

CBP, DCM2, rhodamine-6G, coumarin-47, perylene, and pyrromethane-

546, respectively.

Fig. 9 shows a cross-section of a tunable optically-

pumped laser, in accordance with an embodiment of the present

invention.

Fig. 10 shows a cross-section of an electrically-pumped

laser, in accordance with an embodiment of the present invention.

Fig. 11 shows a cross-section of a stacked, multicolor

electrically-pumped laser, in accordance with an embodiment of

the present invention.

Fig. 12 shows a cross-section of a tunable

electrically-pumped laser, in accordance with an embodiment of

the present invention.

Fig. 13 shows a cross-section of a waveguide reducing

structure, in accordance with an embodiment of the present

invention. Fig. 14 shows emission spectra for an optically-pumped

laser device, in accordance with an embodiment of the present

invention.

Fig. 15 shows the spontaneous emission spectrum in the

substrate normal direction from an exemplary embodiment of an

OVCSEL in accordance with the present invention.

Fig. 16 shows the emission spectrum, at a high

excitation level, from an exemplary embodiment of an OVCSEL of

the present invention.

Fig. 17 shows a high resolution emission spectrum from

an exemplary embodiment of an OVCSEL of the present invention as

a function of excitation near the lasing threshold.

Fig. 18 shows the dependence of output energy on the

input pump energy near the lasing threshold for an exemplary

embodiment of an OVCSEL of the present invention.

Detailed Description

One embodiment of an optically-pumped laser device 100

of the present invention is shown in Fig. 1. The laser device

100 includes a first mirror layer 110 and a layer 112 of active

organic material. The first mirror layer 110 is a "mirror" in

that it is capable of reflecting at least some portion of the

light emitted from the layer 112 of organic material. For example, many of the mirror layers used in the devices of the

present invention are used to transmit laser light of preferred

characteristics such as wavelength, orientation, bandwidth,

brightness, and the like, while reflecting light having non-

preferred characteristics. The mirror layers used in the present

invention include, for example, substrates of glass, quartz,

sapphire or plastic, polished InP layers, distributed Bragg

reflectors, metallic mirrors and the like.

The organic materials used in the present invention are

"active" in that they lase when pumped by any suitable means,

such as by optical or electrical energy, to thereby produce laser

light. The active organic materials include host and dopant

materials, which are deposited by any suitable technique onto

first mirror layer 110. The first mirror layer 110 is optionally

pre-coated with a cladding layer 111 comprising a material having

an index of refraction less than the organic material of layer

112. When the layer 112 is pumped with optical pump energy from

a source such as a nitrogen laser 118, the laser device 100

produces laser beam 116 of a predetermined color.

Layers 111 and 112 are of any suitable thickness. By

way of example, layers 111 and 112 in the embodiment shown in

Fig. 1 are approximately 2 μm and 120 nm, respectively. The

thickness of the cladding layer 111 is preferably chosen to provide optical isolation of the layer 112 from the opague first

mirror layer 110. The thickness of the layer 112 is chosen to

provide uniform pumping and to optimize the ability of the layer

to function as an optical waveguide between the cladding layer

111 and the surrounding environment. For example, when layer 112

comprises CBP, a thickness of 120 nm results in approximately 60%

absorption of pump energy and a waveguiding confinement factor

of approximately 59% at = 485 nm. To further facilitate the

waveguiding capability of layer 112, cladding layer 111 has an

index of refraction less than layer 112 and thus serves to

increase optical confinement in layer 112 in the direction normal

to the surface of the device. As an example, the cladding layer

111 comprises Si0₂ (refractive index, "n", of about 1.5) when the

layer 112 comprises CBP as host organic material (n~1.8) . The

cladding layer 111 may not be required, for example, where the

first mirror layer 110 is intended for laser emission

therethrough. However, in the embodiment shown in Fig. 1 in

which laser emission occurs through the side 113 of the layer

112, the cladding layer 111 is preferred. In addition, layer 112

preferably includes two reflective facets 113 and 114 that are

substantially parallel to each other, thereby forming an optical

resonator in layer 112. Layers 111 and 112 are deposited onto the first mirror

layer 110 by any suitable technique, such as by plasma enhanced

chemical vapor deposition and vacuum thermal evaporation,

respectively. The concentration of the dopant in the layer 112

is typically less than 10% by mass, but can be as low as .01%.

It is generally preferred that the layer 112 is deposited by

thermal co-evaporation of host and dopant materials at a

respective mass ratio of approximately 100:1.

The laser devices of the present invention are grown

on any mirror layer to which the organic layer 112 will adhere

and which preferably has a lower index of refraction (n) than the

organic layer material . Acceptable mirror layer materials

include plastics, glass and silicon coated with Si0₂. A

preferred first mirror layer material is polished InP. A typical

length for a device in accordance with the present invention is

5 mm, although devices of shorter length, such as 0.5 mm, are

possible .

The formation of optically smooth, sharp facets 113 and

114 at opposing edges of the layer 112 is a natural advantage of

vacuum-deposited films. The shape of the facets 113 and 114

mimic the shape of the corresponding facets of the underlying

first mirror layer 110 (or cladding layer 111, if used) . As

such, it is preferred that the sides of ^• the first mirror layer 110 be smooth and parallel to each other. Facet reflectivities

of 7% are obtained via vacuum deposition of layer 112, which is

sufficient to provide the necessary optical feedback. Optical

feedback is also achieved with other structures, such as an

optical grating placed underneath the optically pumped region of

the organic film to thereby form a distributed feedback

structure. As an alternative to forming the facets 113 and 114

by vacuum-deposition of layer 112, layer 112 can be deposited

onto the first mirror layer 110 (or cladding layer 111, if used)

by any other suitable method, and the combination is then cleaved

to form smooth, sharp facets.

Any optically pumped embodiment of the present

invention such as the device 100 is pumped using a light source

that emits light that can be absorbed by the host material

molecular species of layer 112. For example, the device 100 is

optically pumped with a nitrogen laser 118 which generates 500

psec pulses with a wavelength of 337 nm at a 50 Hz repetition

rate. As shown in Fig. 1, the pump beam is focused, such as by

a cylindrical lens 117, into a stripe 115 on the film surface

oriented orthogonal to the facets 113 and 114. The width of the

stripe 115 is any suitable dimension, such as lOOμm, for example.

The pump beam is waveguided in layer 112, thereby producing laser

beam 116, which is emitted from the facet 113 and/or 114 of the device 100. Moreover, through a gain-guiding effect, the

refractive index of the illuminated portion 115 of the organic

layer 112 will be higher than that of the non- illuminated

portion, thereby providing confinement of the optical mode in the

vertical direction.

Fig. 2 shows an embodiment 200 of an optically-pumped

laser in accordance with the present invention in which it is not

necessary to form stripe 115 on layer 112. In this embodiment,

the cladding layer 111 forms a ridge on the first mirror layer

110 on which the layer 112 of organic material is deposited. In

this embodiment, the optical mode is thus confined in both the

z- and y-directions . The width of the layers 111 and 112 should

preferably be narrow enough to support only a single lateral

optical mode (e.g., 1-10 μm) . The thickness of the active

organic layer 112 should be approximately equal to the reciprocal

of the absorption coefficient of the host material at the

wavelength of the light with which the device is pumped. If a

reflective layer (not shown) is placed between the layer 111 and

the first mirror layer 110, the thickness of the organic layer

112 should be approximately half the reciprocal of the absorption

coefficient of the host material at the wavelength of the light

with which the device is pumped. To protect the laser device of

Fig. 2, it is optionally overcoated with a transparent material (not shown) whose index of refraction is lower than that of the

layer 112.

An example of the dependence of the peak output power

of the device 100 to the pump energy density is shown in Fig. 3.

From this relationship, the lasing threshold is clearly

discerned. Each line segment in the graph of Fig. 3 is a linear

fit to empirically measured points. The slopes of the two line

segments emphasize a change in differential quantum efficiency,

from 0.2% (below the lasing threshold) to 10% (above the lasing

threshold) . It is to be noted, however, that measured

differential quantum efficiencies represent a considerable

underestimation since the lasing region in an optically pumped

gain-guided device is only a small fraction of the material which

is being pumped. Hence, most of the pump power is lost in non-

lasing regions. The differential quantum efficiency decreases

to 7% at excitation levels above 10 J/cm² (not shown) ,

corresponding to peak output powers exceeding 4 W.

Fig. 4 (inset to Fig. 3) shows the intensity of the

emission from device 100 as passing through a polarizer as a

function of angle between the plane orthogonal to the film

surface and the plane of the polarizer. The emission is strongly

linearly polarized, as is expected for laser emission. The

degree of polarization measured is 15 dB although it should be noted that this result is limited by the measurement arrangement.

The solid line, which is a fit of the empirically measured

points, follows sin²(), where is the polarizer angle.

In addition to the optically-pumped embodiments

discussed above, the present invention also provides

electrically-pumped organic semiconductor laser embodiments.

Such embodiments make use of electroluminescent materials that

emit light when excited by electric current.

Fig. 5a shows a cross-sectional view of an

electrically-pumped laser device embodiment of the present

invention. Device 300 includes a first electrode 31, a bottom

cladding layer 32, a layer 33 of active organic material, a top

cladding layer 34 and a second electrode 35, which are deposited

in sequence over a first mirror layer 30. The degree of optical

confinement in the active layer 33 depends on factors such as the

indices of refraction of the cladding layers 32 and 34.

The layer 33 of active organic material comprises any

suitable organic electroluminescent material, such as doped Alq₃.

Either one of the cladding layers 32, 34 is composed of a hole

conducting material, such as MgF₂ doped with 10% of TPD. The

other one of the cladding layers is composed of an electron

conducting material, such as Alq₃ or MgF₂ doped with Alq₃. For

either cladding layer 32 or 34, the MgF>_> can be replaced with another alkali halide such as LiF, KF or KI or with a

transparent, low index of refraction, conducting organic

material. Upon passing a current between electrodes 31 and 35,

electrons and holes are injected into the active layer 33 where

the energy is transferred from the conducting host material to

the dopant materials, which emit light.

An optical resonator is formed by the edges of the

device 300, i.e., mirror facets Ml and M2. At least one of the

electrodes 31 and 35 is patterned into a stripe oriented in the

x-direction. That portion of the active material which is

electrically pumped experiences a change in optical gain, thus

forming a waveguide in the lateral direction (i.e., the gain-

guiding effect) .

In the electrically-pumped laser device shown in Fig.

5a, the index of refraction of the active layer 33 is preferably

higher than that of the cladding layers 32 and 34. This ensures

that most of the mode overlaps with the gain layer. Preferably,

the indices of refraction of the cladding layers 32 and 34 should

be substantially equal to provide optimal optical confinement

within layer 33.

The cladding layers 32 and 34 should be thick enough

to substantially prevent absorption of the optical mode at the

electrodes 31 and 35 and to allow for efficient current injection. For single-mode operation, the thickness of the

active layer 33 should be substantially equal to the lasing

wavelength divided by twice the index of refraction of the active

layer. For higher modes, the thickness of the active layer 33

should be accordingly greater.

Optical confinement in layer 33 is also achieved, for

example, by photo-bleaching the layer 33 to define a waveguide

therein. Using this technique, once the active layer 33 has been

deposited, a photomask is applied which partially covers the

active layer 33 and the layer is exposed to intense UV light in

an 0₂ environment. The unmasked portions of the active layer 33

are thus bleached, lowering their index of refraction. The

masked portion of the active layer 33 retains its original,

higher index of refraction and thus results in optical

confinement therein.

As another example, optical confinement is achieved by

patterning the first mirror layer or bottom cladding layer to

achieve index guiding. For example, photo-bleaching can be used

on the bottom cladding layer 32 to create an index modulated

distributed Bragg reflector under the active layer 33.

An optical resonator is optionally formed in the z-

direction. In this case, the top and first electrodes also serve

as optical mirrors, or, if the electrodes are transparent to light, high reflectivity mirrors are added on both sides of the

structure. The maximum of the optical mode of this structure

should spatially match the position of the optical layer.

Furthermore, the wavelength of the optical mode should overlap

with the gain spectrum of the dopant material.

Fig. 5b shows a further embodiment of an electrically-

pumped laser device in accordance with the present invention.

In this embodiment, the first electrode 31 extends beyond the

laser structure so as to provide a contact surface.

Additionally, an insulator 37 is provided on a side of the laser

structure and a contact 36 extending from the second electrode

35 to the surface of the first mirror layer 30 is deposited on

the insulator.

The present invention includes organic vertical-cavity

surface-emitting laser (OVCSEL) structures, in which laser

cavities are used to produce a clear threshold in output power,

a well-defined laser beam, cavity modes, and spectral line

narrowing of emission above threshold to less than 1 . The high

gain in these films is sufficient to generate lasing even in

cavities with active regions less than 500 nm thick, thereby

minimizing the amount of active organic material required for

operation. Fig. 6 shows a cross-section of a laser cavity

structure 1100 in accordance with an embodiment of the present

invention. In this structure, an active organic material layer

1110 is disposed between a first mirror layer 1111 and a second

mirror layer 1112 to thereby form a cavity of thickness, t. The

organic material in layer 1110 is "active" in that it lases when

pumped by optical or electrical means to thereby produce laser

light. The first and second mirror layers 1111, 1112 each

reflect a substantial amount of the light produced by the layer

1110 so that only coherent laser light passes through layers

1111, 1112, the laser light having a desired wavelength and

characterized by a narrow bandwidth. The first and second mirror

layers 1111, 1112 each preferably reflect at least 90%, more

preferably 95%, and most preferably 98%, of the light produced

by the pumping of layer 1110. If the first and second mirror

layers 1111, 1112 reflect substantially the same percentage of

light emitted from layer 1110, the passage of light through both

first and second mirror layers 1111, 1112 will be substantially

equal, as shown in Fig. 6. If, for example, the second mirror

layer 1112 reflects a greater percentage of light than first

mirror layer 1111, however, then the passage of light will be

primarily through the first mirror layer 1111 provided that the absorption characteristics of the first and second mirror layers

are equal .

Fig. 7 shows an optically-pumped OVCSEL structure 1200,

in accordance with an embodiment of the present invention. A

first mirror layer 1111, a layer of active organic material 1110

and a second mirror layer 1112 are disposed on a substantially

transparent substrate 1113. The layer of active organic material

1110 is pumped by incident optical pump energy 1115. The source

of optical pump energy (not shown) is any suitable source of

intense light, such as a nitrogen laser.

The first and second mirror layers 1111, 1112, are any

suitable reflective materials or structures. A preferred

structure for the first mirror layer 1111 is a distributed Bragg

reflector ("DBR") dielectric mirror stack. DBR mirrors are

commercially available and consist of /4 thick dielectric

layers, where represents the wavelength of the DBR mirror

reflective stopband. DBR mirrors thus render the ability to

control the OVCSEL output spectrum, which in turn narrows the

linewidth of such output. DBR mirrors are typically

characterized by reflectivities in excess of 99%. The second

mirror 1112 is preferably a DBR mirror or a layer of reflective

metal or alloy such as silver, platinum, aluminum, magnesium-

aluminum alloy, or combinations thereof. Metal mirrors typically have reflectivities in excess of 90%, but absorb more light than

do DBR mirrors. When the second mirror 1112 comprises a metal,

the OVCSEL structure 1200 preferably includes an organic buffer

layer 1114 to reduce quenching of the organic material in the

layer 1110 at the organic / metal interface with the layer 1112.

In the embodiment shown in Fig. 7, the combined reflectivity and

absorption of the second mirror layer 1112 is greater than that

of the first mirror layer 1111, thus resulting in laser emission

1116 through the first mirror layer 1111.

The substrate 1113 is any suitable transparent

substrate such as quartz, glass, sapphire or plastic. The

substrate 1113 is limited to those materials which are

transparent to the wavelengths of incident optical pumping energy

and of laser light produced by layer 1110.

In all embodiments of the present invention, the active

organic material comprises host and dopant molecules . The pump

energy imparted to layer 1110 is absorbed by the host molecule

and is non-radiatively transferred by a dipole-dipole transition

to the dopant molecule. For this to occur, the emission spectrum

of the host should overlap with the absorption spectrum of the

dopant. The resulting efficient "Fδrster" energy transfer

requires only a low concentration of dopant molecules, which in

turn reduces the lasing threshold, increases laser efficiency, and extends operational lifetime. The host materials used in the

present invention are selected from any materials that provide

good charge transport and are able to transfer energy to a dopant

material via Fόrster energy transfer or carrier capture. In

addition, the rate of energy transfer to the dopant must be

faster than the non-radiative recombination in the host. The

dopant materials used in the present invention are any highly

efficient luminescent molecules having luminescence (lasing)

emission in the same spectral range as the transparency region

of the host:dopant system. Exemplary host-dopant systems for use

as the active organic material in the present invention, and some

of their associated lasing characteristics are set forth in Table

I. The formulae for the chemicals known as DCM (Exciton Inc. of

Dayton, Ohio), Alq₃ , CBP, DCM2 (Exciton Inc. of Dayton, Ohio),

rhodamine-6G, coumarin-47, perylene, and pyrromethane-546 are

shown in Figs. 8A-8H, respectively. The organic materials used

in the present invention are deposited by any suitable technique,

such as by vacuum thermal evaporation.

Table I. Exemplary active organic materials for use in the OVCSELs of the present invention.

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The present invention includes embodiments in which the

laser emission is tunable to a specific wavelength. Fig. 9 shows

an example of an optically-pumped, tunable OVCSEL 1250 in

accordance with an embodiment of the present invention. In this

embodiment, the thickness, t, of the layer 1110 of active organic

material changes monotonically from the left edge 1117 to the

right edge 1118 of the structure. In an alternative embodiment,

the thickness of the layer 1110 changes in a stepwise fashion for

the emission of discrete wavelengths . By changing the thickness

of the OVCSEL cavity, the emission wavelength is tunable by as

much as 50 nm or more, due to the wide gain spectrum of the

lasing material in layer 1110. Such thickness variation is

achieved, for example, by masking the substrate with a sliding

shadow mask during growth of layer 1110. Alternatively, an

effective thickness variation is achieved by tilting the

substrate with respect to a pump beam source . The tunable OVCSEL

is pumped by a pump beam 1115, which excites the active organic material of layer 1110 at a point Xo, which is at a distance d

away from the right edge 1118. The wavelength of laser emission

1116 is a function of the cavity thickness t and the index of the

organic material in layer 1110. By changing the position of

point Xo, a different section of the OVCSEL structure 1250

(having a different thickness, t) is excited by pump beam 1115,

thus resulting in a different wavelength of emission 1116.

Changing the position of Xo by varying d is accomplished by, for

example, moving the OVCSEL structure 1250, moving the position

or angle of the pump beam 1115, or both. As an example, the

inventors have changed the laser emission of an optically-pumped

Alq₃:DCM laser of the present invention from 598 to 635 nm by

changing the thickness of layer 1110 from 430 to 500 nm.

Fig. 10 shows an electrically-pumped OVCSEL structure

1300, in accordance with an embodiment of the present invention.

A first mirror layer 1111, a layer of active organic material

1110 (comprising sub-layers 1110a, 1110b and 1110c) and a second

mirror layer 1112 are disposed on a substantially transparent

substrate 1113. If the first and second mirror layers 1111, 1112

are not able to function as electrodes, or alternatively if it

is preferable to have separate electrodes, a first electrode 1120

is disposed between the first mirror layer 1111 and the layer of

active organic material 1110, and a second electrode 1121 is disposed between the layer of active organic material 1110 and

the second mirror layer 1112. In this embodiment, the organic

material in layer 1110 is electroluminescent such that it is

pumped to produce laser light when an electric current is passed

therethrough. As is known in the art, the organic material in

the layer 1110 (and in all other electrically-pumped embodiments

of the present invention) typically consists of three sub-layers:

a hole transporting layer ("HTL") 1110a, an emissive layer ("EL")

1110b, and an electron transporting layer ("ETL") 1110c. First

and second electrodes 1120, 1121 (in the embodiment shown in Fig.

10, first and second electrodes are anode and cathode,

respectively) are substantially transparent to the light emitted

by layer 1110b, and preferably comprise indium-tin-oxide or any

other transparent conducting material.

In one embodiment of the present invention, multiple

electrically-pumped OVCSEL structures are placed in a stacked

arrangement 1350, as shown in Fig. 11, for the emission of laser

light of various colors. The stacked arrangement 1350 includes

the structure 1300 as shown in Fig. 10, but also includes a third

mirror layer 1211, a third electrode 1220, sublayers 1210a,

1210b, and 1210c (corresponding to HTL, EL and ETL layers,

respectively) of a second active organic material, a fourth

electrode 1221, and a fourth mirror layer 1212 as shown in Fig. 11. As in the OVCSEL structure 1300, the electrodes 1120, 1121,

1220 and 1221 are only necessary in the stacked arrangement 1350

if the respective mirror structures 1111, 1112, 1211 and 1212

cannot function as electrodes, or alternatively if it is

preferable to have electrodes separate from the mirror

structures . Although only two OVCSEL structures are shown in the

stacked arrangement 1350, the present invention includes

embodiments in which three or more OVCSEL structures are stacked

in a unitary structure. The stacking of OVCSEL structures

facilitates the lasing of multiple colors, each from a respective

OVSEL, alone or in any combination.

The present invention includes electrically-pumped

OVCSELs that are tunable to specific wavelengths. Fig. 12 shows

an example of such a tunable OVCSEL 1351 in accordance with an

embodiment of the present invention. As described for the

optically-pumped tunable OVCSEL 1250, the laser emission

wavelength of OVCSEL 1351 is tunable by varying the thickness t

of the OVCSEL cavity. The variation in t is accomplished by

controlling the distance tl between the second electrode 1121 and

the second mirror layer 1112. The distance tl is varied by the

controlled movement of the second mirror layer 1112 towards or

away from the second electrode 1121, or vice-versa. Optical lens

1130 is disposed between the second electrode 1121 and the second mirror layer 1112 to permit the variation in tl without losing

control of the laser light emitted from layer 1110b.

All embodiments of the present invention optionally

include guiding and cladding layers around the organic layers to

help minimize waveguiding losses in the organic layers. For

example, waveguide optical losses in organic layers of thickness

150 nm are often as high as 1000 cm^-1 or more where metal

electrodes are used in electroluminescent devices. A structure

1400 as shown in Fig. 13 is used to minimize such losses.

Structure 1400 includes guiding layers 1161 and 1162 immediately

adjacent to the organic layer (s) 1110, and cladding layers 1160

and 1163 immediately adjacent to the guiding layers 1161 and

1162, respectively. As is known in the art and previously

discussed, if layer 1110 comprises an electroluminescent

material, it may actually comprises multiple sublayers (i.e.,

HTL, EL and ETL layers) . The guiding layers 1161, 1162 are

highly transparent to minimize optical losses, and are

characterized by a high refractive index. The cladding layers

1160, 1163 are also transparent but possess a lower refractive

index than the guiding layers 1161, 1162. The cladding layers

1160, 1163 also generally have higher conductivities than the

guiding layers 1161, 1162, as one of the primary purposes of

cladding layers 1160, 1163 is to conduct electrical current. The structure 1400 reduces the confinement of emitted light in

organic layer (s) 1110, thus reducing optical losses to as low as

10 cm^-1 or less. The structure 1400 is optionally used with any

of the optically- or electrically-pumped embodiments of the

present invention. For example, when used with the optically-

pumped embodiment 1200 of the present invention, the structure

1400 is substituted for the layer of active organic material 1110

in Fig. 7. Likewise, when used with the electrically-pumped

embodiment 1300, the structure 1400 is substituted for the sub-

layers 1110a, 1110b and 1110c such that the structure 1400 is

sandwiched by the electrodes 1120 and 1121. When used with the

electrically-pumped embodiment, the guiding and cladding layers

1160, 1161, 1162, 1163 must be sufficiently conductive to provide

charge carriers to the conductive layer (s) 1110 from the

electrodes 1120, 1121. It is preferred, however, that the

conductivity of guiding layers 1161, 1162 is less than that of

cladding layers 1160, 1163, which have a conductivity less than

that of the surrounding electrodes 1120, 1121. The preferred

material for the guiding and cladding layers 1160, 1161, 1162,

1163 is indium-tin-oxide, the refractive index, conductivity and

transparency of which is varied, for example, by varying oxygen

content . The present invention is further described with

reference to the following non-limiting examples.

Example 1

A laser device as shown in Fig. 1 was formed. Alq₃ and

DCM (molecular ratio of approximately 50:1) were co-evaporated

under a high vacuum (5xl0^~7 Torr) onto a glass slide. The

resulting 3000 thick layer 112 of Alq₃ / DCM, having an index

of refraction (n) of 1.7, forms a slab optical waveguide with

glass (n=1.4) as a cladding layer on one side, and air (n=l) on

the other. This slab optical waveguide in conjunction with the

reflective facets 113, 114 of the layer 112 form an optical

resonator.

A nitrogen laser 118 operating at 500 psec pulses with

a wavelength of 337 nm at a 50 Hz repetition rate was used to

generate the optical pump beam. Lateral confinement of the

optical mode was achieved by gain-guiding induced by the optical

pump beam. Bright red laser emission was clearly visible from

the edge 113. Diffraction of the output beam was faintly

observed. The output laser beam included several transverse

modes which diverged in the direction orthogonal to the device

surface. The peak intensity of the red laser emission at the

output facets is 10⁸ W/cm² (corresponding to a measured peak power exceeding 30W) at a pump level of 200μj/cm². No

degradation of the Alq₃ / DCM film was observed after several

hours of operation (which corresponds to at least 10⁶ laser

pulses. This indicates that Alq₃ / DCM films enjoy a high degree

of photochemical stability and are well-suited for use in

electrically-pumped organic lasers.

Example 2

A laser was formed by vacuum-depositing a 120 nm

organic film containing perylene-doped CBP onto the (100)

crystallographic surface of a polished InP first mirror layer.

The first mirror layer was pre-coated with a 2 μm layer of Si0 ,

deposited by plasma enhanced chemical vapor deposition. The

laser was optically pumped with 500 psec pulses generated by a

nitrogen laser at = 337 nm, with a 50 Hz repetition rate. The

pump beam was focused onto a 50 μm width region on the organic

film to form the gain region.

Fig. 14 shows the edge emission spectra of the

optically-pumped laser at pump levels below (4 nJ) and just above

the threshold (5 nJ) . Lasing appears as a spectrally narrow (2

nm, FWHM) peak at = 485 nm. The inset in Fig. 14 shows the

output pulse energy of the laser as a function of pump pulse energy, clearly indicating the presence of a lasing threshold at

a pump pulse energy of 5 nJ (which corresponds to an energy

density of 5 μj/cm². Moreover, the laser exhibited an

operational lifetime in excess of 10⁵ laser pulses.

Example 3

A laser was formed by vacuum-depositing an organic film

containing coumarin-47-doped CBP onto the (100) crystallographic

surface of a polished InP first mirror layer. The first mirror

layer was pre-coated with a 2 μm layer of Si0₂, deposited by

plasma enhanced chemical vapor depsition. The laser was

optically pumped with 500 psec pulses generated by a nitrogen

laser at = 337 nm, with a 50 Hz repetition rate. The pump beam

was focused onto a 50 μm width region on the organic film to form

the gain region.

The optically-pumped laser resulted in a lasing

wavelength of approximately 455 nm and a lasing threshold of 15

μj/cm². Moreover, the laser exhibited an operational lifetime

on the order of 10³ laser pulses.

Example 4

A laser was formed by vacuum-depositing an organic film

containing coumarin-30 -doped CBP onto the- (100) crystallographic surface of a polished InP first mirror layer. The first mirror

layer was pre-coated with a 2 μm layer of Si0₂, deposited by

plasma enhanced chemical vapor depsition. The laser was

optically pumped with 500 psec pulses generated by a nitrogen

laser at = 337 nm, with a 50 Hz repetition rate. The pump beam

was focused onto a 50 μm width region on the organic film to form

the gain region.

The optically-pumped laser resulted in a lasing

wavelength of approximately 510 nm and a lasing threshold of 13

μJ/cm . Moreover, the laser exhibited an operational lifetime

on the order of 10² laser pulses.

Example 5

A laser was formed by vacuum-depositing an organic film

containing DCM2 -doped Alq₃ onto a polished InP first mirror

layer. The first mirror layer was pre-coated with a 2μm layer

of Si0₂, deposited by plasma enhanced chemical vapor deposition.

The laser was optically pumped with 500 psec pulses generated by

a nitrogen laser at = 337nm, with a 50 Hz repetition rate.

The optically-pumped laser resulted in a lasing

wavelength of approximately 670 nm and a lasing threshold of 2.5

μj/cm². Moreover, the laser exhibited an operational lifetime in excess of 10° laser pulses and a differential quantum

efficiency of approximately 30%.

Example 6

A laser was formed by vacuum-depositing an organic film

containing DCM-doped Alq₃ onto a polished InP first mirror layer.

The first mirror layer was pre-coated with a 2μm layer of Si0₂,

deposited by plasma enhanced chemical vapor deposition. The

laser was optically pumped with 500 psec pulses generated by a

nitrogen laser at = 337nm, with a 50 Hz repetition rate.

The optically-pumped laser resulted in a lasing

wavelength of approximately 650 nm and a lasing threshold of

approximately 3 μj/cm². Moreover, the laser exhibited an

operational lifetime in excess of 10⁶ laser pulses and a

differential quantum efficiency of approximately 30%.

Example 7

As one example of the embodiment shown in Fig. 7, an

OVCSEL 1200 was formed comprising an active layer 1110 of tris-

(8-hydroxyquinoline) aluminum ("Alq₃") doped with DCM laser dye.

The thickness of the active layer 1110 was 500 nm. The

concentration of DCM laser dye in the layer 1110 was 3% by

weight. The active layer 1110 was deposited onto the first mirror layer 1111 (a DBR mirror in this example) by thermal

evaporation at 5 x 10^"7 Torr. The buffer layer 1114 comprised

Alq₃ and was deposited over the active layer 1110, and a silver

mirror layer 1112 was deposited over the buffer layer 1114. The

thicknesses of the buffer layer 1114 and the DBR mirror layer

1111 were 20 nm and 200 nm, respectively. The DBR mirror layer

1111 had a &gt;99% reflective stopband between 600 nm and 700 nm,

while the reflectivity of the silver mirror was calculated to be

91%.

The OVCSEL 1200 was optically pumped using a nitrogen

laser ( = 337 nm) , which generated 500 ps pulses at a 50 Hz

repetition rate. The pump beam was aimed incident through the

DBR mirror 1111 and was focused to a spot approximately 100 m

across on the organic film surface. At the wavelength of the

exciting laser (i.e., = 337 nm) the DBR mirror 1111 exhibited

a transmittance of about 80%. In this example, the substrate

1113 comprised quartz, which is transparent to a pump beam

characterized by = 337 nm.

The emission spectrum in the substrate normal direction

(with a 15 full angle acceptance cone) was analyzed by a

spectrograph using a charge-coupled device camera with a

wavelength resolution of 1 A. To avoid material degradation, all

measurements were performed in a dry nitrogen environment . Fig. 15 shows the spontaneous emission spectrum of the

OVCSEL 1200 just below the lasing threshold. A cavity mode was

observed at = 635 nm. At &lt; 600 nm and &gt; 700 nm, the

spontaneous emission is filtered by the modulation in the DBR

transmission spectrum resulting in the broad satellite peaks

observed. The spectrum above the lasing threshold, corresponding

to an energy of E_TH = 300 J/cm², is completely dominated by the

high gain, spectrally narrow laser emission, as shown in Fig. 16.

Fig. 17 shows the high resolution emission spectra from

the OVCSEL 1200 at increasing excitation levels near the lasing

threshold. Pump energies and the spectral full width at half

maximum are indicated. The transition from the 12 A wide

spontaneous emission peak, spectrally filtered by the microcavity

below threshold, to the resolution-limited, &lt; 1 A full width

spectral line due to laser emission above threshold, is clearly

observed. The spectral width of the peak below threshold is

related to the finesse (the ratio of microcavity mode spacing to

a single mode linewidth) of the microcavity, with additional

broadening due to the presence of several transverse modes . Mode

competition above threshold confines lasing to only a few of the

transverse modes, leading to a concomitant reduction in the

emission linewidth. The left inset of Fig. 17 shows (0.4 ± 0.1)

broad emission line of an OVSEL with a cavity thickness of 475 nm. Accounting for the (0.2 + 0.1) instrument resolution, the

Gaussian full-width-at-half-maximum of the lasing line is

calculated to be (0.2 ± 0.1).

Fig. 18 shows the dependence of the laser output power

on input excitation, clearly indicating a threshold at a pump

energy density of E_TH = 300 J/cm². This threshold is two orders

of magnitude higher than that for similar edge emitting organic

semiconductor lasers as a consequence of higher optical losses

in the microcavity structure (500 cm^-1) , and a short gain length

(500 nm) . The Alq₃:DCM material gain at threshold in OVCSELs is

estimated to be g_TH = 500 cm^-1, which is comparable to the

internal gain in InGaAs/GaAs quantum well structures.

The lasers of the present invention are applicable to

a wide variety of applications, including telecommunications,

printing, optical downconversion, semiconductor circuit etching,

thermal processing (e.g., marking, soldering and welding),

spectroscopy, vehicular control and navigation, measurement

devices, optical memory devices, displays, scanners, pointers,

games and entertainment systems and sensors.

The present invention provides for organic lasers with

desired properties such as narrow bandwidth emission, the minimal

use of active organic materials, and- the facilitation of wavelength tuning and electrical pumping. The vertical-cavity

surface-emitting structures of the present invention represent

the first known successful demonstration of lasing action in a

small molecular weight organic semiconductor microcavity

structure associated with a clear threshold in the output power,

a well defined laser beam, cavity modes, and spectral line

narrowing of emission above threshold to less than 1 A. Those

with skill in the art may recognize various modifications to the

embodiments of the invention described and illustrated herein.

Such modifications are meant to be covered by the spirit and

scope of the appended claims.