Abstract:
Electrically pumped surface emitting organic laser device having a multi-layer of organic materials disposed between a highly reflective microcavity mirror and a highly reflective mirror to thereby form a coupled microcavity. More specifically, the organic laser device includes a substrate; a bottom mirror over the substrate; a layer of spacer over the bottom mirror; a coupling mirror over the spacer layer; an anode over the coupling mirror; an active layer over the anode; a cathode over the active layer; and a top mirror over the cathode. The combination of the electrode and the mirror leads to low optical absorption and highly reflective electrical contacts at organic-electrode interfaces. Electroluminescent emission efficiency is improved due to the realization of efficient electron-injection and hole-injection. A low loss organic laser device with a coupled microcavity structure is realized that can produce surface emitting laser output under electrical pumping.

Description:
GOVERNMENT RIGHTS 
       [0001]    The work leading to this invention was supported by the National Natural Science Foundation of China under grants 10174077, 60376029, and Jilin Province Science and Technology Research Projects 20050108. The China Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to the field of light emitting devices. More specifically, the invention relates to electrical pumped organic laser devices. 
         [0004]    2. Description of Prior Art 
         [0005]    There has been much interest in the realization of electrically pumped organic laser devices (OLDs) as known in the art, since lasing in optically pumped organic thin films has been demonstrated using a variety of optical resonators, including optical waveguides, planar microcavities, and distributed feedback (DFB) structures (N. Tessler et al., Nature 382, 695 (1996); Ruidong Xia et al., Org. Electron. 4, 165 (2003); M. Reufer et al., Appl. Phys. Lett. 84, 3262 (2004), each of which is incorporated herein by reference). As used herein, the term “organic” includes of polymers, small molecular weight organic materials, and other kinds of organic light emitting materials that can be used to fabricate organic opto-electronic devices. 
         [0006]    As laser gain media, organic materials are intrinsically quasi-four-level systems. They are low cost materials that have high fluorescent quantum efficiencies and low absorption losses. Optically pumped stimulated emission, gain, and lasing have been observed in various kinds of organic materials with emission wavelengths spanning the visible spectrum. It also shows that the lasing threshold, emission wavelength, output power of organic lasers have more temperature stable than conventional inorganic laser diodes. (V. G. Kozlov et al., J. Appl. Phys. 84, 4096 (1998), incorporated herein by reference). Therefore, OLDs are believed to be novel visible lasers, which show a number of applications in display, optical storage, optical communications et al. 
         [0007]    There are some patents related to organic lasers. (U.S. Pat. No. 6,498,802, No. 6,160,828, No. 7,242,703, No. 6,853,660, No. 6,674,776, No. 5,881,089, No. 6,996,146, each of which is incorporated herein by reference). However, efforts to make electrically pumped OLDs have been unsuccessful up to now. There are still challenges to overcome such as low carrier mobility of organic films and much higher optical loss associated with electrical pumping. The low carrier mobility of organic films makes OLDs hard to achieve high current densities required, and limits the thickness of organic films to thin layers, which resulting in high optical losses in OLDs with waveguide and DFB structures. (V. G. Kozlov et al., J. Appl. Phys. 84, 4096 (1998); McGehee et al., Adv. Mater. 12, 1655 (2000), each of which is incorporated herein by reference). Recently, very low laser thresholds have been achieved in organic waveguide and microcavity structures under optical pumping, which means that electrically pumped OLDs can operate at a low current density that matching low carrier mobility of organic films. (T. W. Lee et al., J. Appl. Phys. 93, 1367 (2003); M. Berggren et al., Nature 389, 466 (1997); X. Liu et al., Appl. Phys. Lett. 84, 2727 (2004), each of which is incorporated herein by reference). Therefore, to develop an effective electrically pumped OLD, the key problem at present is how to lower the optical loss of the laser structure. (S. Lattante et al., Appl. Phys. Lett. 89, 031108 (2006); P. Görrn et al., Appl. Phys. Lett. 91, 041113 (2007), each of which is incorporated herein by reference). 
         [0008]    One of the effective structures for electrically pumped OLD is microcavity that are formed by depositing one or multi-layer organic materials between two mirrors (either dielectric stacks or metallic) separated by a few hundred nanometers. To achieve a low threshold current, microcavity OLDs typically utilize short gain region. With such a thin gain region, microcavity OLDs have a very small single pass optical gain, thereby requiring high reflectivities equal to or greater than 97% at the interfaces between organic layers and electrodes to achieve lasing. In an electrically pumped OLD, electron carriers and hole carriers need to be injected from two electrodes, generally from an anode and a cathode, respectively. How to lower electron-injection barrier at cathode/organic interface, and hole-injection barrier at anode/organic interface is one of the critical issues for OLDs. Anode materials should employ high work function transparent conducting oxides (TCOs) such as indium tin oxide (ITO) for good hole-injection. Cathode materials should use low work function metals such as aluminum (Al), lithium (Li) or double layer structure such as Al/ITO for good electron-injection. Metal mirrors are frequently used in microcavities as they can provide suitable reflectance and excellent electrical contact. As known in the art, most thick metal mirrors have a reflectance less than 96%. However, to develop a microcavity OLD, it is essential to employ two highly reflective mirrors to construct a high quality factor (high Q) cavity. Therefore, distributed Bragg reflectors (DBRs) are the best choice among highly reflective mirrors for microcavity OLDs. In microcavity OLDs, electrodes are essentially disposed between organic layers and mirrors. In order to lower the optical absorption, microcavity OLDs should use thin and transparent electrodes. ITO film, a familiar anode, averagely has a transmittance of 85% and an absorptance of 0.5% at the thickness of 30 nm. The familiar cathode, Al metal film with the thickness of 10 nm, averagely has a transmittance of 42% and an absorptance of 37% in the visible spectra region. 
         [0009]    To realize low loss microcavity OLDs, one challenge is how to obtain an adequate electrical contact and a reflectance equal to or higher than 97% at organic-electrode interface, especially at organic-metal interface. Another challenge is how to lower electrode-induced optical loss. There is a need, therefore for an alternative microcavity OLD that avoids these limitations. 
       SUMMARY OF THE INVENTION 
       [0010]    Considering the above-mentioned problems, it is an object of this invention to provide a low-loss microcavity organic laser device arrangement that is particularly suitable to operate under electrical pumping. Organic laser device has multi-layers of organic materials disposed between a highly reflective microcavity mirror and a highly reflective mirror to thereby form a coupled microcavity within a stacked arrangement. More specifically, the organic laser device of the present invention comprise a substrate; a bottom mirror over the substrate; a layer of spacer over the bottom mirror; a coupling mirror over the spacer layer; an anode over the coupling mirror; an active layer over the anode; a cathode over the active layer; and a top mirror over the cathode. The active layer of the present invention comprises an anode buffer layer, a hole-transporting layer, a gain zone, an electron-transporting layer, and a cathode buffer layer. The combination of the electrode and the mirror according to the present invention leads to low absorption and highly reflective electrical contacts at organic-electrode interfaces. Electroluminescent emission efficiency is improved due to the realization of efficient electron-injection and hole-injection. According to the present invention, a low loss organic laser device with a coupled microcavity structure is realized that can produce surface emitting laser output under electrical pumping. 
       ADVANTAGES 
       [0011]    It is an advantage of the present invention that it provides for a high reflectance electrical contact at organic-electrode interface. It is a further advantage of the present invention that low loss microcavity OLDs can be achieved and emit lasing under electrical pumping. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]    The above and other objects, features and advantages of the present invention will be clearer from the following detailed description about the non-limited embodiments of the present invention taken in conjunction with the accompanied drawings, in which: 
           [0013]      FIG. 1A  shows a cross section of an electrically pumped OLD structure in accordance with an embodiment of the present invention. 
           [0014]      FIG. 1B  shows a cross section of an active layer in accordance with an embodiment of the present invention. 
           [0015]      FIG. 1C  shows a cross section of a gain zone in accordance with an embodiment of the present invention. 
           [0016]      FIG. 2  shows the measured reflectance spectra of a top mirror (dot) and a thin metal cathode combined with the top mirror (line); the measured reflectance spectra of passive cavity (dash) and an anode combination with a structure of glass/passive cavity/ITO (open circle) in accordance with an embodiment of the present invention. 
           [0017]      FIG. 3  Measured reflectance spectra (line) of the OLD from the substrate side and the top side compared with the modeled reflectance spectra (dot) in accordance with an embodiment of the present invention. The arrows point to the cavity modes. 
           [0018]      FIG. 4  shows current density versus bias voltage in accordance with an embodiment of the present invention. The inset shows emission spectra below and above threshold, measured with a 15° full angle acceptance cone. J th  is the current density at threshold. 
           [0019]      FIG. 5  shows FWHM and emission intensity of EL spectra as a function of injection current density in accordance with an embodiment of the present invention. Emission intensity obtained is spectrally integrated between 605 nm and 631 nm. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0020]    The present invention is directed to electrically pumped organic laser devices (OLDs) with a low loss and high Q microcavity structure. A microcavity is a resonator that has at least one dimension on the order of a single optical wavelength. The electrically pumped OLDs according to the present invention have a coupled microcavity (CMC) structure that consists of two planar Fabry-Perot microcavities. The one without organic materials is passive cavity. The other is laser cavity that has organic emitting materials inside. The two monolithic microcavities sharing a common mirror are border upon each other. Each individual cavity can support a single longitudinal mode. The mode interaction in such a coupled microcavity gives rise to a pair of optical modes, hereafter called AL for “long-wavelength mode” and γ s  for “short-wavelength mode”. The intensities of the two cavity modes depend on the mismatch between the passive cavity and the laser cavity (L. Pavesi et al., Phys. Rev. B 58, 794 (1998), incorporated herein by reference). CMC structure can be considered as a special single microcavity when looking the passive cavity as a special highly reflective mirror. In this way the laser cavity is sandwiched between a highly reflective microcavity mirror and a highly reflective mirror. Compared with normal microcavity, CMC structure can significantly suppress the TE-leaky modes (R. P. Stanley et al., Appl. Phys. Lett. 65, 2093 (1994); P. Bienstman et al., IEEE J. Quantum Elect. 36, 669 (2000); P. Michler et al., Appl. Phys. Lett. 70, 2073 (1997), each of which is incorporated herein by reference). Advantageously, CMC is suitable for the formation of a low loss microcavity. 
         [0021]      FIG. 1A  shows a cross section of an electrically pumped OLD structure  100  according to the present invention. The OLD  100  is constructed of a substrate  110 , a bottom mirror  120 , a spacer layer  130 , a coupling mirror  140 , an anode  150 , an active layer  160 , a cathode  170 , and a top mirror  180 .  FIG. 1B  shows a cross section of an active layer  160  according to the present invention. The active layer  160  generally includes an anode buffer layer (ABL)  162 , a hole-transporting layer (HTL)  163 , a gain zone  164 , an electron-transporting layer (ETL)  165 , a cathode buffer layer (CBL)  166 .  FIG. 1C  shows a cross section of a gain zone  164  in accordance with an embodiment of the present invention. The gain zone  164  is constructed of an electron-barrier layer (EBL)  164   a,  an organic gain layer (OGL)  164   b,  and a hole-barrier layer (HBL)  164   c.    
         [0022]    When a forward bias voltage is applied across the organic layers of the OLD  100  according to the present invention, electrons are injected into the ETL  165  from the cathode  170 , and holes are injected into the HTL  163  from the anode  150 . The migration of the injected charges towards the oppositely charged electrodes results in the formation of electron-hole pairs, which are typically referred to as excitons, in the gain zone  164 . The photons produced by the radiative decay of excitons, can be amplified under the mechanism of stimulated emission in laser cavity. When the injection current is above the threshold current, and the optical gain is higher than optical loss, the surface emitting lasing  190  from the gain zone  164  can be output from the mirror that comparatively has a low reflectance. The OLD  100  can be top emitting or bottom emitting. As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Substrate  110  may be flexible or inflexible, and transparent or opaque. When laser emission  190  is out from the topside, the substrate  110  can either be a suitable transparent substrate such as quartz, glass, sapphire or plastic or an opaque substrate such as silicon. If laser emission  190  is out from the bottom side, the substrate  110  includes suitable materials that are substantially transparent to visible light. 
         [0023]    The passive cavity is sandwiched between the bottom mirror  120  and the coupling mirror  140 . The laser cavity is sandwiched between the coupling mirror  140  and the top mirror  180 . The coupling mirror  140  is the shared mirror of the passive cavity and the laser cavity. Generally mirror  120 ,  140  and  180  are made up of a periodic arrangement of alternating high index and low index dielectric materials. The high-index materials can be zirconium dioxide (ZrO 2 ), tantalum pentoxide (Ta 2 O 5 ), titanium dioxide (TiO 2 ), zinc sulfide (ZnS), Zinc Selenide (ZnSe), and other dielectric materials that have refractive indices more than 1.8. The low-index materials can be silicon dioxide (SiO 2 ), magnesium difluoride (MgF 2 ), lithium fluoride (LiF), yttrium fluoride (YF 3 ), barium fluoride (BaF 2 ) and other dielectric materials having refractive indices less than 1.6. The spacer layer  130  is disposed between the bottom mirror  120  and the coupling mirror  140 , and can be constructed of a suitable dielectric material such as SiO 2 . The spacer layer  130  is the cavity layer of the passive cavity that can be adjusted in thickness and refractive index so that the resulting passive cavity resonates at a desired wavelength. As the cavity length of the OLD  100  is very short in the lasing direction, increased cavity reflectance is needed to accommodate the correspondingly reduced per-pass gain. Generally, the mirror  180  and the passive cavity that are formed by the mirror  120  and the mirror  140 , have a reflectance of at least 97%, preferably 99%, at the laser wavelength. 
         [0024]    Anode  150  employs transparent conducting materials with a thickness on the order of lambda/4n or less, where lambda is the laser wavelength; n is the index of refraction of the anode  150 . The suitable materials for anode  150  include indium tin oxide (ITO), zinc tin oxide (ZTO), fluorine tin oxide (FTO), and indium vanadium oxide (IVO) etc. Anode  150  is formed over the coupling mirror  140 . Mirror  120 , mirror  140 , spacer layer  130 , and anode  150  can be deposited by any suitable techniques such as plasma enhanced chemical vapor deposition (PECVD), electron-beam deposition, sputtering deposition, or thermal vapor phase deposition generally at a substrate  110  temperature higher than 200 Celsius degree. Top mirror  180  can be deposited by sputtering deposition, high-vacuum thermal evaporation or electron-beam deposition generally at a substrate  110  temperature less than 80 Celsius degree to avoid damage to organic materials. 
         [0025]    The ABL  162  can increase planarization of the anode  150 , and serve to improve film formation property of subsequent organic layers. The ABL  162  can facilitate injection of holes from the anode  150  into the HTL  163 . Suitable materials for use in ABL  162  include, but are not limited to, a triarylamine derivatives doped with an organic electron acceptor material, and inorganic oxides such as vanadium oxide (VO x ), molybdenum oxide (MoO x ), nickel oxide (NiO x ), and tungsten oxide (WO x ), etc. The HTL  163  facilitates the migration of holes towards the gain zone  164 . HTL  163  can be one layer or multilayers. HTL  163  may be intrinsic (undoped) organic materials, or doped ones that can be used to enhance conductivity. The HTL  163  can include, for example, triaryl amines derivatives or other suitable materials as known in the art, such as 
         [0000]    N,N′-bis(3-naphthalen-2-yl)-N,N′-bis(phenyl)benzidine (NPD),
 
4,4′,4″-tris(N-naphthyl-N-phenylamino)triphenylamine (2T-NATA),
 
4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine (m-MTDATA),
 
poly(N-vinylcarbazole) (PVK),
 
poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS), etc.
 
         [0026]    The gain zone  164  is disposed between HTL  163  and ETL  165 .  FIG. 1C  depicts a gain zone  164  consisting of an organic gain layer (OGL)  164   b  sandwiched between an electron-barrier layer (EBL)  164   a,  and a hole-barrier layer (HBL)  164   c.  EBL  164   a , which is optional and used as a barrier for the passage of electrons to the anode  150 , can limit electrons within the gain zone  164 . EBL  164   a  can include suitable organic materials and inorganic materials having a large bandgap such as LiF and YF 3 . HBL  164   c,  which is optional and used as a barrier for the passage of holes to the cathode  170 , can limit holes within the gain zone  164 . HBL  164   c  can include suitable organic materials such as bis(2-methyl-8-quinolinolato)(4-phenylphenolato)Aluminum(III) (BALQ), 2,9-dimethyl-4,7-diphenyl-phenanthroline (BCP), and other suitable organic materials as known in the art. HBL  164   c  can also include inorganic materials having a large bandgap such as LiF and YF 3 . The EBL  164   a  and HBL  164   c  may also be used to block excitons from diffusing out of the gain zone  164 . OGL  164   b  can be one layer or multi-layers. OGL  164   b  generally includes one or a plurality of host materials doped with one or a plurality of guest dopants. A suitable host material can efficiently transfer energy to a highly luminescent guest dopant. Host materials can include, BCP, 4,4′-N,N′-dicarbazole-biphenyl (CBP), tris(8-hydroxyquinolato) aluminum (ALQ), and other suitable materials as known in the art. Guest dopants can be selected from highly fluorescent dyes or highly phosphorescent dyes such as 
         [0000]    3-(2′-benzothiazolyl)-7-(diethylamino)coumarin (Coumarin 6),
 
4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM),
 
       Rubrene, 
     Perylene, 
       [0027]    10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H, 5H, 11H-[1]benzo-pyrano[6,7,8-ij]quinolizin-11-one (C545T),
 
4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB),
 
and other suitable materials as known in the art. OGL  164   b  can also comprise a single organic material, such as poly(p-phenylenevinylene) (PPV) derivatives, polyfluorene (PF) derivatives, etc. ETL  165  facilitates the migration of electrons towards the gain zone  164 . ETL  165  can be one layer or multilayers. ETL  165  can include, for example, ALQ, 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butyl phenyl)-1,2,4-triazole (TAZ), 1,3-bis(N,N-t-butyl-phenyl)-1,3,4-oxadiazole (OXD-7), 2,9-dimethyl-4,7-diphenyl-phenanthroline (BCP), and other suitable materials as known in the art. CBL  166  can serve to facilitate injection of electrons from the cathode  170  into the ETL  165 . Suitable materials for use in CBL  166  include, but are not limited to, alkaline or alkaline earth metal doped organic layers, alkali halide salts, rare-earth fluoride, and transition element fluoride, such as LiF, YF 3 , and BaF 2 , etc. The cathode  170  is formed over CBL  166 . Desirable materials of the cathode  170  are metal materials having a low work function (&lt;4.0 eV) and good film-forming properties to ensure good contact with the underlying organic layer, such as aluminum (Al), lithium (Li), silver (Ag), calcium (Ca), magnesium (Mg), and combinations or alloys of these materials. The organic and metal materials used in the present invention can be deposited by any suitable technique, such as by vacuum thermal evaporation.
 
         [0028]    Generally for a thin metal film, the sum-up of its transmittance T, reflectance R, and absorptance A is unity, i.e. A+R+T=1. According to the principle of induced transmission filter first described by Berning and Turner, the absorbance of a metal film is not solely a function of its optical parameters but can depend very significantly on the optical environment of that film. (P. H. Berning et al., J. Opt. Soc. Am. 47, 230 (1957). S. Y. Zheng et al., Can. J. Phys. 61, 361 (1983). Dobrowolski J. A. et al., Appl. Opt. 31, 5988 (1992), incorporated herein by reference). By the proper design of multilayer coatings on thin metal film, absorptance or reflectance can be enhanced or eliminated due to optical interference over a specified wavelength region. This principle has been used to increase the contrast ratios of display devices, and can also be used to construct a high reflective electrical contact with a low absorption. 
         [0029]    The cathode  170  of the present invention employs metal film with a thickness of about 5-15 nm. It is found that a cathode  170  combined with a suitable top mirror  180  can provide an integrated optical and electrical performance such as a high reflectance and a low absorption at a certain wavelength. Because the lower absorptance is associated with the higher reflectance, the optical loss from cathode  170  can be sufficiently reduced with a highly reflective top mirror  180 . Experiments show that such a structure can form a good electrical contact at the active layer  160 /the cathode  170  interface. 
         [0030]    A standing wave electromagnetic field with periodic intensity maxima and minima is established in the laser cavity. The overall thickness of active layer  160  is adjusted so that the cavity length of the laser cavity equals to an integral number of half wavelength of the laser emission. The interfaces at active layer  160 /anode  150  and at active layer  160 /cathode  170  are positioned near the standing wave minima, while the gain zone  164  is disposed at positions corresponding with the antinodes of the standing wave. This arrangement can further lower the optical absorption at lasing wavelength from the anode  150  and cathode  170 , and significantly increase the microcavity effect of the OLD  100  due to Purcell effect. Electrically pumped OLDs  100  according to the present invention can emit a circularly symmetric laser beam in a direction perpendicular to the substrate. Electrically pumped OLDs  100  of the present invention can be fabricated in extremely small sizes, and readily made into two-dimensional laser arrays. Electrically pumped OLDs  100  of the present invention have various applications including laser sources, integrated optoelectronic circuits, optical interconnection, optical computing, optical switching, image processing, sensing, spectroscopy, and optical memory. 
       EXAMPLE 1 
       [0031]    As one example of the embodiment shown in  FIG. 1 , the OLD  100  was prepared over a glass substrate  110  by depositing the bottom mirror  120 . Then a γ/2-thick SiO 2  layer was deposited onto the bottom mirror  120  using as the spacer layer  130 , followed by a coupling mirror  140 . The bottom mirror  120  and the coupling mirror  140  are comprised of γ/4-thick TiO 2 /SiO 2  layers with 4.5 pairs and 4 pairs, respectively. The designed Bragg wavelength A for the passive cavity is 595 nm. On top of the coupling mirror  140 , an ITO layer with the thickness of 54 nm was deposited as the anode  150 . The TiO 2 , SiO 2 , and ITO films were deposited by electron beam evaporation at a substrate temperature of 200 Celsius degree in an oxygen pressure of 2×10 −2  Pa. The following layers were deposited in the following sequence by thermal evaporation or sublimation under a vacuum of 5×10 −4  Pa: 1) the ABL  162  layer over the ITO, 1 nm thick, including MoO 3 ; 2) a HTL  163  layer, 17 nm thick, including 2T-NATA; 45 nm thick, including NPB; 3) the OGL  164   b  layer, including 35 nm of NPB doped with ca. 1.5 wt. % DCM, and 40 nm of ALQ doped with ca. 2 wt. % DCM; 4) the ETL  165  layer, including 58 nm of ALQ; 5) the CBL  166  layer, including 1 nm of LiF; 6) the cathode  170 , including about 10 nm of Al. After the deposition of these layers, the top mirror  180  were deposited over the cathode  170  (Al layer), which consisting of 7.5 pairs of ZnS/MgF 2  quarter-wave layers, and is designed to offer a maximum reflectance of 99.5% at 590 nm. The ZnS and MgF 2  layers were deposited in a vacuum of 1×10 −3  Pa by electron beam evaporation with a substrate temperature less than 80 Celsius degree to avoid damage to organic layers. OLD  100  constructions can be denoted as Glass substrate/bottom mirror/SiO 2 /coupling mirror/ITO (54 nm)/MoO 3  (1 nm)/2T-NATA (17 nm)/NPB (45 nm)/NPB:DCM (35 nm)/ALQ:DCM (40 nm)/ALQ (58 nm)/LiF (1 nm)/Al (10 nm)/top mirror. 
         [0032]    The OLD  100  samples were electrically pumped by a semiconductor parameter analyzer that can generate full wave rectified sinusoidal pulse waveform at a repetition rate of 100 Hz. EL spectra and reflectance spectra were measured with an Avantes fiber optic spectrometer normal to the OLD  100  surface.  FIG. 2  shows the measured reflectance spectra of the top mirror  180  (dot) and the Al cathode  170  combined with the top mirror  180  (line) for one embodiment of the present invention. It shows that compared with single Al thin film, the Al cathode  170  and the top mirror  180  combination system shows an increase in reflectance due to optical interference effect. The maximum reflectance greater than 99% come around 618 nm, which means that a minimum absorptance less than 1% can be obtained at the same wavelength. In contrast to the top mirror  180 , the cathode  170  in combination with the top mirror  180  has a narrowed high reflective stop band and a red shift reflective peak, which can be optimized to locate at the desired wavelength by proper structure design.  FIG. 2  also shows the measured reflectance spectra of passive cavity (dash) and the anode  150  combination structure of glass/passive cavity/ITO (open circle) in accordance with an embodiment of the present invention. The resonance peak of the passive cavity is about 595 nm. The reflectance in the IS stop band is greater than 99%, which changed little when capped with the ITO anode  150 . In the OLD  100 , the Bragg wavelength γ designed for the laser cavity is about 600 nm, which is red shift in contrast to that of the passive cavity. The overall thickness of organic layers is selected to form γ/2 laser cavity with the OGL  164   b  layer situated around the antinode and the two organic layer/electrode interfaces located near the node of the confined cavity electromagnetic field. 
         [0033]      FIG. 3  shows measured reflectance spectra (line) of the OLD  100  from the substrate side and the top side compared with the modeled reflectance spectra (dot) by a transfer matrix method (M. Born, et al., “Principles of Optics”, Cambrige University, Cambrige, 1999) in accordance with an embodiment of the present invention. The arrows point to the cavity modes. Two coupled cavity modes can be observed in the passive cavity with a short-wavelength mode locates at γ s =591 nm and long-wavelength mode γ L  618 nm. While in the laser cavity the mode around 591 nm is too weak. Only the cavity mode at 618 nm can be clearly observed. 
         [0034]      FIG. 4  shows current density of the OLD  100  as a function of the voltage in accordance with an embodiment of the present invention. It shows that the current density increases linearly with voltage over the voltage range from 0 V to 32 V, indicating that good electrical contacts are formed in the interfaces between active layer  160 /anode  150  and active layer  160 /cathode  170 . In the OLD  100 , light emission was not output until injection current density reached a value of about 430 mA/cm 2 , corresponding to a turn-on voltage of 17 V. The lasing is out from the top side.  FIG. 5  shows the full width at half maximum (FWHM) and emission intensity of EL spectra as a function of injection current density in accordance with an embodiment of the present invention. Emission intensity obtained is spectrally integrated between 605 nm and 631 nm. A clear laser threshold behavior is observed at the threshold current density (J th ) of 612 mA/cm 2 , above which integration intensity has a sharp increase, and FWHM decreases from 2.6 nm to 2.0 nm. The inset in  FIG. 4  shows emission spectra of the OLD  100  below and above threshold, measured with a 15° full angle acceptance cone. The emission spectrum below the J th  is dominated by a single cavity mode at 618 nm with a FWHM of 2.6 nm, which corresponds to a cavity quality factor Q=237. At current density of about 1283 mA/cm 2 , an average output power of 572 nW was obtained at the lasing wavelength of 618 nm. 
       EXAMPLE 2 
       [0035]    An OLD  100  is constructed in the manner described in Example 1, except the following changes. The substrate  110  is silicon. The bottom mirror  120  and the coupling mirror  140  are comprised of γ/4-thick Ta 2 O 5 /SiO 2  layers with 5.5 pairs and 4 pairs, respectively. The spacer layer  130  used is a γ/2-thick SiO 2  layer. The top mirror  180  consists of 7.5 pairs γ/4-thick ZnS/MgF 2  layers. The designed Bragg wavelength γ for all the mirrors is 500 nm. The ABL  162  layer including VO x . Both the EBL  164   a  layer and the HBL  164   c  layer are using 0.3 nm of LiF. The OGL  164   b  layer, including 30 nm of NPB doped with ca. 1 wt. % C545T, and 45 nm of ALQ doped with ca. 1.5 wt. % C545T. The CBL  166  layer including 1 nm of YF 3 . The cathode  170  layer is a 10:1 atomic ratio of magnesium and silver film (MgAg). The cathode  170  layer capped with the top mirror  180  has a reflectance of 99% at the wavelength of 515 nm. The completed OLD  100  structure can be denoted as Si substrate/bottom mirror/SiO 2 /coupling mirror/ITO (43 nm)/VO x  (1 nm)/NPB (47 nm)/LiF (0.3 nm)/NPB:C545T (30 nm)/ALQ:C545T (45 nm)/LiF (0.3 nm)/ALQ (42 nm)YF 3  (1 nm)/MgAg (10 nm)/Top mirror. A Keithley 2400 source meter was used to measure the current-voltage characteristics. The lasing output could be obtained from the topside with an emission wavelength of 516 nm and a FWHM of 1 nm. The threshold current density is 892 mA/cm 2 . 
       EXAMPLE 3 
       [0036]    An OLD  100  is constructed by a glass substrate  110 . The bottom mirror  120  and the coupling mirror  140  are comprised of γ/4-thick Ta 2 O 5 /SiO 2  layers with 4.5 pairs and 4 pairs, respectively. The spacer layer  130  used is a γ/2-thick SiO 2  layer. The top mirror  180  consists of 9.5 pairs γ/4-thick ZnSe/YF 3  layers. The designed Bragg wavelength γ for all the mirrors is 645 nm. The ABL  162  layer including VO x . Both the EBL  164   a  layer and the HBL  164   c  layer are using 0.2 nm of YF 3 . The OGL  164   b  layer, including 36 nm of NPB doped with ca. 1 wt. % DCJTB, and 40 nm of ALQ doped with ca. 2.5 wt. % DCJTB. The completed OLD  100  structure can be denoted as Glass substrate/bottom mirror/SiO 2 /coupling mirror/ITO (58 nm)NO x  (1 nm)/NPB (60 nm)/YF 3  (0.2 nm)/NPB: DCJTB (36 nm)/ALQ: DCJTB (40 nm)/ YF 3  (0.2 nm)/ALQ (61 nm)/ LiF (1 nm)/Al (10 nm)/Top mirror. A Keithley 2400 source meter was used to measure the current-voltage characteristics. The lasing output could be obtained from the substrate  110  side with two cavity modes at 635 nm and 651 nm. The threshold current density is 1439 mA/cm 2 . 
         [0037]    The above embodiments are provided for the purpose of example only, and are not intended to limit the present invention. It is to be understood by those skilled in the art that there may be various modifications or replacements to the embodiments without departing from the scope and the spirit of the present invention, and they shall fall into the scope defined by the appended claims. 
       PARTS LIST 
       [0038]      100  organic laser diode
   110  substrate
   120  bottom mirror
   130  spacer layer
   140  coupling mirror
   150  anode
   160  active layer
   162  anode buffer layer
   163  hole-transporting layer
   164  gain zone
   164   a  electron-barrier layer
   164   b  organic gain layer
   164   c  hole-barrier layer
   165  electron-transporting layer
   166  cathode buffer layer
   170  cathode
   180  top mirror