WO1998003566A1 - Polymers conjugues utilises comme materiaux pour lasers a semi-conducteurs - Google Patents

Polymers conjugues utilises comme materiaux pour lasers a semi-conducteurs Download PDF

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Publication number
WO1998003566A1
WO1998003566A1 PCT/US1997/012026 US9712026W WO9803566A1 WO 1998003566 A1 WO1998003566 A1 WO 1998003566A1 US 9712026 W US9712026 W US 9712026W WO 9803566 A1 WO9803566 A1 WO 9803566A1
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solid state
laser
conjugated
lasing layer
ppv
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PCT/US1997/012026
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Maria Diaz-Garcia
Benjamin Schwartz
Fumitomo Hide
Alan J. Heeger
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The Regents Of The University Of California
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/14—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range characterised by the material used as the active medium
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
    • C08G61/00—Macromolecular compounds obtained by reactions forming a carbon-to-carbon link in the main chain of the macromolecule
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/0632—Thin film lasers in which light propagates in the plane of the thin film
    • H01S3/0635—Thin film lasers in which light propagates in the plane of the thin film provided with a periodic structure, e.g. using distributed feed-back, grating couplers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region

Definitions

  • This invention relates to solid state lasers and, more particularly, to the use of conjugated polymers as active materials for solid state lasers.
  • Solid state photonic devices are a class of devices in which the quantum of light, the photon, plays a role. They function by utilizing the electro-optical and/or opto-electronic effects in solid state materials. Because the interband optical transition (in absorption and/or in emission) is involved in photonic phenomena and because photon energies from near infrared (IR) to near ultraviolet (UV) are of interest, the relevant materials are semiconductors with band gaps in the range from 1 to 3 eV. Typical inorganic semiconductors used for photonic devices are Si, Ge, GaAs, GaP, GaN and SiC etc.
  • Photonic devices are often classified into three categories: light sources (light emitting diodes, lasers, diode lasers etc.), photodetectors (photoconductors, photodiodes etc.) and energy conversion devices (photovoltaic cells) [S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981)]. All three are important. Because photonic devices are utilized in a wide range of applications, they continue to provide a focus for research laboratories all over the world.
  • Conjugated polymers are a novel class of semiconductors which combine the optical and electronic properties of semiconductors with the processing advantages and mechanical properties of polymers.
  • Semiconducting polymers typically have band gaps in the range from 1 to 3 eV.
  • the molecular structures of a few important examples of semiconducting polymers are shown in Figure 1. Because of the sp 2 p z bonding of these planar conjugated macromolecules, each carbon is covalently bonded to three nearest neighbors (two carbons and a hydrogen); and there is formally one unpaired electron per carbon. Thus, the electronic structure (semiconductor or metal) depends on the number of atoms per repeat unit.
  • the repeat unit of poly(paraphenylene vinylene), PPV contains eight carbons; PPV is a semiconductor in which the fundamental p z -band is split into eight sub-bands.
  • the energy gap of the semiconductor, the ⁇ - ⁇ * gap is the energy between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.
  • conjugated polymer materials become soluble in common organic solvents and can be processed from solution at room temperature into uniform, large area, optical quality thin films [D. Braun and A.J. Heeger, Appl. Phys. Lett. 58, 1982 (1991)]. Because of the large elongation to break which is a characteristic feature of polymers, such films are flexible and easily fabricated into desired shapes that are useful in novel devices [G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger, Nature 357, 477 (1992).]
  • Solid state lasers require pumping a photoluminescent, solid material with suitable gain, said material contained within a resonant structure.
  • the pumping can be done either optically (optically pumped lasers) or through carrier injection (carrier injection lasers).
  • the photoluminescent material has gain if the stimulated emission is strong enough to exceed any losses from, for example, absorption from the ground state to an excited state, photoinduced absorption, or scattering. If the luminescent medium exhibits gain, Light Amplification by Stimulated Emission of Radiation (LASER) can be achieved if the optical path length in the medium exceeds the gain length.
  • the gain length, L g of a laser material is defined as the distance required for the amplitude to increase by e +1 .
  • conjugated polymers exhibit relatively high photoluminescence (PL) efficiencies with emission that is shifted sufficiently far from the absorption edge that self-absorption is minimal. In such a case, stimulated emission, essential to the development of lasers, might be expected throughout the lifetime of the excited state.
  • Ultrafast spectroscopic studies of poly henylenevinylenes) (PPVs) have revealed that stimulated emission is readily observed in solutions and dilute blend films [M. Yan, L. J. Rothberg, E. W. Kwock and T. M. Miller, Phys. Rev. Lett. 75, 1992 (1995)J. M. Leng et al., Phys. Rev. Lett. 72, 156 (1994); J. W. Blatchford, Phys. Rev.
  • the emission is at longer wavelengths than the onset of significant absorption (the Stokes shift). Because of the spectral Stokes shift between the abso ⁇ tion and the emission, there is minimal self- abso ⁇ tion of the emitted radiation [H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns and A.B. Holmes, Nature 347, 539 (1990); D. Braun and A.J. Heeger, Appl. Phys. Lett. 58 1982 (1991)]. Thus, in semiconducting luminescent polymers, self-abso ⁇ tion need not make the materials lossy. Moreover, since the abso ⁇ tion and emission are spectrally separated, pumping the excited state via the ⁇ to ⁇ * transition does not stimulate emission. Thus, by pumping the ⁇ - ⁇ * transition, one can achieve an inverted population.
  • Gain narrowing and amplified spontaneous emission (as needed for laser emission) in neat solid films of semiconducting luminescent polymers, neither diluted nor blended, has not been previously demonstrated. More specifically, gain narrowing and amplified spontaneous emission in thin solid films of semiconducting luminescent polymers, neither diluted nor blended, with thickness less than 10 ⁇ m have not been previously observed.
  • conjugated polymers possess unique properties as laser materials. These conjugated polymers have gain lengths in the micron or even sub-micron regime and therefore exhibit laser action with low pumping threshold as thin solid films with thicknesses in the micron or even sub-micron regime.
  • this invention takes the form of a solid state lasing layer made up of conjugated polymer.
  • a conjugated polymer useful in the practice of this invention is a conjugated polymer which has a ground state and an excited state and which, in the form of a nondiluted thin film, meets the criteria of; i. having a strong abso ⁇ tion in the ground state with an abso ⁇ tion coefficient of at least about 10 4 cm "1 and an abso ⁇ tion depth not greater than about l ⁇ m), ii. having an efficient luminescence emission from the excited state; this emission being shifted to lower energy relative to the ground state abso ⁇ tion, and iii. providing stimulated emission which is not overwhelmed by photoinduced abso ⁇ tion, such that the thin film exhibits gain narrowing and amplified spontaneous emission.
  • conjugated polymers which meet these criteria are ftmctionalized conjugated polymers.
  • a "functional ized conjugated polymer” is a polymer having repeat units making up a conjugated backbone and, in addition, nonconjugated moieties covalently bonded to the backbone and extending out from the backbone. These nonconjugated moieties should be at least about 5 atoms long. Preferably these nonconjugated moieties provide at least about 5 carbon and/or oxygen atoms for each conjugated repeat unit in the backbone.
  • nonconjugated moieties may extend from copolymeric units in the polymer which are separate from the conjugated units but more commonly are attached to the conjugated repeat units as substituents. In both cases, but especially in the case of nonconjugated moieties extending from separate copolymeric units, the nonconjugated moiety should not appreciably disrupt the overall conjugated nature of the conjugated polymer.
  • this invention employs these ftmctionalized conjugated polymer-based lasing layers in the form of a thin film having a thickness of less than 10 microns, or even 5 microns or even 1 micron.
  • ftmctionalized conjugated polymer-based lasing layers are high efficiency materials. They can exhibit gain narrowing and amplified spontaneous emission at pump energies per pulse of less than 100 micro- Joules ( ⁇ J) , and in some cases of less than 1 ⁇ J.
  • this invention provides lasers employing these ftmctionalized conjugated polymer-based solid state lasing layers.
  • the layers exhibit gain narrowing and amplified spontaneous emission.
  • These lasers can take the forms of, for example, a thin film laser, an optically pumped laser or an injection laser. These lasers may include features or aspects known to enable or enhance their performance.
  • Examples of these features or aspects are employing mirrors external to the lasing layer to provide feedback to the lasing layer; presenting the solid state lasing layer is in the form of an optical waveguide; presenting the solid state lasing layer is in the form of an optical waveguide in combination a substrate which is patterned to provide feedback through distributed feedback; positioning at least one dielectric layer adjacent to the lasing layer and providing gain path extension in the lasing layer through mismatch reflection and waveguiding; and employing a microcavity within which the lasing layer is located.
  • these lasers can take the form of a polymer light- emitting diode or a polymer light-emitting electrochemical cell.
  • Figure 1 provides the molecular structure of a few examples of semiconducting luminescent polymers including representative ftmctionalized conjugated polymers.
  • Figure 2 is a graph of the PL decay of a ftmctionalized conjugated polymer, a soluble derivative of PPV, BuEH-PPV.
  • Figure 3 is a schematic diagram of an apparatus capable of testing a layer of polymer for gain narrowing.
  • Figure 4 is an emission spectrum of BuEH-PPV; the solid curve is the PL spectrum as emitted at low pump energies (for example, at 0.09 ⁇ J per pulse); the dashed curve is the gain narrowed emission spectrum obtained at higher pump energy (for example at 10 ⁇ J per pulse).
  • Figure 5 is a graph showing the gain narrowing transition for BuEH-PPV; the width of the emission spectrum is plotted vs pump energy (energy per pulse).
  • Figure 6a is a plot of emission line width as a function of pump pulse energy (on a log scale) for various BCHA-PPV/PS blend films at different BCHA-PPV concentrations: 8.4% (squares), 15% (open circles), 30% (crosses) and neat (100%) (open triangles).
  • Figure 6b is a plot of the threshold for gain narrowing as a function of the fraction of BCHA-PPV in the blend.
  • Figure 7 is a plot of the line width of the PL spectrum of BuEH-PPV at high pump energy as a function of the film thickness.
  • Figure 8 is a plot of emission line width as a function of pump pulse energy (on a log scale) for MEH-PPV neat films cast on glass from different solvents: tetrahydrofuran (open circles), / * »-xylene (crosses), and chlorobenzene (squares).
  • Figure 9 is a schematic cross-sectional view of a laser device of the present invention showing typical structure capable of achieving distributed feedback (DFB).
  • DFB distributed feedback
  • Figure 10 is a plot of linewidth vs pump energy (per pulse) data for a BuEH-PPV film on a distributed feedback grating (solid squares); see Figure 9 for a diagram of the distributed feedback grating structure.
  • Figure 10 open diamonds
  • the threshold for gain narrowing was reduced by more than a factor of twenty for the BuEH film spin-cast onto the distributed feedback grating.
  • Figure 11 is a plot of emission spectra from a microcavity: DBR/BuEH- PPV PVK/Ag at various pump energies.
  • the PVK layer is ⁇ 340 nm thick.
  • Figure 11a is a plot of the ratio of integrated intensities (mode ratio) of the -552 nm peak to the -645 nm peak (squares) and of the -690 nm peak to the -645 nm peak (triangles) in the system described with reference to Figure 11.
  • Figure 12 is a plot of the external observation angle dependence of emission spectra of a microcavity DBR/BuEH-PPV PVK/Ag at pumping energies above the lasing threshold. PL spectra are normalized to those obtained below threshold.
  • the PVK layer is -900 nm thick.
  • Figure 12a is an expanded view of the lasing peak near 550 nm of the system described with reference to Figure 12.
  • the present invention employs solid bodies of ftmctionalized conjugated polymers as active lasing layers.
  • Derivatives of conjugated polymers with bulky side chains such as MEH-
  • the abso ⁇ tion coefficient is reduced to values between 10" cm '1 and 10 5 cm * .
  • This invention employs a sub-class of semiconducting, luminescent conjugated polymers as materials for use in solid state lasers.
  • solid films can be fabricated in which stimulated emission is sufficiently strong relative to competing processes that the materials exhibit gain with a gain length less than 10 micrometers (10 ⁇ m) or even less than 1 micrometer (1 ⁇ m).
  • Laser action with low threshold is observed for films with thickness below 10 micrometers (10 ⁇ m) or even less than 1 micrometer (1 ⁇ m).
  • Criteria are established for achieving thin solid films of semiconducting polymers in which the gain lengths are less than 10 micrometers (10 ⁇ m) or even less than 1 micrometer (1 ⁇ m), and for achieving lasing from thin solid films of semiconducting polymers in which the gain lengths are below 10 micrometers (10 ⁇ m) or even less than 1 micrometer (1 ⁇ m).
  • the conjugated polymers which are employed in this invention can be defined by their ability to satisfy three criteria: Criterion 1 is satisfied by all known conjugated polymers.
  • Criterion 2 is defined by the photoluminescence (PL) and includes, for example, the poly(phenylene) and substituted derivatives, poly(phenylenevinylene) and substituted derivatives, polyfluorene and substituted derivatives, poly(p-pyridine) and substituted derivatives, and poly(p-pyridal vinylene) and substituted derivatives.
  • Some other conjugated polymers do not exhibit significant PL, for example, poly(acetylene) in either the cis- or trans isomeric forms. Still other conjugated polymers exhibit only relatively weak PL as neat, nondiluted solid films, including for example polythiophene and its substituted derivatives.
  • Preferred materials are those with PL quantum efficiencies greater than 10%, more preferred are those with PL quantum efficiencies greater than 20%, still more preferred are those with PL quantum efficiencies greater than 30%, even more preferred are those with PL quantum efficiencies greater than 40%, and yet more preferred are those with PL quantum efficiencies greater than 50%.
  • the experimental methods for PL measurement are well known and long established in the prior art.
  • Criterion 3 defines the specific sub-class of conjugated polymers which are materials for use as solid state lasers. Materials which satisfy criteria 3 are identified by exhibiting laser action (lasing); that is, by exhibiting gain narrowing.
  • Gain narrowing experiments can be and usually are performed with the PL material at room temperature. Although less convenient, measurements on materials at low temperature are also possible. For such measurements it is convenient to use the frequency doubled output (532 nm) of a 10 Hz, Q-switched Nd:YAG laser (for example the Spectra Physics Quanta-Ray DCR-3) which serves as the excitation source.
  • the pump source laser must emit pulses of sufficient energy to exceed the threshold for laser action. As shown in the Examples, the required energy per pulse ranges from less than 0.1 ⁇ J per pulse to greater than 10 mJ per pulse. Although the use of 10 ns pulses is not critical, the pump pulse width should be greater than the luminescence decay time.
  • the PL decay time is typically several hundred picoseconds to one thousand picosecond (one thousand picoseconds equals 1 ns).
  • the PL decay of a soluble derivative of PPV, BuEH-PPV, is shown in Figure 2.
  • 101 is a neodymium-YAG laser capable of emitting specific length pulses at 355 or 532 nm.
  • Puls are passed through a hydrogen cell 102 to yield Raman-shifted pulses which are separated into various spectral lines by divider 103.
  • Ten ns pulses 104 conveniently focused to a spot size of -1.5 mm, are incident on the sample 105 at near normal incidence, for example ⁇ 30° angle from the normal to the film surface.
  • the emission 106 that passes through a -2 mm aperture 107 placed normal to the front face of the sample is collected, frequency dispersed by a single monochromator 108, and detected by, for example, a CCD array 109.
  • the emission spectrum of a representative polymer, BuEH-PPV is shown in Figure 4.
  • the dashed curve is the PL spectrum as emitted at low pump power.
  • the broad luminescence with characteristic vibronic side band structure is typical of conjugated polymers which satisfy criterion 2.
  • BuEH-PPV exhibits gain narrowing as shown in Figure 4.
  • the gain narrowing transition for BuEH- PPV is shown in Figure 5.
  • the threshold for gain narrowing that is, the pump energy required to reduce the linewidth to one-half of its un-narrowed value
  • the threshold for gain narrowing is at approximately 0.2 ⁇ J per pulse.
  • Functionalized conjugated polymers are representative of polymers which satisfy all three of these criteria. These functionalized conjugated polymers are polymers having repeat units making up a conjugated backbone and, in addition, at least one nonconjugated moiety extending from the polymer. The at least one nonconjugated group, taken as a whole should provide at least about 5 carbon and/or oxygen atoms of chain length for each conjugated repeat unit extending from the backbone. These nonconjugated moieties may extend from copolymeric units in the polymer which are separate from the conjugated units but more commonly are attached to the conjugated repeat units as substituents.
  • These polymers may include, for example, functionalized poly(phenylenes), functionalized poly(phenylenevinylenes), functionalized poly(fluorene), ftmctionalized poly(p-pyridine), and ftmctionalized poly(p- pyridalvinylene).
  • the nonconjugated groups which are present in these conjugated polymers as functionalizing groups contain at least about five carbon and/or oxygen atoms per conjugated unit.
  • nonconjugated groups can include hydrocarbyls, substituted hydrocarbyls, hydrocarbyloxys and poly(oxyalkylene)s. They may be straight chain but more typically are branched.
  • hydrocarbyl refers to an organic radical primarily composed of carbon and hydrogen which may be aliphatic, alicyclic, aromatic or combinations thereof. It includes, without limitation “alkyls”, “alkenyls”, and “aryls” and "cycloalkyls”.
  • substituted hydrocarbyl refers to a hydrocarbyl group having from 1 to 3 substituents selected from the group consisting of hydroxy, acyl, acylamino, acyloxy, alkoxy, alkenyl, alkynyl, amino, aminoacyl, aryl, aryloxy, carboxy, carboxyalkyl, cyano, cycloalkyl, guanidino, halo, heteroaryl, heterocyclic, nitro, thiol, thioaryloxy, thioheteroaryloxy, and the like.
  • Preferred substituents include hydroxy and cyano.
  • hydrocarbyloxy refers to an organic radical primarily composed of carbon, oxygen and hydrogen which may be aliphatic, alicyclic, aromatic or combinations thereof. It includes, without limitation “alkoxys”, “ “alkoxyalkyls” and “aryloxys”.
  • poly(oxyalkylene) refers to a poly ether having on average from about 2 to about 100 oxyalkylene units where the alkylene portion is most typically a 2 or 3 carbon alkylene, that is ethylene or propylene.
  • alkoxy refers to the group alkyl-O-.
  • alkoxy groups include, by way of example, methoxy, ethoxy, n-propoxy, /so-propoxy, w-butoxy, tert- butoxy, sec-butoxy, n-pentoxy, H-hexoxy, 1,2-dimethylbutoxy, and the like.
  • alkoxyalkyl refers to the group -alkylene-O-alkyl which includes by way of example, methoxymethyl (CH 3 OCH 2 -), methoxyethyl (CH 3 -O- CH 2 CH 2 -) and the like.
  • alkenyl refers to alkenyl groups preferably having from 2 to 8 carbon atoms and more preferably 2 to 6 carbon atoms and having at least 1 and preferably from 1 -2 sites of alkenyl unsaturation.
  • alkenyl groups include ethenyl
  • alkyl refers to monovalent alkyl groups preferably having from 1 to 8 carbon atoms and more preferably 1 to 6 carbon atoms. This term is exemplified by groups such as methyl, ethyl, n-propyl, wo-propyl, w-butyl, wo-butyl, n- hexyl, and the like.
  • aryl refers to an unsaturated aromatic carbocyclic group of from 6 to 14 carbon atoms having a single ring (e.g., phenyl) or multiple condensed rings (e.g., naphthyl or anthryl). Preferred aryls include phenyl, naphthyl and the like.
  • aryloxy refers to the group aryl-O- where the aryl group is as defined herein including optionally substituted aryl groups as also defined herein.
  • cycloalkyl refers to cyclic alkyl groups or cyclic alkyl rings of from 3 to 8 carbon atoms having a single cyclic ring or multiple condensed rings which can be optionally substituted with from 1 to 3 alkyl groups.
  • Such cycloalkyl groups include, by way of example, single ring structures such as cyclopropyl, cyclobutyl, cyclopentyl, cyclooctyl, 1-methylcyclopropyl, 2-methylcyclopentyl, 2- methylcyclooctyl, and the like, or multiple ring structures such as adamantanyl, and the like.
  • Suitable cycloalkyl rings include single ring structures such as cyclopentane, cyclohexane, cycloheptane, cyclooctane, and the like, or multiple ring structures such as bicyclo[2.2.1]heptane, bicyclo[3.2.1]octane, and the like.
  • Preferred cycloalkyl rings include cyclopentane, cyclohexane, cycloheptane and bicyclo[3.2.1]octane.
  • the functionalized conjugated polymers employed in the present invention may be present as neat materials. That is in a single polymer, undiluted and not mixed with other materials.
  • the active lasing layers can be a mixture of two or more functionalized conjugated polymers in any proportion.
  • Such blends should contain an adequate proportion of conjugated material to permit a high level of lasing activity in the solid state.
  • the higher the proportion of functionalized conjugated polymer the more intense the laser response.
  • the optical path in the gain medium can be enhanced by multiple reflections at the surface of the film so as to cause the emitted light to propagate in an optical waveguide, or it can be enhanced by providing feedback as in a resonant cavity where the light passes back and forth many times within the cavity prior to exiting.
  • the feedback serves at least two functions; to increase the path length of the radiation in the gain medium, and to provide a resonant structure that determines the specific frequencies which will be amplified above the threshold for laser emission.
  • resonant structures oscillate in specific modes with an associated spectral redistribution of the emission, increasing the intensity at some wavelengths at the expense of others [A. Yariv, Quantum Electronics, 3rd edn, (Wiley, New York, 1989 )].
  • the path length in the medium can be greater than the gain length without feedback from a resonant structure.
  • Resonant waveguide structures can be created by using distributed feedback (DFB). This is done by inducing a periodic spatial variation either of the refractive index or of the gain of the laser medium. Laser oscillation in the periodic structure created has feedback provided by backward Bragg reflection [H. Kogelnik and C.V. Shank, Appl. Phys.Lett. 18, 152 (1971)].
  • DFB distributed feedback
  • a variety of thin film DFB lasers have been demonstrated by using dye- doped polymers (e.g. Rhodamine 6G doped polymethylmethacrylate (PMMA)) [A.A. Zlenko, A.M.
  • the films were built in such a way that they constituted waveguide structures (for example, by choice of substrate with appropriate index of refraction). Waveguiding mechanisms have been often used to create the interference patterns needed to achieve DFB [see D. Shamrakov and R. Reisfeld, Chem. Phys. Lett. 213, 47 (1993)].
  • these polymer materials are in the form of thin films which exhibit gain narrowing.
  • these polymers are in the form of thin films which exhibit gain narrowing, and which are neat and not diluted.
  • these polymers are in the form of thin films which exhibit gain narrowing, which are neat and non diluted, and which are thinner than 10 microns.
  • these polymers are in the form of thin films which exhibit gain narrowing, which are neat and non diluted, and which are thinner than 5 microns.
  • these polymers are in the form of thin films which exhibit gain narrowing, which are neat and non diluted, and which are thinner than 1 micron.
  • Even more preferred configurations present ftmctionalized conjugated luminescent polymers which satisfy criteria 1 and criteria 2 in configurations such that they exhibit gain narrowing as thin solid state films, neat and nondiluted, at pump energies (energy per pulse) less than 10 ⁇ J. Most preferred configurations present functionalized conjugated luminescent polymers which satisfy criteria 1 and criteria 2 such that they exhibit gain narrowing as thin solid state films, neat and nondiluted, at pump energies (energy per pulse) less than 1 ⁇ J.
  • the active layer in thin film lasers comprises a functionalized conjugated polymer, said thin film lasers being either optical lasers (optically pumped) or injection lasers (electronically pumped by electrical current injection).
  • the thin film in these thin film lasers is an optical waveguide and in which the emitted light is guided within the optical waveguide over lengths longer than the gain length within the polymer.
  • the thin film lasers of this invention are either optical lasers or injection lasers with feedback wherein the feedback is realized through external mirrors.
  • the thin film lasers of this invention are either optical lasers or injection lasers with feedback wherein the feedback is realized through distributed feedback.
  • the thin film lasers of this invention are either optical lasers or injection lasers with feedback wherein the feedback is realized through dielectric mismatch reflection.
  • the thin film lasers of this invention are either optical lasers or injection lasers with feedback wherein the feedback is realized through fabrication of the thin film laser material within a microcavity.
  • Such devices can take the form of a polymer light-emitting diode (LED) or a polymer light-emitting electrochemical cell (LEC).
  • Light-emitting diodes have been fabricated using semiconducting, luminescent polymers as the active materials.
  • Polymer light-emitting diodes (LEDs) have been fabricated in two configurations; the sandwich cell configuration and the surface cell configuration. These confiurations work well with the present polymers.
  • Thin film devices in the sandwich (multi-layer thin film) configuration comprise an active luminescent, semiconducting material laminated between two planar electrodes. One of the electrodes is made semi-transparent, thereby allowing the emitting light to exit from the device.
  • Inorganic materials such as ZnS:Mn
  • organic materials such as organic dye molecules [C.W. Tang and S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913; J. Appl. Phys. 65 (1989) 3610] and conjugated polymers [H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Bums and A.B. Holmes, Nature 347, 539 (1990); D. Braun and A.J. Heeger, Appl. Phys. Lett. 58 1982 (1991)] have been used in this general type of electroluminescent device.
  • Electrodes are pre-prepared onto a substrate with a gap in between [U. Lemmer, D. Vacar, D.Moses, A.J. Heeger, T. Ohnishi and T.Noguchi , Appl. Phys. Lett. 68, 3007 (1996)].
  • That substrate can be polycrystalline, glass, plastic or a single crystal of, for example, silicon.
  • that substrate can be a free standing film of a semiconducting luminescent polymer.
  • the injecting electrodes can be fabricated on the substrate by, for example, thermal evaporation, electron beam evaporation, sputtering, chemical deposition and so on.
  • the electrodes can be patterned either with a shadow mask or by means of photolithographic techniques well known in the art.
  • the electrodes can be fabricated in an interdigitated pattern as demonstrated, for example, for light-emitting electrochemical cells (LECs) [Q. Pei, G. Yu, C. Zhang, Y.Yang and A.J. Heeger, Science, 269, 1086 (1995)].
  • a solution of BuEH-PPV (chemical structure is shown in Figure 1) i p- xylene, at a concentration of 1 % w/v, was prepared by stirring at room temperature in a sealed environment where the oxygen level was minimized by a constant flow of high purity nitrogen. The solution was vigorously stirred until the polymer was dissolved.
  • a glass substrate for film preparation was obtained by cutting a commercially available microscope slide (7.5 cm x 2.5 cm x 1 mm) into a 1.5 cm x 1.5 cm square. It was cleaned by sonication in reagent grade acetone for 20 minutes followed by sonication in reagent grade 2-propanol for 20 minutes. The substrate was then dried in an oven at 170°C and introduced into the nitrogen chamber via a vacuum port.
  • the nitrogen chamber was equipped with a spin casting apparatus which allows film preparation to be carried out in the nitrogen environment. Approximately 70 ⁇ l of solution was poured onto the substrate and the substrate was spun at a speed of 750 revolutions per minute (rpm). As soon as the film was dry, an additional 70 ⁇ l of solution was poured onto the substrate and the substrate was again spun at a speed of 750 rpm until the film was dry. This gave a high optical quality film with a thickness of 210 nm, as determined with a Dektak surface profilometer. The sample was stored in the nitrogen chamber until immediately before the optical characterization experiment described below.
  • the -10 ns pulses focused to a spot size of -1.5 mm, were incident on the samples at an angle of approximately -30°.
  • the energy of the pulses was selected with calibrated neutral density filters that were held in the beam path.
  • the emission that passed through a ⁇ 2 mm aperture placed normal to the front face of the sample was collected, frequency dispersed in a single monochromator (Spex), and detected by a Peltier-cooled CCD array (Photometries).
  • PL spectra were recorded under optical pumping at two different pump energies and are presented in Figure 3.
  • the dotted curve shows the broad PL spectrum at 0.09 ⁇ J per pulse.
  • Example 2 In this example, we used the sample identical to that described in
  • Example 1 The sample was characterized optically in a manner analogous to that described in Example 1 except that PL spectra were recorded for a range of energies from 0.05 to 200 ⁇ J/pulse.
  • the linewidth defined as full width at 1/e height, was determined. This definition of the linewidth is more suitable (compared to other definitions such as FWHM or full-width at half-maximum) for comparison of disparate PL spectra with varying magnitudes of vibronic features.
  • the evolution of the linewidth as a function of the pump pulse energy is shown on a log scale in Figure 4. The dramatic collapse of the line width (from 130 nm to 9 nm) at very low pumping energies ( ⁇ 1 ⁇ J) indicates that these undiluted thin films (210 nm thickness) exhibit laser action with a very short gain length.
  • BuEH-MEH copolymers are polymerized from varying ratios of monomers of BuEH-PPV and MEH-PPV, and hence have a mixture of these functionalized side chains attached to the PPV backbone. These copolymers were synthesized with 5 different monomer ratios as listed in Table 1.
  • HEH-PF and BDOO-PF are polymers with PF backbone structures and appropriate functionalized chains attached thereto.
  • CN-PPP is a polymer with a PPP backbone structure with the appropriate functionalized side chain attached thereto.
  • MEH-PPV, HEH-PF, BDOO-PF, and CN-PPP were obtained from UNI AX Co ⁇ ., Santa Barbara, California, and were used as received. The other polymers in Table 1 were synthesized at the Institute for Polymers and Organic Solids at the University of California, Santa Barbara.
  • the polymers were dissolved in the solvents listed in Table 1 : tetrahydrofuran (THF), p-xylene, and chlorobenzene (CB). Because of the affinity of THF for atmospheric moisture, it was thoroughly dried in a distillation apparatus immediately before being used for any sample listed in Table 1.
  • THF tetrahydrofuran
  • CB chlorobenzene
  • Polymer solutions were prepared in different solvents (see Examples 13 and 14). Sample preparation was carried out in a manner similar to that described in Example 1.
  • the polymer concentration in the appropriate solvent was varied in the range 0.5 - 5 %w/v and spin casting spin speed in the range 750 - 4800 rpm, respectively. For each substrate, spin casting was carried out once or twice depending on desired thickness.
  • the frequency doubled output of the Nd:YAG laser can be used to pump a high pressure H 2 cell, from which the first anti-Stokes line (435 nm) emerges.
  • the appropriate excitation wavelength was chosen in order to get the maximum linear absorption.
  • the pump wavelength for each polymer is listed in Table 1. All polymers listed in Table 1 presented well-defined energy thresholds on the order of 1 ⁇ J/pulse, above which gain narrowing occurred, and below which the broad spontaneous emission spectra were seen. The energy thresholds for gain narrowing and the final linewidths after gain narrowing are listed in Table 1. In addition, for reference, the peaks in the spontaneous emission spectra are also listed in same Table.
  • the energy threshold for gain narrowing is over 1000 times lower than that of DCM in the solid state.
  • Thick films of BCHA-PPV suspended in polystyrene (PS) were made in the manner described below.
  • BCHA-PPV was dissolved in THF at a concentration of 3 % w/v.
  • the BCHA-PPV/THF solution, PS, and additional THF were combined and stirred to yield solutions with BCHA-PPV/PS concentration of 8.4%, 15%, 30%, and 100%, and PS/THF concentration of 20 % w/v.
  • the resulting mixtures were mixed thoroughly, drop cast on glass substrates, and allowed to dry in a nitrogen environment.
  • the films were characterized optically as in Examples 1-3.
  • Figure 6 shows the emission line width as a function of pump pulse energy (on a log scale) for various BCHA-PPV/PS blend films at different BCHA-PPV concentrations: 8.4% (squares), 15% (open circles), 30% (crosses) and neat (100%) (open triangles).
  • a significant decrease in the energy threshold is observed at higher BCHA-PPV concentrations, reaching a minimum of 1.3 ⁇ J for the neat film.
  • This Example demonstrates the advantage offered by the high density of chromophores in solid-state films of conjugated polymers, neat and undiluted, as compared to films in which the density of chromophores is deliberately lowered by dilution in an inert matrix.
  • Thin films of BuEH-PPV of different thickness were prepared by spin casting onto glass substrates from solutions in THF as in Examples 1-3.
  • the resulting film thicknesses ranged between 27 and 252 nm.
  • the films so fabricated were characterized optically as in Examples 1-3, with the pump wavelength at 435 nm and pump energies ranging between 0.05 and 200 ⁇ J per pulse. In this range of pump energies, films with thickness below 106 nm showed broad PL spectra with vibronic side bands in all cases. No gain narrowing was observed. Films with thickness greater than 126 nm showed broad PL spectra at low energies ( ⁇ 0.1 ⁇ J per pulse). At higher pump energies (> 0.4 ⁇ J per pulse) the PL spectrum exhibited gain narrowing.
  • the line width of the PL spectrum at high pump energy as a function of the film thickness is shown in Figure 7. A well defined thickness cutoff is evident around 116 ⁇ 10 nm.
  • This example demonstrates that for neat, undiluted thin films of BuEH- PPV on glass there is a thickness cutoff of 116 ⁇ 10 nm, below which gain narrowing is not observed. As shown in Examples 8, 9 and 10, this thickness cutoff originates from the thin semiconducting polymer film functioning as an optical waveguide.
  • Thin films of BCHA-PPV and MEH-PPV of different thickness were prepared as in Examples 1-3. Films thickness ranged between 160 nm and 650 nm for BCHA-PPV and between 52 nm and 405 nm for MEH-PPV.
  • the films so fabricated were characterized optically as in Examples 1-3, with the pump wavelength at 532 nm and pump energies ranging between 0.05 and 200 ⁇ J per pulse. Both BCHA-PPV and MEH-PPV exhibited the same type of thickness dependence as BuEH-PPV in Example 6. A well defined thickness cutoff was evident around (215 ⁇ 60) nm in BCHA-PPV and around (70 ⁇ 17) nm in MEH-PPV.
  • Example 8 Free standing films of BuEH-PPV were made in the manner described below. BuEH-PPV was dissolved in THF at a concentration of 1 % w/v. The solution was drop cast on glass substrates and allowed to dry in a nitrogen environment. The films were removed from the glass substrate and characterized optically as in the Example 1-3. The refractive index (n) and the thickness of the free standing film was determined from modal waveguide characterization by the prism coupling technique in a standard Metricon apparatus.
  • This cutoff thickness may be calculated through the following expression:
  • ⁇ is the wavelength of the guided light and c, f and s refer to cover, film and substrate respectively.
  • h-. ⁇ 120 nm for a BuEH-PPV film on glass.
  • the film fabrication procedure is analogous to those in Examples 1-3, and the resulting film thickness was 440 nm.
  • the optical characterization experiment is as described in Examples 1-3.
  • the PL spectrum recorded in the standard configuration showed only limited gain narrowing. However, exciting the film through the sapphire led to strong gain narrowing.
  • Waveguiding in the substrate provides some confinement (through the evanescent wave) of the emitted photons in the active medium, that is big enough to exhibit gain narrowing.
  • the film fabrication procedure is analogous to those in Examples 1-3
  • the same type of experiments were performed as in Example 11 , and the same type of behavior was observed, namely a dependence of the lasing behavior on the side of sample being pumped.
  • This example demonstrates that waveguiding in the substrate is the dominant process for the observation of gain narrowing in BCHA-PPV films deposited over sapphire. This conclusion may be generalized to the usage of any other polymer of Table 1, deposited over any type of substrate that has a refractive index higher that of the polymer.
  • Example 13 The experiments of Example 13 were repeated using MEH-PPV films cast from THF, ⁇ -xylene, and chlorobenzene (CB).
  • the distributed feedback (DFB) structure 120 shown in Figure 9 was made by spin casting a 200 nm thick film of BuEH-PPV 121 on sub-micron period DFB gratings 122 in silicon dioxide layer 123.
  • the gratings were prepared using a holographic setup.
  • a 1 ⁇ m thick film 123 of silicon dioxide was deposited on a silicon substrate 124 by plasma enhanced chemical vapor deposition.
  • a 700 A thick film of photoresist was then spin-cast onto the substrate.
  • the photoresist was then exposed to the interference pattern of two cw UV beams from a He-Cd laser.
  • the grating pattern was transferred into the silicon dioxide by plasma etching with CF 4 gas. Etch depths were varied from 2 nm to 20 nm.
  • the threshold for gain narrowing occurred at approximately 10 J; the linewidth vs pump energy (per pulse) data are shown in Figure 10 (solid squares).
  • the data for a thin film waveguide structure without DFB are also shown on Figure 10 (open diamonds), the threshold for gain narrowing was reduced by more than a factor of twenty for the BuEH film spin- cast onto the DFB grating.
  • This Example shows that low threshold distributed feedback lasers can be fabricated using semiconducting polymers as the gain material.
  • Microcavities consisted of simple layered structures DBR/PVK BuEH-PPV/Ag or DBR/BuEH-PPV/PVK/Ag where DBR stands for a highly reflective Distributed Bragg Reflector (one mirror of the microcavity) and Ag (silver) formed the other mirror of the microcavity.
  • DBR Distributed Bragg Reflector
  • Ag silver
  • Film thicknesses were measured with a Dektak profilometer. Solvents were chosen so that they would not dissolve any underlying polymer films during spin-casting of bilayers. BuEH-PPV was dissolved at concentrations from 1 to 1.5% w/v in p-xylene and spin-cast to thicknesses of from -470 nm. PVK films with thicknesses from ⁇ 90 nm to ⁇ 900 nm were spin case from cyclohexanone with concentrations varying from 1.5 to 10% w/v. Film quality was generally good, with no macroscopic inhomogeneities.
  • the transmittance of the silver layer ( ⁇ 60 nm, deposited by thermal evaporation at 2 x 10 "6 torr) at 435 nm was 11%. Microcavities were processed and stored in inert environment until immediately before use; the optical experiments were carried out under ambient conditions.
  • the DBR mirrors (type BBD1), obtained from CVI Laser Inc., have nominally greater than 99% reflectivity at normal incidence from 488 - 694 nm.
  • the DBR consists of 1/4-wavelength dielectric layers (> 60) with alternating high ( « 2.2) and low ( « 1.4) index.
  • the high reflectivity over such a broad wavelength range results from the many layers with different layer thicknesses (chirped DBR) such that longer wavelengths are reflected deeper inside the stack. As a result, each microcavity supported several modes.
  • microcavities have relatively high Q and efficiently couple emitted photons to only a few cavity modes [J. Gruner, F. Cacialli and R.H. Friend, J. Appl. Phys. 80, 207 (1996); V. Cimrova and D. Neher, J. Appl Phys. 79, 3299 (1996); V. Cimrova, U. Scherf and D. Neher, Appl. Phys. Lett. 69, 608 (1996); M. Berggren, O. Inganas, T. Granlund, S. Guo, G. Gustafsson, and M.R. Andersson, Synth. Met. 76, 121 (1996)].
  • the emission peaks are quite narrow ( ⁇ 7 nm) even below threshold. Above threshold, the emission peak nearest the gain maximum (550 nm for BuEH-PPV) grew in intensity more rapidly than the other emission peaks as the pump energy increases. Thus, following N. Tessler, G.J. Denton and R.H. Friend, Nature, 382, 695 (1996), we defined the mode ration as the ratio of the integrated power of the lasing mode to that of one of the other spontaneous emission modes.
  • the emission modes from a BuEH-PPV microcavity ( ⁇ 340 nm of PVK between the BuEH-PPV and Ag mirror) are shown in Figure 11. All pump energies were corrected for the transmission of the silver. There was a strong emission peak around 552 nm, close to the gain maximum of BuEG-PPV, and observable emission from four other microcavity modes. An abrupt increase in the mode ratio Figure 1 la was observed near 0.1 ⁇ J/pulse, the lasing threshold. Note that the mode ration approaches 100 at pump energies that are above threshold but still fairly low ( ⁇ 60 ⁇ J/pulse). This demonstrates that microcavities can produce single-mode emission with a narrow peak when pumped above threshold.
  • Varying the thickness of PVK in microcavities allowed control over the wavelengths of the allowed modes with respect to the gain maximum of BuEH-PPV.
  • the mode ratio is sensitive to the positions of the cavity modes. If the PVK thickness is adjusted such that resonance occurred at the maximum gain wavelength of BuEH-PPV, the normalized mode ratio was greatly enhanced, leading to effectively single-mode emission above threshold (cf. Figure 1 1). Single-mode operation was only realized, however, when the lasing peak was located within ⁇ 5 nm of the gain maximum. BuEH-PPV microcavities with lasing peak at wavelength less than -547 nm or greater than - 554 nm showed mode ratios of only about 5, even well above threshold. Hence, to obtain single-mode microcavity lasers, the cavity must be optimized to better than about 5 nm.
  • the lasing threshold was ground 0.1 ⁇ J/pulse. There was no indication that thresholds were lower for samples with a PVK buffer layer between the BuEH-PPV and Ag. Thus, although the metal electrode limits the Q of the microcavity, PL quenching by proximity to the metal is not of major importance.
  • the emission spectra shown in Figure 12 were collected at three different angles at a pump energy (- 10 ⁇ J/pulse) well above the lasing threshold (see Figure 12a).
  • the spectra are scaled such that the intensities of the main peak at below-threshold pumping levels were normalized to the same value so that the directionality that is inherent in microcavity structure has been divided out.
  • Figure 12 shows that above threshold, the normalized intensity of the main peak was reduced by -35% relative to its below-threshold value as the collection angle was increased from 0 to 3.2 degrees.
  • the directionality was enhanced above the lasing threshold.
  • This Example demonstrates that the light emitted from optically pumped semiconducting polymer microcavities is laser light.

Abstract

L'invention concerne des laser à semi-conducteurs utilisant des polymères conjugués. Un polymère conjugué utile dans la mise en pratique de cette invention est un polymère conjugué présentant un état fondamental et un état excité et qui, sous la forme d'une couche mince non diluée, satisfait les critères consistant: (i) à présenter une forte absorption à l'état fondamental avec un coefficient d'absorption d'au moins 104 cm-1 et une profondeur d'absorption non supérieure à 1νm, (ii) à présenter une émission de luminescence efficace dans l'état excité, cette émission étant décalée vers un état d'énergie plus faible par rapport à l'absorption de l'état fondamental, et (iii) à produire une émission stimulée non perturbée par l'absorption photoinduite, de sorte que la couche mince présente une réduction du gain et une émission spontanée amplifiée. Ces polymères conjugués présentent des longueurs de gain de l'ordre du micron ou même inférieur au micron, et par conséquent une action laser avec un seuil de pompage faible, sous la forme de couches minces solides dont les épaisseurs sont de l'odre du micron ou même inférieur au micron.
PCT/US1997/012026 1996-07-19 1997-07-11 Polymers conjugues utilises comme materiaux pour lasers a semi-conducteurs WO1998003566A1 (fr)

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US7608856B2 (en) 2004-05-03 2009-10-27 Merck Patent Gmbh Electronic devices comprising organic semiconductors
DE102008045662A1 (de) 2008-09-03 2010-03-04 Merck Patent Gmbh Optoelektronische Vorrichtung
DE102008045663A1 (de) 2008-09-03 2010-03-04 Merck Patent Gmbh Fluorverbrückte Assoziate für optoelektronische Anwendungen
DE102008045664A1 (de) 2008-09-03 2010-03-04 Merck Patent Gmbh Optoelektronische Vorrichtung
US7901766B2 (en) 2003-09-04 2011-03-08 Merck Patent Gmbh Electronic devices comprising an organic conductor and semiconductor as well as an intermediate buffer layer made of a crosslinked polymer
US7989071B2 (en) 2004-05-04 2011-08-02 Merck Patent Gmbh Organic electronic devices
US8637853B2 (en) 2007-10-24 2014-01-28 Merck Patent Gmbh Optoelectronic device
CN109320693A (zh) * 2018-09-13 2019-02-12 南方科技大学 共轭聚合物点及其制备方法和应用、可饱和吸收体及其制备方法和应用

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US5408109A (en) * 1991-02-27 1995-04-18 The Regents Of The University Of California Visible light emitting diodes fabricated from soluble semiconducting polymers
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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7592414B2 (en) 2000-08-01 2009-09-22 Merck Patent Gmbh Materials that can be structured, method for producing the same and their use
US7901766B2 (en) 2003-09-04 2011-03-08 Merck Patent Gmbh Electronic devices comprising an organic conductor and semiconductor as well as an intermediate buffer layer made of a crosslinked polymer
US7608856B2 (en) 2004-05-03 2009-10-27 Merck Patent Gmbh Electronic devices comprising organic semiconductors
US7989071B2 (en) 2004-05-04 2011-08-02 Merck Patent Gmbh Organic electronic devices
US8637853B2 (en) 2007-10-24 2014-01-28 Merck Patent Gmbh Optoelectronic device
DE102008045662A1 (de) 2008-09-03 2010-03-04 Merck Patent Gmbh Optoelektronische Vorrichtung
DE102008045663A1 (de) 2008-09-03 2010-03-04 Merck Patent Gmbh Fluorverbrückte Assoziate für optoelektronische Anwendungen
DE102008045664A1 (de) 2008-09-03 2010-03-04 Merck Patent Gmbh Optoelektronische Vorrichtung
CN109320693A (zh) * 2018-09-13 2019-02-12 南方科技大学 共轭聚合物点及其制备方法和应用、可饱和吸收体及其制备方法和应用
CN109320693B (zh) * 2018-09-13 2021-03-30 南方科技大学 共轭聚合物点及其制备方法和应用、可饱和吸收体及其制备方法和应用

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