Multimode laser action was observed from optically excited 2,5-dioctyloxy poly(para-phenylene-vinylene), DOO-PPV, micro-rings coaxially deposited around glass optical fibres of elliptical cross-section. The laser emission was found to be dependent upon the incident angle of the excitation and exhibited linewidths of approximately 1.2 Å, quality factors (Q) exceeding 5000, and thresholds below 0.3 μJ pulse. Such elliptical organic micro-ring lasers offer increased tailorability in emission properties over more conventional analogues of circular cross-section. Also discussed is the potential for such low-threshold lasers to serve as integrated sources for fibre lasers and amplifiers

Ballato, John

Gregory, R, V.

Korade, P. R.

English

Old Dominion University

Old Dominion UniversityODU Digital CommonsChemistry & Biochemistry Faculty Publications Chemistry & Biochemistry2007Elliptical Micro-Ring Organic LasersP. R. KoradeJohn BallatoR, V. GregoryOld Dominion UniversityFollow this and additional works at: https://digitalcommons.odu.edu/chemistry_fac_pubsPart of the Optics Commons, and the Organic Chemistry CommonsThis Article is brought to you for free and open access by the Chemistry & Biochemistry at ODU Digital Commons. It has been accepted for inclusionin Chemistry & Biochemistry Faculty Publications by an authorized administrator of ODU Digital Commons. For more information, please contactdigitalcommons@odu.edu.Repository CitationKorade, P. R.; Ballato, John; and Gregory, R, V., "Elliptical Micro-Ring Organic Lasers" (2007). Chemistry & Biochemistry FacultyPublications. 130.https://digitalcommons.odu.edu/chemistry_fac_pubs/130Original Publication CitationKorade, P. R., Ballato, J., & Gregory, R. V. (2007). Elliptical micro-ring organic lasers. Journal of Optics A: Pure and Applied Optics, 9(7),L9-L11. doi:10.1088/1464-4258/9/7/l01IOP PUBLISHING JOURNAL OF OPTICS A: PURE AND APPLIED OPTICSJ. Opt. A: Pure Appl. Opt. 9 (2007) L9–L11 doi:10.1088/1464-4258/9/7/L01RAPID COMMUNICATIONElliptical micro-ring organic lasersP R Korade1, John Ballato1 and R V Gregory21 Center for Optical Materials Science and Engineering Technologies (COMSET) and theSchool of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA2 Department of Chemistry and Bio-chemistry, Old Dominion University, Norfolk,VA 23529-3034, USAReceived 16 May 2007, accepted for publication 6 June 2007Published 21 June 2007Online at stacks.iop.org/JOptA/9/L9AbstractMultimode laser action was observed from optically excited 2,5-dioctyloxypoly(para-phenylene-vinylene), DOO-PPV, micro-rings coaxially depositedaround glass optical fibres of elliptical cross-section. The laser emission wasfound to be dependent upon the incident angle of the excitation and exhibitedlinewidths of approximately 1.2 Å, quality factors (Q) exceeding 5000, andthresholds below 0.3 μJ/pulse. Such elliptical organic micro-ring lasers offerincreased tailorability in emission properties over more conventionalanalogues of circular cross-section. Also discussed is the potential for suchlow-threshold lasers to serve as integrated sources for fibre lasers andamplifiers.Keywords: polymers and organics, micro-rings, lasers, micro-cavity andmicro-disk lasersSince their first realization over a decade ago [1], organiclasers, particularly in micro-cavity geometries, have enjoyedsteady inquiry by both the materials science and optical physicscommunities [2–7]. This is largely due to the potentialfor these components to exhibit low-threshold lasing andacceptably high-quality factors, Q, while possessing greatermanufacturability at lower cost than inorganic counterparts.In particular, considerable advances have been made indevice design to understand and optimize performance andchemistry to control the excited-state dynamics and emissionwavelengths and quantum efficiencies. Continued progresstowards practical application would be benefited by furtherenhancing the tailorability of the lasing properties. To thisend, this work focused on the fabrication and spectroscopiccharacterization of organic micro-ring lasers of ellipticalcross-section. Such oval micro-cavity lasers are generatedby the solution deposition of a light-emissive pi-conjugatedpolymeric film on an elliptical optical fibre. As is shown, asidefrom the realization of low thresholds, the lasing spectrumof the elliptical micro-rings was dependent upon the incidentangle of the optical excitation. This angle dependence betweenpump incidence and laser wavelength provides an added degreeof control over the performance of micro-cavity lasers.The pi-conjugated polymer used in this work was 2,5-dioctyloxy poly(para-phenylene-vinylene), DOO-PPV. TheDOO-PPV was synthesized following previously reportedprocedures [8], dissolved in toluene at 70 ◦C to varyingconcentrations ranging from 1 to 5 mg ml−1 in 1 mg ml−1increments, and doubly filtered through a 0.25 μm PTFE(Teflon) syringe filter in order to remove any suspended orundissolved particles that would lead to optical scattering inthe micro-ring.Micro-rings were fabricated by first taking a 4–5 cm lengthof the elliptical glass fibre and cleaning its surface in ethanolto remove any dust or particulates on the fibre surface. In atransfer pipette, the DOO-PPV/toluene solution was depositedonto elliptical optical fibres by drip-coating, where a dropof solution is placed onto the glass fibre surface and surfacetension is used to yield a smooth uniform coating. The toluenein the solution subsequently evaporates at room temperature,leaving a thin DOO-PPV film of approximately 500 μm inthickness, depending upon the number of drops of solutionused.The elliptical optical fibre was drawn at ClemsonUniversity’s optical fibre draw laboratories [9]. A cylindricalrod of phosphate glass was ground to have optical flats ontwo parallel sides. Uncoated glass optical fibre was drawnat approximately 1000 ◦C, with long and short axes of thesubstantially elliptical cross-section being approximately 150and 300 μm, respectively. Surface tension effects during thefibre drawing did lead to some rounding off of the parallel flats.This can be lessened to some degree by a lower-temperature1464-4258/07/070009+03$30.00 © 2007 IOP Publishing Ltd Printed in the UK L9Rapid CommunicationFigure 1. Emission from the DOO-PPV micro-ring as a function ofexcitation power.draw but, for the proof-of-concept efforts of this work, thisovality was not considered to be consequential to the lasingperformance of the micro-rings.The surface roughness of the micro-rings was character-ized by using atomic force microscopy (AFM). The AFMused was a Nanoscope Dimension™ 3100 Atomic Force Mi-croscope (Digital Instruments) operated in the tapping (non-contact) mode.Photo-excitation of the micro-rings was performedby using a frequency-doubled Nd:YAG regenerative laseramplifier producing 100 ps pulses at 1 kHz repetition rate. Thepump beam was focused using a cylindrical lens into a 6 mm×150 μm stripe, which was applied perpendicular to the micro-ring. Micro-ring samples of 0.1–0.6 mm of lateral length werephoto-excited inside a sample holding chamber where vacuumwas applied with a continuously running vacuum pump tominimize photo-degradation of the polymer. The emissionsfrom the micro-ring samples were collected from the end ofthe glass optical fibre and spectrally analysed using a CCDcamera with spectral resolution of 0.1 Å. All measurementswere made at room temperature and the excitation beam wasalways perpendicular to the coaxial micro-ring and ellipticaloptical fibre.AFM analysis (images not shown) was performed on themicro-ring as a function of DOO-PPV solution concentration.For the films drip-cast from the 2 mg ml−1 solution, the rootmean square (rms) roughness was measured to be 18.7 nm,whereas the 3 and 5 mg ml−1 films exhibited an rms roughnessof 20.6 and 40.7 nm, respectively. Accordingly, the higher-concentration solutions yielded rougher surfaces. As relatesto subsequent use in a laser oscillator/amplifier system, thesurfaces formed from the 2 and 3 mg ml−1 solutions areconsidered to be optically smooth on the basis of the Rayleighand Fraunhofer criteria.The Rayleigh criterion defines a surface as being smoothwhen its roughness is less than λ/8 cos(θ), where λ is the free-space wavelength of the light being used and θ is the incidentangle of light to the micro-ring surface, which, for this presentconsideration, is selected as zero degrees where a maximumin scattering would occur. In the present case, where λ is630 nm corresponding to the peak emission from DOO-PPV,Figure 2. Micro-ring emission intensity as a function of pump powerfor excitation normal to the minor (a) and major (b) ellipse axes.Dashed lines are only a guide-to-the-eye for approximating the lasingthreshold.the surface is smooth up to a maximum value of approximately79 nm. Similarly, the Fraunhofer criterion defines a surfaceas being smooth when its roughness is less than λ/32 cos(θ),which here would be below approximately 20 nm. Whereassolutions of all concentrations meet the Rayleigh criterion,only micro-rings made from the 1, 2, and 3 mg ml−1 solutionsmeet the Fraunhofer criterion. Hence, desirous of meetingboth conditions in order to minimize any extrinsic scatteringfrom the structures, only micro-rings fabricated from solutionswith concentrations of 2–3 mg ml−1 were used in the laserinvestigations.Figure 1 shows the photoluminescence spectra of theelliptical organic micro-ring as a function of pump energy. Thefluorescence is fairly broadband, as is known for DOO-PPV,and exhibited two peaks with maxima at 525 and 625 nm.As the pump energy is increased, the luminescence linenarrows as the system becomes more highly excited. Forexcitation intensities higher than 0.3 μJ/pulse, multimodelaser emissions were observed. As is shown in figure 2, thelasing characteristics of the micro-ring depended on whetherthe excitation was normal to the ellipses’ major or minor axis.The exact threshold for lasing is critical to the waythe sample was prepared and the experiment was carriedout. The typical laser threshold observed was in the rangeof 0.15–0.5 μJ/pulse (1.5–5 mJ cm−2). These values arecomparable to the reported values for dye doped lasers [1] andalso correspond approximately to the threshold observed in acylindrical micro-cavity [10].The above-noted angular threshold dependence suggestedthat other laser parameters might also be affected by theangle of the pump beam with respect to the micro-ring axes.Accordingly, the laser spectrum was analysed at a series ofangles relative to the major axis. As is shown in figure 3, themultimode nature of the laser was preserved and there was anoticeable shift in the spectral envelope of the lasing modesshifting to shorter wavelengths as the angle of excitation wasrotated towards the minor (short) axis of the ellipse. TheL10Rapid CommunicationFigure 3. Emission from the DOO-PPV micro-ring as a function ofangle of excitation with respect to the fibre major axis: (a) 10◦,(b) 30◦, and (c) 50◦.five (5) most intense peaks in each spectrum was evaluatedand averaged yielding linewidths of approximately 1.2 Å andquality factors, Q, exceeding 5000.It was noted in the experimental section that the micro-ringemissions were collected from the end of the glass optical fibreand analysed. This is unexpected and the practical importanceof this should not be overlooked. The ability to collect themicro-ring emissions from the end of the substrate optical fibreimplies that the micro-ring modes couple to the propagatingmodes of the elliptical bulk glass fibre. This is a departure fromusing a fibre of circular cross-section as the substrate for themicro-rings. In that case, the orthogonality between the micro-ring lasing modes and the propagating modes of the circularfibre preclude couplings, unless there is some perturbationto break this degeneracy [11, 12]. The ability to couple themicro-ring laser emission directly into the propagating modeof the fibre may permit a novel approach to low-thresholdpumping of active optical fibres and fibre lasers [13]. In thiscase, inexpensive broad-area pump diodes of reduced beamquality could be used to excite the micro-ring whose high-Q laser emissions then couple into an active fibre yieldingexcitation of the doped core. The micro-ring’s excitation angle-dependent laser emission could be used as a further way totailor the excitation to optimize absorption by the dopant.Lastly, since these micro-rings are easily deposited on the glassfibre, it is certainly possible during the fiberization process (justlike the protective polymer coatings) that long lengths of theintegrated micro-ring pump sources (i.e. moving from micro-rings to polymeric laser coatings) may be generated to providea distributed excitation at lower power levels than by directend-pumping.Micro-rings of the pi-conjugated polymer DOO-PPV weredeposited onto glass optical fibres of elliptical cross-section.Upon optical excitation, laser emissions were observed fromthe organic micro-ring above a threshold of approximately0.3 μJ/pulse. The micro-ring laser output exhibited linewidthsof approximately 1.2 Å and quality factors of above 5000.The spectral peak position of the lasing modes was observedto be a function of the angle of excitation with respect tothe ellipse axes, which can be used to fine-tune the laserwavelengths. Micro-ring laser emissions were found to coupleinto the propagation modes of the glass optical fibre due toits ellipticity, which breaks the symmetry associated with theorthogonality of propagating modes between the micro-ringand fibre. This may permit a novel way to convert a low-qualitypump sources into more efficient excitation of an active fibrelaser or amplifier.AcknowledgmentsThe authors wish to acknowledge experimental contributionsand measurements by R Polson and Z V Vardeny of theDepartment of Physics at University of Utah. Dr Paul Pitman,Wade Hawkins and Kigre Incorporated are also acknowledgedfor their assistance, as is the financial support from the USDefense Advanced Research Projects Agency (grants N66001-01-1-8938 and N66001-03-1-8900).References[1] Kuwata-Gonokami M, Jordan R, Dodabalapur A, Katz H,Schilling M and Slusher R 1995 Polymer microdisk andmicroring lasers Opt. Lett. 20 2093–5[2] Hide F, DiazGarcia M, Schwartz B, Andersson M, Pei Q andHeeger A 1996 Semiconducting polymers: a new class ofsolid-state laser materials Science 273 1833–6[3] Kozlov V, Bulovic V, Burrows P and Forrest S 1997 Laseraction in organic semiconductor waveguide anddouble-heterostructure devices Nature 389 362–4[4] Frolov S, Vardeny Z V and Yoshino K 1998 Plastic microringlasers on fibres and wires Appl. Phys. Lett. 72 1802–4[5] Kozlov V and Forrest S 1999 Lasing action in organicsemiconductor thin films Curr. Opin. Solid State Mater. Sci.4 203–8[6] McGehee M and Heeger A 2000 Semiconducting (conjugated)polymers as materials for solid state lasers Adv. Mater.12 1655–68[7] Scherf U, Riechel S, Lemmer U and Mahrt R 2001 Conjugatedpolymers: lasing and stimulated emission Curr. Opin. SolidState Mater. Sci. 5 143–54[8] Frolov S, Shkunov M, Vardeny Z V and Yoshino K 1997 Ringmicrolasers from conducting polymers Phys. Rev. B56 R4363–6[9] http://comset.clemson.edu[10] Kawabe Y, Spiegelberg C, Schulzgen A, Nabor M F,Kippelen B, Mash E A, Allemand P M,Kuwata-Gonokami M, Takeda K andPeyghambarian N 1998 Whispering-gallery-mode microringlaser using a conjugated polymer Appl. Phys. Lett. 72 141–3[11] Baktur R 2005 Coupling of a microring laser into acoaxially-oriented optical fibre PhD Dissertation ClemsonUniversity[12] Baktur R, Pearson L W and Ballato J 2007 Theoreticaldetermination of lasing resonances in microrings J. Appl.Phys. 106 043102[13] Ballato J and Carroll D 2005 Optical fibre amplifier with fullyintegrated pump source US Patent Specification 6,947,208L11

Elliptical Micro-Ring Organic Lasers

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Elliptical Micro-Ring Organic Lasers

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