Amplified spontaneous emission in the spiropyran-biopolymer based systemJaroslaw Mysliwiec, Lech Sznitko, Stanislaw Bartkiewicz, Andrzej Miniewicz, Zacaria Essaidi, Francois Kajzar, and Bouchta Sahraoui Citation: Applied Physics Letters 94, 241106 (2009); doi: 10.1063/1.3155203 View online: http://dx.doi.org/10.1063/1.3155203 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/94/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Low threshold amplified spontaneous emission from dye-doped DNA biopolymer J. Appl. Phys. 111, 113107 (2012); 10.1063/1.4728218 Biopolymer based system doped with nonlinear optical dye as a medium for amplified spontaneous emissionand lasing Appl. Phys. Lett. 99, 031107 (2011); 10.1063/1.3610566 Lasing effect in a hybrid dye-doped biopolymer and photochromic polymer system Appl. Phys. Lett. 96, 141106 (2010); 10.1063/1.3377912 A hybrid laser system consisting of a frequency-doubled, narrow-line-width, distributed-feedback dye laseroscillator and a high saturation-fluence Ce:LiCaAlF 6 crystal amplifier Appl. Phys. Lett. 82, 3391 (2003); 10.1063/1.1576294 Amplified spontaneous emission and optical gain spectra from stilbenoid and phenylene vinylene derivativemodel compounds J. Appl. Phys. 86, 6155 (1999); 10.1063/1.371668

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Amplified spontaneous emission in the spiropyran-biopolymer basedsystem

Jaroslaw Mysliwiec,1,a� Lech Sznitko,1 Stanislaw Bartkiewicz,1 Andrzej Miniewicz,1

Zacaria Essaidi,2 Francois Kajzar,2 and Bouchta Sahraoui21Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology,Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland2Laboratoire POMA FRE CNRS 2988, Universite d’Angers, 2 Bd. Lavoisier, 49 045 Angers, France

�Received 2 March 2009; accepted 26 May 2009; published online 16 June 2009�

Amplified spontaneous emission �ASE� phenomenon in the 6-nitro-1� ,3� ,3�-trimethylspiro�2H-1-benzopyran-2 ,2�-indolin� organic dye dispersed in a solid matrix has beenobserved. The biopolymer system deoxyribonucleic acid blended with cationic surfactant moleculecetyltrimethyl-ammonium chloride served as a matrix. ASE appeared under sample excitation byUV light pulses ��=355 nm� coming from nanosecond or picosecond neodymium doped yttriumaluminum garnet lasers and has been reinforced with green ��=532 nm� light excitation followedUV light pulse. The ASE characteristics in function of different excitation pulse energies as well assignal gain were measured. © 2009 American Institute of Physics. �DOI: 10.1063/1.3155203�

Photochromic reaction in spiropyrans �SP� was observedby Fisher and Hirsberg1 in 1952. Since that time many po-tential applications involving this optically bistable moleculehas been shown, e.g., displays, filters or optical storagedevices.2 The basic form of the colorless �closed form� spiro-pyran consists of two heterocyclic rings linked by a tetrahe-dral carbon, which enforces an orthogonal orientation be-tween the two heterocycles. In this form the �-electron donot interact with one another, therefore the absorption spec-trum is a sum of the spectra of two parts of the molecule.3

Upon UV irradiation, spiropyran isomerizes to its meta-stable, highly polar, and planar colored merocyanine �MR�form �cf. inset of Fig. 3�. The colored MR �open form� is ahighly conjugated structure having a characteristic absorp-tion band in the visible region due to extended conjugationof the �-electron system and exhibits fluorescence. The equi-librium between the closed and open forms depends onphotochemical and thermal conditions. The photoinducedchange in molecular dipole moment can be exploited to con-trol an electrical conductivity of a polymer containing pho-toswitchable SP-MR system4 or for optical recording withvisible light.5

In this report, we show that the SP molecule doped tobiopolymer, based on deoxyribonucleic acid �DNA�, pre-serves its photochromic properties and shows efficient fluo-rescence upon light excitation. Photochromic compounds arevery sensitive to their environment.6 Variation in the polarity,free volume, and rigidity of a polymer matrix in whichphotochromic molecules are embedded, can affect theirphotochromic as well as luminescent properties. The strongluminescence of MR allowed us to observe the amplifiedspontaneous emission �ASE� phenomenon in this dye-polymer system. In ASE phenomenon spontaneously emittedphotons are amplified by coherent stimulated emission. ASEis a polarized and coherent emission centered around the partof fluorescence spectrum with the highest gain and appearsas gain narrowing.

The DNA-based solid polymeric matrix is a very prom-ising material for photonic applications due to the uniqueproperties derived from DNA double-helix structure andtransparency in the whole visible spectral range.7,8 It hasbeen shown that in the systems consisted of DNA with dyeslike photochromic disperse red 1 �DR1� or fluorescentrhodamine �SRh�, very fast dynamic diffraction gratingrecording9 or strong fluorescence enhancement,10 respec-tively, can be observed. ASE has already been reported byKawabe et al.11 in DNA-heksadecyltrimethylammonium�HTMA� doped with hemicyanine dye DMASDPB and inDNA-HTMA doped with rhodamine 6G.12

For the sample preparation we used commerciallyavailable SP dye: 6-nitro-1� ,3� ,3�-trimethylspiro�2H-1-benzopyran-2 ,2�-indolin� �Aldrich� and a purified salmonroe DNA �SIGMA� complexed with cationic surfactantcetyltrimethyl-ammonium chloride �CTMA�. The procedureof conversion of pure DNA material into DNA-CTMA com-plex was described elsewhere.13 Such a complex unlike thepure deoxyribonucleic acid is well soluble in many organicsolvents including alcohols and can be processed into rela-tively good optical quality thin films. Films were depositedon glass substrates by casting or by spin-coating depositionmethod.

In order to obtain around 2% solution of SP in DNA-CTMA w/w in the dry mass, we prepared separately butanolsolution of DNA-CTMA and butanol solution of SP andmixed them together. Then DNA-CTMA:SP solution wascast onto a glass plate and dried over 24 h in air at roomtemperature. Thickness of the so obtained films was around3 �m as estimated from interference fringes in light trans-mission measurements by Tolansky method.

Before starting the ASE experiment we characterize theabsorptive and emissive properties of the DNA-CTMA:SPsystem. All photoluminescence measurements were carriedout on a Hitachi F-4500 fluorescence spectrophotometerwithin the range of 300–800 nm at room temperature. Thespectral resolution was maintained at 1 nm both for excita-tion and emission. In Fig. 1 we show an example of theexcitation and emission luminescence spectra of the 2%

a�Author to whom correspondence should be addressed. Electronic mail:jaroslaw.mysliwiec@pwr.wroc.pl.

APPLIED PHYSICS LETTERS 94, 241106 �2009�

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DNA-CTMA:SP sample. First, the fluorescence emissionfrom DNA-CTMA:SP has been measured using 360 nm ex-citation light. A broad luminescence band centered at 600 nmwas clearly visible. Then, the SP molecule excitation spectrahave been measured by looking at fluorescence emission at600 nm wavelength. Excitation spectrum consists of twobands: one is centered at 400 nm while the other at 550 nm.The latter band is a characteristic for absorption of the meta-stable colored MR. The difference between the wavelengthof maximum absorption and the fluorescence �the Stokesshift� was found to be 50 nm. In order to prove that only MRform is luminescent we measured the photoluminescencespectra of DNA-CTMA:SP sample before any irradiation andsample preirradiated with UV light ��400 nm�. As is shownin the inset of Fig. 1 the fluorescence enhances with UVirradiation dose. Therefore one can conclude that irradiationof SP with UV light enhances the content of open MR formin the DNA-CTMA matrix. The absorption spectra of thefilm irradiated by UV light allowed us to estimate the absorp-tion coefficient of MR at 532 nm to amount to 1535 cm−1.

Investigation of the ASE phenomenon in DNA-CTMA:SP films has been performed using nanosecond andpicosecond neodymium doped yttrium aluminum garnet�Nd:YAG� lasers as it requires relatively high pump powersin the single pass geometry. Linearly polarized light of wave-length �=355 nm was generated via two second-harmonicgeneration crystals acting on fundamental frequency light ofNd:YAG laser �� f =1064 nm, 6 ns pulse duration, and 11 Hzrepetition rate�. The excitation 355 nm light inciding nor-mally onto the sample generated luminescence of the film.Luminescence light emerging from the sample edge was col-lected by a lens and directed into an optical fiber. Afterreaching certain threshold of excitation laser energy an ASEappeared. A signature of ASE is gain narrowing of the fluo-rescence spectrum, characterized by a relatively smooth linedominating the luminescence spectrum. ASE spectrum wasanalyzed with an Ocean Optics spectrometer �model LS-1�equipped with diode array detector and coupled to a com-puter for data acquisition.

As an inset of Fig. 2, results of the normalized sponta-neous fluorescence and ASE coming from the DNA-CTMA:SP are presented. The sample was excited by the

355 nm 6 ns pulse for two excitation energy densities: belowand just above the ASE threshold. A luminescence band cen-tered at 676 nm showing characteristic narrowing is emerg-ing from the sample above the ASE threshold light excitationenergy density. The emission linewidth converged from��FWHM=63 nm for spontaneous fluorescence to ��FWHM=6 nm for ASE. In Fig. 2 the results of integrated intensityof fluorescence emission in function of excitation energydensity are presented. From the plot the threshold for ASEprocess was estimated to be �th=8.6 mJ /cm2 which is rela-tively high when compared to rhodamine 6G having ASEthreshold of 2 mJ /cm2.12 Configuration of our experimentalsetup �light picked up at the sample edge� causes that belowthe ASE threshold, the intensity of a spontaneous lumines-cence was close to zero, as shown in Fig. 2. Lower thresholdenergies can be obtained for dyes having larger Stokes shiftsbecause of reduced self-absorption

Gain occurs when the stimulated emission of photonsexceeds the reabsorption or loss due to scattering, then thematerial will lase. ASE gain is the increase in the ratio oflight intensity emitted to incident intensity per unit length ofthe gain material that is optically pumped. In order to mea-sure the exponential gain coefficient for ASE process we usethe method of Shank.14 We calculated the average exponen-tial gain � to be 6 cm−1 in DNA-CTMA:SP �2%� thin layer.

In order to check the temporal stability of the ASE emis-sion of the studied sample we monitored the ASE peak in-tensity in function of time of irradiation. All experimentswere conducted in air atmosphere at room temperature. Inthis measurement the sample was excited either by 355 nmpicosecond pulses or 355 and 532 nm picosecond pulses al-most simultaneously with 10 Hz repetition rate. Excitationenergy density was set just above the ASE appearancethreshold. In Fig. 3 the plot of the ASE intensity changesduring prolonged exposure to the UV �355 nm� pulsed exci-tation light is shown. We can observe that the reduction inthe ASE intensity signal to 50% of its initial value occursafter around 1000 pulses. However, modifying sample exci-tation to that of both UV �355 nm� and green �532 nm� lightpulses, an important improvement has been noticed. TheASE signal intensity was significantly larger as compared to

FIG. 1. �Color online� Emission and excitation spectra of fluorescent SP dyein DNA:CTMA matrix �a�. Fluorescence spectrum has been collected using360 nm wavelength excitation. Excitation spectrum has been collected ob-serving fluorescence at 600 nm. UV irradiation �inset� enhances number ofMR forms in the film and results in stronger luminescence observed upon530 nm excitation.

FIG. 2. Integrated fluorescence of ASE intensity in function of ns pulsedlaser excitation energy density. As an inset of the normalized spontaneousfluorescence emission and ASE for two excitation energy intensities: belowand just above the ASE threshold is shown. The linewidth of the depictedemission converges from ��FWHM=63 nm to ��FWHM=6 nm.

241106-2 Mysliwiec et al. Appl. Phys. Lett. 94, 241106 �2009�

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that obtained for single UV pulse excitation �cf. inset of Fig.4�. The reduction in the ASE intensity signal to 50% oc-curred after around 2000 pulses for the same sample �cf. Fig.4�. Such a behavior is strictly related to the mechanism of theSP isomerization and emission coming from the metastableMR form. In the former case upon UV irradiation, SPisomerizes to its metastable MR form. The photochemicalring opening occurs in the subnanosecond range15 which forsome time remains in its excited singlet state and then radia-tively and nonradiatively deactivates to its ground state.However 355 nm light is a highly energetic light which couldproduce some photochemical pathway for dye decomposi-tion.

Photochromism of SP-MR system in DNA-CTMA leads,after UV excitation, to production of fluorescent MR mol-ecule. At low light level excitation this process is fully re-versible. We observed that fluorescence efficiency and ASEsignal increased when for the sample excitation 355 and 532nm pulses were used �cf. inset of Fig. 4�. The study of pho-tochemical ring opening and ring closure of the SP/MRcouple have been reported by Görner.16 Based on this prin-

ciple, we assume that the effect of enhancement is due to thefact that photocoloration occurs in the triplet manifold. MRtriplet state is rather short-lived ��10 �s�, therefore thepresence of short UV light pulses keeps the high concentra-tion of trans-MR in the sample. The picosecond pulses of532 nm laser light depopulate significantly the reservoir oftrans-MRs via stimulated emission which needs to be re-populated with subsequent UV light pulse. This process bothslows down the photodegradation rate and enhances the lu-minescence efficiency and ASE signal of the whole DNA-CTMA:SP system.

Taking into account the measured absorption coefficientfor MR in the film ��=1535 cm−1� and assuming that quan-tum yield of formation of MR triplet from SP is 0.04 �thelowest value reported in Ref. 17 for ethanol solution� thequantum yield of ASE could be estimated. The number of532 nm excitation photons was measured to be 1.51015

and those emitted into two directions in ASE process �at 676nm� amounted to 6.81011 photons. Then the value for thelower limit of emission quantum yield is estimated to 0.005in the studied experimental conditions.

In conclusion, the ASE in biopolymer loaded with a SPmolecule system has been observed and characterized. Thereported ASE parameters here are comparable to that ob-tained earlier by Kawabe et al.11 in hemicyanine dye. Webelieve that proper doping of DNA-CTMA matrix with SPderivatives and improvement of the dye photostability bringsperspectives of lasing applications and construction of or-ganic solid-state dye lasers.

The authors wish to thank for financial support The Eu-ropean Commission through the Human Potential Pro-gramme �Marie-Curie RTN BIMORE, Grant No. MRTN-CT-2006-035859� and The Wroclaw University of Technology.Thanks are due to Dr. Krystyna Palewska for luminescencemeasurements.

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FIG. 3. The ASE peak intensity changes in function of time while pumpedby the picosecond pulsed laser with 10 Hz repetition rate-irradiation onlywith picosecond pulses of UV light �355 nm�. Inset shows photoisomeriza-tion of the SP “off” form upon UV irradiation to its metastable, highly polar,and colored MR “on” form.

FIG. 4. The ASE peak intensity in function of time, while pumped by thepicosecond laser with 10 Hz repetition rate under simultaneous irradiationwith 355 and 532 nm pulses. Inset shows the difference of ASE for twopulse �355 and 532 nm� and single �355 nm� pulse illumination.

241106-3 Mysliwiec et al. Appl. Phys. Lett. 94, 241106 �2009�

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