Study On Two-photon Induced Upconversion Amplified Spontaneous Emission(ASE) From Organic Crystals

Organic materials that exhibit strong two-photon absorption and efficient two-photon fluorescence are attractive for applications such as data storage,optical limiting,two-photon photopolymerization,fluorescence microscopy,and two-photon pumped(TPP) lasing.TPP lasing are currently of considerable interest for upconversion lasers,as the gain medium can be pumped at red or near-infrared wavelengths,for which laser diodes are inexpensive,and need no phase matching.So far,two-photon pumped lasing have been achieved with dye solution,conjugated polymer and dye-doped molecular solids as gain media.On account of their high purity and their rigorously defined structure,high-quality single crystals are more advantageous than polymer films and solutions.Crystal materials possess higher thermo-stability compared with amorphous materials,which are in thermodynamically metastable phase and tend to transit to a more stable phase at low temperature(glass transition temperature,T_g).Unlike solution-processed thin films, single crystals do not display grain boundary defects that may serve as carrier traps. Furthermore,a single crystal is excellent in optical properties,typically the low light propagation loss due to less light scattering.For the first time,the TPP lasing was achieved,also called amplified spontaneous emission(ASE),of DBADSMB organic crystals,in our group.The lasing behavior is proved by the significant thresholding effect,spectral narrowing and laser intensity dependence on the length of pumping area.Because of the amplification of photon of a certain frequency near the center of the gain profile of the crystal by stimulated emission from inversed population,strong and stable laser power density has been achieved.First,two-photon induced fluorescence method was used in the two-photon absorption cross-section measurements for four kinds of compound used in this work. Measurements were performed by use of ultrashort～100-fs Ti:sapphire pulsed excitation over the range 700-850 nm.TPE spectra were obtained by comparison with a fluorescein calibration standard(Fluorescein in H₂O pH=11).From quadratic illumination power dependence,indicating genuine two-photon processes. Implications of these measurements for two-photon laser-scanning microscopy are discussed.To date,the number of reported novel MPP lasing dyes is below one tenth of the total number of reported novel two-photon absorbing chromophores,partly because of quenching effect.Motivated to overcome this limitation,a special molecular design for a TPA chromophore is required not only to ensure a large two-photon activity,but more importantly,to overcome fluorescence quenching at high concentration,which is generally more serious in crystal.We employed 1,4-bis(α-cyano-4-diphenylaminostyryl)-2,5-dipheny lbenzene(S1:CN-DPASDB), 1,4-Bis[1-cyano-2-(4-(diphenylamino)phenyl)Vinyl]benzene(S2:CNDPADSB),and another CN-OPV(S3:CN-DPDSB) to realize TPP up-converted lasing in chapter 3. The 3D supramrolecular networks not induced the high quantum efficiency but also conduct the formation of high-quality crystals with the uniaxially oriented molecular packing.We have investigated lasing properties from organic crystals with enhanced solid-state emission,pumped by one and two photon for S1 and S2,and single photon pumped for S3.From the measurements of two photon absorption cross section, absolute fluorescence quantum efficiency and crystal morphology we discuss the factors controlling the TPP lasing.Two-photon induced amplified spontaneous emission(ASE) characteristics of needle-like crystals of N-(4-(4-(4-(diphenylamino)styryl) styryl)phenyl)-N- phenylbenzenamine(Ph-TPA2) were studied in chapter 4.The FWHM of the two-photon excited fluorescence can reach 6.5nm and ASE threshold reached 0.29 mJ·pulse-1·cm-2 for the crystal pumped with femtosecond laser.Under two-photon excitation,the up-converted light emission was confined within the crystal and self-waveguided along the needle axis then radiated from the tips of the needles.The needlelike crystal of Ph-TPA2 is shown to be a promising candidate for frequency up-converted organic semiconductor laser.