| Laser crystal materials play fundamental roles in modern laser physics, quantum electronicsand optoelectronics. Understanding and ingenious utilization of these activated crystals haveled to a veritable revolution in the progress of photonic technology and devices. Since the firstdemonstration of lasing in the solid state organic semiconductors in1990s, great progresseshave been achieved. Over the past decade, researchers have been always looking for the organicsemiconductors with both higher stimulated emission cross section and higher carrier mobilities,for optoelectronic devices. Single crystals of conjugated organic molecules are, undoubtedly,the materials with the highest degree of order and purity among the variety of different forms oforganic semiconductors, which make their carrier mobility much higher (at least2order) thantheir amorphous counterparts. It has been reported that organic crystals with combination ofrelative high carrier mobility and high quantum efficiency. These features make them amazingfor optoelectronic applications. However, the reseach of organic single crystal-based devicesare advancing behind, the technology on growth of high quality organic single-crystals is notmature, the knowledge on relationship between optoelectronic properties and structures is notdeeply understood, and many challenges still exist in fabrication of crystal device. This thesisdescribes a number of studies on organic semiconductors crystal growth, properties andfabrication techniques, focused around using them for laser devices. The main reseaches arelisted as follows:1. Novel growth methods, such as tam-assisted release regulation, temperature gradientliquid phase epitaxy, have been developed for preparation of high quality organic crystalgrowth. With the tam-assisted release regulation physical vapor transport technique, we havesuccessfully prepared large size single crystals, with molecule-level smooth surface and opticalquality. We have further prepared organic crystals with tunable color emission, form blue, togreen and red with high quantum yield by change doping. High efficient F rster energy transferfrom donor molecules to acceptor molecules is demonstrated. Moreover, we have successfullyproduced well-defined polygon microsize thin crystals directly grown onto a substrate with temperature gradient liquid phase epitaxy.2. The two-photon excited stimulated emission is a frequency upconversion process, withwhich one can achieve laser frequency upconversion without phase matching condition. Andthe lasing wavelength is tunable. We have firstly observed two-photon excited amplifiedspontaneous emission in organic crystals, which shows great potentials for novel compactminiaturized all-solid-state laser devices, two-photon pumped (TPP) laser. Furthermore,two-photon absorption and frequency upconverted stimulated emission are undoubtly observedin other organic crystals, such as9,10-distyryl anthracene derivative crystals andcyano-substituted styrene oligomers organic crystals. The relationship between the two photonabsorption, upconverted amplified spontaneous emission (ASE) and crystal structures havebeen systemically studied. Highly polarized and directional ASE under both of the single-andtwo-photon pumping condition is realized in crystals of CNDPASDB. Its polarization contrastwas estimated to be~0.93. This large ratio is due to the unified unidirectional configuration ofthe molecular long axis in crystal, beneficial to the stimulated emission with a low threshold.The relationship of two-photon absorption, upconverted emission and the crystal structure issystemly studied. Futhermore, characteristics of single-and two-photon amplified spontaneousemission are comparatively studied. The polarization distribution between the two-photon andone-photon excitation in CNDPASDB crystal are investigated. Very different behaviors areobserved that the former generates a cos4θ distribution, while the later gives rise to cos2θdistribution in different orientation. A phenomenon, similar as stimulated resonance Ramanscattering (SRRS), is observed in CNDPASDB crystal. However, a pronounced difference withSRRS is, here the Stokes shift is not fixed, but decreased as the excitation wavelength increaseand the FWHM is much broader. A model describing competition between the stimulatedemission and vibration relaxations has been proposed to interpret the underlying mechanism.Finally, ultrafast dynamics of two-photon-pumped ASE from a single crystal is studied by thetime-resolved fluorescence upconversion technique. Formation and kinetic behavior oftwo-photon excitation of amplified spontaneous emission is discussed.3. To address the problem that the crystals are difficult to process because of their friable,delicate properties, we have proposed thechniques, such as laser processing and improved photolithography, to microfabricate structures in organic crystals.(1) Laser interferenceablation technology is employed to fabricate micro-nano structures in organic crystals. Withthis method, high quality grating structure on organic crystals are successfully prepared.Distributed feedback (DFB) structures have been designed and fabricated on the organic singlecrystalline thin film materials. DFB lasing is demonstrated from the organic single crystals forthe first time. We adopt a double-layered laser configuration and demonstrate lasing both fromone-and two-dimensional distributed feedback. The thin single crystals are further extendedonto mechanically bendable poly(ethylene terephthalate)(PET) substrates and presented aproof-of-concept “flexible’’ organic single-crystal DFB laser. Femtosecond lasers machining isproposed to fabricate complex structures in organic crystal. We present the preparation ofnanometer-size organic crystals using femtosecond laser induced forward transfer (Fs-LIFT)method.(2) We have improved semiconductor processes to fabricated organic crystal structures.We introduced polyvinyl alcohol (PVA) as protector and SU-8resist as an etch mask during theetching process. Whispering-gallery mode (WGM) resonators of BP1T and BP2T crystallinematerials have been fabricated through a combination method with improved lithography anddry etching. Crystalline microresonators with different geometries over a large area aretop-down fabricated with submicrometer spatial resolution. WGM lasing oscillation isdefinitively observed from circular, hexagonal, pentagonal and square organic single crystallineresonators.(3) A simple, melt-processed route to the development of two-photon upconversionlaser is presented. We have fabricated surface emitting lasers, consisting melt-processedDPAVB glass film as active media and two designed distributed Bragg reflectors.Wavelength-tunable upconversion lasing with a threshold as low as of150μJpulse-1cm-2isachieved upon pumped with near-IR (800nm) radiation. Lasing from multimode tosingle-mode oscillation is demonstrated. Continuously tunable single mode oscillation wasobtained at wavelength from514nm to523nm. Ultrafast pulse emission with23ps is achievedin this device. Our system offers a versatile candidate for the future application of photonicdevices. |
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