A new family of lasing dyes from an old family of fluors

We report the development of a new series of laser dyes and describe the performance of these materials. This work was spurred by the realization that few laser dyes have an optimally shaped fluorescence emission for lasing action. Most of the dipole strength of the transition is concentrated in the 0.0 band, where lasing cannot occur, and very little is concentrated in the vibrational satellites 0–1 and 0–2 where lasing is possible. The new dyes are unsymmetrical materials which may be considered as a hybrid of a rhodamine molecule and a fluorescein molecule (fig. 1), and we consider them to be cyclized merocyanines. One dye in particular, 6-dimethylamino-9-o-carboxyphenylxanthen-3H-one (labelled SNH-8), has given power output and time stability better than the preferred lasing dye Rhodamine 6G. The effects of dye structure and of acid-base transitions on lasing activity will be discussed briefly. The theory of band-shaping and the detailed optical parameters of the new dyes will be presented separately.     

Properties and the nature of electric-field-induced variations in optical absorption spectra of polymethine dyes in polymer matrices

The effect of a permanent electric field on the absorption spectra of ionic (cationic and anionic) and intraionic symmetric and asymmetric polymethine dyes in poly-N-epoxypropylcarbazole, polyvinylbutyral, and polystyrene polymer films is studied. It is found that the electric field causes an increase in absorption of symmetric dyes in the short-wavelength region and a decrease in their absorption in the long-wavelength region. Asymmetric dyes exhibit opposite properties. These properties are common for these groups of dyes irrespective of their chemical structure and ionic nature. They are retained both in photoconducting polymers and in polymers that are incapable of charge photogeneration. These effects also take place in liquid solutions, although in this case they are much weaker. Various mechanisms of the influence of an electric field on the spectra, such as the electron transfer, Stark and Kerr effects, intermolecular interactions, and dye isomerization, are discussed. It is shown that the spectral effects are caused by the redistribution of the electron density in a chromophore of the dye in the ground state resulting in a change in the probability of vibronic transitions.     

Broadband tunability of dye lasers

A double compartment dye cell to hold two or more dyes has been found to increase the frequency range of dye lasers. Results are compared with those obtained by mixing the same two dyes in a single cell. The dye cells are transversely pumped by a nitrogen laser. Broadband tunability results from dyes having similar absorption ranges but different Stokes shifts.     

Inorganic phosphors in lead–silicate glass for white LEDs

Using spectrophotometry and stationary and kinetic fluorimetry, we have shown that xanthene dye fluorescein forms complexes with polycationic derivative of fullerene in aqueous solutions mainly due to electrostatic interactions. It is found that efficient quenching of singlet excited states of dye occurs in the structure of these complexes due to the transfer of excitation or electron from dye to fullerene. As a result, the photodynamic activity of the newly formed complex is much higher than that of fluorescein and fullerene derivative. This effect makes it possible to predict the formation of new-generation hybrid photodynamic preparations using dyes excited only into a singlet state; as a result, directed searches for these dyes are significantly facilitated.     

Comparative photophysical and energy transfer studies of C480:C535 binary dye mixture in solid and liquid environments

The spectral behaviour of the individual dyes (C480 and C535) is discussed. The absorption and fluorescence spectral profiles of the dyes, in solid and liquid environments, are identical. Dye doped polymer rods of donor dye (C480) without the acceptor dye (C535), acceptor dye (C535) without the donor dye (C480) and donor dye with acceptor dye at different acceptor concentrations are synthesized. Energy transfer technique is studied, in detail, using these rods. The results obtained are compared with their respective liquid mixtures. Optical gain of the acceptor dye with and without the donor dye is determined experimentally, in both solid and liquid media. The gain of the acceptor with donor increases and then decreases with increase in acceptor concentration. Both solid and liquid media show similar trends. But, the gain coefficient in the solid matrix is less than that in the liquid medium. The photobleaching of the dye doped polymer rod under nitrogen laser excitation is studied.     

Spectral-Kinetic Properties of Laser Dyes in Active Elements on the Basis of Sol-Gel Matrices

The absorption and photoluminescence spectra as well as the photoluminescence lifetimes of laser dyes (Phenylamine 430, Rhodamine 6G, Rhodamine 6G zwitterion, Rhodamine 4C, and Rhodamine 101) in new matrices are studied. These matrices represent the hybrid of two types of porous glasses (microporous and sol-gel glass). Chemical transformations of the dye molecules in the matrices are not found. The dye dimers mentioned above are also absent for concentrations up to 10^-4 M. The luminescence efficiencies of the dyes in the matrix and in the ethanol solutions are compared. The difference in concentration dependences of the photoluminescence spectrum of Rhodamine 6G in the matrix and ethanol solution is found and discussed. The collective emission of the dyes in new matrices is observed at a power density of exciting radiation of 10²⁵ cm^-2s^-1 and a concentration of 10^-4 M. The energies and collective emission spectra of the dyes in the matrix are compared with those in the solutions.     

An enhancement of efficiency of a solid-state dye-sensitized solar cell due to cocktail effect of N719 and black dye

The solar cell performance of the black dye, N719 dye and the cocktail of two dyes on TiO₂ films were studied by mean of the utilization as light harvesting electrodes in solid-state FTO|TiO₂|dye|CuI|Cr–FTO cells. The power conversion efficiencies of 3.8% and 3.0% are obtained when N719 and black dye were used. When the mixture of 1:1 of two dyes was used, the conversion efficiency rises to 4.6%. In the mixture of N719 and black dye, the N719 dye acts as the aggregation preventer and a co-absorber on TiO₂ surfaces. The increased absorption of light by the two dyes results in increase of electron injection thus enhancing both the short-circuit current density and the open circuit voltage contributing to increased power conversion efficiency of the cell.     

Effect of Matrix Treatment on Spectroscopic Properties of HCl Catalysed Sol-Gel Glasses Containing Coumarin Laser Dyes

Coumarin 1, Coumarin 2 and Coumarin 120 are embedded in transparent sol-gel glass samples prepared by sol-gel process using dip method. The sol-gel matrix is given dip treatment with Methanol /Distilled Water (50/50vol) for 1 to 16h before dipping into dye solution. The effect of dipping time of matrix in Methanol/ Distilled Water on spectroscopic properties of coumarin dye doped glass samples has been studied. The Optical Density (OD) at absorption maximum wavelength and Fluorescence Intensity (FI) at fluorescence maximum wavelength of all coumarin dyes increase with the time of dipping of the sol-gel sample. These absorption/fluorescence properties of coumarin dyes in sol-gel glass matrices are compared with its respective properties in methanolic solution in acidic environment. The cause of these changes in OD/FI with dipping time is discussed by taking into account the absorption / fluorescence of dye in acidified methanol.     

Laser dye mixtures

Properties necessary to obtain efficient laser action from laser dyes are reviewed. It is concluded that the dye added (booster dye) to a laser dye (active dye) solution to increase its efficiency need not necessarily be a laser dye. This is illustrated by considering different spectral locations of a triplet-triplet absorption band present in the booster dye relative to the location of the laser action region (part of the fluorescence spectrum) of the active laser dye. Conditions required for booster dyes to form efficient dye mixtures are given. These observations are used to explain why specific well known laser dye mixtures show efficient, little, or no laser action at all.     

PHOTOCHROME DYE TRACING IN WATER FLOWS

The properties and uses of photochromic dyes are presented, and the history of the use of various photochromic dyes in fluid flow experiments is discussed. The mechanisms and theory of the changes in properties of various types of photochromic dyes are presented. In response to the need for a water-soluble photochromic dye, a tri-arylmethane dye, Acid Violet 19, was selected for use in two-phase air-water experiments. Its optical and chemical properties are given, and the dependence of its rate constant on various parameters is studied. Details of the use of Acid Violet 19 in fluid flow experiments are presented.     

Theoretical analysis of S1-state lifetime measurements of dyes with picosecond laser pulses

Various methods for the determination of the S₁-state lifetime of dye solutions (laser dyes and modelocking dyes) are analysed. A general model of interaction of laser light with dye molecules is presented and reduced to a dye energy level scheme of six levels. Fluorescence emission, light amplification and absorption recovery techniques are investigated theoretically and their limitations revealed. The determination of the S₁-state lifetime of saturable absorbers by single picosecond pulse bleaching experiments is very thoroughly discussed. The influence of various laser and dye parameters on the bleaching experiments are analysed numerically. The results are compared with isotropic steady state two- and three-level dye models.     

Dye assisted enhanced transmission in near field optical lithography

Surface plasmon lithography using embedded-amplitude masks has received considerable attention in recent times for its ability to produce high density features with resolution beyond diffraction limit. However plasmon damping caused due to intrinsic metal absorption restricts the achievable aspect ratio of the fabricated features. One possible way to rectify this issue is to use a gain medium to amplify the surface plasmons and thereby increase their propagation length. In this context this paper proposes a novel concept of employing dye medium to enhance plasmon propagation in mask based surface plasmon lithography, so as to obtain higher transmission depth in the writing medium. The proposed concept is supported by numerical simulations and the results obtained indicate a 14.5 fold field enhancement in presence of dye (gain) medium.     

The plasmon–exciton interaction in layered nanostructures with two-dimensional J-aggregates

The transformation of the electronic excitation energy in a plane-layered nanostructure with two-dimensional J-aggregates of a cyanine dye has been studied theoretically. The dependences of the plasmon–exciton interaction energy on the system parameters have been determined. In the case of small values of the Rabi frequency, the rates of nonradiative energy transfer from surface plasmon–polaritons of the metal substrate to molecular excitons of J-aggregates have been calculated in terms of the perturbation theory. The dispersion laws for hybrid plasmon–exciton states have been determined, and it has been shown that the Rabi splitting can range up to 100 meV.     

Energy transfer in flashlamp pumped organic dye lasers

The pumping characteristics of dye lasers using multiple dyes are discussed. It turns out that varying the dye concentration may change considerably the portion of the pump light spectrum which is effective in creating population inversion. Thus the effect of an energy transfer dye depends strongly on the concentration of the lasing dye. Multiple dye systems are shown to have larger tuning ranges than single dye systems, and should exhibit improved tuned linewidth as well as better dye stability. Experimental results are presented for a common dye combination (Rhodamine 6G and Coumarin 6). Measurements were made both by directly mixing the auxiliary dye with the lasing dye, and also by using the auxiliary dye as a fluorescent spectral filter placed between the pump light source and the laser cell; the results are compared.     

Pulse formation in synchronously pumped infrared dye lasers

Pulse formation in synchronously pumped dye lasers with fast relaxing dyes is treated by computer simulation. The influence of spontaneous emission and chirp, as well as cavity mismatch, on the pulse characteristics and stability of generation are discussed.     

三氮吲哚Li嗪对增感染料吸收光谱和荧光谱的影响

本文研究了三氮吲哚Ｌｉ嗪（ＴＡＩ）对三种卤化银乳剂增感染料水溶液的吸收光谱和荧光光谱的影响，研究表明ＴＡＩ能使这些染料的各吸收峰强度增强，对容易形成了Ｊ聚集体的染料来说，更有利于其Ｊ态光谱吸收；当三氮吲哚Ｌｉ嗪的浓度达到一定值后，它能侃染料Ｈ聚集体分解成单分子，利用单分子光谱吸收，结果说明染料的荧光发射也随溶液中ＴＡＩ浓度的增大而增强，本文还讨论了产生这种现象的原因。     

Large optical-frequency shift of molecular radiation via coherent coupling to an off-resonance plasmon

We demonstrate coherent optical coupling between molecular and plasmon resonances that are well separated in energy. In the presence of metallic nanoparticles, the second harmonic spectrum of organic dyes no longer peaks at the absorption wavelength but is instead blueshifted by 25 nm towards the localized plasmon resonance. The phase of the light generated by the dyes displays a large modulation across the plasmon resonance and no change across the molecular one. The second harmonic signal contributed by the nanoparticles, which is peaked at the plasmon frequency when no molecules are present, similarly displays a shift towards the molecular resonance in their presence. A model based on the interplay of the nonlinear optical near fields is able to account for these observations.     

Dopant-enhanced ablation of nitrocellulose by a nitrogen laser

The photoetching behavior of pure nitrocellulose and of nitrocellulose dyed with stilbene-420, coumarin-120 and rhodamine 6G by 337 nm nitrogen laser pulses has been studied. Ablation with a low power nitrogen laser is hereby reported for the first time. A two step photochemical mechanism is proposed to account for the ablation of the pure material. With the addition of dyes strongly absorbing at 337 nm the photoetching rate of nitrocellulose can be increased significantly. This increase is proportional to the molar extinction coefficient of the dye at 337 nm and its concentration in the polymer. The photoetching mechanism and the energy transfer processes from the dye to the polymer are discussed in detail.     

Hexagonal microlasers based on organic dyes in nanoporous crystals

Molecular sieves, such as nanoporous AlPO₄-5, can host a wide variety of laser active dyes. We embeded pyridine-2 molecules as a representative of a commercially available dye which fits into the channel pores of the host matrix. Many efficient dye molecules, such as rhodamines, do not fit into the pores. But modifying the structure of the dyes to appear like the used templates allows us to increase the amount of encapsulated dyes. The properties of resulting microlasers depend on size and shape of the microresonators, and we discuss a model for microscopic hexagonal ring resonators. In terms of pump needed to reach lasing threshold molecular sieve microlasers are comparable to VCSELs. For dyes that fit into the pores we observed a partial regeneration of photo-induced damage. Received: 24 September 1999 / Revised version: 26 October 1999 / Published online: 23 February 2000     

Modeling of light absorption and scattering by metal nanoparticles coated with organic dye J-aggregate

The results of the theoretical study of optical properties of composite nanoparticles consisting of a metal core (Ag, Au, Cu, Al, Ni, Cr) and a J-aggregate shell of organic dye are presented. Light extinction, absorption, and scattering coefficients in colloidal solutions were calculated within the model based on the Mie theory modified taking into account dimensional phenomena and complemented by calculations of complex dielectric functions of the metal core and J-aggregate shell. The model adequately explains the features observed in light absorption and scattering spectra by hybrid nanoparticles, associated with the plasmon resonance in the metal core and with electronic excitation of the J-aggregate. The strong dependence of the results on geometrical parameters of nanoparticles and dielectric constants of core and shell materials was demonstrated. Methods for controlling the effects of the plasmon-exciton interaction in the system and optical properties of composite materials developed based on nanoparticles under study are discussed.