Condensations between 3-X-2,4-dimethylpyrroles (X = H, CH3, C2H5, and CO2C2H5) and acyl chlorides gave derivatives of 3,5,3′,5′-tetramethylpyrromethene (isolated as their hydrochloride salts): 6-methyl, 6-ethyl, 4,4′,6-trimethyl, 4,4′-diethyl-6-methyl, and 4,4′-dicarboethoxy-6-ethyl derivatives for conversion on treatment with boron trifluoride to 1,3,5,7-tetramethylpyrromethene–BF2 complex (TMP–BF2) and its 8-methyl (PMP–BF2), 8-ethyl, 2,6,8-trimethyl (HMP–BF2),2,6,-diethyl-8-methyl (PMDEP–BF2), and 2,6-dicarboethoxy-8-ethyl derivatives. Chlorosulfonation converted, 1,3,5,7,8-pentamethylpyrromethene–BF2 complex to its 2,6-disulfonic acid isolated as the lithium, sodium (PMPDS–BF2), potassium, rubidium, cesium, ammonium, and tetramethylammonium disulfonate salts and the methyl disulfonate ester. Sodium 1,3,5,7-tetramethyl-8-ethylpyrromethene-2,6-disulfonate–BF2 complex was obtained from the 8-ethyl derivative of TMP–BF2. Nitration and bromination converted PMP–BF2 to its 2,6-dinitro-(PMDNP–BF2) and 2,6-dibromo- derivatives. The time required for loss of fluorescence by irradiation from a sunlamp showed the following order for P–BF2 compounds (10−3 to 10−4 M) in ethanol: PMPDS–BF2, 7 weeks; PMP–BF2, 5 days; PMDNP–BF2, 72 h; HMP–BF2, 70 h; and PMDEP–BF2, 65 h. Under similar irradiation PMPDS–BF2 in water lost fluorescence after 55 h. The dibromo derivative was inactive, but each of the other pyrromethene–BF2 complexes under flashlamp excitation showed broadband laser activity in the region λ 530–580 nm. In methanol PMPDS–BF2 was six times more resistant to degradation by flashlamp pulses than was observed for Rhodamine-6G (R-6G). An improvement (up to 66%) in the laser power efficiency of PMPDS–BF2 (10−4 M in methanol) in the presence of caffeine (a filter for light <300 nm) was dependent on flashlamp pulse width (2.0 to 7.0 μsec).

Pyrromethene–BF2 complexes as laser dyes:1.

This paper describes the results of an investigation of the interrelated physical parameters that determine laser threshold for three organic dyes: rhodamine B, rhodamine 6G, and fluorescein. Not only are these dyes (rhodamine 6G in particular) among the most widely used and important laser dyes, but insights into the threshold condition derived from our studies of these particular dyes should be generally applicable to future dye laser research. Our detailed characterization of threshold for these dyes and our novel insights result from the use of triplet state spectral data in a thorough and quantitative way for the first time, coupled with the physically realistic approximation that the triplet population is proportional to the singlet excited state population. For the case of self‐tuning of the laser emission wavelength, solutions to the threshold equations are presented that establish relations between all the parameters affecting lasing: critical inversion, emission wavelength, extrinsic losses, dye concentration, length of active medium, and triplet to excited singlet population ratio. For reasonably high extrinsic cavity losses, the critical inversion is a simple power law function of extrinsic loss, and the emission wavelength is self‐tuned in such a manner that the ratio of extrinsic losses to intrinsic singlet absorption losses is substantially constant. There exists a region of low, but still physically realizable, cavity losses, where laser action is determined exclusively by intrinsic characteristics of the dyes (triplet absorption, singlet emission, and absorption). Asymptotic limits on the maximum wavelength of emission and on the minimum critical inversion appear in this region, beyond which there is no point in reducing extrinsic cavity losses. The fraction of triplet state molecules also determines long wavelength cutoffs for an externally tuned laser. Methods are outlined for measuring the ratio of triplet state to excited singlet state populations for dyes with known triplet absorption spectra and for determining semiquantitatively the triplet effects in dyes where this information is not known. A final interesting conclusion is that molecular modifications of rhodamine 6G cannot be expected to improve its threshold characteristics as a laser dye by much more than a factor of 4.

Organic Dye Laser Threshold

A cw laser using a 2.5×10−4 M solution of rhodamine 6G in water containing a deaggregating agent (1.5% Triton‐X100) is described. The laser consists of a 4.5‐mm transverse‐flow hemispherical cavity excited longitudinally by the 5145‐A output of a 1‐W argon ion laser. With mirror reflectances of 97.5% and 99.5%, a 200‐mW threshold excitation was measured. Power output was approximately 30 mW at 597 nm with 960 mW excitation power.

cw OPERATION OF AN ORGANIC DYE SOLUTION LASER

THE layer of sodium atoms present in the atmosphere at a height of about 90 km has been extensively studied by observations of the resonance scattering of sunlight in daytime and twilight1. Rocket measurements of the daytime height distribution of the resonance radiation2,3 have shown that the simple photochemical model based on the equilibrium between Na and NaO must be modified. In recent discussion of models involving a dust layer as a source of sodium4, attention has been drawn to the need for data on the night time abundance and distribution of atomic sodium. We report preliminary measurements at night by a new optical radar.

Atmospheric Sodium measured by a Tuned Laser Radar

Optofluidic dye lasers refer to a class of liquid dye lasers, usually on a microfabricated device, in which the adaptive nature of the liquid gain medium allows the dynamical control of the laser properties. Miniaturizing liquid dye lasers onto a microfluidic device not only results in compact, easy-to-maintain and safe dye laser systems, but also provides unprecedented optical performances such as precise spatial mode control, low threshold, and automatic fluidic tuning. Equally important, such on-chip liquid laser sources represent an important component for “lab-on-a-chip” systems.

This thesis studies the implementations of optofluidic dye lasers on polydimethylsiloxane (PDMS) based microfluidic devices. Replica molding soft lithography was used to fabricate monolithic PDMS devices which contain both wavelength-scale optical structures and large-sized microfluidic channels. We have demonstrated narrow linewidth single mode DFB lasers, simultaneous operation of integrated DFB laser arrays with a single pump, multiple color lasing from the same DFB cavity, continuous mechanical wavelength tuning over a 60nm range, microfluidic wavelength tuning, single mode liquid-core microring lasers using Vernier effect, liquid-cladding evanescent gain DFB lasers, and monolithic integration with PDMS microfluidic circuits. Typical laser thresholds achieved are well within the reach of commercial high power laser diodes, thus enabling the implementations of compact tunable laser sources for portable “lab-on-a-chip” devices. The impressive performances, diverse geometries and applications clearly demonstrate the power of optofluidic integration and adaptation.

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Optofluidic dye lasers

Stimulated emission spectra of two organic dyes, chloro-aluminum phthalocyanine (CAP) and 3,3′-diethylthiatricarbocyanine iodide (DTTC) are compared. Giant-pulse ruby laser excitation was used in both cases. An end pumping configuration employed with DTTC resulted in narrow beam divergences and high conversion efficiencies. For CAP, the oscillating transition is one which terminates on an excited vibrational level of the ground electronic state. For DTTC, stimulated emission at the lowest concentrations occurs at the peak of the Franck–Condon-shifted fluorescence band but moves to longer wavelengths as the concentration is increased. The transient behavior of the CAP laser, pumped in a transverse geometry, was observed and compared with computer solutions of the rate equations. Polarization measurements of the laser beams were also made. An analysis is given of requirements for achieving optimal pumping by means of flashlamps.
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 [The paper concludes with an addendum reporting recent observations of stimulated emission from dye solutions lasing at wavelengths ranging from blue (4385 A) to orange (6000 A)].

Laser-pumped stimulated emission from organic dyes: experimental studies and analytical comparisons

The results of recent theoretical and experimental studies on flashlamp‐pumped organic‐dye lasers are presented. The importance of rapid pumping is considered in detail. Constructional aspects of a suitable flashlamp assembly are given, and lasing characteristics of various organic dyes excited by this lamp are summarized.

Flashlamp‐Pumped Organic‐Dye Lasers

Locking of the output of a flashlamp‐pumped Rhodamine 6G dye laser to the D lines of sodium vapor was achieved by means of the large Faraday rotation that occurs in the vicinity of the resonance lines. The laser beam was observed to consist spectrally of two sharp components symmetrically disposed on either side of the D1 and D2 lines. The laser doublet splitting varies with magnetic field.

V. L. Moruzzi

Papers

Laser-pumped stimulated emission from organic dyes: experimental studies and analytical comparisons

Flashlamp‐Pumped Organic‐Dye Lasers

Frequency‐locking of organic dye lasers to atomic resonance lines