Lasers and Dyes

With the move to smaller, shorter wavelength lasers, new dyes had to be found that could absorb the light energy. Initially, the dye was incorporated into a 1 pm thick polymer film [36] (often polyimide), spin-coated onto the ITO electrodes this was not without its problems, as the resistive polymer layer dictated that a higher voltage was required to cause dielectric reorientation of 

Dyes that dissolve in the liquid crystal and have high extinction coefficients at the laser emission wavelength are required. If the device is for projection applications, the dye should not absorb at the wavelengths that may be used to project the final image-infrared dyes and lasers are well suited for this. The dye will experienee very high temperatures for very short time periods when addressed by the laser and should therefore be stable to heat and light. Dichroic dyes appear to give smaller spot widths than isotropic dyes, and so dyes with a modest order parameter (5=0.5-0.6) are desirable [23]. 

For the commonly used He-Ne lasers, blue dyes based on anthraquinone [38, 39] (10) are particularly useful. 

Dyes absorbing at 0.75 -1.2 pm tend to be of low solubility in liquid crystals, less stable, and often ionic or organometallic, thus undesirably drawing current during 

The use of dyes dissolved in the liquid crystal is obviously more efficient in the reverse mode effect, as the random orientation of the dye in the unwritten state provides a better chanee for the dye to absorb the light energy. 

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Atomic Fluorescence Spectrometry. A spectroscopic technique related to some of the types mentioned above is atomic fluorescence spectrometry (AFS). Like atomic absorption spectrometry (AAS), AFS requires a light source separate from that of the heated flame cell. This can be provided, as in AAS, by individual (or multielement lamps), or by a continuum source such as xenon arc or by suitable lasers or combination of lasers and dyes. The laser is still pretty much in its infancy but it is likely that future development will cause the laser, and consequently the many spectroscopic instruments to which it can be adapted to, to become increasingly popular. Complete freedom of wavelength selection still remains a problem. Unlike AAS the light source in AFS is not in direct line with the optical path, and therefore, the radiation emitted is a result of excitation by the lamp or laser source. 

However, even for lasers with a very sophisticated stabilization system, the residual uncompensated fluctuations of nd cause frequency fluctuations which are large compared with this theoretical lower limit. With moderate efforts, laser linewidths of Avj = 10 - 10 s" have been realized for gas lasers and dye lasers. With very high expenditure, laser linewidths of a few Hertz can be achieved [6.40,41]. There are, however, several proposals as to how the theoretical lower limit may be approached more closely. 

Fig.11.9. Synchronous mode locking of a cw dye laser. In order to match the ca-city lengths of argon laser and dye laser (which are not drawn to scale) an optical delay line can be used in the dye laser resonator (courtesy of coherent radiation)...

In order to achieve a reasonable signal strength from the nonlinear response of approximately one atomic monolayer at an interface, a laser source with high peak power is generally required. Conuuon sources include Q-switched ( 10 ns pulsewidth) and mode-locked ( 100 ps) Nd YAG lasers, and mode-locked ( 10 fs-1 ps) Ti sapphire lasers. Broadly tunable sources have traditionally been based on dye lasers. More recently, optical parametric oscillator/amplifier (OPO/OPA) systems are coming into widespread use for tunable sources of both visible and infrared radiation. 

In bofh CW and pulsed lasers fhe dye solution musf be kepf moving to prevenf overheating and decomposition. In a pulsed laser fhe dye is continuously flowed fhrough fhe confaining cell. Alternatively, magnetic stirring may be adequate for low repetition rates and relatively low power. In a CW laser fhe dye solution is usually in fhe form of a jef flowing rapidly across fhe laser cavify. 

Because of the tunabiUty, dye lasers have been widely used in both chemical and biological appHcations. The wavelength of the dye laser can be tuned to the resonant wavelength of an atomic or molecular system and can be used to study molecular stmcture as well as the kinetics of a chemical reaction. If tunabiHty is not required, a dye laser is not the preferred instmment, however, because a dye laser requires pumping with another laser and a loss of overall system efficiency results. 

Laser removal of tattoos and of colored birthmarks has been widely studied. A high power pulsed laser at a wavelength absorbed by the pigment is used to vaporize the pigment and to bleach the colored area. Ruby, Nd YAG, and dye lasers are favored for this purpose. 

Selenium and selenium compounds are also used in electroless nickel-plating baths, delayed-action blasting caps, lithium batteries, xeroradiography, cyanine- and noncyanine-type dyes, thin-film field effect transistors (FET), thin-film lasers, and fire-resistant functional fluids in aeronautics (see... 

The crystalline mineral silicates have been well characterized and their diversity of stmcture thoroughly presented (2). The stmctures of siHcate glasses and solutions can be investigated through potentiometric and dye adsorption studies, chemical derivatization and gas chromatography, and laser Raman, infrared (ftir), and Si Fourier transform nuclear magnetic resonance ( Si ft-nmr) spectroscopy. References 3—6 contain reviews of the general chemical and physical properties of siHcate materials. 

Frequency-Modulation Spectroscopy. Frequency-modulation spectroscopy (tins) is a high sensitivity null-background infrared technique for measuring absorbances down to 10 with fast acquisition speeds. Fms involves frequency-modulating a laser source at COq to produce a carrier frequency having sidebands at cJq where is an integral multiple of the modulation frequency. Dye lasers and many other single-line sources can... 

A new cyanide dye for derivatizing thiols has been reported (65). This thiol label can be used with a visible diode laser and provide a detection limit of 8 X 10 M of the tested thiol. A highly sensitive laser-induced fluorescence detector for analysis of biogenic amines has been developed that employs a He—Cd laser (66). The amines are derivatized by naphthalenedicarboxaldehyde in the presence of cyanide ion to produce a cyanobenz[ isoindole which absorbs radiation at the output of He—Cd laser (441.6 nm). Optimization of the detection system yielded a detection limit of 2 x 10 M. 

Organic colors caused by this mechanism are present in most biological colorations and in the triumphs of the dye industry (see Azinedyes Azo dyes Eluorescent whitening agents Cyanine dyes Dye carriers Dyes and dye intert diates Dyes, anthraquinone Dyes, application and evaluation Dyes, natural Dyes, reactive Polymethine dyes Stilbene dyes and Xanthenedyes). Both fluorescence and phosphorescence occur widely and many organic compounds are used in tunable dye lasers such as thodamine B [81-88-9], which operates from 580 to 655 nm. 

The cyanine class of dyes is also useful in biological, medical, laser, and electro-optic appHcations. Dyes marketed as Povan [3546-41-6] (5) and Dithiazanine [7187-55-5] (6) are useful anthelmintics, and Indocyanine Green [3599-32-4] (7) is an infrared-absorbing tracer for blood-dilution medical diagnoses. "Stains-AU." is a weU-studied biological stain (8) and Merocyanine 540 s photochemotherapeutic activity is known in some detail (9). Many commercially available red and infrared laser dyes are cyanines (10). 

Data on duorescence, phosphorescence, excited-state lifetimes, transient absorption spectra, and dye lasers are tabulated in Ref. 16. The main nonduorescent process in cyanine dyes is the radiationless deactivation Sj — Sg. Maximum singlet-triplet interconversion ( 52 ) methanol for carbocyanines is about 3% (maxLgrp > 0.03), and the sum [Lpj + st] I than 0.10. 

After condensation, the clusters are transported by the He-flow through a nozzle and a differential pumping stage into a high vacuum chamber. For ionization of the clusters, we used excimer and dye laser pulses at various wavelengths. The ions were then mass analyzed by a time-of-flight mass spectrometer, having... 

Lick Observatory. The success of the LLNL/AVLIS demonstration led to the deployment of a pulsed dye laser / AO system on the Lick Observatory 3-m telescope (Friedman et al., 1995). LGS system (Fig. 14). The dye cells are pumped by 4 70 W, frequency-doubled, flashlamp-pumped, solid-state Nd YAG lasers. Each laser dissipates 8 kW, which is removed by watercooling. The YAG lasers, oscillator, dye pumps and control system are located in a room in the Observatory basement to isolate heat production and vibrations from the telescope. A grazing incidence dye master oscillator (DMO) provides a single frequency 589.2 nm pulse, 100-150 ns in length at an 11 kHz repetition rate. The pulse width is a compromise between the requirements for Na excitation and the need for efficient conversion in the dye, for which shorter pulses are optimum. The laser utilizes a custom designed laser dye, R-2 perchlorate, that lasts for 1-2 years of use before replacement is required. 

Figure 6. Tempcraiure dependence of the fluorescence lifetime of BMPC in 1 1 ethanol-mcihanol. Measurements were carried out at the LENS laboratory of Florence by a picosecond apparatus using as an excitation source (at 380 nm) a dye laser pumped by a frequency-doubled cw Nd-YAG laser and recording the fluorescence time jirofiles by a streak camera. Since the overall insuumental response time was 75-80 ps, decays with t>200 ps, observed at T<130 K, were analyzed without deconvolution. At 177, 178 and 193 K, the lifetimes were roughly estimated as i=(FWHM -77 ), where FWHM was the width at half maximum of the decay. Because of the rather high sample absorbances (An,x=2), self absorption may have reduced the lifetimes to some extent.

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