Q-Switch Dyes

The above discussion of the Q-switching process immediately gives some requirements for a good Q-switching dye. First, this dye must exhibit a saturable absorption at the laser wavelength. Second, the residual losses by excited state absorption should be as low as possible. Third, the photochemical stability (and, of course, also the chemical stability in the dark) should be as high as possible. Fourth, the 

There are many dyes which fulfill the above requirements, few of which have been used until now. The first dyes used with a ruby laser were phthalocyanines the free base as well as chloro-aluminum- and vanadium-phthalocyanine in 1-chloronaphthalene or nitrobenzene 22 . While these dyes show little residual absorption 14 and good photochemical stability, the solubility is relatively low and the solvents are not well suited to operation in a laser resonator. 

An essentially incomplete list of other dyes suitable for operation at the ruby laser wavelength can be found in 1 . Some merocyanine dyes were investigated as Q-switches in 26 . 

Q-switch dyes for neodymium lasers are not so numerous since there are only relatively few dyes known which exhibit absorption due to an electronic transition 

Recently, a new Q-switch for neodymium lasers has become available 29 which is extremely stable and shows energy-dependent Q-switching similar to the phthalocyanines by a rapid intersystem crossing into the triplet state 30 . This dye is a nickel-complex, bis-[p-dimethylamino]-dithiobenzylnickel, and has a strong absorption band around 1 / in several solvents. 

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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. 

PMMA finds ordn usage in several areas in ballistic or impact shields for missiles or airplanes also as windows, windshields or canopies in aircraft (Refs 7 and 22) as a Laser Q switch host using an organic Ni complex dye (Ref 22) and in proplnts as fuel (with A1 and NG as cofuels — Ref 20) and Amm perchlorate or K perchlorate as oxidizers (Refs 2, 4, 8—11, 13,14 16—20). Also see under Aeroplex Propellants in Vol 1, A108-R and under Composite Propellants in Vol 3, C464-L to C474-L Refs 1) Beil 2, [398] and (1279 1283 ... 

The 6 Nd YAG lasers pump the DM0, preamplifier and power amplifier (Fig. 19, Friedman et al., 1998). The YAG lasers are built from commercially available flashlamp/laser rod assemblies, acousto-optic Q-switches and frequency doubling crystals (LBO and KTP). Most of the mirror mounts and crystal holders are commercial. Nd YAGs are frequency doubled to 532 nm using a nonlinear crystal. The Nd YAG rod and nonlinear crystal are both in the pump laser cavity to provide efficient frequency conversion. The 532 nm light is coupled out through a dichroic and fed to multimode fibers which transport the light to the DM0 and amplifier dye cells. 

Manusciatti W, Fitzpatrick RE, Goldman MP (2000) Treatment of facial skin using combinations of CO Q-switched alexandrite, flashlamp-pumped pulsed dye, and Er YAG lasers in the same treatment session. Dermatol Surg 26 114-120 Jordan R, Cummins C, Burls A (2000) Laser resurfacing of the skin for the improvement of facial acne scarring a systematic review of the evidence. Br J Dermatol 142 413-423... 

Third-order susceptibilities of the PAV cast films were evaluated with the third-harmonic generation (THG) measurement [31,32]. The THG measurement was carried out at fundamental wavelength of 1064 nm and between 1500 nm and 2100 nm using difference-frequency generation combined with a Q-switched Nd YAG laser and a tunable dye laser. From the ratio of third-harmonic intensities I3m from the PAV films and a fused quartz plate ( 1 thick) as a standard, the value of x(3) was estimated according to the following equation derived by Kajzar et al. [33] ... 

Cyanines have been widely used as laser dyes, and as saturable absorbers in modelocked and Q-switched laser systems. 8, 50) The propensity of most cyanines to photooxidize which makes them useful in photographic film and as saturable absorbers makes them less than desirable as fluorophores in other applications. The use of... 

Light pulses with halfwidths of 10" sec have been generated mainly by Q-switched solid-state lasers but can be obtained principally with all high-gain laser transitions, as for instance CO2 lasers " ), nitrogen lasers ) (X = 3300 A), or dye lasers ). 

The second category comprises the flash photolysis experiments using the short high power light pulses from Q-switched lasers, furthermore all investigations of time-dependent behavior of excited dye molecules, which play an important role as active material in dye lasers or as saturable absorbers in passive Q-switched giant pulse lasers. 

In the years since 1964, when the first passive Q-switching with organic dyes was accomplished, the use of organic dyes in laser technology has become increasingly important. Some of the most important developments in the laser field would not have been possible without organic dyes, and one can foresee even further possibilities for the use of dyes in laser technology which could be realized in the near future. 

Applications making use of the nonlinear absorption of dyes are passive Q-switching in solid-state lasers, pulse shaping, pulse intensity measurements of high-power ultrashort pulses, optical isolation between amplifier stages of high power solid-state lasers, and pulse width measurements of ultrashort pulses by the two-photon-fluorescence (TPF) method. 

The most important application of the nonlinear absorption characteristics of dye solutions is the so-called passive Q-switching of solid-state lasers, in particular ruby lasers emitting at 694.3 nm and neodymium lasers emitting at 1.064 /tm. 

Fig. 7. Experimental arrangement of a giant-pulse laser (Q-switching by dye solution). AM, active material (e.g. ruby crystal rod), F, flashlamp, Mj, 2, resonator mirrors, DC, dye cell...

The result of a computer solution of the rate equations is shown in Fig. 8 for a typical example, in this case the cyanine dye 3,3 -diethylthiadicarbocyanine bromide (DTDC) in methanol solution as a Q-switch for a ruby laser, with To =... 

Peak laser powers of over 500 MW/cm2 are easily obtained with dye solutions as passive Q-switches. 

Until recently a general drawback of this passive Q-switching scheme was the difficulty of obtaining an exact synchronization of the giant pulse with other events in more complex experiments. This difficulty does not exist with active Q-switching in which an electro-optic device, e.g. a Kerr-cell or Pockels-cell, is used instead of a dye cell, and one is able to determine exactly the time at which... 

Fig. 8. Laser intensity S and dye population density difference between ground state and excited state, mo—mi, versus time for a typical example of a Q-switched ruby laser. (From Ref. D)...

One drawback of dye lasers as compared to solid-state lasers is the short fluorescence lifetime rp or energy storage time, which implies a quick inversion decay when pumping stops. For this reason one cannot Q-switch a dye laser. On the other hand, dye lasers can be mode-locked by saturable absorbers 52> in much the same way as solid-state lasers, and many investigations have shown that one can obtain psec pulse in this way over a wide spectral region 53,54)... 

Photochromic or phototropic dye stuffs are used as the basis of photochemical high speed memory with an erasable image. They can also be used as automatic variable density filters for example as Q-switches in high intensity lasers. For thermochromic substances colour change is observed on change of temperature (spirans, bianthrones). 

This technique is known as the passive Q-switch. The dye acts as an absorber for weak light, so that the population of excited atoms or molecules in the active material can increase until its maximum level is reached. The dye cell is in fact a high-speed shutter. 

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