Argon Ion Laser Laura Rossi Photon Physics Course 2005-2006

General Description THREE PRIMARY COMPONENTS: -Resonator Structure -Plasma Tube -Power Supply The basic ion laser consists of three primary components, a resonator structure, a plasma tube and a power supply. The resonator structure holds the two reflective mirrors in precise alignment, thus forming a resonant optical cavity. One of these mirrors is totally reflective in the wavelength of operation and is referred to as the high reflector. The other mirror, the output coupler, is partially transmissive to allow a fraction of the light energy stored in the cavity to escape as output power. The plasma tube provides optical gain within the resonator, causing it to act as an oscillator The power supply provides a DC voltage and an adjustable current across the plasma tube to sustain a controlled arc discharge through the fill-gas of the plasma tube. In the discharge, the ionized atoms of the fill gas are excited through multiple collisions with the accelerated electrons . Stimulated emission from the various excited states to the ground state of the ionized argon atom produces the required laser action. Depending on physical conditions of the discharge, a fraction of the noble gas atoms may be double ionized. Stimulated emission from these states is possible as well, giving rise to UV laser action.

General Description Continuous Wave (CW) 25 spectral lines in VIS and 10 in UV Two primary wavelengths: 488.0 nm (blue) & 515.5 nm (green) Argon gas pressure ~ 0.1 Torr High plasma temperature Significant cooling is needed Argon output power : 3-5 W

Ar+: three level system Three level scheme in which the upper of the three levels is the upper laser level Because the pumping requirements necessary to achieve sufficient gain to reach Isat are much less stringent than for solid state lasers

DOPPLER BROADENING DOMINANT Broadening Mechanism DOPPLER BROADENING DOMINANT H«D @ 488.0 nm H=4.5x108 Hz D=2.7x109 Hz H«D The doppler effect essentially spreads the emission over a much broader range of frequencies than would normally occur for homogeneous broadening, especially if that homogeneous broadening were determined by natural broadening.

Typical Argon Ion Lasers Parameters Stimulated emission cross section, ul = 2.6x10-16 m2 Inversion density, Nul = 2x1015/m3 Small-signal gain coefficient, go= 0.5/m Single-pass gain, exp(ulNulL) = 1.05-1.65

Mode Operations BROADBAND OPERATION SINGLE LINE OPERATION BROADBAND OPERATION: high reflector and output coupler are used. These mirrors have optimized reflection and transmission for a number of lines not too widely separated in wavelength (usually 70 nm). Different sets of all.lines optics are used to cover different groups of lasing lines. With a given set, the laser operates at a number of wavelengths simultaneously. SINGLE LINE OPERATION is achieved by use of an intracavity prism assembly(BREWSTER angle prism and high reflector). The dispersive effect of the prism is utilized to have only one laser line at time perfectly aligned perpendicular to the high reflector, allowing only this line to lase. Different lines may be accessed by tilting the prism assembly with respect to the longitudinal axis of the laser

SINGLE FREQUENCY OPERATION Mode Operations SINGLE FREQUENCY OPERATION

Applications MANY APPLICATIONS: -Phototherapy of eyes -Laser printers Van’t Hoff Laboratorium: Static and Dinamic Light Scattering (SLS & DLS) Fluorescence Recovery After Photobleaching (FRAP) PHOTOTHERAPY OF THE EYE involve the dissolution of small streamers of blood that develop within the eyes of people with diabetes. To remove these streamers , a physician directs an argon laser at them through the lens of the eye.The blue and green wavelengths are highly absorbed by the blood streamers and thus dissolve them, but they are not absorbed by the transparent regions of the eye through which the beam passes before reaching the streamers. The high intensity of the argon laser and the blue and green wavelegths make it suitable for use in printers. The laser can be focused to a small spot, and the high intensity enables rapid scanning rates for printing purposes.

All fluorescent dyes emit light of one wave length (e. g All fluorescent dyes emit light of one wave length (e.g. green) after they have absorbed light of another wave length (e.g. blue). However, if a very high intensity blue light is delivered to the dye, the dye will "photobleach" meaning that the high intensity light has bring them into a permanent non-fluorescent state. This phenomenon has lead to an interesting method called Fluorescence Recovery After Photobleaching (FRAP). The idea behind this method is to use FRAP to measure the ability of a colloidal particle to move around over time. To do this, a fluorophore must be covalently attached to the particles. During a typical measurement, after the short intense bleach pulse, a second (probe) leaser with lower intensity, continuously excites the remaining fluorophores. The probed intensity (1), which after the bleaching pulse drastically drops (2), is restored after a certain time(3-4), due to the Brownian motion of the particles which diffuse in and out from the bleached area. If now we repeat the experiment by using, for instance, a vertically polarized bleaching pulse, we will create an anisotropy in our sample due to the fact that the dyes with the absorption dipole moment parallel to the laser pulse will have a higher probability of adsorbing the radiation and getting bleached. We can measure this anisotropy by exiting the sample with a probe laser alternatively polarized parallel and perpendicular respect to the bleach pulse, and comparing the two signals. The orientational rearrangement due to rotational diffusion will cause the decay of the anisotropy in time. By measuring this characteristic decay time  we can calculate the diffusion coefficient D0r for our colloidal particle, which for non interacting spheres is given by:  = 1/(6D0r) On our experiment the bleach pulses (5 ns) was produced by a frequency-doubled Nd-YAG laser with wavelength λB=532 nm. The probe beam was produced by a CW Ar-ion laser (Spectra Physics 2000) with wavelength λA=514.5. Because these working wavelengths are close to the absorption maximum of RITC dye, rhodamine-labelled colloids will be used for rFRAP measurements. =1/(6D0r)

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Pumping Pathways TRANSFER FROM BELOW Nq=0qN00q q» u TRANSFER FROM BELOW: generally occurs for gas lasers in which the level q is an excited state with a long lifetime. The level q thus accumulates energy according to Nq=0qN00q and, because of its long lifwtime, serves as storage state. In the argon ion laser level q is the grownd state of Ar+. 1 q» u intermediate level q has a lifetime q that is much longer than the lifetime u of level u. ù 2 0q pumping rate q» u IONIZING TRANSITION