Department of Chemistry Physical Chemistry Göteborg University

&RQVWUXFWLRQ RI D SXOVHG G\H ODVHU

3OHDVH REVHUYH WKDW WKH VDIHW\ SUHFDXWLRQV RQ SDJH  PXVW EH IROORZHG RWKHUZLVH WKHUH LV D ULVN RI H\H GDPDJH

Renée Andersson -97, Leif Holmlid -99

1 ,QWURGXFWLRQ

The active medium in a dye laser is an organic dye dissolved in mostly an organic solvent such as methanol, ethanol and dimethylsulfoxid and others. A characteristic property of a dye is the ability of absorbing some wavelengths in the visible region. Also typical for a dye is a high absorbing power characterised by the oscillator strength close to 1 and the high flourescence quntum yield.

Large polyatomic organic molecules, such as dye molecules, dissolved in a solution exhibit broad, structureless absorption and emission spectra due to the large number of vibronic transitions present underneath the Franck-Condon envelope. In this laboration we will use a very common dye, Rhodamine 6G. The absorption and emission spectra of the dye are shown in fig. 1.

This dye absorbs strongly in the blue-green region, emitting red-shifted light with a maximum at 570 nm. The important radiative transitions appear between two singlet electronic levels, S0 and S1.

Fig. 1 The absorption and emission spectra of Rhodamine 6G

2 0HFKDQLVP

The dye laser is a four level laser where the vibrational and electronic levels form the system, fig. 2. Absorption causes transitions from the lower vibrational state of the S0 state to the upper vibrational states of S1 (1). The populations in the initially excited states are relaxed rapidly through collisions to the lower vibrational states of the S1 state where a population inversion is built up and from which stimulated emission occurs to the upper vibrational states of S0 (2). The vibrationally excited dye molecule is then rapidly relaxed by collisions to the vibrational ground state of S0.

E

1 (3)

(1) (2) 1

(4) 0

)LJ(QHUJ\OHYHOVFKHPHIRUDG\HPROHFXOHVKRZLQJSURFHVVHVLPSRUWDQWLQODVHUDFWLRQ

The resulting laser gain profile is a slightly contracted version of the emission band. This means that, by using suitable wavelength-selecting elements inside the laser cavity a continuously tuneable output of light can be obtained.

There are a few processes that are competing with the stimulated emission causing a less efficient laser action.

First, the population in the lower states of S1 can by intersystem crossing be transferred to the metastable triplet state T1 (3) lying just below the S1 state in energy. T1 can be depopulated by non-radiative processes (4), i.e. a radiative transition between the T1 and S0 states is spin-forbidden and appears as phosphorescence relaxing on a longer time scale. The use of short pump pulses prevents this by letting the laser action take place before any substantial part of the population is transferred to the triplet state. It is also possible to quench the triplet state by introducing oxygen into the system.

Second, the non-radiative processes heat the dye and high lying vibrational states in the S0 state are populated which lowers the laser efficiency. Also, when the dye is heated the refractive index changes. This is prevented by circulating the dye outside the cuvette.

3 There is a wide range of laser dyes that have been developed, covering the whole of the visible and parts of the UV and near IR regions. Commercial dye lasers are available using a further wavelength-selecting elements in the laser cavity.

([SHULPHQWDO

0DWHULDOIRUFRQVWUXFWLQJDG\HODVHU

• Nd:YAG laser operating at 532 nm • 90° prism • cylindrical lens • cuvette with circulating dye (Rhodamine 6G) • grating • output coupler

3HUIRUPDQFH Try to design the dye laser with the available elements. Define your laser cavity with a grating, an output coupler and with the active medium in between. A tip is to focus the pump beam as a rod on the cuvette at 90° relative to the dye laser cavity direction. When you believe your set-up is going to work, ask the assistant for approval EHIRUHstarting the Nd:YAG laser.

Start the Nd:YAG laser according to the instructions of the assistant. The frequency doubled light with a wavelength at 532 nm is used in this laboration. Align the elements in order to get a laser beam out of your dye laser.

When the dye laser is working, measure the wavelength with the spectrometer: • Put the optical fibre close to the dye laser beam: be careful, the intensity coming into the spectrometer may be too high!! • Try to tune the dye laser wavelength by turning the grating. • Check if there is any change in colour of the laser and measure the wavelength.

3UHFDXWLRQV

• Observe that the Nd:YAG laser beam gives a pulsed very energetic light. • Always wear laser protecting glasses, however GR QRWplace your eye close to the laser beam. • Be careful where the beam hits and observe that reflections can be dangerous. Therefore, take off your watches and rings!

4 5HSRUW

• Give a brief summary of the principles of the dye laser. • Describe the set-up of your dye laser in words and picture. • Give the tuning region of the dye laser. • Suggest one way of narrowing the band width of the dye laser.

______

$FRPSOHWHG\HODVHU

The laboratory supervisor will give you a short demonstration of a commercial dye laser with the following data:

5 6