Photon Physics ‘08/’09 Thijs Besseling Dye Lasers Photon Physics ‘08/’09 Thijs Besseling

Dye Laser Introduction Organic dye dissolved in liquid as gain medium High density gain medium Large quantum yield Broad emission and gain spectrum: tunable laser output short pulse output - Solid and vapor dye lasers also exist Strong absorption in ultraviolet and visible spectrum Molecule structure: shielded form surrounding solvent: decay primarily radiative (non collisional) so very high quantum yield: ratio of radiating excited molecules to total numer of excited molecule

Energy levels Singlet and triplet state, rotational & vibrational levels Shifted excited state S1 Similar to four level laser Intersystem crossing Energy states comparable to atom electronic configuration Because of rotations and vibrations: different sub levels: broadening of the spectrum Shift in the excited state: emission has lower energy thus higher larger wave lengths

Absorption and emission Ground state absorption: reduced gain at shorter wavelengths Triplet absorption: “Flowing the dye” Although the emission and absorption spectra are shited, because they are so broad, there is overlap: Ground state absorption Spontaneous emission spectrum: fluorescence spectrum Sigma ul: Stimulated emission cross section Triplet absorption: not inherent to the system, can be minimized or eliminated

Laser pumped pulsed tunable dye laser Pumped by a Neodymium YAG laser Freqnecy tuning by use of etalon (Fabry Perrot cavity), and rotatable grating Or pumped by flaslamp: electric glow discharge lamp that produces extremely intense, incoherent, full-spectrum white light for very short durations Provides tunable ultra narrow-frequency pulsed laser output Pumped by Nd:YAG laser or by flashlamp

Continuous wave tunable dye laser Three mirror cavity with thin flowing dye region: jet stream Emission linewidth of less than 1 kHz Mode locked dye laser: pulses as short as 6 fs Dye flows in thin cuvette: jet stream to overcome triple state absorption To focus the light on this thin region, it is pumped under the brewster angle Standing wave cavity

Characteristics Laser wavelengths 320-1200 nm Gain bandwidth 25-30nm Small signal gain coefficient 500/m Single pass gain 1.5-150 Output power up to 109 W/pulse, up to 2 W (cw) Most dyes are very toxic, often carcinogenic and many are corrosive Solvent are often highly toxic and extremely flammable - Large range, large bandwidth, high gain, ‘user unfriendly’

Applications Research experiments with Important for spectroscopy Narrow-band tunable laser emission Ultra fast optical pulses Important for spectroscopy Medicine Dermatology Reduced need by development of tunable solid-state lasers

Recent experiment Micro fabricated liquid dye lasers: opto-fluidics technology Trying to achieve complete “lab-on-a-chip” systems Liquid dye laser on a micro-chip. Liquid not only as gain medium but also manipulated to tune the optical properties of the laser. The small size reduced safety risk and handling problems. Experiment: absorbance measurements -> concentration of fluids

Thank you!