Excitation sources

Three different laser systems were used for the majority of the lasing studies, a nanosecond optical parametric oscillator, a femtosecond optical parametric amplifier system and a microchip laser.

3.5.1.1.Optical parametric oscillator

For most of the experiments described in this thesis an optical parametric oscillator (OPO) was used as a widely-tunable pump source. An OPO contains a non-linear crystal that, when illuminated with laser light of a specific energy (specific wavelength), produces a pair of photons with lower energies (longer wavelengths) than the original pump photon. The sum of the energies of the two photons (called signal and idler photons) must be equal to the energy of the initial photon in order for the energy to be preserved in the system. This can be written as

i s

p

v

v

v

=

+

where νp is the frequency of the pump photon and νs, νi are the frequencies of

the signal and idler photons respectively.

In addition, momentum must also be preserved for the photons participating in this interaction, as described in the following equation

0

=

−

−

=

∆k

k

k

k

where the corresponding wavevectors for the pump, signal and idler photons must result in a zero overall momentum difference. In order for that to happen within a non-linear crystal, a technique called phase matching is used, which utilises the different refractive index of the crystal material at different electric field polarisations to allow for a pair of signal and idler photons to be produced from a given pump photon. This pair of photons can be tuned across a wide spectrum by changing the phase-matching condition, always keeping in mind the energy conservation principle.

The OPO used in these experiments (Panther OPO, from Photonic Solutions) is pumped using the 3rd harmonic of an Nd:YAG laser (355 nm), which has an

energy of 100 mJ per pulse. The OPO produces a wide range of output wavelengths from 410 nm up to 2500 nm (expandable into the UV using double crystals that allow the extraction of wavelengths as low as 220 nm) at energies of up to 14 mJ per pulse, with a pulse duration of 4 ns at a repetition rate of 20 Hz. This OPO uses a BBO nonlinear crystal that is angle-tuned, meaning that the rotation of the crystal in relation to the path of the pump beam running through it changes the phase-matching condition to produce a different pair of signal and idler wavelengths. The BBO crystal is placed within a cavity that double passes the idler wavelengths (infra-red reflecting mirrors) to achieve self-seeding of the parametric process and increased conversion efficiency. A compensator crystal at the output of the cavity ensures that the wavelengths produced by angle-tuning of the nonlinear crystal exit the cavity in the same direction.

The OPO was incorporated in the setup seen in Figure 3.6, which allows for the energy of the OPO beam to be varied using a set of two cube polarisers (to maintain a constant polarisation of the beam hitting the sample) mounted on rotating base plates, and also provides enough space on the optical table to insert various elements such as focusing optics, vacuum chambers, neutral density filters and an observation screen to observe the emission patterns from organic lasers.

Figure 3.6 Photophysical measurements setup that uses the optical parametric oscillator for optical excitation of organic semiconductor samples.

The photoluminescence from the sample was recorded using a Jobin-Yvon Triax 180 spectrograph connected to a cooled CCD detector array that collected sample fluorescence through an optical fibre.

3.5.1.2.Optical parametric amplifier system

The pump light for the femtosecond experiments comes from a diode-pumped mode-locked Ti:Sapphire femtosecond laser emitting at 800 nm. This light is then passed through a Hurricane regenerative amplifier to increase the peak power of the beam. The amplified beam is then used as the pump source for an optical parametric amplifier (OPA) by Spectra Physics. Upon entering the OPA the beam is split into two parts, with the weakest part is tightly focused on a sapphire plate to generate a white-light continuum extending from 450 to 750 nm. The main part of the 800 nm beam is used to optically pump a Beta Barium Borate (BBO) nonlinear crystal that can be rotated to satisfy the phase-matching equation for a different pair of signal and idler photons, thus selecting what wavelength from the white-light continuum spectrum that is also incident on the BBO crystal will be amplified. Further nonlinear processes such as sum and difference frequency mixing are possible that, along with second and third- harmonic generation crystal can extend the tuning range of the system. The pump beam resulting from the above amplification stages has a pulse duration of 100 fs at a repetition rate of 5 kHz and can be tuned from 200 nm to 10 µm with energies in the range of tens of microjoules.

3.5.1.3.Microchip laser

At the beginning of this PhD, the microchip laser represents the most compact pump source for organic semiconductor lasers.[6, 7] Monolithic microchip lasers combine compact dimensions with high output efficiency, moderate output power and have been studied extensively, making them very robust and reliable.[8]

A typical microchip laser is based around a Neodymium-doped optical material, usually a crystal such as Yttrium Aluminium Garnet (Nd:YAG) or Yttrium Orthovanadate (Nd:YVO4) that is optically excited by an infrared laser diode

emitting at 808 nm. The gain medium is Neodymium and the host material acts as a spacer for the molecules, allowing for population inversion and lasing that occurs at 1064 nm. These lasers can be Q-switched to achieve nanosecond pulses at kHz repetition rates that lead to higher optical powers. They can also include frequency doubling and tripling nonlinear crystals to produce 532 nm and 355 nm respectively, allowing microchip lasers to be used as optical pumping sources for a wide range of organic semiconductors. Two such microchip lasers were used in this work to pump organic distributed feedback lasers with nanosecond pulses.