8.2 Emission dynamics in Alq 3 :DCM
8.2.2 DFB-lasers
The time-integrated operation of Alq3:DCM lasers was characterized in detail in section 5.2.
Fig. 8.7 shows the emission transient of a 2nd order DFB laser with a film thickness of 180 nm27. Similar to the case of ASE on a planar substrate the laser output shows a classic gain-switched characteristic which consists of a single optical pulse at low excitation fluence and multiple relaxation oscillation peaks at high excitation density 264. Due to the slightly larger excitation spot diameter (~ 200 µm) the threshold fluence is lower than in the experiments above (EP≈ 15 µJ/cm2), corresponding to a threshold gain of merely gth≈ 22 cm-
1 (N
exc≈ 1.9⋅1018 cm-3). Together with the time delay owing to the excitation transfer from
Alq3 to DCM this results in a comparatively long pulse peak delay up to 160 ps. The
combination of the slow rise time and a cavity decay time of τcav ≈ 7.5 ps yields rather long
pulses up to ∆t≈ 30 ps (FWHM).
As the excitation density is increased, the delay time falls below 25 ps and the pulse duration decreases to ∆t≈ 5 ps. Therefore the first emission burst terminates before the excitation transfer is entirely completed and laser oscillation evolves one more time. For increasing excitation fluence up to four peaks are observed. Simultaneously the oscillation frequency, determined by the inverse of the separation between the first two oscillation peaks, increases from 8 GHz up to 45 GHz as shown in the inset of Fig. 8.7.
Large and by, the dynamic evolution of the laser emission is an analogue of the ASE in a planar waveguide. The most important difference between the two is the slower timescale of the laser dynamics. The reasons for the slower dynamics of the laser are similar to the case of MeLPPP. Owing to the feedback the threshold material gain is considerably lower in a DFB laser. Therefore the photons have to travel a longer optical path before significant amplification occurs; the laser needs a comparatively long time to switch on. Secondly, the cavity lifetime is increased, slowing down the decay of the photon density. As long as the photon density is large the repeated build-up of an excess population is prevented. The cavity lifetime therefore sets an upper limit to the relaxation oscillation frequency 265. Last but not
least, the excitation transfer is prolonged. In the transient at 22 µJ/cm2, for example, the first emission burst is not terminated earlier than 90 ps after the pulsed excitation. The evolution of a second emission burst after 200 ps clearly indicates that a considerable energy transfer occurs later than 90 after the excitation pulse. With an exponential energy transfer, having a time constant of 9.5 ps, such a behavior cannot be explained.
In summary, the kinetics of laser emission in composite materials is dominated by the energy transfer process. Depending on the pump level single or multiple-pulse emission can be achieved. 0 50 100 150 200 250 300 10-1 100 101 102 103 17 µJ/cm2 22 µJ/cm2 50 µJ/cm2 79 µJ/cm2 Int e ns ity (ar b . uni ts ) Time delay (ps) 0 50 100 150 0 25 50 O sci llat io n f requ enc y (G H z) Excitation Fluence (µJ/cm2)
Fig. 8.7: Transient of the surface emission from a 2nd order 1D-DFB laser for various excitation
densities. The sample is excited with a 150 fs long pulse at the time 0 ps. The transients are smoothed with a low pass Fourier filter, the experimental noise level is at an intensity of 10-1.
98
9 Devices and applications of organic solid-state lasers
Two alternative concepts for the realization of electrically operated organic solid- state lasers are investigated. Firstly, the tunability of the organic solid-state material is exploited in a hybrid design using a compact external pump source. Secondly, the advances towards direct electrical pumping of organic semiconductor diode lasers are described. For the first time optically pumped lasing is demonstrated in a diode laser structure. Future challenges are discussed.
Owing to their low laser threshold and the large spectral tuning range organic semiconductor lasers have great potential for many applications, e.g. for laser-based analytical techniques. Furthermore the inherent mechanical flexibility together with the availability of large-area fabrication techniques might eliminate the need for mechanic scanning procedures in laser survey, data storage or sensor applications. Anyway, electrically driven laser operation in combination with a compact design will be required for any of these applications. In principle, two alternative concepts exist which take advantage of the exceptionally high gain provided by organic semiconductors. In the first concept the organic DFB laser is merely used as a tunable solid-state laser, which is optically pumped by a compact primary laser. Secondly, the rapid development of organic light-emitting diodes and their high efficiency motivate work towards direct electrical operation of organic injection lasers. Their promises, challenges and recent advances towards their realization are discussed.
9.1 Very compact tunable organic solid-state laser
In chapter 5.4 it has been demonstrated that spectrally narrow surface emission can be obtained from second order 1D-DFB lasers using flexible, nanopatterned substrates covered with an organic semiconductor. By variation of the thickness of an Alq3:DCM layer tuning of
the emission wavelength over 44 nm can be achieved. In these experiments –and also in the experiments reported by other groups- the organic film is pumped by a rather bulky and expensive laser system28. Such lasers are well suited for basic studies of the laser properties, however, they do not take advantage of the low laser threshold in organic semiconductor lasers. For most applications much more compact and inexpensive pump-lasers are desirable. The low laser threshold resulting from the distributed feedback concept and the high gain allow the realization of a very compact and, in principle, widely tunable all solid state laser. For this purpose a diode pumped passively Q-switched Nd3+ :YAG laser with an integrated frequency tripler (Uniphase NanoUV-355) is used as pumping source. The inset of Fig. 9.1
28 Typical laser systems include frequency-doubled or –tripled Nd3+:YAG lasers, Nitrogen lasers and the various
shows a scheme of the laser configuration. The pump laser produces sub-nanosecond pulses with a pulse energy of ~100 nJ at a wavelength of 355 nm and a 10.8 kHz repetition rate. Due to its laser wavelength and its short pulse duration this laser is optimally suited to pump Alq3
and accumulate a very large population in the DCM dopant molecules. Since the microchip laser furthermore produces a high-quality beam, exhibiting narrow beam diameter and divergence, the organic laser can be pumped without need of intermediate focussing optics. Accordingly the whole optical arrangement has a length of only ~10 cm and a volume of ~70 cm3.
0
1
2
3
In
te
nsi
ty (a
rb
. un
its)
Pump Energy (nJ)
Diode Laser Nd:YAG Microlaser Frequency Tripler Organic Laser ~ 10 cmFig. 9.1: The inset shows the scheme of the very compact all solid state laser using the organic semiconductor film as active medium. The pump source is a commercially available diode- pumped, self-pulsating, frequency tripled Nd:YAG-microchip laser (Uniphase NanoUV-355). The organic semiconductor laser can be placed directly in front of the pump laser. Thus the total length of this very compact solid state "dye" laser is less than 10 cm. The main part of the figure shows the input-output characteristics at the laser wavelength when the beam is focused to a 125 µm diameter spot.
Fig. 9.1 shows the input-output characteristics of a laser utilizing a 300 nm thick Alq3:DCM
film. The observed laser threshold amounts to EP= 2 nJ when the pump laser is focussed to an
excitation spot with a diameter of 125 µm. The threshold pump energy is by a factor of 50 lower than the maximum pulse energy provided by the compact pump laser. In ambient atmosphere the durability of the organic laser is limited to some 105 pulses. However, the lifetime is increased by several orders of magnitude by placing the organic sample in vacuum. Similar results could be achieved by a proper sealing of the active medium. It was mentioned above that the incorporation of the dye into a solid matrix reduces the fluorescence yield as excited states are quenched. Here, the very same process has a positive effect. The repetition rate of conventional dye lasers working with a liquid dye solution is limited by the build-up of a long-lived triplet state population and its parasitic absorption. In the solid state triplets are
100 9 Devices and applications of organic solid-state lasers
rapidly quenched so that a repetition rate 10.8 kHz is easily achieved. Compared to commercial dye lasers without circulating dye solution this is an increase by a factor of 1000. So far, different laser wavelengths were reached by a variation of the film thickness in different samples. The maximum tuning range is limited by the range of film thicknesses giving rise to monomode laser operation. The use of a tapered grating with spatially varying periodicity Λ and/or an evaporated film with a thickness gradient can provide a possibility for continuous tuning of the laser wavelength over the entire gain spectrum on one and the same substrate.