单分子单晶晶体中的激光振荡

This Communication reports for the first time on the clear evidence about laser  oscillation in monolithic molecular single crystals of a thiophene/phenylene  co-oligomer (P6T). The laser oscillations are characterized by naturally formed crystal facet cavities with molecular-scale flatness. The multi-mode laser oscillation in a  Fabry-Pérot resonator is characterized by sharply resolved spectral lines with their full  width at half maximum down to only 38 pm. The laser oscillation is characterized by  the presence of a well-defined threshold.  

In this Communication, we describe how we have solved the historic open  question raised by Karl in regard to monolithic molecular crystals. We have definitively  observed laser oscillation by optically pumping a thin-plate single crystal with parallel  crystal faces on either end, which together function as a Fabry-Pérot cavity. The key  point is that we have carefully fabricated the single crystals by using a new method of  crystal growth.Most importantly, this method produces crystal  faces, and thus, Fabry-Pérot cavities, of optically high flatness. This ensures strong  self-cavity optical confinement in these crystals. We emphasize here that, unlike thin  films, monolithic single crystals are hard to equip with a resonator without hampering  its operation. Another key point lies in selecting a molecular semiconductor in which  thiophenes and phenylenes are suitably hybridized at the molecular level.Since the crystallographic structure of the single crystals that we used has been  fully determined, the relationship between that structure and the  photo-pumping geometry can be uniquely defined.

The features present in the scanning electron microscope (SEM) photograph in  Fig. 1c are fully consistent with the crystallographic data for P6T.[Hotta, 2004 #294]  Note that the pair of crystal faces paralleling the ac-plane constitutes an optical  resonator with its length defined as the separation between the two ends (measured  along the b-axis). In the case of the sample shown in Fig. 1b and designated as Sample A, for instance, the resonator length is 0.462 mm. These crystal end faces cross the other pair of crystal faces (i.e., the ab-plane) at right angles. Thus, these two pairs of  crystal faces with the molecular-scale flatness can provide strong, two-dimensional self-cavity optical confinement. From the single crystals that we grew, we carefully selected two more samples with different resonator lengths (Sample B: 0.560 mm; Sample C: 1.02 mm). The part of each crystal surrounded by the two pairs of faces was  entirely photo-pumped to detect the photoemission and record its spectrum (see Fig. 1e  for the schematic experimental setup).

Fig1c

Figure 2 shows the laser-induced emission spectrum of Sample A at the excitation  (incident) laser fluence of 1 mJ/cm² . The photoemission was highly directional and  exhibited superior output stability after thousands of shots with the excitation pulse laser.  As seen in the figure, a progression of extremely narrow emission lines clearly appears  around 689 nm. The full width at half maximum (FWHM) of the individual sharp lines  is limited to only 38 pm (as shown in the inset of Fig. 2), close to our experimental limit of 35 pm. These narrow lines arise regularly at an interval of 121 pm. A frequency  power spectrum obtained with a fast Fourier transform of the emission spectrum (Fig.  2b) indicates that a major periodic peak occurs at a frequency of 8.30 nm^-1,  corresponding to the interval 121 pm. There are no other lower frequency peaks with  intensities comparable to that peak.

Figure 3 shows photopumping intensity dependence of peak intensities of  photoluminescence (PL) spectra from the same sample. A threshold is clearly noted at  ∼750 µJ/cm² . Concomitantly, the spectral profiles dramatically change below and above the threshold (compare Figs. 2a and the inset of Fig. 3). Below the threshold emission  spectra are weak and featureless (see the inset of Fig. 3), reflecting a nature of a spontaneous emission. The strongly gain-narrowed and intensified spikes in Fig. 2a  present a striking contrast to this emission. The spectra taken under a weak excitation  imply that the center wavelength of the laser oscillation of 689 nm agrees with that of  the second vibronic peak (assigned to the 0-1 transition).