Organic semiconductor distributed feedback (DFB) laser as excitation source in Raman spectroscopy

As an application of organic semiconductor distributed feedback (DFB) lasers we demonstrate their use as excitation sources in Raman spectroscopy. We employed an efficient small molecule blend, a high quality resonator and a novel encapsulation method resulting in an improved laser output power, a reduced laser line width and an enhanced power stability. Based on theses advances, Raman spectroscopy on selected substances was enabled. Raman spectra of sulfur and cadmium sulfide are presented and compared with the ones excited by a helium-neon laser. We also fabricated a spectrally tunable organic semiconductor DFB laser to optimize the Raman signals for a given optical filter configuration. ©2013 Optical Society of America OCIS codes: (140.3490) Lasers, distributed-feedback; (140.7300) Visible lasers; (180.5655) Raman microscopy; (300.6450) Spectroscopy, Raman. References and links 1. C. V. Raman and K. S. 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Introduction
Since the first reports in 1928 [1,2], Raman spectroscopy has played an increasingly important role in chemometrics [3,4], material science [5][6][7][8], life science [9][10][11][12], quality control in industrial processes [13,14] and recently in nanostructure characterization [15][16][17]. The advent of laser as an intense and monochromatic source of excitation light improved the Raman signals dramatically [18][19][20]. Enabled by the broad spectral gain and the efficient energy conversion in active materials, organic semiconductor lasers are particularly useful for spectroscopic applications. They can cover the whole visible spectrum with only few active materials [21,22]. Relying on distributed feedback (DFB) resonators, it allows the simple fabrication of laser devices with narrow bandwidth and low laser threshold, both being favorable for Raman spectroscopy. Recently, organic semiconductor DFB lasers (DFB-OSL) have been applied for high resolution absorption and transmission spectroscopy [23][24][25]. As a promising candidate of excitation source for Raman spectroscopy, however, the DFB-OSL has to be improved through advanced fabrication techniques. In this contribution, we report Raman spectroscopy of sulfur (S 8 ) and cadmium sulfide (CdS) excited by DFB-OSLs. We show that these lasers can replace a bulky helium-neon (He-Ne) gas laser in Raman spectroscopy setup. The influence of the laser exciting power and laser spectral width was investigated.

Device design and characterization setup
There are some basic requirements on laser sources which must be met for Raman spectroscopy. These are adequate laser power, high wavelength stability, high spatial mode quality and moderately narrow line width [19]. To meet these criteria, we prepared a DFB-OSL with a high slope efficiency, a low divergence emission and a long-term stability. The organic semiconductor tris(8-hydroxyquinoline) aluminum (Alq 3 ) doped with the laser dye 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) forms a very efficient and stable Foerster energy transfer system [26] and was chosen as the active gain material for our laser devices. To match the Alq 3 :DCM red emitter, we applied a glass grating (Visolas GmbH) with a one dimensional corrugation period of 400 nm as the second order distributed feedback resonator for a bottom emitting device. Compared to polymer substrates, the glass grating substrates have fewer fabrication defects and may avoid possible chemical reactions. The laser devices built on the glass gratings delivered higher slope efficiency, lower degradation rate and an improved beam profile. After the cleaning procedure, Alq 3 and 2.3% by weight of DCM were thermally co-evaporated in a high vacuum chamber with a thickness of 210 nm onto the glass grating substrate. An encapsulation was achieved by bonding a glass lid through ultraviolet curable optical adhesive (Norland, NOA68) [27,28]. Figure 1(a) shows the encapsulated DFB-OSL when pumped on an area of 2 × 10 −3 cm 2 by a diode pumped passively Q-switched solid state laser (Crylas, FTSS355-Q2) with a wavelength of 355 nm. For a convenient coupling of the laser beam into the confocal microscope for Raman application, we utilized the bottom emission from device. The laser line width was investigated through measuring the full width at half maximum (FWHM) of the laser spectra at different pump pulse energies by a spectrometer (Acton, SpectraPro 300i, grating 1800 g/mm) connected to an intensified CCD-camera (Princeton Research, PiMax 512) at an optical resolution of 0.025 nm. As shown in Fig. 1(b), the FWHM of the laser spectra with the maximum at 629.3 nm grows from 0.11 nm to 0.26 nm, when the UV pump enhances from 60 nJ pulse −1 to 1.29 µJ pulse −1 . This behavior is attributed to higher lateral modes which are excited for higher pump pulse levels [29]. The DFB-OSL exhibits a slope efficiency of 7.6% and a laser threshold of 28 nJ pulse −1 at an pump area of 540 µm × 425 µm as shown in Fig.  1

(c).
For Raman scattering measurements, the UV pump pulse energy was adjusted during the operation through a variable neutral density filter and controlled by a pulse energy meter (Coherent, LabMax-TOP). The maximal pump pulse energy could reach 3.2 µJ pulse −1 . Under this pumping condition, our encapsulated organic semiconductor DFB laser achieved a maximal output pulse energy of 160 nJ pulse −1 , which corresponds to an averaged laser power of 780 µW at a pump repetition rate of 10 kHz. The FWHM of the laser spectrum was measured as 0.50 nm. The Raman spectroscopy setup is shown in Fig. 2(a). It consists of a UV pump laser, the DFB-OSL, an inverted confocal microscope (Nikon, Eclipse TE2000-U) and a detection system. We inserted a clean-up bandpass filter (Semrock, F94-633L) with a bandwidth of 2.4 nm at the entrance of the confocal microscope to suppress ambient light and photoluminescence of the organic gain material. Making use of a 45°-tilted dichroic beam splitter (Semrock, F78-630), the organic laser emission is guided to excite the Raman scattering of the sample on a horizontal translation stage. Compared to more conventional laser, e.g., a He-Ne laser, the laser radiation of the DFB-OSL is emitted perpendicular to the substrate as a slightly divergent fan parallel to the orientation of the grating grooves as shown in Fig. 1(a). Applying a collimation lens and a 40 × objective with a numerical aperture of 0.6, the emitted laser stripe could be further focused to a length of less than 100 µm on the sample as shown in the inset of Fig. 2(a). The laser power on the sample was 350 µW. A longpass filter (Semrock, F76-633) was employed in the setup to further suppress the laser reflection and the Rayleigh scattering of the analytes. The collected Raman signals were analyzed and recorded by a spectrograph (Horiba, Triax 320, grating 300 g/mm) connected to a liquid-nitrogen-cooled CCD-Camera (Horiba, Symphony LN 2 detector). This combination of the detection system gave a spectral resolution of 0.26 nm, corresponding to a Raman shift resolution of 9.2 cm −1 for our excitation wavelength at approximately 630 nm. At a temperature as low as 140 K, the dark counts of the CCD-camera were efficiently suppressed. The life time of the DFB-OSL is an issue in general and especially for Raman measurements requiring long integration times. We thus investigated the laser degradation characteristic through monitoring the excitation power while maintaining the pump pulse energy constant. As shown in Fig. 2(b), the laser emission from one pumped spot of the organic semiconductor laser decays only 10% within 30 min at a pump pulse energy of 3.2 µJ pulse −1 and a high repetition rate of 10 kHz. This stability of one single pumped spot is sufficient to meet the demands of Raman spectroscopy.

Raman experiments and discussion
For the Raman experiments we applied sulfur as analyte. Sulfur (S 8 ) is a strong Raman scatterer with dominant bands at ~219 Δcm −1 and ~473 Δcm −1 [30,31]. Currently, the Raman spectroscopy for sulfur and sulfocompounds attracts a lot of interest for in situ detection of marine organisms and minerals influenced by sulfidic environments [32][33][34]. Using an integration time of 50 s for spectra acquisition, we acquired Raman spectra of the sulfur sample (Sigma-Aldrich, >99.5%) excited by the DFB-OSL with an emission peaked at 629.3 nm. The Raman peaks at 219 Δcm −1 and 473 Δcm −1 can be easily identified from the recorded Raman spectra. With increased excitation power, the Raman signal intensity grew as well and was linearly proportional to the laser power as shown in Fig. 3(a). Above 120 µW excitation power the weak Raman band at 438 Δcm −1 can also be identified. Compared to the sulfur Raman spectrum excited by the He-Ne laser (Coherent, Model 200 single frequency stabilized HeNe laser) with a random polarization, we find a good correspondence in the Raman peaks at 219 Δcm −1 , 246 Δcm −1 , 438 Δcm −1 and 472 Δcm −1 . The polarization of the excitation source has no influence on the Raman lines of sulfur (S 8 ) due to the symmetry of the orthorhombic crystal [31]. As shown in Fig. 3(b), the width of the Raman lines is comparable with those excited by the He-Ne laser. The missing peaks at 154 Δcm −1 and 86 Δcm −1 in the excited Raman spectrum are explained by the filter function of the beam splitter and the longpass filter. The lower emission wavelength of the organic laser results in a small overlap of the attenuation range with Raman shifts smaller than 167 Δcm −1 as shown in Fig. 2(a). To realize wavelength tunability and to better match our optical filters we fabricated a continuously tunable DFB-OSL via evaporating a wedge-shaped active material layer onto the glass grating substrate. The fabrication details can be found in the work of S. Klinkhammer et al. [35]. Due to the correlation between the active layer thickness and the DFB-OSL emission wavelength, a specific organic laser emission wavelength can be obtained by pumping at a spatially defined position on the wedged layer. Figure 4(a) shows a photograph of the laser sample built on a glass grating substrate with a corrugation period of 400 nm after coating with a wedge-shaped film of Alq 3 :DCM. It reveals a spatially rendering color which evolves from wedge-shaped thin films interference effects. The evaporated film thickness changes linearly from 190 nm to 380 nm as measured on a reference sample. We scanned the laser sample with a spatial resolution of 250 µm and a spectrum was taken at each point. The laser emissions vary from 617 nm to 653 nm continuously, as shown in Fig.  4(b). Using this approach we could tune our laser exactly to match the He-Ne laser wavelength and to optimally use the existing optical filters. For an integration time of 50 s the Raman peaks at 153 Δcm −1 and 81 Δcm −1 turned up and the Raman intensities are comparable with the ones excited by the He-Ne laser, as shown in Fig. 4(c). Only slight differences can be observed regarding the width of the Raman lines. Since the pump area on the organic laser sample (540 µm × 425 µm) for the tunable laser covered a slightly wedge-shaped organic layer, the laser line width grows to 0.62 nm for an UV excitation pulse energy of 3.2 µJ pulse −1 . In turn, this leads to slightly broader lines than in case of excitation with the He-Ne laser.
We have also employed the DFB-OSLs to investigate the Raman spectra of other analytes. Figure 5 shows the Raman lines of cadmium sulfide (Crystal GmbH) excited by the nonwedged DFB-OSL with a laser emission at 629.3 nm. We can identify the two prominent Raman peaks at 307 Δcm −1 and 600 Δcm −1 , which originate from the CdS longitudinal optical (LO) phonon mode and the second longitudinal optical (2LO) phonon mode, respectively [36,37]. In addition we could observe the other multiphonon scattering modes at 216 Δcm −1 , 249 Δcm −1 , 327 Δcm −1 and 346 Δcm −1 [38]. The Raman spectrum is comparable with the one excited by the He-Ne laser and corresponds well with the Raman database of the unoriented bulk wurtzite-type crystals of CdS [39].

Conclusion
In conclusion, we have successfully applied organic semiconductor DFB lasers as excitation sources in Raman spectroscopy. Compared to the previous fabricated organic laser devices, our new lasers have been improved in terms of laser output efficiency, laser beam divergence and laser life time. Efficient laser excitation power up to 350 µW and a long-term stability were observed. The excited Raman spectra of sulfur and cadmium sulfide are demonstrated as examples. Our work paves the way to apply organic semiconductor DFB lasers in Raman spectroscopy. We foresee the advantages of organic lasers in resonance Raman spectroscopy and surface enhanced Raman spectroscopy due to their high tunability in visible spectrum range. Furthermore, the integration of organic lasers in lab-on-a-chip devices could enable the miniaturized Raman detection schemes for biomedical analysis.