IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 10, MAY 15, 2007
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Laser Diode-Pumped Organic Semiconductor Lasers Utilizing Two-Dimensional Photonic Crystal Resonators Christian Karnutsch, Marc Stroisch, Martin Punke, Member, IEEE, Uli Lemmer, Jing Wang, and Thomas Weimann
Abstract—Two-dimensional photonic crystal lasers based on the small molecule organic semiconductor tris-(8-hydroxyquinoline) aluminum (Alq3 ) doped with 4-Dicyanomethylene-2-methyl-6-(pdimethylaminostyryl)-4H-pyran (DCM) are optically pumped with a conventional low-cost pulsed (In)GaN laser diode. We compare photonic crystal resonators providing first- and second-order distributed feedback and find threshold values of 1.9 and 3.2 kW/cm2 , respectively. Such inorganic–organic hybrid laser systems open up a way to inexpensive, tunable, and all solid-state lasers in the full visible wavelength range. Index Terms—Alq3 : DCM, organic semiconductor photonic crystal, solid-state laser, tunable laser.
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I. INTRODUCTION RGANIC semiconductors have attracted much interest as active electroluminescent materials over recent years. Their spectrally broad emission range and high efficiency has prompted intense research towards organic laser devices. Optically pumped organic semiconductor lasers have been realized based on spin-coated conjugated polymer films as well as evaporated thin films of small organic molecules [1]–[8]. These organic lasers were pumped by other gas or solid-state lasers such as frequency-doubled/tripled Nd : YAG-lasers, nitrogen lasers, and complex femtosecond laser systems, resulting in a versatile but expensive laser source. However, for many applications, e.g., for laser-based analytical techniques and sensors, much more compact and inexpensive all solid-state pump laser sources are desirable. The recent evolution of blue-violet emitting inorganic laser diodes renders these devices attractive as pump sources for conjugated polymer lasers [9], [10], [11]. In this letter, we report on laser diode-pumped photonic crystal lasers utilizing thin films of the small molecule organic semiconductor tris-(8-hydroxyquinoline) aluminum
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Manuscript received December 28, 2006; revised February 15, 2007. This work was supported by the German Federal Ministry for Education and Research BMBF (FKZ 13N8168A). The work of M. Punke was supported by the Deutsche Forschungsgemeinschaft and the State of Baden-Württemberg through the DFG-Forschungszentrum “Center for Functional Nanostructures” (CFN). C. Karnutsch, M. Stroisch, M. Punke, and U. Lemmer are with the Light Technology Institute (LTI), Universität Karlsruhe (TH), 76131 Karlsruhe, Germany (e-mail:
[email protected]). J. Wang and T. Weimann are with Physikalisch-Technische Bundesanstalt, 38116 Braunschweig, Germany. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2007.895894
Fig. 1. Top: Schematic cross-sectional view of an investigated organic semiconductor photonic crystal laser. Bottom: SEM picture of an investigated firstorder two-dimensional photonic crystal DFB substrate.
Alq doped with 2.2 wt% of the laser dye 4-Dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) as the active gain medium. Low-threshold laser emission around 650 nm has been achieved using first- and second-order distributed feedback (DFB) in a two-dimensional photonic crystal resonator. II. DEVICE STRUCTURE AND OPTICAL SETUP A schematic cross-sectional view of the investigated organic semiconductor lasers is shown in the upper part of Fig. 1. To realize a laser resonator, we used two-dimensional photonic nm crystal lattices. Square lattices with periods of nm were fabricated in order to provide first- and and second-order feedback for the laser emission of DCM at about m m 650 nm. The gratings with a total size of were fabricated via dry etching of a 600-nm-thick silicon dioxide layer on a silicon wafer [12]. The height of the resulting nm, with a radius of nm. A SiO pillars was scanning electron microscope (SEM) picture of an investigated first-order two-dimensional photonic crystal lattice is depicted in the lower part of Fig. 1. The organic semiconductor thin film 360 nm) was deposited on top of the substrate (thickness by thermal co-evaporation of Alq and DCM (2.2 wt%) in a mbar . The resulting organic high vacuum chamber film exhibits a higher refractive index than the surrounding air–substrate, thus forming a planar waveguide.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 10, MAY 15, 2007
Fig. 3. Emission spectra of a second-order Alq : DCM two-dimensional photonic crystal laser at various currents supplied to the (In)GaN laser diode. Inset: Input–output characteristics of the laser shown in the main part of this figure.
Fig. 2. Pumping scheme used in our experiments. CL: collimating lens. DM: dichroic mirror. MO: microscope objective. PC: nitrogen purge chamber. EF: emission filter. FL: focusing lens. Inset: current–light output characteristics of the pulsed (In)GaN laser diode used for our experiments.
Our pumping scheme is shown in Fig. 2. We used a connm, TopGaN) and ventional (In)GaN laser diode ( a pulsed power supply (Sacher Lasertechnik, PP-9A) to produce laser pulses with a pulsewidth of 20 ns at a repetition rate of 16 Hz. When using such short pulsewidths, the laser diode can be driven at high peak current levels of more than 3 A resulting in an optical peak power of up to 3 W (see inset of Fig. 2). The output of the laser diode was collimated (aspheric mm) and then reflected into a microscope objeclens, tive (Spindler&Hoyer, 20/0.35) using a dichroic mirror (Omega Optical, 410 DRLP). The resulting pump spot on the organic laser surface was roughly circular with a diameter of 90 m. During the measurements, the organic lasers were kept in a nitrogen purge chamber to prevent degradation. The surface emission from the organic laser sample was collected and collimated by the microscope objective, sent through an emission filter (Omega Optical, 435 ALP), and focused into an optical fiber (Ocean Optics, P400-3-VIS/NIRFC), which was connected to a spectrometer equipped with an intensified charged-coupled device (CCD) camera (Princeton Instruments, PiMax:512).
III. MEASUREMENT RESULTS AND DISCUSSION Fig. 3 shows the emission spectra of a second-order laser at various currents supplied to the (In)GaN laser diode. For peak current values up to 1400 mA, a broad photoluminescence emission spectrum spanning from 550 to 700 nm is observed. Starting at a current of 1500 mA, a sharp laser peak at a wavelength of 647 nm evolves. The corresponding pump intensity at
Fig. 4. Emission spectra of a first-order Alq : DCM two-dimensional photonic crystal laser at various currents supplied to the (In)GaN laser diode. Inset: Input–output characteristics of the laser shown in the main part of this figure.
threshold is 3.2 kW/cm . For higher pump powers, the laser peak dominates the emission spectrum. The inset of Fig. 3 shows the input–output characteristic of the organic laser device. Fig. 4 shows the data obtained for a first-order photonic nm. In this case, crystal laser with a grating period of only scattered light is detected since the laser operates in the plane of the waveguide with no outcoupling perpendicular to the waveguide. The onset of lasing at a wavelength of 645 nm 1400 mA. The corresponding is detected for a current of pump intensity is only 1.9 kW/cm and is thus about a factor of two lower than the threshold of the second-order photonic 1700 mA, a second crystal resonator. Above a current of nm evolves at the short-wavelength edge laser peak of the photonic stopband.
KARNUTSCH et al.: LASER DIODE-PUMPED ORGANIC SEMICONDUCTOR LASERS
IV. CONCLUSION In summary, we have realized a very compact all solid-state laser system using a low-cost pulsed (In)GaN laser diode. While a second-order photonic crystal laser bears advantages for applications where a surface-emitting laser is needed, a first-order laser enables an even reduced threshold, which can be favorably used, e.g., in sensing schemes relying on the evanescent field. The proposed lasers could be made mechanically tunable by either using a wedge-shaped organic thin-film with varying thickness or by use of grating matrices, i.e., by spatially varying the lattice period. Both concepts alter the emission wavelength when the photonic crystal laser is moved mechanically in a way that the pump laser diode excites a different region of the photonic crystal laser. Such inorganic–organic hybrid laser systems could provide the basis for innovative portable analysis systems. In addition, the first-order system is a promising candidate for an even cheaper pumping scheme based on light-emitting diodes, which could lead to extremely low-cost and versatile laser sources emitting throughout the entire visible wavelength range. ACKNOWLEDGMENT The authors would like to thank P. Perlin and M. Leszczyn´ ski (TopGaN, Warsaw) for providing the GaN laser diodes and T. Woggon for useful discussions. REFERENCES [1] V. G. Kozlov, G. Parthasarathy, P. E. Burrows, V. B. Khalfin, J. Wang, S. Y. Chou, and S. R. Forrest, “Structures for organic diode lasers and optical properties of organic semiconductors under intense optical and electrical excitations,” IEEE J. Quantum Electron., vol. 36, no. 1, pp. 18–26, Jan. 2000.
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