With two-photon absorption induced polymerization arbitrary three dimensional nano- and microstructures can be patterned directly into photoresists. We report on the fabrication of a low threshold organic semiconductor distributed feedback laser using the technique of two-photon absorption induced polymerization. A surface grating with 400 nm periodicity and 40 nm height modulation was fabricated by two-photon absorption induced polymerization in the organic-inorganic hybrid material ORMOCER®. With structuring several stacked layers acting as a planar basis for the nanostructure microscopic substrate tilt can be compensated simply. This enabled us to uniformly nano-structure the surface grating over an area of 200×200 μm2.
©2009 Optical Society of America
Since its first proposal  the two-photon absorption (TPA) induced polymerization has become an established processing technique for the fabrication of three dimensional structures in the micro- and nanometer scales [2, 3, 4]. First commercial fabrication systems are currently introduced . The development progress made in synthesizing the bio-compatible, TPA processable organic-inorganic hybrid material class ORMOCER®  has increased the use of TPA for rapid prototyping in research areas like life sciences  as well as integrated photonic circuits [8, 9].
Current lab-on-a-chip applications with laser based sensing schemes rely on external laser sources. Therefore such sensing setups require extensive alignment efforts to couple the laser light into the chip’s optical system. Integrating the laser directly onto the chip would solve this problem. Recent approaches for integrating distributed feedback (DFB) laser sources on chips for optical sensor applications [10, 11, 12, 13] require the combination of lithography and imprinting techniques to create structures with feature sizes in the micrometer as well as the sub-micrometer scale. Although usable for wafer scale low-cost production, these imprinting solutions are not suitable for low-cost prototyping or small volume applications. Two-photon induced polymerization on the other hand is perfectly suited for the latter as it offers the possibility to combine both feature scales in an one-step process with the ability for rapid prototyping.
The fabrication of a microcavity laser by two-photon polymerization was first demonstrated by S. Yokoyama in 2003 by structuring a resin containing a dendrimer host and the laser dye 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyril)-4H-pyrane (DCM) . Later in 2005 S. Klein  proposed changing the refractive index of a gain medium using TPA to fabricate a distributed feedback laser. They dispersed the dye Rhodamine 6G as active material in a resin whose refractive index can be increased by TPA. Unfortunatly these two methods have the drawback of having to embed the laser dye inside the resin. This causes a high self-absorption of the laser light in the integrated micro-optical system if it is written in the same process. For the integration of additional low loss optical structures like waveguides these approaches therefore require further processing. Additionally the laser dyes suffer from photo-bleaching during the TPA as they are optically excited [16, 17].
In this work we describe the rapid prototyping of a DFB laser resonator based on a surface grating structured in an ORMOCER® material using two-photon polymerization.
2. Fabrication of DFB lasers by TPA induced polymerization
For the active material layer we used the organic semiconductor tris- 8- hydroxyquinoline aluminum (Alq3) doped with the laser dye DCM. The active material was evaporated onto the resonator. The resulting slab waveguide DFB resonator structure is known to form an efficient laser device .
For the DFB resonator structure the organic-inorganic hybrid material ORMOCER® US-S4 (from Microresist Technology GmbH ), also referred to as Ormocomp was used. This UV curable hybrid polymer material has been designed for molding micro-optics. Because of its organic-inorganic hybrid nature, ORMOCER® combines the qualities of polymers, glasses and silicones. Compared to other polymer materials it exhibits a high optical transparency in the near UV region. As an interesting feature an Ormocomp nanostructure can be directly used as a master for further replications by molding in the same material. Due to the low process temperatures needed low cost polymeric substrates can be used.
2.2. Sample fabrication
For fabricating the sample we used a standard 170 μm thick glass cover slip as the substrate. The substrate was cleaned with subsequent ultrasonic cleansing in acetone and isopropanol baths. To remove any remaining organic residues the substrate surface was plasma-etched with an oxygen plasma. For improving the resin adhesion the sample was then heat treated with a pre-bake step for 2 minutes at 80°C on a contact hotplate. We then processed the ORMOCER® by spincoating a droplet of the resin at 3000 rpm for 30 s. This formed a 50 μm thick layer. No additional adhesion promoter was used. The structuring of the material by TPA induced polymerization was done by exposing the sample to the focused beam of a femtosecond laser (Coherent Mira 900D titanium:sapphire laser). This laser emits 150 fs long pulses at a repetition rate of 76 MHz. The central wavelength of the output pulses was tuned to 800 nm. Figure 1 shows a schematic of the experimental setup we used. To get reproducible results the average pulse energy was controlled by using a motorized gradient neutral density filter (GF) in a closed loop control setup. This is important as the pulse energy directly affects the voxel size .
The laser beam was coupled into a standard optical microscope (Zeiss Axioplan) using free space optics. With the microscope it was then focused onto the sample surface through a Zeiss Apochromat 100x oil immersion objective with a numerical aperture of 1.4. To completely illuminate the microscope objective rear aperture the laser beam is expanded by a 7x Linos beam expander (BE). In this way the focal area was minimized. To pattern the DFB resonator the grating lines were defined by moving the sample relatively to the focused laser beam using a three axis nanometer positioning table (PI P-563.3CD). The exposure dosage can be controlled using a mechanical beam shutter.
With this setup single voxels with a lateral size of 490 nm and a longitudinal size of 2.3 μm could be reliably patterned. By translating the positioning stage with a speed of 1 mm/s and using a high repetition rate laser system the voxels transform into a line shape with a smooth surface. However, this was not sufficient to form a second order laser resonator surface grating for the visible wavelength region. To achieve a grating period smaller than the lateral voxel dimension we placed the voxel-lines to overlap each other by part.
Due to fabrication tolerances of the substrate holder the cover slip is inevitable tilted at a microscopic angle. This causes a vertical movement of the laser focal volume relative to the resin layer when the substrate position is changed horizontally. Therefore, when patterning large areas the structure tends to be written partly inside the substrate. This issue would normally require an additional electromechanical leveling of the substrate surface.
Instead of an active leveling, we compensated the angular tilt of the substrate by writing a baseplate providing a coplanar starting plane for the DFB resonator structure. This baseplate consisted of six layers composed of parallel lines with a periodicity of 400 nm written continuously with a voxel-line width of 570 nm in diameter and 2.5 μm in height at 2 mW time averaged laser power. For this the positioning stage was moved with a translation speed of 1 mm/s. The vertical distance between these layers was 600 nm with the first baseplate layers being placed below the interface between substrate and resin (see figure 2). We did this to additionally promote the adhesion of the structured area on the substrate. Omitting this step caused the structure to be washed off from the substrate during developing. The interface location was estimated at low laser pulse energies by measuring the fluorescence signal of the Ormocomp material. As depicted in figure 3 the signal exhibits a strong shift in intensity depending on the material in which the beam is focused. The signal was detected using a CCD camera. Due to the limited sensitivity of the camera the interface position accuracy was limited to 1 μm.
The DFB resonator grating was written subsequently as a seventh layer 600 nm on top of the baseplate. For this layer a voxel-line width of 490 nm × 2.3 μm was chosen by setting the time averaged laser power to 1.8 mW. Immediately after the TPA exposure step the sample was heat treated with a 5 min long post-bake step at 80 °C for further improvement of the resin adhesion. Figure 4 shows the finished surface grating after being developed for 5 min with Ormodev . This image was taken with an atomic force microscope (Zeiss NanoWizard II).
The grating periodicity is 400 nm with a height modulation of 40 nm. The complete structure spans an area of 200 × 200 μm2. To finalize the sample preparation we deposited a 350 nm thick layer of the active material Alq3 doped with 2.5 mol% of the laser dye DCM. Both materials were deposited simultaneously out of seperate crucibles using a thermal vacuum vapor-deposition. The deposition process took place in a vacuum at a pressure of < 10-5 mbar. This Alq3:DCM material system forms an efficient Forster energy transfer system . A scheme of the resulting organic semiconductor DFB laser structure is depicted in figure 2.
3. Optical characteristics
The fabricated DFB laser was characterized by using an actively Q-switched diode pumped neodymium: yttrium orthovanadate (Nd:YVO4) laser (AOT-YVO-20QSP) as pump source. The pump laser output pulses were frequency tripled to a wavelength of 355 nm. The pulse duration was 500 ps with a repetition rate of 1.3 kHz. The experimental optical setup we used is shown schematically in figure 5. We varied the pump pulse energy with a variable neutral density filter and measured it simultaneously with a calibrated gallium arsenide phosphide (GaAsP) photo-diode connected to an oscilloscope (Agilent Infiniium 54810). The pump laser was directed onto the sample using a dichroic mirror and a focusing lens. The elliptical shaped laser spot on the sample had a diameter of 300 × 200 μm2. These dimensions were determined using the moving edge method. To protect the active material from degradation due to photooxidation we kept the DFB laser sample inside a vacuum chamber at a pressure smaller than 5 · 10-3 Pa. The emission of the sample was collected with the focusing lens and coupled into an optical fiber connected to an imaging spectrometer. Figure 6 shows different emission spectra of the sample taken below and above the lasing threshold. The evolution of the lasing mode out of the photoluminescence spectrum with rising pump energy densities can be clearly seen. The lasing wavelength is 634 nm with a spectral linewidth (FWHM) of 0.15 nm. The corresponding characteristic threshold behavior can be seen in figure 7. Lasing operation starts at a threshold energy density of about 175 μJ/cm2. As shown in figure 8 the DFB laser emission shows also a high degree of polarization.
In summary, we have structured an optical surface grating with 400 nm periodicity in an ORMOCER® material by using two-photon absorption induced polymerization. We showed a simple approach to circumvent the problem of substrate tilt which needs no additional electromechanical hardware. The grating was used as a distributed feedback laser resonator for an organic semiconductor laser device combined with Alq3:DCM as gain medium. Single mode low threshold lasing action of the DFB laser sample under optical pumping could be demonstrated. This proves the applicability of using TPA induced polymerization of organic-inorganic hybrid materials for rapid prototyping of functional nanostructures. Having the possibility to create micro-optics as well, provides the basis for the rapid prototyping of all-integrated optical circuits and lab-on-a-chip systems. By using glass like ORMOCER®s different active and passive on-chip optical components can be fabricated on a single technology platform. As this also includes laser light sources lab-on-a-chip devices may no longer require external laser sources and the alignment processes connected with them . Pumping these integrated lasers with laser diodes [18, 19, 20] or LEDs  enables low-cost lab-on-a-chip applications. In the future even direct electrical pumping might be possible .
The author would like to acknowledge Dr. Bernd Nawracala and the Agilent Technology Foundation for their kind support.
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