Polymeric Thin Films for Application in THz Generation and Detection

1. Polymeric Thin Films for Application in THz Generation and Detection D. Crowley, B. Kubera, J. Shan Department of Physics, Case Western Reserve University, Cleveland, OH 44106 Introduction The terahertz (1012 Hz) or far-infrared region of the electromagnetic radiation corresponds to many fundamental excitations in solids and molecules. THz falls between the microwave and IR regions of the electro magnetic spectrum. Potential applications of this radiation range from imaging to spectroscopy to security, and much more (refer to Figure 1, below). Over the past several years, a new approach has been developed based on the dramatic advances in the production of ultrafast optical pulses from mode locked lasers. In this scheme, the femtosecond laser pulses are used to produce and detect far-IR radiation with controlled electric-field waveforms through the use of a nonlinear crystal.  In this study, we attempt to make polymeric thin films for THz generation. Polymers have shown relatively large nonlinearities and have advantageous phase matching properties for these processes. Lack of phonon modes, like in traditional crystals means less of the spectrum might be cut out. They are also easier and cheaper to produce than conventional inorganic crystals. Results The initial phase of my project was to build and establish a working THz experiment utilizing conventional sources and detectors. This turned out being successful, and I now have a working THz set up employing ZnTe crystals (Figure 5a – below). The second phase is to generate a signal using a polymer films. Several difficulties have arisen here, preventing a signal from being detected. The THz signal is proportional to the thickness of the emitter, and the ZnTe crystal is 1 mm thick. Any successfully poled samples have been three orders of magnitude thinner. Corona poling works great for thin samples, but when they are that thin compared to the crystal, and the signal to noise is 100:1 there is little hope. Even averaging over many runs to pull the signal from the noise came up with nothing. The thicker, samples can not be sufficiently poled using the Corona technique, as the same amount of charge is deposited for a much thicker sample – lowering the applied field by a factor of 10 or more. Any attempt to pole them between ITO has caused sparking, and ultimately resulting in damaged films. Figure 4. a) (above) Disperse Red 1 molecule used in my polymers. b) (right) Picture of one of my earlier stage THz setups. Thin Film Preparation A guest-host mixtures is implemented to create our thin films. This requires dissolving of a dye into some host material. In our case, we used disperse red 1 (DR1) dye (see Figure 4 above), in a PMMA host polymer. DR1 was chosen due to its relatively high dipole moment. First, the PMMA is dissolved in about a 10:1 ratio of solvent: polymer. Once this is dissolved, DR1 is also added and dissolved having about 10% of the mass of the PMMA that is present. Once evenly dissolved, it is time to make the films. There are two methods that I have tried so far. First is spin coating. Where the polymer is laid onto a glass (or ITO) substrate and put onto a spinning device. This whirls about at nearly 1000rpm flinging off excess solution and creating nice, uniform films of thickness order 1-3 microns. These would need to be stacked or folded to achieve desired thicknesses. Secondly, I have tried creating wells of different depths where I just lay the solution in and evaporate off the solvents. This has been successful in creating fairly uniform samples from 50-80 microns. In both cases the samples are baked at around 100 C to expedite the evaporation process. Poling these films is a different story. There are two methods here that I am also trying. Each have similarities though. First the polymer must be heated to around its glass transition temperature (~100 C) – this allows the molecules to move a bit more freely. Next an electric field must be applied to line up the molecules (~50V/um). The sample is then cooled, ideally leaving it in a poled and well ordered state. Better poling results in a larger nonlinear effect to be derived from the material. The two ways I have used is 1) to sandwich the sample between ITO sheets, and apply a voltage or 2) use a needle in what’s known as Corona poling to deposit charge on the surface of the film creating an electric field across it. THz Apparatus The THz experiment implements a Ti:Sapphire mode locked, pulsed laser to create high intensities of near infra red light that can be used to stimulate a nonlinear reaction resulting in THz radiation. The laser pulses will then be split by a beam sampler to send a majority of the power towards the emitter crystal and the rest towards the probing line. Figure 3 (above) shows an example of a basic THz experiment. The pump pulse will cause THz to be emitted from a ZnTe crystal with a process known as optical rectification, which is effectively difference frequency generation. THz emission quickly diverges in all directions, but it can be collected and even collimated by properly positioning a parabolic mirror. Another parabolic mirror is then used to focus the THz down onto the detecting crystal. By inserting a pellicle (effectively a large area, thin film beam splitter transparent to THz) into the THz line it is possible to realign the probing pulse with the THz emission so it can be focused in the same position on the sensor crystal. Moving the delay stage let’s us trace the THz pulse – note Figure 2 (bottom left). The electric field associated with the THz radiation causes an electro optical birefringence that alters the probing pulse’s polarization. A polarizing beam splitter causes different polarizations to travel in two separate paths to a balance detector which acts like a seesaw. Changes to the probe’s polarization by the THz cause the balance to tilt one way or another – allowing us to see a temporal trace of the THz. Figure 3. Picture of a basic optical THz apparatus. Ti:Sapphire mode locked laser is split into two beam lines. Most of the laser pulse is sent along the pump line to generate the THz radiation. The THz is collected by parabolic mirrors, collimated, and then focused back down onto the sensor element where it meets back up with the probing pulse. THz causes a birefringence in the detector causing the probe pulse to shift in polarization. This shift is measured by the balance detectors. Conclusions THz radiation is a new and exciting field lending itself to many useful applications. In order to achieve widespread use there is a need for new emitting and detecting elements which are better and easier to produce. One possible answer is in electrically poled thin film polymers combined with nonlinear optical techniques. While I have yet to acquire any useful results personally. Others have shown that polymers can be just as good of emitters as conventional crystals (Figure 5b – below). Figure 1. a) image of a tooth taken with THz radiation. b) tumor located just under the skin – detected with THz. c) traces of explosives: not only can THz tell something is there – but can figure out exactly what materials are present due to frequency absorption spectrum. d) 3d constructed image from THz of a turkey bone. e) can look through common items to find dangerous objects underneath – here there is a ceramic knife and plastic explosive. f) THz phased image of watermark in money. THz could be used for lots of quality, security, and medical imaging tasks. Figure 2. THz pulse (green) being traced out by a delayed probing pulse (red). As delay stage moves along, probing pulse, which is much shorter than the THz pulse, is effected differently by different parts of the THz waveform. Figure 5. a) 20 scans taken from my THz set up with a trace through the averaged points. b) *example of a comparison polymer film THz signals compared to similar thickness ZnTe. References: *Sinyukov, Alexander M. and Hayden, Michael L. “Efficient electro-optic polymers for THz applications.” J. Phys. Chem. B, Vol. 108, pg 8515. 2004. *Sinyukov, Alexander M. and Hayden, Michael L. et al. “New Materials for Optical Rectification and Electro-optic Sampling of Ultra-short Pulses in the THz Regime.” J. Polymer Sci. B. Polymer Phys. Vol. 41, pg. 2492. 2003 *Schmuttenmaer, Charles A. “Exploring Dynamics in the Far-Infrared with Terahertz Spectroscopy.” Chem. Rev. Vol. 104, pg 1759. 2004. *J. Shan, A. Nahata, and T. F. Heinz, "Terahertz time-domain spectroscopy based on nonlinear optics," J. Nonlinear Opt. Phys. Mater. Vol. 11, No. 1, pg 31. 2002.