Faraday Rotation Measurements on Thin Films of Regioregular Alkyl-Substituted Polythiophene Derivatives † Palash Gangopadhyay,* ,‡ Ramakrishna Voorakaranam, ‡ Alejandra Lopez-Santiago, ‡ Stijn Foerier, § Jayan Thomas, ‡ Robert A. Norwood, ‡ Andre Persoons,* ,‡,§ and Nasser Peyghambarian ‡ College of Optical Sciences, UniVersity of Arizona, 1630 E. UniVersity BlVd, Tucson, Arizona 85721, and Department of Chemistry, Catholic UniVersity of LeuVen, HeVerlee 3001, Belgium ReceiVed: January 28, 2008; ReVised Manuscript ReceiVed: March 3, 2008 The authors describe a magneto-optic measurement system for the determination of Faraday rotation from ultrathin ﬁlms relative to a substrate reference. The setup is intrinsically immune to reciprocal effects such as circular birefringence and thermal ﬂuctuations. Faraday rotation measurements on thin ﬁlms of 3-hexyl and 3-dodecyl derivatives of regioregular polythiophene are reported. The authors show that the Faraday rotation from these ﬁlms is largely dependent on the supramolecular organization within the thin ﬁlms of these polymers. Introduction Faraday rotation 1 is the rotation of the plane of polarization of linearly polarized light due to magnetic ﬁeld induced circular birefringence of a material. In a nonabsorbing or weakly absorbing medium, a linearly polarized monochromatic light beam passing through the material along the direction of the applied magnetic ﬁeld experiences circular birefringence, result- ing in rotation of the plane of polarization. The angle of rotation θ is expressed as θ ) VBL ) π(Δn/λ)L where Δn is the magnitude of circular birefringence (the difference between the refractive indices for left and right circularly polarized light in the medium), λ is the wavelength of light, B is the magnetic ﬂux density applied to the material parallel to the propagation direction of light, and L is the length of the medium. The constant V, the Verdet constant (generally expressed in degree/ tesla-meter) is a materials property and a quantitative measure of the Faraday rotation ability of the material. The Verdet constant is strongly wavelength dependent, decreasing dramati- cally away from resonance, and in the case of a paramagnetic material, V also depends on the frequency of the magnetic ﬁeld used. 2 The Verdet constant is usually measured by determining the amount of polarization rotation that linearly polarized light experiences when incident on a sample under an ac or dc magnetic ﬁeld. Faraday active materials are used in high-end applications such as the optical isolator, 3 an important device that protects lasers from unwanted back reﬂection, highly sensitive magnetic ﬁeld sensors, 4 and satellite altitude monitors, 5 among others. Faraday rotation measurements have also been used to estimate magnetic susceptibilities and carrier densities 6 in semiconductors where the effective mass of the carrier is known. Generally, Faraday rotation is at its strongest in inorganic substances containing paramagnetic ions or in super- paramagnetic and magnetic materials. 7 Unfortunately, these materials are often very expensive and difﬁcult to process, do not allow for miniaturization and hybrid integration, and/or are not suitable for applications at ambient temperatures. Organic or polymeric materials had not been investigated for efﬁcient Faraday rotation until recently. Nevertheless, a signiﬁcant advantage of organic materials would be their ease of processing, limited weight, and the fact that they can be custom designed and synthesized to meet speciﬁc device requirements. Recently, we showed that conjugated polythiophenes are very promising materials for Faraday rotation. 8 We have shown that some of the polythiophene derivatives possess exceptionally high Verdet constants, 200-300 times larger than those of commercially used magneto-optic (MO) materials, such as terbium-doped gallium garnet (TGG) crystals or substituted yttrium iron garnets (YIGs). 9 The potential applications of polymeric MO materials include attractive possibilities of more compact and integrable waveguide optical isolators and high-performance magnetic ﬁeld sensors that can be readily inserted onto photonic integrated circuit platforms for both commercial and military applications. 10 Instrumentation for Faraday rotation measurement (so-called magneto-optic polarimetry) has continued to attract attention over the last few decades largely because of its application as a highly sensitive photonic magnetic ﬁeld sensor. 4 The sensing of weak to extremely weak magnetic ﬁelds of low frequency (∼4 to 60 Hz), i.e., those associated with the surveillance of military activities such as moving vehicles or underwater moving objects causing magnetic ﬁeld ﬂuctuations through interactions with the earth’s magnetic ﬁeld, has been explored. 11 Ongoing investigations aim to improve the sensitivity enough to measure sub-femto-tesla magnetic ﬁelds, a requirement for mapping biomagnetism such as that occurring in the human brain. 12 Potentially, a highly sensitive Faraday rotation measurement system will enable one to measure MO properties of organic materials, possibly to the single-molecule level, opening a hitherto uncharted arena of MO materials. Methods to measure Verdet constants of materials using ac 13 or dc 14 magnetic ﬁelds involving either simple polarimetric or the more complex interferometric 15 approach in both free space and in-ﬁber 16 conﬁgurations have been reported in literature. An estimated noise equivalent rotation measurement sensitivity of 3 × 10 -6 °/Hz has been reported 17 using interferometric techniques and is among the best sensitivities achieved in benchtop applications. In a high power free space common path polarization Sagnac † Part of the “Larry Dalton Festschrift”. * To whom all correspondence should be addressed. Email: palash@ optics.arizona.edu, Andre.Persoons@fys.kuleuven.be. ‡ University of Arizona. § Catholic University of Leuven. J. Phys. Chem. C 2008, 112, 8032–8037 8032 10.1021/jp800829h CCC: $40.75  2008 American Chemical Society Published on Web 05/08/2008