A Simple Experiment for Determining Verdet Constants Using Alternating
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A simple experiment for determining Verdet constants using alternating current magnetic fields Aloke Jain, Jayant Kumar, Fumin Zhou, and Lian Li Department of Physics, Center for Advanced Materials, University of Massachusetts Lowell, Lowell, Massachusetts 01854 Sukant Tripathy Department of Chemistry, Center for Advanced Materials, University of Massachusetts Lowell, Lowell, Massachusetts 01854 ͑Received 20 July 1998; accepted 15 December 1998͒ A simple experiment suitable for a senior undergraduate and graduate laboratory on the measurement of Faraday rotation using ac magnetic fields is described. The apparatus was used to measure the wavelength dependence of Verdet constants for several materials. A concentration-dependent study on ferric chloride water solution was also carried out to demonstrate that the diamagnetic and paramagnetic contributions to Faraday rotation have opposite signs. © 1999 American Association of Physics Teachers. I. INTRODUCTION in noise. The Verdet constants of several materials com- monly available in the laboratory were determined at differ- Faraday rotation1 is the rotation of the plane of polariza- ent wavelengths and compared with published data.8,9 tion of light due to magnetic-field-induced circular birefrin- gence in a material. In a nonabsorbing or weakly absorbing II. EXPERIMENT medium a linearly polarized monochromatic light beam pass- ing through the material along the direction of the applied The experimental setup is schematically shown in Fig. 1. magnetic field experiences circular birefringence, which re- The conventional large electromagnet is replaced with two sults in rotation of the plane of polarization of the incident 500-turn coils ͑wound with enamelled copper wire 1 mm in light beam. The angle of rotation can be expressed as diameter͒. These coils were routinely used in our under- ϭVBLϭ⌬nL/, where ⌬n is the magnitude of circular graduate labs for demonstrating concepts relating to induc- birefringence ͑a difference in refractive index of left and tances and resonant circuits. Each coil has an air core with a right circularly polarized light in the medium͒, L is the length cross section of 4 cmϫ4 cm and a length of 7 cm. Each of of the medium traversed, is the wavelength of light in these coils has an inductance of 0.005 H and a resistance of ⍀ vacuum, and B is the magnetic field. The constant V is a 2.5 allowing a maximum current of 2.5 A. The spacing material property and gives a quantitative measure of the between the coils is 3 cm. The power source to drive each of Faraday rotation ability of the material and is called the Ver- the coils generating the ac field was a car stereo amplifier det constant. Faraday rotation is commonly used in fabrica- capable of supplying 50-W per channel and was bought from tion of optical isolators2 to prevent unwanted backreflections. a discount store for $30.00. A 13.8-V 10-A power supply Faraday rotation measurements can also be used to infer the was used as the power source for the amplifier. Two 1- F ͑ ͒ susceptibility of materials and measure carrier densities3 in capacitors rated for 300 V were connected in series with semiconductors if the effective mass of the carrier is known. each of the coils to resonate the circuits at a convenient drive Many investigations have been carried out to measure the frequency which was chosen to be in the vicinity of 2 kHz. Verdet constants of materials using dc magnetic fields.4–7 The resonant circuit helps cancel current limiting due to the Since Faraday rotation combines many fundamental ele- inductive impedance of the coils and makes it possible to ments of polarization optics with electromagnetism, dc generate larger ac magnetic fields. It is important to bear in magnetic-field-based Faraday rotation experiments have also mind that due to the resonant excitation the voltage across been introduced to the undergraduate physics laboratory to examine the Faraday rotation and determine the Verdet con- stants of samples.5,6 However, the dc magnetic-field-based measurement requires an expensive and bulky electromagnet and a costly high current power supply to produce magnetic fields of the order of a kilogauss or higher due to the small values of the Verdet constants for paramagnetic and diamag- netic materials ͑of the order 10Ϫ3 to 10Ϫ2 min/G cm͒. Mea- surement of the Verdet constants with an ac magnetic field has been recently reported.7 In this paper, we describe a simple experimental setup suitable for a senior or graduate laboratory to investigate Faraday rotation and determine the Verdet constant using an ac magnetic field. The experiment uses a lock-in amplifier to measure the Faraday rotation Fig. 1. Schematic diagram of the setup for measuring the Faraday rotation angle and introduces students to the utility of the phase sen- using ac magnetic field. P: polarizer; S: sample; A: analyzer; D: photode- sitive detection technique to extract a signal normally buried tector. 714 Am. J. Phys. 67 ͑8͒, August 1999 © 1999 American Association of Physics Teachers 714 Iץ the capacitor and inductors can get as high as 200 V and care ϭ ͑Ϫ͒ 2 ͑ ͒ sin 2 A0. 5ץ -should be taken to make sure that these components are ap propriately insulated or contained in a closed box to avoid a possible shock during operation. The sinusoidal output of a Since the angle is very small (Ӷ1°), one can clearly see signal generator was fed into the two input channels of the that when ϭ45° maximum ⌬I can be obtained. With the audio amplifier and the frequency was scanned until reso- analyzer set at 45°, the intensity measured by the detector as nance was obtained at 2.2 kHz. Typically, the ac magnetic expressed by Eq. ͑4͒ can be simplified: field was adjusted to be in the range of 20–40 G rms. The ϭ 1͑ ϩ ͒ 2 ͑ ͒ magnitude of the magnetic field can be controlled by adjust- I 2 1 2 A0. 6 ing the magnitude of the input signal or the frequency of the ϭ ⍀ Since the ac magnetic field is sinusoidal B B0 sin( t) signal generator so as to move slightly away from the reso- and is proportional to B, the rotation angle can be written nance peak. The magnetic field was measured using a Hall ϭ ⍀ ͑ ͒ in the form of 0 sin( t). Then Eq. 6 can be rewritten probe ac gaussmeter. If a gaussmeter is not available, the as magnetic field can still be determined by measuring the Far- ϭ 1͑ ϩ ͑⍀ ͒͒ 2ϭ ϩ⌬ ͑⍀ ͒ ͑ ͒ aday rotation of a material whose Verdet constant is accu- I 2 1 2 0 sin t A0 I0 I sin t . 7 rately known. We shall further elaborate on this in Sec. IV. ͑ By measuring the relative change of the light intensity In this experiment, a polarizer inexpensive polarizers ⌬ with extinction ratios of 1:1000 work fine͒ is used to obtain I/I0 , the amplitude of the angle of rotation can be easily a linearly polarized light beam that passes through the determined to be sample placed between the two coils. The plane of polariza- 2 ⌬ 1 2 0A0 1 I tion of the light beam passing through the sample under the ϭ ϭ . ͑8͒ 0 2 2 2 I influence of the ac magnetic field oscillates periodically. This A0 0 oscillation of the plane of polarization is converted by an- The Verdet constant of the material is then determined using other polarizer ͑as analyzer͒ that is set at 45° with respect to ϭ the relation 0 VB0L, where 0 , L, and B0 are experimen- the input polarizer to an oscillation of light intensity which is tally determined. It is important to point out that it is not detected by a photodetector and a lock-in amplifier ͑Stanford ⌬ Ϫ5 difficult to measure a ratio I/I0 of the order 10 using a Research Systems SR510 single-phase analog lock-in ampli- phase sensitive technique. A rotation of the plane of polar- ͒ fier with a frequency range of 0.5 Hz–100 kHz . The photo- ization of the order of 10Ϫ5 rad is thus measurable by this detector used here is a silicon-based low-power detector technique due to the elimination of broadband noise by the ͑Newport 818-SL͒ which has a broad spectral response of Ϫ5 lock-in amplifier. A rotation of 10 rad corresponds to a 400–1100 nm and a rise time about 2 s. The Faraday rota- Ϫ10 tion angle is experimentally measured and the Verdet con- difference of the order 10 in the refractive index of left stant can be easily determined if the value of the magnetic and right circularly polarized light in the medium. field and sample thickness are known. Section III discusses some of the theoretical aspects underlying the experiment. IV. RESULTS Experiments were performed on a number of materials III. THEORY which are relatively easily available in a teaching laboratory. The materials investigated included distilled water, ethyl al- The linearly polarized light incident on the sample is as- cohol, methyl alcohol, BK7 glass, and aqueous solutions of sumed to be propagating in the z direction and polarized ferric chloride at different concentrations. For liquid along the x axis. The electric field of the light beam can be samples, a glass cuvette with 1-cm path length was used. The expressed in Jones matrix form as thickness of the BK7 glass used in our experiments was 1.2 cm. Lasers were used as light sources for the wavelength- 1 E ϭͩ ͪ A exp͑Ϫitϩikz͒, ͑1͒ dependent experiments simply because they were available, 0 0 0 but a collimated white light source followed by an appropri- where A is the amplitude of the electric field of the beam.