A Faraday rotation experiment can set limits on the magnetic moment of a electrically-neutral, dark-matter particle, and the limits increase in stringency as the candidate-particle mass decreases. Consequently, if we assume the dark-matter particle to be a thermal relic, our most stringent constraints emerge at the keV mass scale. We discuss how such an experiment could be realized and determine the limits on the magnetic moment as a function of mass which follow given demonstrated experimental capacities.

http://absimage.aps.org/image/APR09/MWS_APR09-2009-000941.pdf

Shedding Light on Dark Matter: A Faraday Rotation Experiment to Limit a Dark Magnetic Moment

The Faraday effect is a magneto-optical (MO) phenomenon that causes the plane of linearly polarized light to rotate when passing through a medium subjected to a parallel magnetic field. Informed by the established quantum mechanical model developed by Buckingham and Stephens, we sought to identify molecules that would exhibit large MO responses. Magnetic circular dichroism studies of ferrocenium in the 1970s revealed its potential as an MO material; however, it has not been evaluated in the context of Faraday rotation and thin-film optical applications. Herein, we report near-infrared (NIR) Faraday rotation in thin films of decamethylferrocenium/poly(methyl methacrylate) composites with maximum Verdet constants of -3.45 × 104 deg T-1 m-1 at 810 nm (absorbance = 0.09) and -1.44 × 104 deg T-1 m-1 at 870 nm (absorbance = 0.01). These polymer-metallocene thin films deliver larger Verdet constants than commercially used NIR inorganic Faraday rotators and are facile and inexpensive to produce. The temperature dependence and distinct lineshape of the MO responses observed in decamethylferrocenium radical cations, decamethylmanganocene, and chromocene are in accordance with the quantum mechanical model. The observation of a strong C-term Faraday rotation in solid-state organometallic materials provides the groundwork for the development of high-performance metallocene-based Faraday rotators.

C-Term Faraday Rotation in Metallocene Containing Thin Films.

We present a simple permanent magnet set-up that can be used to measure the Faraday effect in gases, liquids and solids. By fitting the transmission curve as a function of polarizer angle (Malus' law) we average over short-term fluctuations in the laser power and can extract phase shifts as small as ± 50 μrads. We have focused on measuring the Faraday effect in olive oil and find a Verdet coefficient of V = 192 ± 1 deg T-1 m-1 at approximately 20 °C for a wavelength of 659.2 nm. We show that the Verdet coefficient can be fit with a Drude-like dispersion law A / (λ2 - λ0 2) with coefficients A = 7.9 ± 0.2 × 107 deg T-1 m-1 nm2 and λ0 = 142 ± 13 nm.

/pdf/measuring-the-faraday-effect-in-olive-oil-using-permanent-184cspcc37.pdf

Measuring the Faraday effect in olive oil using permanent magnets and Malus' law.

In this paper, we present a sensitive measurement method of the Verdet constant of magneto-optic materials with an auto-balanced photodetector and a lock-in amplifier under weak alternating magnetic fields in the near infrared wavelength region. We describe the theories and formulas for the measurement method by accounting for the characteristics of the auto-balanced photoreceiver and lock-in amplifier. The measurement method was verified by measuring the Verdet constant of the well-known BK7 samples at 1,310 and 1,550 nm wavelengths. This method was used to measure the Verdet constant of the regioregular poly(3-hexylthiophene-2,5-diyl) thiophenes (P3HT) materials prepared at various baking and cooling conditions for the wavelengths of 1310 and 1550 nm.

Sensitive measurement of the magneto-optic effect in the near infrared wavelength region with weak alternating magnetic fields

This thesis addresses optically probed order and dynamics in the chiral cluster magnet Cu2OSeO3. Cu4 triplet clusters form the relevant spin entity for the formation of long-range magnetic order in this Mott insulator. As such, we can envision the magnetic ground state of this material as being a helix of spin clusters. Under an applied magnetic field different metamagnetic phases form, among others and most intriguingly, a topologically protected skyrmion lattice phase. The broken spatial inversion and time-reversal symmetry leads to a variety of optical phenomena in Cu2OSeO3, which are studied with a newly constructed polarization spectroscopy setup. It is shown that the broken spatial inversion symmetry leads to strong dipole active crystal field excitations below the charge transfer gap, and in addition, to natural optical activity. The natural optical activity may be used as a probe for structural chirality, and shows sensitivity to magnetic ordering, evidencing a finite magneto-electric coupling. Under external magnetic fields a strong magneto-optical response is observed, which is discussed to originate from spin cluster formation and the relative ease of magnetic domain reorientation. The strong Faraday-rotation is used to probe the metamagnetic phase transitions and to all-optically map the metamagnetic phase diagram of Cu2OSeO3, including the topologically protected skyrmion lattice phase. Previous work has shown that the long-range spin cluster ordering results in a characteristic spin wave spectrum which can be divided into low-energy external and high-energy internal spin cluster excitations. Here, spontaneous Raman spectroscopy was used to observe multiple spin cluster transitions at the Brillouin zone center, of which the Raman-activity is discussed to originate in the Elliot-Loudon light scattering mechanism. The temperature dependent scattering response of the high-energy internal cluster modes shows a crossover from broad continuum scattering above the critical temperature Tc into well-defined magnetic modes below Tc, which is discussed to originate from a change of the character of the internal cluster modes. Above Tc the internal spin cluster excitations have a localized magnetic excitation character, while in the long-range ordered phase the modes acquire dispersion, and correspond to optical magnons. Finally, using a home built novel time-resolved Raman spectroscopy setup, the photoinduced near-equilibrium spin and lattice dynamics of the helimagnetic phase was probed. Multiple ps-decade spin-lattice relaxation dynamics is observed, evidencing a separation of the order parameter dynamics into disordering of long-range and internal spin cluster order. The observations of the steady-state and time-resolved Raman study exemplify that Cu2OSeO3 can be regarded as a solid-state molecular crystal of spin nature.

Optically probed order and dynamics in the chiral cluster magnet Cu2OSeO3

The utilization of polarized targets in scattering experiments has become a common practice in many major accelerator laboratories. Noble gases are especially suitable for such applications, since they can be easily hyper-polarized using spin exchange or metastable pumping techniques. Polarized helium-3 is a very popular target because it often serves as an effective polarized neutron due to its simple nuclear structure. A favorite cell material to generate and store polarized helium-3 is GE-180, a relatively dense aluminosilicate glass. In this paper, we present a Faraday rotation method, using a new triple modulation technique, where the measurement of the Verdet constants of SF57 flint glass, pyrex glass, and air were tested. The sensitivity obtained shows that this technique may be implemented in future cell wall characterization and thickness measurements. We also discuss the first ever extraction of the Verdet constant of GE-180 glass for four wavelength values of 632 nm, 773 nm, 1500 nm, and 1547 nm, whereupon the expected 1/{\lambda}^{2} dependence was observed.

A Sensitive Faraday Rotation Setup Using Triple Modulation

Faraday rotation has become a powerful tool in a large variety of physics applications. Most prominently, Faraday rotation can be used in precision magnetometry. Here we report measurements of gyromagnetic Faraday rotation on a dense, hyperpolarized $^{3}\mathrm{He}$ gas target. Theoretical calculations predict the rotations of linearly polarized light due to the magnetization of spin-1/2 particles on the scale of ${10}^{\ensuremath{-}7}$ radians. To maximize the signal, a $^{3}\mathrm{He}$ target designed to use with a multipass cavity is combined with a sensitive apparatus for polarimetry that can detect optical rotations on the order of ${10}^{\ensuremath{-}8}$ radians. Although the expected results are well above the sensitivity for the given experimental conditions, no nuclear-spin-induced rotation was observed.

Limits on magnetically induced Faraday rotation from polarized He3 atoms

The utilization of polarized targets in scattering experiments has become a common practice in many major accelerator laboratories. Noble gases are especially suitable for such applications, since they can be easily hyper-polarized using spin exchange or metastable pumping techniques. Polarized helium-3 is a very popular target because it often serves as an effective polarized neutron due to its simple nuclear structure. A favorite cell material to generate and store polarized helium-3 is GE-180, a relatively dense aluminosilicate glass. In this paper, we present a Faraday rotation method, using a new triple modulation technique, where the measurement of the Verdet constants of SF57 flint glass, pyrex glass, and air was tested. The sensitivity obtained shows that this technique may be implemented in future cell wall characterization and thickness measurements. We also discuss the first ever extraction of the Verdet constant of GE-180 glass for four wavelength values of 632 nm, 773 nm, 1500 nm, and 1547 nm, whereupon the expected 1/λ2 dependence was observed.

/pdf/a-sensitive-faraday-rotation-setup-using-triple-modulation-196clkz45u.pdf

A sensitive Faraday rotation setup using triple modulation

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A Sensitive Faraday Rotation Setup Using Triple Modulation

Limits on magnetically induced Faraday rotation from polarized He3 atoms

A sensitive Faraday rotation setup using triple modulation