Circularly polarized light to study linear magneto-optics for ferrofluids:θ-scan technique

Abstract

Circularly polarized light can be divided into two vertically linearly polarized light beams with ±π/2 phase differences. In the presence of an external magnetic field, when circularly polarized light travels through a ferrofluid film, whose thickness is no more than that of λ/4 plate, magneto-optical, magnetic birefringence and dichroism effects cause the transmitted light to behave as elliptically polarized light. Using angular scan by a continuously rotating polarizer as analyzer, the angular (θ) distribution curve of relative intensity (T) corresponding to elliptically polarized light can be measured. From the T − θ curve having ellipsometry, the parameters such as the ratio of short to long axis, and angular orientation of the long axis to the vertical field direction can be obtained. Thus, magnetic birefringence and dichroism can be probed simultaneously by measuring magneto-optical, positive or negative birefringence and dichroism features from the transmission mode. The proposed method is called θ-scan technique, and can accurately determine sample stability, magnetic field direction, and cancel intrinsic light source ellipticity. This study may be helpful to further research done to ferrofluids and other similar colloidal samples with anisotropic optics.

Reveal linear optical effects of the liquid crystal by Stokes-Mueller calculus and θ-scan technology

“…When the long axis of the polarization ellipsoid is a, and the short axis is b, the ellipticity can be described as [15,19]:…”

“…θl and θr correspond respectively to the angle between the long axis of the T-θ curve for the left-and right-handed elliptically polarized light and the x-axis, and the angle ϕ can be obtained as [15],…”

“…The relation between the extinction indices κ and absorbance αd can be described as 𝜅 = 𝜆𝛼𝑑 4𝜋𝑑 ⁄ , where λ is the light wavelength. Therefore, the dichroism Δκ can be expressed as [15],…”

“…In our early work, the LC doped with γ-Fe2O3 NPs can increase the dielectric constant and improve the electro-optical response [13,14]. In addition, the transmittance curves with angle (T-θ) were measured using θ-scanning technology to investigate the anisotropic optical properties of ferrofluids and LCs [15,16]. The results show that using circularly polarized light (CPL) as the incident light, the transmittance curve with angle (T-θ) for a ferrofluids sample indicates linear magneto-optical effects [15].…”

“…In addition, the transmittance curves with angle (T-θ) were measured using θ-scanning technology to investigate the anisotropic optical properties of ferrofluids and LCs [15,16]. The results show that using circularly polarized light (CPL) as the incident light, the transmittance curve with angle (T-θ) for a ferrofluids sample indicates linear magneto-optical effects [15]. In our work presented, the linear optical effects of LC doping with NP were investigated by Stocks-Muller calculus and the θ-scan technology.…”