1. Introduction
Smart materials such as piezoelectric materials, shape memory alloys, magnetostrictive and electrostrictive materials have potentials to be adopted to optical device components like modulator, wavelength-division multiplexer, and optical switch [
1]. Recently, many research groups have shown a great deal of interest in the design and fabrication of compact and lightweight optomechanical switches using smart materials, and in developing new micromachining techniques [
2]. Magnetic and mechanical characteristics of ferromagnetic films with thickness of nano and micro scale have attracted extensive interest because of their fundamental and technological importance [
3-
5]. The magnetostrictive behavior, which is the phenomenon that the specific materials (such as TbDyFe film, etc.) strict under imposed magnetic fields, of a thin film is one of the intensively studied issues due to its wireless controllable function. It has potentials to be adapted to optical device as actuators for components including mirror, multiplexer and optical switch. Giant magnetostrictive properties of rare-earth-transition-metal (RE-TM) alloy thin films make these materials potential candidate for use in microsystem actuators [
6]. Among the RE-TM materials, TbDyFe amorphous thin films exhibit most promising characteristics such as low-field magnetostriction for micro applications, soft magnetic properties, and compensated anisotropy [
7]. It has been shown that during thermal and magnetic annealing, the composition variations and sputtering conditions are main parameters that critically influence the microstructure, and the magnetic and magnetostrictive properties of magnetostrictive films [
8,
9]. These properties of TbDyFe can be applied into magneto-optics devices. Magneto-optics provides the opportunity to combine the advantages of the optical methods; contactless, wide dynamic range, absence of electric connections; with those of magnetic methods that easing the requirements imposed to the overall setup.
In this paper, we report a developed magnetostrictive optical mirror and optical mirror switch matrix design using the micromachining. And, the results of the investigation of magnetic and magneto-optical characteristics of the optical mirror are also discussed for the micro application.
3. Measurement and Characterization Results
The characteristics of fabricated mirror depend on the thickness of the deposited TbDyFe film and magnitude of applied external magnetic field. After the film deposition, the film thicknesses are measured by X-ray diffraction (XRD).
Figure 4 shows the measured X-ray diffraction of TbDyFe films in terms of the diffraction angle-intensity (
θ-I) relation. From the figure, the film thicknesses can be calculated as follows [
8]:
where,
I is the intensity of the upper peak point at each 2
θ,
λ is the X-ray wave length of TbDyFe film, Λ is the film thickness, and
δ is the real part of the refractive index and has a value of 10
-5. Λ can be obtained by fitting
θ-I under the condition of the positive least value of
δ. It presents the deposited thicknesses as a function of deposition time. The thickness calculated results are presented in
Figure 5. Under the suggested sputter conditions, it takes about 12 hours to deposit 2μm thickness TbDyFe film.
The most sensitive form of ellipsometry is the oblique-incidence reflectivity difference (OI-RD) technique [
10]. It is a polarization-modulated nulling ellipsometry that directly measures the difference in fractional reflectivity change. To relate the structural and kinetic information on a thin film or the modified surface layer on a substrate to the experimentally measured optical response and to reach the full potential of the OI-RD technique, one typically resorts to mean-field models of optics for multilayer films [
11]. Using a classical multi-layer model to describe the optical response from the surface of a homogeneous substrate covered with a thin film, Zhu and co-workers have shown the fractional reflectivity change Δ
Reflectivity as the
Equation (2) [
12,
13].
where,
θ is the incidence angle of the sample;
ε0,
εd, and
εs are the optical dielectric constants of the ambient, the thin film, and the substrate, respectively; d is the thickness of the film (or the modified surface layer),
λL is the He-Ne probe laser wave length. Changes at the surface other than thickness, such as mass density, chemical makeup, and morphology, are represented by the corresponding change in
εd. The optical arrangement and experimental procedures for obtaining fractional reflectivity changes have been described in detail by Thomas
et al. [
10].
In
Figure 6, we display reflectivity difference in response to adsorption of one monolayer of TbDyFe of 0.5μm thickness on Si substrate versus the incidence angle.
We have computed reflectivity difference by using
Equation (1). The calculated incidence-angle dependence agrees quantitatively with the experimental data.
The magnetization and the magnetostrictions are also measured to characterize the magnetic properties of the TbDyFe mirror. The magnetization of the mirror under the external magnetic field is observed using VSM (Vibrating Sample Magnetometer).
Figure 7 shows the measured magnetization. Magnetization values increase with the increase of deposited TbDyFe film thickness although coercive force at each thickness shows almost same value. Magnetostriction is the changing of a material's physical dimensions in response to changing its magnetization. In other words, a magnetostrictive material will change shape when it is subjected to a magnetic field. Most ferromagnetic materials exhibit some measurable magnetostriction. The magnetostriction of the mirror is determined by measuring differences of the curvature of the film coated cantilever beam using an optical cantilever method. The curvatures of the length and width directions of the mirror due to external magnetic fields are measured by detecting deflected laser signals through a position sensitive detector. The measurement set up for magnetostriction is illustrated in
Figure 8.
The measured curvatures are converted into magnetostrictions as follows [
14]:
where,
Y is Young's modulus,
ν is Poisson's ratio,
t is thickness,
R is the radius of curvature, and the subscript
s and
f denote the value for substrate and film, respectively. Since the deflection is proportional to the amount of magnetostriction, the magnetostriction can be calculated using the deflection. The measured results of the magnetostriction are shown in
Figure 9. From the figure, it can be seen that the deflection amounts increase with increasing TbDyFe film thicknesses; and the maximum measured deflection is about 330μm when the film thickness is 2μm.
And, the magneto-optical characteristics are determined by measuring the deflected angles of the emitted light under the variation of the magnitude of applied magnetic field.
Figure 10 shows the magneto-mechanical characteristics of the fabricated magnetostrictive mirror in terms of magnetic field and deflected angle variations when the TbDyFe film thickness is 2μm. To investigate the defection characteristics of the fabricated mirror, two curves are obtained by increasing and decreasing the applied magnetic field amounts. Also, it can be seen that the two curves show very similar deflection trends under same magnetic field variations.