We investigated the optical and magnetic properties of a transparent magnetic garnet with a particular focus on its applications to atomic physics experiments. The garnet film used in this study was a magnetically soft material that was originally designed for a Faraday rotator at optical communication wavelengths in the near infrared region. The film had a thickness of 2.1 μm and a small optical loss at a wavelength of λ=780 nm resonant with Rb atoms. The Faraday effect was also small and, thus, barely affected the polarization of light at λ=780 nm. In contrast, large Faraday rotation angles at shorter wavelengths enabled us to visualize magnetic domains, which were perpendicularly magnetized in alternate directions with a period of 3.6 μm. We confirmed the generation of an evanescent wave on the garnet film, which can be used for the optical observation and manipulation of atoms on the surface of the film. Finally, we demonstrated a magnetic mirror for laser-cooled Rb atoms using the garnet film.

The manipulation of neutral atoms above a solid surface is important in atomic physics from both a fundamental and practical point of view. Surface reflection1–7 is a basic technique in atom optics and can also provide information on atom-surface interactions.8 Atoms trapped above the surface, so-called atom chips,9–11 are now used as the basis for many experiments, such as atom interferometry,12 atomic sensors,13 and atomic clocks.14 There are two main kinds of manipulation technique: magnetic and optical. Magnetic fields above the surface can be produced either by current-carrying wires or patterned magnetic materials and are used to reflect or trap atoms in low-field-seeking states,15 as well as to induce velocity-selective magnetic transitions at the surface.16 Currents are useful particularly when the variable and dynamical control of atoms is required. However, fundamental limitations to the coherence times of trapped atoms due to fluctuations from conductors have been noted previously.17 Magnetic patterns created on magnetic materials can provide stable, less-noisy trapping environments, but dynamical manipulation is not easy. Optical methods, especially evanescent wave (EW) techniques, are also used for atom reflection18,19 and trapping.20,21 There are various parameters to control atoms that are rapidly changeable: intensity, polarization, and frequency. Optical spectroscopy using EW is also useful to observe atoms located near the surface.22,23 A combination of such magnetic and optical techniques at the surface would offer a new tool for use in atomic physics.24 However, the demonstration of such techniques has been rather limited, partly because magnetic materials that are transparent and, thus, allow optical access are not very common. One exception is a previous study on the use of a transparent and magnetically-hard ferrite-garnet film as an atom chip, where magnetic patterns for the magnetic trapping of atoms were optically written.25,26

Magnetic garnets are known to be a unique magnetic substance. They are employed as an optical isolator to provide a large Faraday effect in an absorption-free region.27 These garnets have been highly investigated for magnetic bubble memory28 because magnetic domains can be manipulated by an external magnetic field. In rapidly progressing spintronics studies, yttrium iron garnets (YIG) are widely used to pump a spin current into contacting metal by the ferromagnetic resonance of YIG.29 Magnons in YIG are potentially useful as an interface to exchange quantum information between microwave and optical photons.30 

In this paper, we studied a transparent magnetic garnet film that was originally designed for a commercial Faraday rotator at optical communication wavelengths, from the point of view of atomic physics. The magnetic garnet film we investigated had a small optical loss at a wavelength of λ=780 nm resonant with the D2 transition of Rb atoms. We observed an EW generated above the surface of the film at the total internal reflection of laser light at λ=780 nm. The garnet film was a magnetically soft material and, thus, had a small total magnetization with a zero external magnetic field, while its magnetic domains were magnetized in alternate directions. With an exponentially decaying magnetic field above the surface of the film, we demonstrated a magnetic mirror for cold Rb atoms dropped perpendicularly onto the surface. Our study shows the feasibility of the application of this magnetic material to future atomic physics experiments using magnetic and optical techniques at the surface.

The garnet film used in this study was a thinner version of a commercial Faraday rotator (Type WRC; Adamant Co., Ltd., Tokyo, Japan).31 It was a single crystal whose chemical composition was (GdHoBi)3(GaAlFe)5O12, and was made by the liquid-phase-epitaxial method on a paramagnetic gadolinium gallium garnet (GGG) substrate with a thickness of 467 μm. The thickness of the film was 2.1 μm, and its dimensions were 10×10 mm2. The roughness average of the film surface, Ra, was measured to be on the order of 1 nm using an atomic force microscope. The garnet film was originally designed for use at optical communication wavelengths and is highly transparent in that region. The specified refractive indices of the garnet film are 2.40 and 2.42 at λ=1600 nm and 1310 nm, respectively. The film was of a brownish color to the naked eye, but was still transparent enough at λ=780 nm, as shown in Fig. 1. The GGG substrate was clear, and its refractive index was 1.95 at λ=780 nm.32 The transmittance and the reflectance of a laser beam at λ=780 nm incident to the garnet film on the GGG substrate at an angle of 45° were measured to be 72% and 18%, respectively. The Faraday rotation caused by each magnetic domain of the garnet film was easily detectable in the visible region. When a linearly polarized laser beam at λ=532 nm was incident perpendicularly to the garnet film with stripe magnetic domains (see below), diffraction patterns caused by the spatially periodic modulation of the polarization planes of the laser light were observed for the transmitted laser beam. The Faraday angle was approximately 18°. However, at λ=780 nm diffraction patterns could hardly be observed, indicating a negligible Faraday effect on the polarized light.

FIG. 1.

Photographs of the garnet film attached to one side of a prism, taken (a) under the illumination of room light and (b) using an infrared scope under the illumination of light at λ=780 nm. The brownish garnet film shown in (a) is barely visible in (b).

FIG. 1.

Photographs of the garnet film attached to one side of a prism, taken (a) under the illumination of room light and (b) using an infrared scope under the illumination of light at λ=780 nm. The brownish garnet film shown in (a) is barely visible in (b).

Close modal

Envisaging applications to the optical observation and manipulation of atoms at the surface, we measured an EW at λ=780 nm above the garnet film. The experimental setup is shown in Fig. 2, which is similar to Refs. 33 and 34. The GGG substrate was glued to the mount prism with a refractive-index-matching oil. The mount prism was made of N-BK7, whose index of refraction is 1.51 at λ=780 nm. The prism was able to be rotated so that the incident angle of the laser beam could be changed. Due to increasing refractive indices from the prism to the garnet film through the GGG substrate, the EW was produced at total internal reflection at the surface of the garnet film. The EW above the film was converted into a travelling light beam with another prism (the pick-up prism) mounted symmetrically on the stage. The distance between the surfaces of the garnet film and the pick-up prism was controlled by a piezoelectric actuator. By observing the interference patterns of the reflected beam as a function of the thickness of the air gap between the surfaces, we determined the gap distance.

FIG. 2.

Schematic of the experimental setup for detecting the evanescent wave.

FIG. 2.

Schematic of the experimental setup for detecting the evanescent wave.

Close modal

We first measured the intensity of the laser beam, reflected by the garnet film on the mounted prism without the pick-up prism as a function of the incident angle of the laser, to locate the critical angle θc. We then measured the intensity of the picked-up beam as a function of the air gap, as shown in Fig. 3, where the incident angle of the laser was incremented by 0.13° from θc. The light had a p-polarization. The solid line in Fig. 3 shows an exponential fit to the experimental data. The observed exponentially-decaying behavior is a clear indicator of the existence of the EW above the surface of the garnet film. The penetration depth, which is double the 1/e decay length, was 1.73 μm, and consistent with theory.33,34 The decay curve did not tend to zero, probably because the garnet film scattered a small fraction of the laser light.

FIG. 3.

Intensity of the beam picked up from the evanescent wave as a function of the air gap, which is indicated in the inset (not to scale).

FIG. 3.

Intensity of the beam picked up from the evanescent wave as a function of the air gap, which is indicated in the inset (not to scale).

Close modal

The hysteresis curves of the garnet film were measured with a vibrating sample magnetometer, as shown in Fig. 4, where the contribution from the paramagnetic GGG substrate has been deducted. It can be intimated from the curve that the film was composed of a soft magnetic material with a small remanent magnetization. The direction normal to the surface of the film was the axis of easy magnetization, and the saturation magnetization in the perpendicular direction was 72 mT.

FIG. 4.

Hysteresis curves of the magnetic garnet film.

FIG. 4.

Hysteresis curves of the magnetic garnet film.

Close modal
FIG. 5.

Magneto-optical microscope images of (a) stripe domain, (b) labyrinth domain, (c) bubble domain on the garnet film at no external magnetic field.

FIG. 5.

Magneto-optical microscope images of (a) stripe domain, (b) labyrinth domain, (c) bubble domain on the garnet film at no external magnetic field.

Close modal

The small remanent magnetization indicates that magnetic domains with an alternate magnetization direction perpendicular to the surface cancel out the total magnetization of the film when there is zero external magnetic field. Although various patterns of the magnetic domains were produced spontaneously at a zero external magnetic field, we could control the formation of the patterns using a neodymium magnet (diameter: 19 mm, height: 10 mm, surface magnetic flux density: 0.4 T). We moved the magnet across above the garnet film by hand. By moving the magnet at a speed of about 10 cm/s about 1 cm above the garnet film, we were able to obtain a stripe domain pattern as shown in Fig. 5(a). When the magnet was moved faster, a typical pattern was a labyrinth domain pattern (Fig. 5(b)). When the magnet was moved farther above the garnet film, we sometimes obtained a bubble domain pattern (Fig. 5(c)). We used the stripe domain pattern for the following magnetic mirror experiment. Each stripe was 1.8 μm in width and the period of the magnetic field was 3.6 μm. The pattern did not change for several months in a magneto-optical trapping (MOT) cell.

The magnetic domain patterns were subject to deformation at high temperatures. We observed that the straight domain walls as shown in Fig. 5(a) became corrugated, typically with an amplitude of 2 μm and a period of 30 μm, after a 40-hour heating at 150°C.

The width of the magnetic domain was determined by the garnet film properties and could not be changed at zero magnetic field. However, the domain walls were moved and the domain pattern was changed when the external magnetic field was applied. Figure 6 shows that the period of the strip domain pattern increased notably at an external field of 0.023 T. When the magnetization was saturated above 0.03 T, no patterns were observed, and then a new pattern appeared after the external field was turned off. Up to about 0.01 T, original patterns were maintained after at least a few cycles of the application of the external field.

FIG. 6.

Magneto-optical microscope images of the magnetic domain (a) with no external magnetic field, and (b) with an external field of 0.023 T applied perpendicularly to the film.

FIG. 6.

Magneto-optical microscope images of the magnetic domain (a) with no external magnetic field, and (b) with an external field of 0.023 T applied perpendicularly to the film.

Close modal

We additionally mention a possible application of bubble domains of magnetic garnets. The bubble domains, as shown in Fig. 5(c), can be well separated in an external perpendicular magnetic field close to the saturation. These isolated bubbles may be useful for trapping atoms in localized regions. Each bubble domain has the magnetization in the direction opposite to the external field, and produces the potential well whose dimensions are similar to the bubble radius. It has a depth on the order of the applied field and is located at a distance similar to the bubble radius from the film surface. The magnetic bubbles can be manipulated using external magnetic fields28 and thus the dynamical control of trapped atoms would be possible.

Finally, we demonstrated a magnetic mirror using the garnet film in a standard setup of the vapor cell trap.35 The schematic diagram for atom mirror experiments is shown in Fig. 7. The MOT cell was a rectangular glass cell (inside dimensions: 20×20×100 mm3). It was not baked and the pressure in the cell was 10−6 Pa. The anti-Helmholtz coils (diameter: 42 mm, separation: 30 mm) were placed outside the cell. 85Rb atoms supplied from a Rb dispenser were cooled and caught in a magneto-optical trap 9 mm above the garnet film, and further cooled by polarization gradient cooling. All the laser cooling beams, including the repumping beam (not shown in Fig. 7), did not pass through the garnet film. The number of the atoms was 107, and the temperature of the atoms was around 5 μK. After cooling, the atoms were released and fell onto the surface of the garnet film. The atoms in the ground state F = 3 level was probed after a certain period of time by shining a probe laser briefly and detecting the atoms’ fluorescence using a CCD camera. One image in Fig. 8(a)(d) was the sum of 48 shots in repeated trapping, releasing, and measuring cycles. The images show a clear reflection of the Rb atomic cloud at the surface of the garnet film. About 40% of the atoms were reflected by the magnetic mirror. 85Rb atoms falling from 9 mm above the garnet film are retro-reflected with a mirror field of 4.0 mT when the atoms are in the low-field seeking states (the magnetic quantum number mF1) of the F = 3 level. This mirror field is achieved 1.4 μm above the film surface, assuming each magnetic domain is perpendicularly magnetized to 72 mT. The observed reflectance was consistent with the percentage of the number of the low-field seeking states in the F = 3 level, and thus agreed with our estimation. We further confirmed, in a slightly different setup, that the spin polarization in the F = 3 level produced by optical pumping changed the reflectance; it was made twice or one-fourth as large as the reflectance for non-optically-pumped atoms, depending on the direction of the polarization.

FIG. 7.

Schematic of the experimental setup for the atom mirror experiment.

FIG. 7.

Schematic of the experimental setup for the atom mirror experiment.

Close modal
FIG. 8.

Fluorescence images of the atomic cloud reflected by the garnet film. Images (a) and (b) are taken before the reflection, while images (c) and (d) are taken after the reflection. The black solid line indicates the surface of the garnet film. Times shown in the images represent durations after the release of atoms from the trap.

FIG. 8.

Fluorescence images of the atomic cloud reflected by the garnet film. Images (a) and (b) are taken before the reflection, while images (c) and (d) are taken after the reflection. The black solid line indicates the surface of the garnet film. Times shown in the images represent durations after the release of atoms from the trap.

Close modal

In this study, we characterized a transparent magnetic garnet film for use in atomic physics experiments. The garnet film used in this study was originally designed for commercial Faraday rotators at optical communication wavelengths. It comprised a soft magnetic material, and its magnetic domains were perpendicularly magnetized in alternate directions in a zero magnetic field. The film had a small optical loss for near-infrared light used for heavy alkali atoms. An EW above the garnet film was confirmed at λ=780 nm, and the optical observation and manipulation of atoms appears to be possible with this garnet film. In addition, we demonstrated a magnetic mirror for cold Rb atoms using the film with stripe magnetic domains.

On the basis of the optical and magnetic properties of the garnet film investigated in this paper, we believe that this magnetic material will find many useful applications in atomic physics experiments. We have subsequently begun an experiment to observe the effects of EW on magnetically reflected Rb atoms. These results will hopefully be reported elsewhere. Another of our planned experiments is the application of the garnet film to the study of motion-induced resonance16,36 for atoms passing through a spatially periodic magnetic field above the garnet film. The spatially periodic magnetic field will be used to induce magnetic resonance of moving atoms; therefore we will use a relatively fast atomic beam incident at a grazing angle on the film surface to fulfill the non-adiabatic conditions.5 In this experiment, we plan to use an optical atom mirror with an EW on the garnet film to control the approach of the atoms to the surface.

We appreciate the supply of garnet films from Adamant, Co. Ltd.. This work was supported by MEXT/JSPS KAKENHI Grant Numbers JP20684017 and JP23244082.

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