Advanced Optical Materials

Experimental Demonstration of Magnetic Carriage for Transport of Light Trapped inMagnetizable Mie Spheres

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Full Title: Experimental Demonstration of Magnetic Carriage for Transport of Light Trapped inMagnetizable Mie Spheres

Article Type: Communication

Keywords: Magnetic colloids, Mie scattering, Light transport

Corresponding Author: Rajesh Patel, Ph.D.Bhavnagar UniversityBHAVNAGAR, Gujarat INDIA

Corresponding Author SecondaryInformation:

Corresponding Author's Institution: Bhavnagar University

Corresponding Author's SecondaryInstitution:

First Author: Rajesh Patel, Ph.D.

First Author Secondary Information:

Order of Authors: Rajesh Patel, Ph.D.

Rasbindu Mehta

Order of Authors Secondary Information:

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Experimental Demonstration of Magnetic Carriage for Transport of

Light Trapped in Magnetizable Mie Spheres

Rajesh Patel and R. V. Mehta

Department of Physics, Maharaja Krishnakumarsinhji Bhavnagar University, Bhavnagar,

364002, India.

Correspondence: rvm@bhavuni.edu; +91-278-2422650

Abstract

Colloidal dispersion of micron sized magnetizable particles surrounded by a ferrofluid has a

unique characteristics that the refractive index contrast between the micron sized particles

and the ferrofluid is a function of applied static magnetic field. This field dependent contrast

can be used to tune Morphology Dependent Resonance in the micro spheres and this may

lead to trapping of the incident light. We show here that such trapped light can be transported

to a distance. The microspheres surrounded by the ferrofluid mimics like a magnetic carriage

for transporting the trapped light which may be released by switching off the applied field.

Measurements of the released intensity is carried out as a function of distance for three

different size of the spheres, two different wavelengths and eleven different exposure time

of the incident light. The technique is simple, operates at room temperature and amenable for

photonic applications.

Key words: Magnetic colloids, Mie scattering, Light transport

Complete Manuscript

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A bidispersed magnetic colloid containing micron sized magnetizable spheres and

nanomagnetic particles is an interesting as well as intriguing scattering medium[1-3]

. Optical

wave propagation in strongly scattering medium or in partially ordered systems exhibits

several novel and useful phenomena. Strong and weak localization, photonic Hall effects,

anisotropic diffusion coefficients are some of the examples [4-6]

. The most fascinating

phenomenon amongst these is that of storing and retrieval of light [7-9]

. Successful attempts

were also made to transport the stored light at some distance [10]

. Scully and his group have

trapped laser signal (with help of a writer pulse) in ultra cold rubidium atoms. Then the writer

laser was switched off. After a fraction of millisecond another reader laser was switched on

which was at a distance of 6 millimeters away from the writer laser and the signal pulse was

received. We demonstrate here, a new technique which is comparatively simpler, cost

effective and operates at room temperature.

In most of the above work scatterers were nonmagnetic particles surrounded by a

nonmagnetic medium. When scatterers are magnetic or the surrounding medium is

magnetically active they exhibit new photonic effects [11-14]

. When both the scatterers as well

as the medium are magnetizable, the system manifests several intriguing possibilities. We

show here that such a scattering system can transport the stored light at a distance. Earlier,

we have shown that such a ferrodispersion exhibits several magnetically tunable photonic

effects like weak localization, zero scattering, photonic bandgaps, optical capacitors etc.[15-19]

.

The most intriguing effect is trapping and release of the trapped light with help of externally

applied magnetic field [15]

. The experiment was performed in the following configuration. A

linearly polarized light was allowed to pass through a diluted sample of MMS. The later was

subjected to a static magnetic field. It was observed that when the direction of propagation

and the electric vector of the incident light are transverse to the direction of the applied field,

the emergent light from the sample disappears at a critical value of magnetic field. The light

again reappears when the field is slightly more or less than this value. The system was then,

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subjected to the critical field and was exposed to the incident light for some time and then the

light shutter was closed. Under this condition the field was switched off. Almost immediately

a flash of light having same frequency and state of polarization as that of the incident light

was observed. The findings were attributed to the trapping of light at the critical field and

release of it when the field is removed. The details of these findings are described elsewhere

[15]. In the present work, we show that the trapped light can be carried to a distance and can be

retrieved at this distance. We have also studied role of size of MMS, wavelength of incident

light and exposure time and results are analyzed in terms of morphology dependent resonance

(MDR) induced by the applied magnetic field.

Methods of preparation stable suspensions of micron sized magnetite particles and the

ferrofluid are described in earlier papers [13,16]

. Commercially available magnetite powder was

first washed with dilute nitric acid to remove impurities. The powder was then washed with

double distilled water and acetone. The dried powder was mixed with kerosene and ball-

milled in presence of oleic acid. Using fractional sedimentation suspensions containing 1, 2 ,

and 3 µm sized particles were obtained. The particles were found to be almost spherical.

Ferrofluid was synthesized by coprecipating nanomagnetic particles of magnetite and coating

these particles with oleic acid. Again kerosene was used as base liquid. Aggregation, if any,

was removed by centrifuging the fluid at 12000 rpm. Average particle size of nanomagnetic

particles were determined using X-ray diffraction and were found to be ~10 nm. Saturation

magnetization of the fluid was 200 Gauss. Each Suspension of MMS was mixed with the

ferrofluid and diluted with kerosene as per the requirement. These samples were

homogenized by ultrasonification and no sedimentation was observed during the

experimental measurements. A sample under the investigation was filled in a rectangular

glass cell having 2 mm path length. All the samples were found to be transparent at this path

length.

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The schematic of the experimental setup is shown in Fig. 1a. A 5mW diode pumped

solid state (DPSS) green laser (λ = 532 nm) and a He-Ne 10 mW laser (λ=632 μm ) were used

as light sources. A Glan − Thomson polarizing prism was used to convert unpolarized light

into a polarized one and axis of the polarizer was arranged so that E-vector of the light

incident on the glass cell having 2 mm light path remain perpendicular to the direction of the

applied field. Electromagnet was driven by a constant current power supply. Magnetic field

was measured using a Hall probe. The rectangular cell filled with the ferrodispersion was

fixed between the pole pieces of the magnet and was mounted on x-y-z platform and its

position can be read by a micrometer screw. The translation of the stage in the direction of

the magnetic field was controlled by a motor. Emerging light from the sample was detected

by a CCD camera. This Camera was also mounted on a translation stage. Provision was made

to introduce another glass cell having 1 cm light path. Distance between the sample cell and

the glass cell was kept ~10 cm.

This cell was filled with Rh B-650 fluorescent dye solution in methanol. This

fluorescent dye solution has excitation wavelength equal to 532 nm while its emission

wavelength is 650 nm. Initially, laser beam polarized with its electric vector perpendicular to

the directions of propagation and the field was allowed to incident on the sample. The field

was gradually increased to the critical value so that the emergent light disappears. In this

condition, the sample was exposed to the incident light for some controllable time‘t’. Then,

the light shutter was closed but the field was kept on. The electromagnet along with the

sample cell and CCD camera were displaced to a distance ‘x’ mm( Fig.1b). In this position

the glass cell filled with the dye solution was introduced in the front of the cell. Details of

observation are described in the next section. For photometric measurements of retrieved

intensity the cell containing the dye solution was not used.

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Figure 2a shows the transmitted light from the sample when the light from the DPPS

laser was incident on it and the applied field is zero. The intensity of the CCD image

decreases with increase in the field (Fig. 2b). The magnetic field was increased to the critical

value such that emerging light from the sample cell disappears (Fig.2c). The shutter which

controls the incidence of the laser beam was closed after four minutes. Next the sample cell-

magnet assembly and the dye solution cell were displaced in the field direction at a distance

’x’. At this position the magnetic field was switched off. Almost immediately the CCD

recorded an orange flash (Fig. 2d). This observation confirms the presence of optical waves

having wavelength ~532 nm in the sample. The finding suggests that the stored light having

wavelength 532 nm is transported to a distance ‘x’. This deduction was further confirmed by

photometric measurements. The cell containing the dye solution was removed and CCD

camera was replaced by photodiode. Intensity of the flash was recorded at different values of

‘x’. Experiments were repeated for two wavelengths and different sizes of MMS. Variation

of the intensity of retrieved light with distance for the red and green laser beams is shown in

the figure 3. It is observed that the stored light with 10 mW He-Ne laser can be transported

to a greater distance than that for 5 mW DPSS laser. Variation of the intensity of retrieved

light is found to increase with exposure time (storage time) (Figure 4). Since, the intensity

increases with the time of exposure the stored intensity will also increases with the time.

Consequently, it can survive against losses for longer distance. This distance was increased

up to 9 mm when the exposure time was 20 minutes (Figure 5). Values of the critical field

observed for two wavelengths and different particle sizes are shown in Table 1. In the

following section we shall discuss the above results in light of magnetically induced

morphological resonance in MMS.

Rigorous solution of scattering of electromagnetic waves by a sphere of arbitrary size and

refractive index was derived by Mie and Lorentz. Excellent books are available that treat

different aspects and applications of Mie theory [20,21]

. According to Mie theory scattered

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intensity depends on two intensity functions

( ) ( ) ( ) 2

(1)

Here, suffix 1, 2 represent the two orthogonal states of linear polarization respectively,

perpendicular and parallel to the scattering plane defined by the direction of propagation and

the direction of applied magnetic field. α ( = πd/λ , d is diameter) is the size parameter and θ

is the scattering angle. mMMS ( =

) is the relative refractive index of the scatterer (ms) with

respect to that of surrounding ferrofluids (mf). The scattering coefficients

( ) ( ) in turn depend on Mie coefficients an , bn and partial derivatives of

Legendre polynomials πn ( cosθ) and τn (cosθ). Detailed expressions are given in Ref. [1].

The Mie coefficients depend on the size parameter α and m MMS. It is known that refractive

index of a ferrofluids is a function of applied field. Using Langevin theory of paramagnetism

it has been shown that[22,23]

( ) (2)

Here, m∞ is the saturation value of refractive index, m0 is the refractive index at zero field

and Langevin function L (ξ ) = (cothξ -

)) , ξ =

where µ is the magnetic moment of

nanomagnetic particles, H is the applied magnetic field , k is Boltzmann constant and T is the

absolute temperature. Accordingly, mMMS will also depend on the applied field. Mie

calculations have shown that as the particle size increases, scattering in the forward direction

increases and sharp fluctuations in intensities (ripples) are observed. The ripples are due to

resonances and lead to a number of nonlinear optical effects like glare spots, lasing in micro

drops, and stimulated Raman scattering etc.[24,25]

. It was also predicted that a very high

resonance field gradient near the surface of the sphere may be useful for trapping of light[24]

.

The origin and physical interpretation of this phenomenon is explained on the ‘effective

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potential model’. The optical energy is temporarily trapped near the surface of the sphere in

‘dielectric potential well’. Such a resonant state is referred as ‘quasi bound state’ [24,25]

. The

energy enters and exits the well by tunneling through a centrifugal barrier. The top and

bottom of the well decides the upper and lower bound of the resonance level. The widths of

the resonance which are inversely proportional to the decay time of the quasi bound state are

determined by the rate at which energy tunnel through the outer barrier of the well. The lower

levels have longer life time and hence narrower width since; it has to pass through a longer

barrier. The shape of the potential well depends upon size, shape and refractive index of the

scatterers,[24-29]

. Quality factor Q (λ/dλ) can be > 108. Light trapping observed in the present

case is attributed to MDR. The novelty of the present technique is that MDR can be tuned by

applying static magnetic field of moderate magnitude and a small electromagnet is required

for this purpose. Moreover resonance is sustained as long as field is present. This

characteristic along with the long life time facilitates transport of light. Once the light is

trapped in a MMS, it may remain within it depending on life time of the resonant state.

During this time there may be continuous losses due to total internal reflections[24]

. This

losses being continuous it will increase with time. If, Nt is the number of wave packets

trapped during the exposure time and NL is the number of packets that are lost during this

time, then total number available at an instant will be ( Nt– NL). Once the incident light is shut

off Nt will remain constant while NL will increases with time. Consequently after time T the

available number will be only (Nt – TNL). Obviously, it will take a longer time T to transport

the magnetic carriage to a larger distance. If, Nt > TNL then and then only flash will be

observed. In other words, larger the exposure time (Nt) longer will be the transport distance.

In conclusion, we have shown that refractive index contrast between microns sized

magnetizable spheres and the surrounding ferrofluids can be used to tune MDR by varying

the applied magnetic field. Tuning is sustained as long as the field is present and the light

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remained trapped within the magnetic spheres. This light can be transported to some distance

by transporting the spheres along with the magnetic field. At a controllable distance the

trapped light can be released by switching off the field. For a given system the transport

distance depend on the exposure time of laser light. The technique of loading and off loading

the light is simple, less expensive and operates at room temperature. It is possible to increase

the transport distance by using higher power lasers. The technique will be useful to develop

devices based on MDR.

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Figure 1a: Schematic of the basic experimental set up. 1 Laser source. 2 Shutter,

3 Shutter control. 4 Iris diaphragm. 5. Sample. 6. Electromagnet. 7. Power

supply for the electromagnet. 8 RhB-650 dye with micrometer screw stage. 9

Photo Detector. 10 CCD camera. 11Storage Oscilloscope. 12 Computer .

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Figure 1b: Schematics of experimental assembly to observe the trapped light at a

distance ‘x’ Cell containing‘RhB-650’ dye solution was introduced to confirm the

emission of excitation wavelength from the system. For photometric

measurements this cell was removed.

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Figure 2: From right images indicate (a) Light transmitted from the sample when exposed to green laser and field is zero (b) transmitted light under a field lower than critical field (c) under critical field light disappears. (d)The cell-magnet assembly is displaced, a glass cell filled with RhB-650 solution is introduced in front of the CCD camera and the field is switched OFF. Fluorescence of orange colour confirms incidence of green light having wavelength of ~532 nm emitted from the sample.

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Figure.3: Normalized intensity of the retrieved flash at different distances. The

sample was exposed for 4 minutes to the incident light.

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Figure 4: Variation of retrieved intensity with storage time.

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Figure 5: Variation of retrieved intensity with distance when exposure time He-Ne

laser beam was 20 minutes.

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Table 1

Critical field for light trapping observed for different wavelengths and different sizes

Wavelength(nm) Size (µm) Critical Field(Oe)

532 3 150 2 200 0.5 300

633 3 100 2 160

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