PHYSICAL REVIEW E 84, 051403 (2011) Enhanced transmission with tunable Fano-like profile in magnetic nanofluids Junaid M. Laskar, Baldev Raj, and John Philip * SMARTS, Metallurgy and Materials Group Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamilnadu, India (Received 10 March 2011; revised manuscript received 27 July 2011; published 3 November 2011) We observe a Fano-like resonance in a magnetically polarizable nanofluid. Under an external magnetic field, the transmittance spectrum of a ferrofluid emulsion containing droplet size of ∼220 nm shows an enhanced peak with a Fano-like profile, which is attributed to a localized waveguide resonance from random array of tubes with charged inner surface that are formed by the alignment of the droplets. Furthermore, by varying the magnetic field, the Fano profile is tuned and an opaque emulsion is turned into a transparent one. This finding may have interesting applications in tunable photonic devices. DOI: 10.1103/PhysRevE.84.051403 PACS number(s): 82.70.−y, 73.20.Mf, 41.20.Jb, 42.79.Dj I. INTRODUCTION A resonance exhibiting a distinctly asymmetric line shape, well known as a Fano resonance [1–4], is a ubiquitous phenomenon observed in various condensed matter systems such as quantum dots [5], carbon nanotubes [6], graphene [7], etc. Such an asymmetric resonance originates from coupling between the localized and continuum states. Manifestation of a Fano resonance is elegantly demonstrated in transmis- sion, reflection, or absorption spectra in numerous physical systems such as nanoscale structures [8,9], metamaterials [9,10], subwavelength apertures such as photonic circuits [11], metallic films [12,13], etc. The observation of the enhanced transmission with the Fano shape in perforated metallic films has attracted enormous attention due to their application in molecular sensing, spectroscopy, photonic devices, etc., besides the aspect of fundamental understanding [12–15]. The observed enhanced transmission in perforated periodic and aperiodic metallic systems are attributed to surface plasmon [16] and localized waveguide resonances [17,18], respectively. However, the exact origin of enhanced transmission and the Fano resonance is still not very clear [15,19–21]. Tuning the Fano resonance in the microwave region by controlling the cavity-lead opening and utilization of this aspect to probe decoherence is also demonstrated [22,23]. The resonances in various other Fano systems are also tuned by controlling the discrete energy levels and the interference be- tween distinct many-body excitations with an external electric field [5,7,24,25]. But all these Fano resonances are observed only in well-engineered structures. Analogies between the propagation of electromagnetic waves in strongly scattering media and that of electrons in condensed matter have inspired great interest in topics such as the localization of light, photonic band gaps, the photonic Hall effect, etc. [26]. Soft-matter systems like nanofluids have been a topic of intense research during the last decade due to their interesting properties and technological applications [27–29]. The questions we tried to address here are the following: Can we realize a Fano resonance in soft matter with aperiodic- or random-tube-like structures? Is it possible to tune the Fano profile using external stimuli like a magnetic field? Here, we report tunable enhanced transmission with a Fano-like * philip@igcar.gov.in profile in a magnetically polarizable oil-in-water emulsion (considered as smart soft matter) with a droplet diameter of about 200 nm. We tune the transmission through this soft-matter system with an external magnetic field where the aligned droplets form random tubes whose diameter and length vary with the magnetic field strength. II. THEORETICAL BACKGROUND AND EXPERIMENTAL METHOD The ferrofluid (ff) emulsion used in our studies is octane oil droplets (average diameter d ∼ 220 nm, polydispersity < 2%) containing magnetic (Fe 3 O 4 ,d ∼ 6.5 nm) nanoparticles dispersed in water [30]. The oil droplets are electrostatically stabilized with an anionic surfactant of sodium dodecyl sulphate. When the droplet double layer is very thin (κ a < 5), the electrostatic force profile follows the equation F r (r ) = 4πεψ 2 0 a 2  κ r + 1 r 2  exp[−κ ({r − 2a}], (1) where a is the droplet radius, r is the droplet separation distance, ε is the dielectric permittivity of the suspending medium, ψ 0 is the electrical surface potential, and κ is the inverse Debye length that essentially depends on the electrolyte concentration (C s ) and can be represented as [31] κ −1 =  1 4π   2L 2 B C s  −0.5 , (2) where L B is the Bjerrum length. On applying an external field H 0 , the strength of interaction between the droplets increases, which is described by the coupling constant [32]  = πμ 0 d 3 χ 2 H 2 0 72k B T , (3) where χ and k B T are the magnetic susceptibility and thermal energy, respectively. When > 1, the emulsion undergoes a disorder-order transition, where linear chain-like structures are formed due to head-on aggregation of oil droplets along the field direction [32,33]. To obtain insight into the implications of field-induced structures, the transmitted light intensity is recorded as a function of applied external field at different ramp rates using an automated light-scattering setup [34]. 051403-1 1539-3755/2011/84(5)/051403(7) ©2011 American Physical Society