Ali Haddadpour

Combination of scanning probe technology with photonic nanojets

Light focusing through a microbead leads to the formation of a photonic nanojet functional for enhancing the spatial resolution of traditional optical systems. Despite numerous works that prove this phenomenon, a method to appropriately translate the nanojet on top of a region of interest is still missing. Here, by using advanced 3D fabrication techniques we integrated a microbead on an AFM cantilever thus realizing a system to efficiently position nanojets. This fabrication approach is robust and can be exploited in a myriad of applications, ranging from microscopy to Raman spectroscopy. We demonstrate the potential of portable nanojets by imaging different sub-wavelength structures. Thanks to the achieved portability, we were able to perform a detailed optical characterization of the resolution enhancement induced by the microbead, which sheds light into the many contradictory resolution claims present in literature. Our conclusions are strongly supported by rigorous data analysis and by numerical simulations, all in perfect agreement with experimental results.

Highly compact magneto-optical switches for metal-dielectric-metal plasmonic waveguides.

We introduce highly compact resonant-cavity-enhanced magneto-optical switches for metal-dielectric-metal (MDM) plasmonic waveguides. The field profile of the fundamental mode of a MDM waveguide in which the metal is subject to an externally applied static magnetic field is asymmetric. The static magnetic field induced asymmetry, which enhances or reduces the coupling between the waveguide and a side-coupled resonator, and the relatively large induced wave vector modulation are used to design a Fabry-Perot cavity magneto-optical switch, consisting of a MDM waveguide side-coupled to two MDM stub resonators. The on and off states correspond to either the presence or the absence of the externally applied static magnetic field.

Tunable spatial mode converters and optical diodes for graphene parallel plate waveguides

We introduce compact tunable spatial mode converters between the even and odd modes of graphene parallel plate (GPP) waveguides. The converters are reciprocal and are based on spatial modulation of graphenes conductivity. We show that the wavelength of operation of the mode converters can be tuned in the mid-infrared wavelength range by adjusting the chemical potential of a strip on one of the graphene layers of the GPP waveguides. We also introduce optical diodes for GPP waveguides based on a spatial mode converter and a coupler, which consists of a single layer of graphene placed in the middle between the two plates of two GPP waveguides. We find that for both the spatial mode converter and the optical diode the device functionality is preserved in the presence of loss.

Magneto-optical switches in metal-dielectric-metal plasmonic waveguides

The magneto-optical effect has been used to control the propagation of surface plasmon polaritons in plasmonic waveguides. Here we investigate single-interface metal-dielectric and metal-dielectric-metal plasmonic waveguides in which either the dielectric or the metal is a magneto-optical material. We derive the dispersion relation of these waveguides, and investigate the effect of an externally applied static magnetic field. We find that in metal-dielectric-metal waveguide structures in which the dielectric is a magneto-optical material, the symmetry of the structure prohibits any non-reciprocal propagation in the system. Moreover, the induced change in the propagation constant of the supported modes in the presence of an externally applied static magnetic field is relatively small. In addition, we find that using a magneto-optical metal in a single-interface metal-dielectric plasmonic waveguide results in non-reciprocal propagation of the plasmonic modes along the interface. We also find that in metal-dielectric-metal plasmonic waveguides in which the metal is a magneto-optical material, the propagation constant of the supported modes is dependent on the relative direction of the applied magnetic fields to the upper and lower metal regions. If the applied magnetic fields to the two metal regions are equal and in the same direction, the induced changes in the propagation constants of the modes propagating in the positive and negative directions are the same. On the other hand, if the directions of the applied external magnetic fields are opposite, the propagation constants of the modes propagating in the positive and negative directions are different. We finally investigate Fabry-Perot cavity magneto-optical switches.

Microcavity enhanced directional transmission through a subwavelength plasmonic slit

We show numerically that a compact structure, consisting of multiple optical microcavities at both the entrance and exit sides of a subwavelength plasmonic slit, can lead to greatly enhanced directional transmission through the slit. The microcavities can increase the reflectivity at both sides of the slit, and therefore the resonant transmission enhancement. In addition, the microcavities can greatly improve the impedance matching, and therefore the coupling between free-space waves and the slit mode. An optimized structure with two microcavities on both the entrance and exit sides of the slit leads to ~16 times larger transmission cross section per unit angle in the normal direction compared to the optimized reference slit without microcavities. We also show numerically that the operation frequency range for high emission in the normal direction is broad.

Photonic Crystals for Plasmonics: From Fundamentals to Superhydrophobic Devices

In the last couple of decades we have been witnessing an enormous technological advancement in the field of micro-technology to the extent that nowadays we talk about nanotechnology. Faster computers, LCD based mobiles, nanoparticles for UV absorption in suntan lotions are just few of many examples where nanotechnology plays a fundamental role. The merit of this is mainly in the advance of the fabrication methods. Present techniques such as Focused Ion Beam (FIB) lithography guarantee a resolution of less than 10 nanometers which is about five times more precise than ten years before. Also Photonic Crystals (PhCs), among the others, take advantage from this extremely high resolution level allowing a downscale that permits the realization of structures which in principle can work at vey high energy. Historically PhCs were known as Bragg mirrors and only in 1987 (Yablonovitch, 1987; Sajeev, 1987) with the works of Yablonovitch and Sajeev the term Photonic Crystals was introduced. Nowadays, besides their natural application as filters in particular under full band gap conditions, PhCs see a number of applications: optical fibers (Birks et al., 1997; Zhao et al., 2010), vertical cavity surface emitting lasers (Yokouchi et al., 2003), high reflection coatings, temperature sensors (Song et al., 2006), high efficiency solar cells (Bermel et al., 2007), electric field detectors (Song & Proietti Zaccaria, 2007), non-linear analysis (Malvezzi et al., 2002; Malvezzi et al., 2003), just to name a few. Many are the techniques for the fabrication of PhCs, for example by means of focused-ion beam (Cabrini et al., 2005), two-photon fabrication (Deubel et al., 2004), laser-interference (Proietti Zaccaria et al., 2008a) or waver-fusion techniques (Takahashi et al., 2006). Here we shall focus on the role that PhCs can play for another exciting discipline known as Plasmonics. It refers to the capability of some devices of sustaining an optical surface mode, namely an electromagnetic wave travelling at the interface between two different materials such as a dielectric and a metal. Such a wave originates from the coupling of incident photons on the interface with

Tunable graphene-based mode converters and optical diodes

We introduce compact tunable spatial mode converters for parallel-plate waveguides consisting of two graphene monolayers operating in the mid-infrared wavelength range. We show that such structures can also be used to design graphene-based optical diodes.

Multiwavelength Resonant Absorption Enhancement and Highly Directional Absorption with Aperiodic Multilayer Structures

We show that a graphene monolayer between two aperiodic multilayer structures can achieve near total resonant light absorption at multiple tunable wavelengths, and also that aperiodic multilayer structures can lead to highly directional absorption.

Nanophotonics: devices for manipulating light at the nanoscale

Abstract: This chapter reviews nanophotonic devices for manipulating light at the nanoscale. The chapter first discusses wavelength-scale nanophotonic devices, such as photonic crystals and ring resonators. It then discusses several topics associated with manipulating light at subwavelength scales with nanometallic structures: extraordinary optical transmission through subwavelength apertures, optical nanoantennas, plasmonic focusing, near-field optical microscopy, and plasmonic waveguides. Finally, the chapter discusses the application of nanophotonics in enhancement of nonlinear processes and photovoltaics.