Ashok Kodigala

Lasing action from photonic bound states in continuum

In 1929, only three years after the advent of quantum mechanics, von Neumann and Wigner showed that Schrödinger’s equation can have bound states above the continuum threshold. These peculiar states, called bound states in the continuum (BICs), manifest themselves as resonances that do not decay. For several decades afterwards the idea lay dormant, regarded primarily as a mathematical curiosity. In 1977, Herrick and Stillinger revived interest in BICs when they suggested that BICs could be observed in semiconductor superlattices. BICs arise naturally from Feshbach’s quantum mechanical theory of resonances, as explained by Friedrich and Wintgen, and are thus more physical than initially realized. Recently, it was realized that BICs are intrinsically a wave phenomenon and are thus not restricted to the realm of quantum mechanics. They have since been shown to occur in many different fields of wave physics including acoustics, microwaves and nanophotonics. However, experimental observations of BICs have been limited to passive systems and the realization of BIC lasers has remained elusive. Here we report, at room temperature, lasing action from an optically pumped BIC cavity. Our results show that the lasing wavelength of the fabricated BIC cavities, each made of an array of cylindrical nanoresonators suspended in air, scales with the radii of the nanoresonators according to the theoretical prediction for the BIC mode. Moreover, lasing action from the designed BIC cavity persists even after scaling down the array to as few as 8-by-8 nanoresonators. BIC lasers open up new avenues in the study of light–matter interaction because they are intrinsically connected to topological charges and represent natural vector beam sources (that is, there are several possible beam shapes), which are highly sought after in the fields of optical trapping, biological sensing and quantum information.

Exceptional points in three-dimensional plasmonic nanostructures

Exceptional points (EPs) are degeneracies in open wave systems where at least two energy levels and their corresponding eigenstates coalesce. We report evidence of the existence of EPs in three-dimensional (3D) plasmonic nanostructures. The systems are composed of coupled plasmonic nanoresonators and can be judiciously and systematically driven to EPs by controlling symmetry-compatible modes via their near-field and far-field interactions. The proposed platform opens the way to the investigation of EPs for enhanced light-matter interactions and applications in communication, sensing, and imaging.

Hybridized metamaterial platform for nano-scale sensing

Plasmonic/metamaterial sensors are being investigated for their high sensitivity, fast response time, and high accuracy. We propose, characterize and experimentally realize subwavelength bilayer metamaterial sensors operating in the near-infrared domain. We measure the figure-of-merit (FOM) and the bulk sensitivity (S) of the two fundamental hybridized modes and demonstrate both numerically and experimentally that the magnetic dipolar mode, degenerate with the electric quadrupolar mode, has higher sensitivity to a variation of the refractive index compared to the electric dipolar mode. In addition, the hybridized system exhibits a four fold increase in the FOM compared to a standard dipolar plasmonic system.

Engineering resonance dynamics of plasmon hybridized systems

The ability to control resonances is crucial in advancing applications of plasmonics ranging from chemical and biological sensing at the single molecule level to on-chip communication via fully optical interconnects. To this end, a method employing an effective Hamiltonian formalism is described to study and tailor resonances of plasmonic systems at optical frequencies. Using this method, we compute the complex poles of the scattering matrix and investigate resonance dynamics of coupled plasmonic bars. We show that symmetry breaking, by tailoring near-field interactions in the whole complex plane, provides a very large degree of tunability, including a controllable negative coupling regime.

Integrated metaphotonics: symmetries and confined excitation of LSP resonances in a single metallic nanoparticle

Using numerical simulations, we demonstrate that the dipolar plasmonic resonance of a single metallic nanoparticle inserted in the core of a dielectric waveguide can be excited with higher order photonic modes of the waveguide only if their symmetry is compatible with the charge distribution of the plasmonic mode. For the case of a symmetric waveguide, we demonstrate that this condition is only achieved if the particle is shifted from the center of the core. The simple and comprehensive analysis presented in this contribution will serve as basis for applications in integrated nanophotonic/metamaterials devices, such as optical filters, modulators and mode converters.

Mechanically stable conjugate and suspended lasing membranes of bridged nano-cylinders

We present two different fabrication approaches for a suspended lasing membrane with intricate sub-micron patterning for an InGaAsP/InP platform. One approach involves a hydrogen silsesquioxane (HSQ) electron beam lithography resist as a dry etch hard mask and another with an added chromium (Cr) hard mask. The Cr hard mask process allows for fine control over patterned dimensions in comparison to the HSQ mask. This is crucial to both membrane stability and device performance. Both approaches are heavily susceptible to dry etch requirements and the etching window used for membrane release. The techniques presented here are of practical interest to the design of membrane based devices with applications in microfluidic biosensors and flexible laser membranes.

Bound states in the continuum lasers (Conference Presentation)

In 1929, von Neumann and Wigner showed that Schrodinger’s equation can have, somewhat surprisingly, bound states above the continuum threshold [1]. These bound states represent the limiting case of quasi-bound states with an infinite lifetime, i.e., resonances that do not decay. It was recently realized that bound states in the continuum (BICs) are intrinsically a wave phenomenon and are thus not restricted to quantum mechanics. Since then, they have been shown to occur in many different fields of wave physics such as acoustics and photonics. In photonics’ terminology, BICs are eigenmodes of an open system with an infinite radiation quality factor, Qrad. To take advantage of this unique property to design high quality resonant cavities, most investigations have focused on dielectric structures that, unlike their plasmonic counterparts, are not limited by their material quality factor, Qmat [3-5]. To investigate the properties of BICs, various platforms have been used such as 1D gratings [3], waveguide arrays [4], and 2D photonic crystal slabs [5]. In this contribution, we have designed a high quality cavity based on a BIC and harnessed its novel properties to achieve a compact low-threshold nanophotonic laser. [1] J. von Neumann and E. Wigner, “On some peculiar discrete eigenvalues” Phys. Z, 465 (1929). [2] C. Linton et al., “Embedded trapped modes in water waves and acoustics” Wave Motion 45, 16 (2007). [3] D. C. Marinica et al., “Bound states in the continuum in photonics” Phys. Rev. Lett. 100, 183902 (2008). [4] Y. Plotnik et al., “Experimental observation of optical bound states in the continuum” Phys. Rev. Lett. 107, 183901 (2011). [5] C. W. Hsu et al., “Observation of trapped light within the radiation continuum” Nature 499, 188 (2013).

Integrated and steerable vortex laser using bound states in continuum (Conference Presentation)

Steering the beam of a wave source has been demonstrated using mechanical and non-mechanical techniques. While mechanical techniques are bulky and slow, non-mechanical techniques rely on breaking the symmetry of the refractive index profile either using asymmetric structure or injecting a non-uniform current. In this contribution, we theoretically and experimentally demonstrated a new type of topological steering of light sources in which the phase offset is provided by Floquet-Bloch phase in periodic structure. It was shown that in periodic structures, there exist singular states in the radiation region of the band diagram that exhibit diverging quality factor. Thus light sources can operate at these states with lower power threshold. The existence of these singular states are topologically protected, and their momentum are very sensitive to any small perturbations, which is used to control the steering angle. By uniformly controlling some parameters in the system, such as a physical dimension or injecting current uniformly, the beam of the light source steers. Our experimental demonstrations open new paradigm in the implementation of light steering with applications in data communications, bio imaging and sensing.

Novel lasing using bound states in the continuum

We have designed a high quality factor cavity that is based on a bound state in the continuum and harnessed its properties to demonstrate a novel type of surface emitting laser in the c-band (~1550nm). We have experimentally demonstrated lasing action in this compact nanophotonic laser at room temperature with a very low threshold power.

Cramér-Rao bounds for metasurfaces susceptibilities

Over the past fifteen years, a lot of efforts have been focused on understanding the effective properties of metamaterials [1]. In the last few years, metasurfaces in particular have been widely investigated [2]. Several homogenization methods dedicated to them have been proposed but, due to the topic’s complexity, none have yet to be widely accepted. We considered a specific homogenization method dedicated to metasurfaces, namely Generalized Sheet Transition Conditions (GSTC, [3]). This method was chosen because it is compatible with retrieval from reflection and transmission coefficients. In this method, metasurfaces are characterized by electric and magnetic susceptibilities. In the literature, retrieved effective parameters have been shown to violate causality around resonances and this has been attributed to spatial dispersion [4]. In order to determine if spatial dispersion is the only source of this phenomenon, we have investigated the statistical properties of estimators that have been put forward for these susceptibilities. We have thus computed the Cramér-Rao lower bounds on the variance of these estimators. We have shown that this bound increases substantially around resonances making retrieval possible only for very high Signal-to-Noise Ratio (SNR, [5]). Therefore, in experiments, issues arising from spatial dispersion and noise compound and result in non-physical effective parameters. To mitigate this, we have proposed a least-squares estimator for susceptibilities that has a better performance with respect to noise. Sensitivity to noise is particularly acute for low-loss metasurfaces. It often results in required SNRs that are unachievable in practice. The present work is thus relevant to the development of loss-compensated metasurfaces for which the issues posed by retrieval will have to be closely considered for accurate and robust device characterization.