Babak Bahari

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.

Nonreciprocal lasing in topological cavities of arbitrary geometries

Topological lasing Resonant cavities that confine light are crucial components of lasers. Typically, these cavities are designed to high specification to get the best possible output. That, however, can limit their integration into photonic devices and optical circuits. Bahari et al. fabricated resonant cavities of arbitrary shape within a hybrid photonic crystal structure. The confinement of light to topologically protected edge states resulted in lasing at communication wavelengths. Relaxing the resonant cavity design criteria should be useful in designing photonic devices. Science, this issue p. 636 Resonant cavities of arbitrary shape can be designed to provide lasing into topically protected edge states. Resonant cavities are essential building blocks governing many wave-based phenomena, but their geometry and reciprocity fundamentally limit the integration of optical devices. We report, at telecommunication wavelengths, geometry-independent and integrated nonreciprocal topological cavities that couple stimulated emission from one-way photonic edge states to a selected waveguide output with an isolation ratio in excess of 10 decibels. Nonreciprocity originates from unidirectional edge states at the boundary between photonic structures with distinct topological invariants. Our experimental demonstration of lasing from topological cavities provides the opportunity to develop complex topological circuitry of arbitrary geometries for the integrated and robust generation and transport of photons in classical and quantum regimes.

Topological terahertz circuits using semiconductors

Topological insulator-based devices can transport electrons/photons at the surfaces of materials without any back reflections, even in the presence of obstacles. Topological properties have recently been investigated using non-reciprocal materials, such as gyromagnetics, or using bianisotropy. However, these effects usually saturate at the optical frequencies and limit our ability to scale down the devices. In order to implement topological devices that we introduce in this paper for the terahertz range, we show that the semiconductors can be utilized via their cyclotron resonance in combination with small magnetic fields. We propose two terahertz operating devices such as the topological tunable power splitter and the topological circulator. This work opens up the perspectives in designing the terahertz integrated devices and circuits with high functionality.

Directive and enhanced spontaneous emission using shifted cubes nanoantenna

Recent studies have demonstrated that nano-patch antennas formed by metallic nanocubes placed on top of a metallic film largely enhance the spontaneous emission rate of quantum emitters due to the confinement of the electromagnetic field in the small nanogap cavity. The popularity of this architecture is, in part, due to the ease in fabrication. In this contribution, we theoretically demonstrate that a dimer formed by two metallic nanocubes embedded in a dielectric medium exhibits enhanced emission rate compared to the nano-patch antenna. Furthermore, we compare the directivity and radiation efficiency of both nanoantennas. From these characteristics, we obtained information about the “material efficiency” and the coupling mismatch efficiency between a dipole emitter and the nanoantenna. These quantities provide a more intuitive insight than the Purcell factor or localized density of states, opening new perspectives in nanoantenna design for ultra-directive light emission.

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.

Functional topological THz devices using semiconductors

We showed that cyclotron resonance of semiconductors can be utilized in Topological devices to break the time-reversal symmetry for unidirectional propagation in THz. To demonstrate, we proposed a tunable power splitter based on topological effect.

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.