Metasurface Light-Emitting Diodes with Directional and Focused Emission.

Phased-array metasurfaces enable the imprinting of complex beam structures onto coherent incident light. Recent demonstrations of photoluminescent phased-array metasurfaces highlight possibilities for achieving similar control in electroluminescent light-emitting diodes (LEDs). However, phased-array metasurface LEDs have not yet been demonstrated owing to the complexities of integrating device stacks and electrodes within nanopatterned metasurfaces. Here, we demonstrate metasurface LEDs that emit directional or focused light. We first design nanoribbon elements that achieve the requisite phase control within typical LED device constraints. Subsequently, we demonstrate unidirectional emission that can be engineered at will via phased-array concepts. This control is further exhibited in metasurface LEDs that directly emit focused beams. Finally, we show that these metasurface LEDs exhibit external quantum efficiencies (EQEs) superior to those of unpatterned LEDs. These results demonstrate metasurface designs that are compatible with high-EQE metal-free LED devices and portend opportunities for new classes of metasurface LEDs that directly produce complex beam structures.

Designing Highly Directional Luminescent Phased-Array Metasurfaces with Reciprocity-Based Simulations.

Phased-array metasurfaces grant the ability to arbitrarily shape the wavefront of light. As such, they have been used as various optical elements including waveplates, lenses, and beam deflectors. Luminescent metasurfaces, on the other hand, have largely comprised uniform arrays and are therefore unable to provide the same control over the wavefront of emitted light. Recently, phased-array control of the wavefront of spontaneous emission has been experimentally demonstrated in luminescent phased-array metalenses and beam deflectors. However, current luminescent metasurface beam deflectors exhibit unidirectional emission for only p-polarized light. In this paper, we use a reciprocal simulation strategy to explain the polarization disparity and improve the directionality of incoherent emission from current quantum-well emitting phased-array metasurfaces. We also design complementary metasurfaces to direct emission from systems where emission originates from alternate quantum mechanical processes.

Ex Vivo Performance of a Flexible Microwave Ablation Antenna.

OBJECTIVE: In this study, we investigate the performance of a flexible microwave ablation antenna for generating localized ablation zones. METHODS: We designed a helical dipole antenna to operate at 1.9 GHz in egg white and liver. Semi-rigid prototypes of the antenna were fabricated and used to perform ablation experiments in egg white and perfused liver. Pulsed and continuous-wave power deliveries at different power levels were used. Flexible prototypes of the antenna were fabricated and used to perform ex vivo ablation experiments in perfused liver. RESULTS: Pulsing was effective in reducing the shaft heating of semi-rigid cables. The antenna was capable of producing substantial ablation zones in perfused liver. Typical diameters (perpendicular to the antenna axis) of generated ablation zones with semi-rigid antennas in egg white and perfused liver were 30 mm and 20 mm, respectively. The flexible antenna had a good impedance match while bent. Average diameter of generated ablation zones by the flexible antenna with 10-W continuous-wave experiments in perfused liver was 26 mm. No significant difference was observed between the performances of semi-rigid and flexible prototypes. CONCLUSION: The flexible helical dipole antenna is capable of maintaining its good impedance match while bent and can generate substantial ablation zones in presence of perfusion. SIGNIFICANCE: The proposed flexible antenna is promising for minimally invasive treatment of tumors that are otherwise inaccessible by rigid antennas. One example is lung where a catheter-based deployment of the flexible antenna into the tumor via airways may substantially reduce risks associated with using rigid antennas.

Light-emitting metalenses and meta-axicons for focusing and beaming of spontaneous emission.

Phased-array metasurfaces have been extensively used for wavefront shaping of coherent incident light. Due to the incoherent nature of spontaneous emission, the ability to similarly tailor photoluminescence remains largely unexplored. Recently, unidirectional photoluminescence from InGaN/GaN quantum-well metasurfaces incorporating one-dimensional phase profiles has been shown. However, the possibility of generating arbitrary two-dimensional waveforms-such as focused beams-is not yet realized. Here, we demonstrate two-dimensional metasurface axicons and lenses that emit collimated and focused beams, respectively. First, we develop off-axis meta-axicon/metalens equations designed to redirect surface-guided waves that dominate the natural emission pattern of quantum wells. Next, we show that photoluminescence properties are well predicted by passive transmission results using suitably engineered incident light sources. Finally, we compare collimating and focusing performances across a variety of different light-emitting metasurface axicons and lenses. These generated two-dimensional phased-array photoluminescence waveforms facilitate future development of light sources with arbitrary functionalities.

The Performance of Higher Frequency Microwave Ablation in the Presence of Perfusion.

OBJECTIVE: In this paper, we investigate the impact of perfusion on the performance of microwave ablation across a large frequency range. METHODS: We designed multiple microwave ablation antennas to operate in liver tissue at discrete frequencies in the range 1.9-18 GHz. We performed electromagnetic simulations to calculate microwave power absorption patterns. Five-minute, 25 W ablation experiments were performed at each frequency in perfused and nonperfused ex vivo porcine livers, and thermal lesion dimensions were measured. RESULTS: The volume of greatest microwave power absorption shrinks by two orders of magnitude as the frequency is increased from 1.9 to 18 GHz. Mean thermal lesion volumes are consistent across the frequency range for a given perfusion state and are about three times smaller under active perfusion. Typical thermal lesion diameters (perpendicular to the antenna axis) were 24 mm and 16 mm for nonperfused and perfused ablations, respectively. No significant differences in axial ratio were observed among different frequency groups in active-perfusion experiments. CONCLUSION: Higher-frequency microwave ablation produces thermal lesions with volumes comparable to those achieved at lower frequencies, even in strongly perfused environments. SIGNIFICANCE: Higher-frequency microwave ablation is appealing because it allows for more flexibility in antenna design. A critical issue concerning the feasibility of higher frequency microwave ablation, considering its strong dependence on heat diffusion to grow thermal lesions, is its performance in strongly perfused environments. This paper shows that higher frequency microwave ablation achieves thermal lesions comparable to those from microwave ablation performed at conventional frequencies in both non- and strongly perfused environments.