Chase T. Ellis

Spatially indirect radiative recombination in InAlAsSb grown lattice-matched to InP by molecular beam epitaxy

A photoluminescence (PL) spectroscopy study of the bulk quaternary alloy InAlAsSb is presented. Samples were grown lattice-matched to InP by molecular beam epitaxy and two different growth temperatures of 450 °C and 325 °C were compared. Interpolated bandgap energies suggest that the development of this alloy would extend the range of available direct bandgaps attainable in materials lattice-matched to InP to energies as high as 1.81 eV. However, the peak energy of the observed PL emission is anomalously low for samples grown at both temperatures, with the 450 °C sample showing larger deviation from the expected bandgap. A fit of the integrated PL intensity (I) to an I∝Pk dependence, where P is the incident power density, yields characteristic coefficients k = 1.05 and 1.18 for the 450 °C and 325 °C samples, respectively. This indicates that the PL from both samples is dominated by excitonic recombination. A blue-shift in the peak emission energy as a function of P, along with an S-shaped temperature depe...

Active tuning of surface phonon polariton resonances via carrier photoinjection

Surface phonon polaritons (SPhPs) are attractive alternatives to infrared plasmonics for subdiffractional confinement of infrared light. Localized SPhP resonances in semiconductor nanoresonators are narrow, but that linewidth and the limited extent of the Reststrahlen band limit spectral coverage. To address this limitation, we report active tuning of SPhP resonances in InP and 4H-SiC by photoinjecting free carriers into nanoresonators, taking advantage of the coupling between the carrier plasma and optic phonons to blueshift SPhP resonances. We demonstrate state-of-the-art tuning figures of merit upon continuous-wave excitation (in InP) or pulsed excitation (in 4H-SiC). Lifetime effects cause the tuning to saturate in InP, and carrier redistribution leads to rapid (<50 ps) recovery of the resonance in 4H-SiC. This work demonstrates the potential for this method and opens a path towards actively tuned nanophotonic devices, such as modulators and beacons, in the infrared, and identifies important implications of coupling between electronic and phononic excitations.Infrared surface phonon polariton tuning is achieved by photoinjecting free carriers into resonators.

Resonant quantum efficiency enhancement of midwave infrared nBn photodetectors using one-dimensional plasmonic gratings

We demonstrate up to 39% resonant enhancement of the quantum efficiency (QE) of a low dark current nBn midwave infrared photodetector with a 0.5 μm InAsSb absorber layer. The enhancement was achieved by using a 1D plasmonic grating to couple incident light into plasmon modes propagating in the plane of the device. The plasmonic grating is composed of stripes of deposited amorphous germanium overlaid with gold. Devices with and without gratings were processed side-by-side for comparison of their QEs and dark currents. The peak external QE for a grating device was 29% compared to 22% for a mirror device when the illumination was polarized perpendicularly to the grating lines. Additional experiments determined the grating coupling efficiency by measuring the reflectance of analogous gratings deposited on bare GaSb substrates.

The effect and nature of N–H complexes in the control of the dominant photoluminescence transitions in UV-hydrogenated GaInNAs

Due to its 1 eV band gap and GaAs-matched lattice constant, GaInNAs has long been considered for use in four-junction multi-junction solar cells; but, material quality issues have impeded its use in highly efficient devices. Here, we present an analysis of GaInNAs samples partially hydrogenated via a UV-activated process in which nitrogen-related alloy fluctuations, impurities, and defects have been passivated; remarkably, removing completely the ‘s-shape’ dependence of the photoluminescence while keeping intact the effects of nitrogen substitution, i.e., the band gap of the alloy prior to passivation. Hydrogenation of the optical samples by a UV-activated process has resulted in GaInNAs photoluminescence dominated by the free-excitonic band gap transition, rather than radiative recombination processes from the shallow localized centers that result due to unavoidable alloy fluctuations. This behavior is unique since these centers dominate the low temperature photoluminescence even in the highest quality dilute nitrides. Density functional theory calculations show that the hydrogenation of the N and Ga atoms eliminates the defect levels from the band gap through the formation of H–N centers that act as donors; while at high concentration of hydrogen, Ga–H2–N complexes reside within the continuum. The formation of these hydrogen complexes, along with the incumbent change of the band structure, explains the reduction of emission from the localized centers upon hydrogenation.

The role of N-H complexes in the control of localized center recombination in hydrogenated GaInNAs (Conference Presentation)

A significant improvement in the quality of dilute nitrides has recently led to the ability to reveal depletion widths in excess of 1 μm at 1 eV [1]. The real viability of dilute nitrides for PV has been recently demonstrated with the reporting of a record efficiency of 43.5% from a 4J MJSC including GaInNAs(Sb) [2]. Despite the progress made, these materials remain poorly understood and work continues to improve their lifetime and reproducibility. We have investigated the possibility of improving the functionality of GaInNAs using hydrogenation to selectively passivate mid-gap defects, while preserving the substitutional nitrogen. Temperature dependent photoluminescence measurements of the intrinsic region of a GaInNAs p-i-n solar cell show a classic “s-shape” associated with localization prior to hydrogenation. No sign of this “s-shape” is evident after hydrogenation, despite the retention of substitutional nitrogen as evidenced by the band absorption of 1 eV. The absence of an “s-shape” at low-temperature in hydrogenated GaInNAs is rather curious since, even in high quality nitrides this behavior is due to the emission of isoelectronic centers created via N-As substitution [3]. The potential origins of this behavior will be discussed. The promise of this process for GaInNAs solar cells was demonstrated by a three-fold improvement in the performance of a hydrogenated device with respect to an as-grown reference [4]. [1] “Wide-depletion width GaInNAs solar cells by thermal annealing,” I. R. Sellers, W-S. Tan, K. Smith, S. Hooper, S. Day and M. Kauer, Applied Physics Letters 99, 151111 (2011) [2] “43.5% efficient lattice matched solar cells,” M. Wiemer, V. Sabnis, and H. Yuen, Proc. SPIE 8108, 810804 (2011) [3]“Probing the nature of carrier localization in GaInNAs, epilayers using optical methods,” T. Ysai, B. Barman, T. Scarce, G. Lindberg, M. Fukuda, V. R. Whiteside, J. C. Keay, M. B. Johnson, I. R. Sellers, M. Al Khalfioui, M. Leroux, B. A. Weinstein and A. Petrou. Applied Physics Letters 103, 012104 (2013) [4] “Improved performance in GaInNAs solar cells by hydrogen passivation by hydrogen passivation,” M. Fukuda, V. R. Whiteside, J. C. Keay, A. Meleco, I. R. Sellers, K. Hossain, T. D. Golding, M. Leroux, and M. Al Khalfioui, Applied Physics Letters 106, 141904 (2015)

Strong Coupling Effects Between IR-Inactive Zone Folded LO Phonon and Localized Surface Phonon Polariton Modes in SiC Nanopillars

While plasmonics have a broad range of technological applications including infrared photovoltaics and photodetectors, plasmonic metals are subject to high optical losses in the long-wave infrared spectral regime. In order to reduce optical losses in the infrared, alternatives to plasmonic metals are being explored. One promising alternative employs polar dielectric materials, which exhibit a highly-reflective, optically-metallic spectral band (Reststrahlen band), bounded by the LO and TO optical phonons, and are capable of supporting plasmonic-like resonance in the infrared. In polar dielectrics, plasmonic-like resonances, known as surface phonon polariton (SPhP) resonances, arise from a coupling between incident light and collective oscillations of bound lattice charges, which are mediated by the optical phonons. In this study, we have examined the SPhP resonances of SiC nanopillars with constant height of 950 nm and width in the range of 200–400 nm, as a function of their aspect ratio (AR=Length/Width=0.5–16). As the nanopillar width is decreased, we have found that localized SPhP resonances redshift towards the zone folded LO (ZFLO) phonon that is normally not infrared active. However, as localized SPhP resonances are spectrally tuned through the ZFLO mode, we have found that the latter mode becomes infrared active. Furthermore, reflectance measurements have revealed strong coupling between the ZFLO and both the monopolar and dipolar localized SPhP resonances.

Symmetry Breaking and Active Fano Resonance Tuning in Dolmen Nanostructures

Polar-dielectrics have garnered a lot of attention as an alternative to the metal plasmons to support sub-diffractional confinement of light in the mid-infrared to THz regime due to their very low optical losses (Caldwell JD, Lindsay L, Giannini V, Vurgaftman I, Reinecke TL, Maier SA, Glembocki OJ: Nanophotonics 4(44), 2015). The surface phonon polariton (SPhP) in SiC, a polar dielectric, is formed when the optical phonon vibration in the crystal lattice couple with the incident radiation. Recently coupled phononic SiC nanostructures have received enormous attention both in basic research and applied research (Caldwell JD, Glembocki OJ, Francescato Y, Sharac N, Giannini V, Bezares FJ, Long JP, Owrutsky JC, Vurgaftman I, Tischler JG, Wheeler VD, Bassim ND, Shirey LM, Kasica R, Maier SA: Nano Lett 13:3690, 2013; Ellis CT, Tischler JG, Glembocki OJ, Bezares FJ, Giles AJ, Kasica R, Shirey LM, Owrutsky JC, Chigrin DN, Caldwell JD: Sci Rep 6(32959), 2016). In this work, we fabricate a metasurface in the form of SiC nanostructured dolmen structures from a 4H-SiC substrate, which enables creation of new optical modes via periodicity induced symmetry breaking that are active in the Reststrahlen band region (window between the transverse and longitudinal optical phonons). The polarisation dependent far field studies reveal that the nanostructures exhibit very strong dipolar and quadrupolar resonances and the interaction between the radiant, broader dipolar modes with the narrow, sub-radiant quadrupolar modes gives rise to a unique asymmetric Fano type of resonance. By controlling the geometry and spacing of the features in the dolmen we notice that the resonances can be highly tunable, resulting in exceptional optical properties highly desirable to enhance the performance of various nanophotonic applications.

Low-Loss Phonon Polaritons in Nanostructured Dielectrics

Plasmonics provides great promise for nanophotonic applications. However, the high optical losses inherent in metal-based plasmonic systems have limited progress. Thus, it is critical to identify alternative low-loss materials. One alternative is polar dielectrics that support surface phonon polariton (SPhP) modes, where the confinement of infrared light is aided by optical phonons. SiC nanopillar arrays support such modes, exhibiting a dipolar resonance transverse to the nanopillar axis and a monopolar resonance associated with the longitudinal axis dependent upon the SiC substrate. Both exhibit exceptionally narrow linewidths (7–24 cm−1), with quality factors of 40–135, which exceed the theoretical limit of plasmonic systems, with extreme subwavelength confinement of (λres3/Veff)1/3 = 50–200. These observations promise to reinvigorate research in SPhP phenomena and their use for nanophotonic applications. Another approach is the use of hyperbolic materials, which have been a focus of the nanophotonics community for their potential to realize sub-diffractional imaging and focusing of light, and novel optical properties, such as a negative index of refraction. The recent observation that hexagonal boron nitride (hBN) is a natural, high efficiency hyperbolic material has led to a surge in research within this field. Due to the low-loss nature, van der Waals bonding and extreme crystal anisotropy, the hyperbolic polaritons within hBN are not only promising for novel applications within the mid-infrared, but is also extremely well suited for fundamental investigations into their resonant behaviors. We have used scattering near-field optical microscopy (s-SNOM) to directly probe the local surface electromagnetic fields of three-dimensionally confined nanostructures of hBN, reporting the first experimental observation of frequency dependent internal angular reflection within a hyperbolic nanostructure, a phenomenon previously theoretically predicted.

High-Order Multipole Resonances in Cuboidal Surface Phonon Polariton Nanoresonators

It has been demonstrated that nanoresonators fabricated on the surface of polar dielectric materials, such as silicon carbide, are able to sustain plasmonic-like effects in the mid- to long- wave infrared spectral range with impressive figures of merit (Hillenbrand R, Taubner T, Keilmann F, Nature 418:159–162, 2002; Caldwell JD, Glembocki OJ, et al., Nano Lett 13:3690–3697, 2013; Wang T, Li P, et al., Nano Lett 13:5051–5055, 2013). Such phenomena is achieved by exploiting the TO and LO phonons to resonantly excite collective oscillations of bound lattice (Caldwell JD, Lindsay L, et al., Nanophotonics 4:2192–8614, 2015). The fact that these excitations are mediated by bound charges, rather than free charges - such as the case with plasmonic metals, results in extremely low optical losses and enhanced resonant phenomena. As such, polar dielectric nanoresonators may play a role in improving infrared nanophotonic technologies, such as waveguides, sources, near-field optics, solar cells, chemical sensors, biosensors, and photonic circuitry. However, fully realizing this potential, hinges on the ability to precisely control the near-field behavior of polar dielectric nanoresonators. In this work, we use a combination of optical measurements and finite element method simulations to investigate the far- and near-field resonant behavior of structurally-asymmetric, cuboidally-shaped, 4H-SiC nanoresonators with fixed height (\( h=950\ \mathrm{nm} \)), fixed length (\( l=400\ \mathrm{nm} \)), and varying width (\( w=400-6400\ \mathrm{nm} \)). Overall, we observe over 12 polarization-sensitive resonances that can be tuned across the Reststrahlen band of 4H-SiC (\( 796-964\ {\mathrm{cm}}^{-1} \)) (Ellis, C.T. et al. Scientific Reports 6:32959, 2016) by changing the nanopillar aspect ratio (\( AR=w/l=1-16 \)). Futhermore, we find that these resonances exhibit a wide range of near-field radiation patterns that vary from a simple transverse dipole mode that is preserved for all ARs to complex, high-order multipoles with modal profiles that evolve with aspect ratio.

Sub-diffractional, volume-confined polaritons in a natural hyperbolic material: hexagonal boron nitride (Presentation Recording)

Strongly anisotropic media where principal components of the dielectric tensor have opposite signs are called hyperbolic. These materials permit highly directional, volume-confined propagation of slow-light modes at deeply sub-diffractional size scales, leading to unique nanophotonic phenomena. The realization of hyperbolic materials within the optical spectral range has been achieved primarily through the use of artificial structures typically composed of plasmonic metals and dielectric constituents. However, while proof-of-principle experiments have been performed, the high plasmonic losses and inhomogeneity of the structures limit most advances to the laboratory. Recently, hexagonal boron nitride (hBN) was identified as a natural hyperbolic material (NHM), offering a low-loss, homogeneous medium that can operate in the mid-infrared. We have exploited the NHM response of hBN within periodic arrays of conical nanoresonators to demonstrate ‘hyperbolic polaritons,’ deeply sub-diffractional guided waves that propagate through the volume rather than on the surface of a hyperbolic material. We have identified that the polaritons are manifested as a four series of resonances in two distinct spectral bands that have mutually exclusive dependencies upon incident light polarization, modal order, and aspect ratio. These observations represent the first foray into creating NHM building blocks for mid-infrared to terahertz nanophotonic and metamaterial devices. This talk will also discuss potential near-term applications stemming from these developments.