Theoretical study about the gain in indirect bandgap semiconductor optical cavities

[EN] Indirect bandgap semiconductors such as silicon are not efficient light emitters because a phonon with a high momentum is required to transfer an electron from the conduction to the valence band. In a recent study (M.J. Chen et al., 2006) [6] an analytical expression of the optical gain in bulk indirect bandgap semiconductors was obtained. The main conclusion was that the free-carrier absorption was much higher than the optical gain at ambient temperature, which prevents lasing. In this work, we consider the case in which the semiconductor material is engineered to form an optical cavity characterized by a certain Purcell factor. We conclude that although the optical gain is increased, losses due to free carriers increase in the same way so lasing is also prevented even when creating a high-Q optical cavity. © 2012 Elsevier B.V. All rights reserved.This research has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement number 233883 (TAILPHOX). The authors wish to thank M.J. Chen for his useful comments.Escalante Fernández, JM.; Martínez Abietar, AJ. (2012). Theoretical study about the gain in indirect bandgap semiconductor optical cavities. Physica B: Condensed Matter. 407(12):2044-2049. https://doi.org/10.1016/j.physb.2012.02.002204420494071

Theoretical study of light and sound interaction in phoxonic crystal structures

En esta tesis se realiza un estudio teórico de la interacción luz-sonido en estructuras foxonicas, con las cuales es posible el control de la luz y el sonido a la misma vez. Esta interacción en dichas estructuras se estudia, tanto desde un punto de vista macroscópico (diseño de estructuras para el confinamiento y guiado de ondas electromagnéticas y elásticas) como microscópico (estudio de la interacción fotón-fonón en microcavidades y desarrollo teórico de modelos cuánticos para la comprensión de dicha interacción).Escalante Fernández, JM. (2013). Theoretical study of light and sound interaction in phoxonic crystal structures [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/33754TESI

Transient Photophysics of Bismuth Halide Semiconductors for Optoelectronic Applications

This thesis describes a series of spectroscopic studies of three different semiconductors based on bismuth halides in order to discover their fundamental photophysical properties. The materials investigated — the double perovskite Cs2AgBiBr6, bismuth iodide, and bismuth oxyiodide — have all been demonstrated in thin film photovoltaic devices fabricated at low temperatures. Propelled by the success of lead halide perovskites, this work forms part of the search for defect tolerant, non-toxic and stable materials for next-generation solar cells. I use transient absorption spectroscopy to show that the charge carrier lifetime in Cs2AgBiBr6 thin films is 1.4 μs. This is significantly longer than previous estimates based on time-resolved photoluminescence measurements, which measure the radiatively decaying carriers, but is a less sensitive probe of the high proportion of carriers which recombine non-radiatively. I propose a radiative recombination mechanism via defects, based on the detection of mid-gap electronic states in the transient absorption spectra. Coherent phonon transients are also measured on ultrafast timescales showing strong electron-phonon coupling in the material, which may contribute to the slow recombination in this material. The recombination dynamics of excitons in the layered material bismuth iodide, BiI3, are investigated using temperature dependent photoluminescence and transient absorption. I show that coupling to interlayer and intralayer phonon modes strongly affects the direct exciton decay pathway through scattering and modulation of the band gap energy. In a single crystal of BiI3, the room temperature exciton lifetime is 72 ns, which indicates this material’s potential for photovoltaics if the defects in thin films can be passivated. I show that the excited states of thin films of layered bismuth oxyiodide, BiOI, have a great degree of energetic disorder, most likely due to the presence of self-trapped excitons and defect states. Transient absorption measurements give an excited state lifetime of 47 ps, which would make efficient photovoltaic performance of BiOI thin film absorbers very unlikely. On the basis of my findings, all three materials show strong coupling between phonons and electronic states, but have excited state lifetimes varying over many orders of magnitude. This should guide the direction of future research towards different applications: the most promising candidate for photovoltaic applications is therefore Cs2AgBiBr6, whereas BiI3 and BiOI have more potential for use in nanostructured devices, such as ultrathin photodetectors, which can exploit their anisotropic nature

Optical properties of an ensemble of G-centers in silicon

We addressed the carrier dynamics in so-called G-centers in silicon (consisting of substitutional-interstitial carbon pairs interacting with interstitial silicons) obtained via ion implantation into a silicon-on-insulator wafer. For this point defect in silicon emitting in the telecommunication wavelength range, we unravel the recombination dynamics by time-resolved photoluminescence spectroscopy. More specifically, we performed detailed photoluminescence experiments as a function of excitation energy, incident power, irradiation fluence and temperature in order to study the impact of radiative and non-radiative recombination channels on the spectrum, yield and lifetime of G-centers. The sharp line emitting at 969 meV ($\sim$1280 nm) and the broad asymmetric sideband developing at lower energy share the same recombination dynamics as shown by time-resolved experiments performed selectively on each spectral component. This feature accounts for the common origin of the two emission bands which are unambiguously attributed to the zero-phonon line and to the corresponding phonon sideband. In the framework of the Huang-Rhys theory with non-perturbative calculations, we reach an estimation of 1.6$\pm$0.1 \angstrom for the spatial extension of the electronic wave function in the G-center. The radiative recombination time measured at low temperature lies in the 6 ns-range. The estimation of both radiative and non-radiative recombination rates as a function of temperature further demonstrate a constant radiative lifetime. Finally, although G-centers are shallow levels in silicon, we find a value of the Debye-Waller factor comparable to deep levels in wide-bandgap materials. Our results point out the potential of G-centers as a solid-state light source to be integrated into opto-electronic devices within a common silicon platform

Surface Plasmon Based Engineering of Semiconductor Nanowire Optics

Semiconductor nanowires combine the material properties of semiconductors, which are ubiquitous in modern technology, with nanoscale dimensions and as such, are firmly poised at the forefront of nanotechnology research. The rich physics of semiconductor nanowire optics, in particular, arises from the increased interaction between light and matter that occurs when light is confined to dimensions below the size of its wavelength, in other words, when the nanowire serves as a light trapping optical cavity, which itself is also a source of light. Light confinement is taken to new extremes by coupling to the surface plasmon modes of metallic nanostructures, where light acquires mixed photonic and electronic character, and which may focus light to deep-subwavelength regions amenable to the dimensions of the electron wave. This thesis examines how the integration of plasmonic optical cavities and semiconductor nanowires leads to substantial modification (and enhancement) of the optical properties of the same, resulting in orders-of-magnitude faster and more efficient light emission with colors that may be tuned as a function of optical cavity geometry. Furthermore, this method is applied to nanowires composed of both direct and indirect bandgap semiconductor materials resulting in applications such as light emission from high-energy states in light emitting materials, highly enhanced broadband light emission from nominally non-light emitting (dark) materials, and broadband (and anomalous) enhancement of light absorption in various materials, all the while maintaining the unifying theme of employing integrated plasmonic-semiconductor optical cavities to achieve tailored optical properties. We begin with a review of the electromagnetic properties of optical cavities, surface plasmon-enhanced light emission in semiconductors, and the key physical properties of semiconductor nanowires. It goes without saying that this thesis work resides at the interface between optical physics and materials science

Excitonic resonances in thin films of WSe2: From monolayer to bulk material

We present optical spectroscopy (photoluminescence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono- to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interaction as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral exciton upon increase of temperature is demonstrated.Comment: 12 pages, 5 figure

Engineering Phonon, Photon, Electron and Plasmon interactions in Silicon - Metal Nanocavitiies for Silicon Photonics and Thermoplasmonics

ENGINEERING PHONON, PHOTON, ELECTRON AND PLASMON INTERACTIONS IN SILICON - METAL NANOCAVITIIES FOR SILICON PHOTONICS AND THERMOPLASMONICS Daksh Agarwal Ritesh Agarwal, PhD Silicon photonics offers a cost effective solution to achieve ultrafast data processing speeds. But due to its indirect bandgap structure, making lasers from silicon is extremely difficult. Thus research has focused on nonlinear Raman processes in silicon as a method to achieve optical gain. Silicon nanowires provide an interesting platform for enhancing these nonlinearities because of their small size, geometry and relevant length scales. In the current work Raman measurements done on silicon nanowires reveal that up to twelvefold enhancement in Stokes scattering intensity and fourfold enhancement in anti Stokes scattering intensity can be attained depending on cavity structure and size, and excitation wavelength. In some cavities Stokes intensity depends on the sixth power of pump intensity, indicating extreme nonlinearity. Numerical calculations, done to understand the mechanism of these results indicate that silicon nanowires confine light to highly intense electric field modes inside the cavity which lead to stimulated Stokes and anti Stokes Raman scattering. Cavity modes can also be tuned to enhance the relative emission of either one of anti Stokes or Stokes photons which could enhance cavity cooling. These results would enable the development of smallest monolithically integratable silicon laser with extremely low lasing threshold and could lead to the development of next generation of high speed and energy efficient processors. The intense electric field inside the nanowire could also be used to enhance the degree of plasmon excitation in metallic nanoparticles. Silicon nanowires coated with a 10 nm thick gold film lead to strong plasmon excitation in gold and high cavity absorption which enable the cavity to heat up to temperatures of 1000K at relatively low pump powers. The cavities also give the ability to measure temperature attained during plasmon excitation and control the plasmon resonance wavelength. Because of the strong heating and plasmonic effects, these cavities show enhanced evolution rates of hydrogen, a crucial industrial building block and a promising fuel, in photoreforming reactions of alcohols

Reversible Modulation of Spontaneous Emission by Strain in Silicon Nanowires

We computationally study the effect of uniaxial strain in modulating the spontaneous emission of photons in silicon nanowires. Our main finding is that a one to two orders of magnitude change in spontaneous emission time occurs due to two distinct mechanisms: (A) Change in wave function symmetry, where within the direct bandgap regime, strain changes the symmetry of wave functions, which in turn leads to a large change of optical dipole matrix element. (B) Direct to indirect bandgap transition which makes the spontaneous photon emission to be of a slow second order process mediated by phonons. This feature uniquely occurs in silicon nanowires while in bulk silicon there is no change of optical properties under any reasonable amount of strain. These results promise new applications of silicon nanowires as optoelectronic devices including a mechanism for lasing. Our results are verifiable using existing experimental techniques of applying strain to nanowires

Fabrication and characterization of Silicon Nanowires

In this project work Si nanowires were fabricated on the Si substrate by aqueous method. In this aqueous method Ag is used for electroless chemical etching. The precursors those were taken are AgNO3, HF and H2O2. Si nanowires are fabricated at 55⁰C. The samples were characterized by X-ray diffraction and scanning electron microscope. Result shows morphology of the Si nanowires by scanning electron microscope. X-ray diffraction confirms the phase Si. The XRD analysis confirms the phase of silicon and crystallinity nature of silicon .It is found to be single crystalline with plane (1 0 0).The SEM study shows that the particles were uniform and afterwards the non uniformity arises. At 60 second of electroless deposition, the particles shape became anisotropic. Some of the particles have grown vertically. This kind of non uniform pattern can cause a non uniform distribution of Silicon nanowires. It is confirmed that the the morphology of the nanowires also depends on the resistivity of the wafers. The magnified HRTEM image shows the well-resolved lattice spacing of the silicon nanowire, which depicts the crystalline nature of the silicon nanowires