Thermo-optic single-plasmon blockade in graphene nanoislands (Conference Presentation)

Terahertz light is electromagnetic radiation, similar to visible light. The photons that the terahertz light is comprised of carry a much smaller amount of energy compared to the visible light photons. Unlike visible light, terahertz light can pass through materials like plastic, cardboards, wood etc.; a very useful property which enables it to replace harmful X-rays in many security applications. However, it is not possible to see the terahertz photons with our naked eyes, and it requires special detectors to observe them. A lot of attention has been drawn to terahertz radiation recently because of its potential use in various applications in national security (as mentioned before), and in the biomedical and the semiconductor industries. Essential to any terahertz device is a suitable terahertz source. There are different methods to generate this type of radiation. After the advent of ultrafast lasers, an optical technique was developed which became very popular afterwards. In very simple terms, this technique can be considered as producing an extremely quick disturbance in a suitable material using an extremely quick flash of laser light. Here the phrase `extremely quick' refers to femtosecond time scales where one femtosecond is one millionth of one billionth of a second. The quick electromagnetic disturbance can lead to the emission of a pulse of electromagnetic radiation of a different frequency: terahertz light. Certainly, this process depends on the material in which the disturbance is created, which we will see in a bit more detail below. It is this method of terahertz generation we focus on in this thesis. Let us now have a closer look at this. Only certain materials have this property of converting a flash of laser light efficiently into a flash of terahertz light, for example, some semiconductors. What type of a disturbance can a flash of laser light, (a laser pulse), create in such a material? In the case of semiconductors, the incident light pulse can lead to the excitation of mobile conduction electrons by providing them with the required energy. The semiconductor becomes momentarily a conductor. If it was initially kept under an external voltage bias, a momentary current is thus induced by the light pulse. A time-varying current can act as a source of electromagnetic waves. The emitted flash of light in this case is a terahertz pulse. Similar momentary disturbances can also be produced in certain nonlinear crystals without really exciting electrons from their bound states, but by causing an ultrafast displacement of the bound charges. In both these cases, the emitted light pulse carries information about the material's response to the femtosecond flash of light, which in fact is information about the material per se. For example, we see that the illumination of graphite with femtosecond laser pulses results in the emission of terahertz light pulses. The properties of the emitted terahertz pulse are suggestive of a transient photocurrent produced in the material. Graphite consists of stacks of atomic planes of carbon which are loosely attached to each other. Electric conductivity along a direction perpendicular to these planes is known to be very low as in this case electrons have to jump from one plane to the other. However, in our experiments the emitted terahertz pulses indicate a resultant photocurrent flowing in that direction. Oxidized copper surfaces are known to act as a semiconductor-diode. A semiconductor diode is a device which restricts the electric current to flow through it in only one direction. In the case of oxidized copper surfaces, this is possible by a potential barrier formed at the interface between copper and cuprous oxide. When a femtosecond light pulse excites electrons at such an interface, and frees electrons in it, a quick pulse of current flows across the interface. This transient current emits a terahertz pulse. The same idea can also be applied to different other semiconductor-metal interfaces. We have shown that terahertz pulses can be produced by exciting thin films of germanium and silicon deposited on a gold substrate. If the thin films of these semiconductors prepared on a glass substrate are illuminated with femtosecond light pulses, the emitted terahertz pulses are very feeble. When the thin films of the semiconductors are on a gold substrate, a surprising enhancement of the generated terahertz light from such thin films is observed. The later part of the thesis concentrates on the different possible ways in which the gold substrate can contribute to the enhancement of terahertz radiation from thin films. When coherent laser light is incident on an extremely thin film of a semiconductor material deposited on a metal surface, light reflected from the top and the bottom of the film can result in a complete or partial reduction of the reflected light. It is equivalent to trapping the light inside the film, which leads to enhanced absorption in the thin film. This is sometimes called `coherent optical absorption'. Very strong absorption of the pump light can be achieved in thin films as compared to bulk materials, as a result of this. When light is strongly absorbed by the semiconductor, more electrons will be freed and a stronger transient current can be produced which can result in a stronger terahertz emission. This leads to the counter-intuitive result that less material emits more terahertz light. The concentration of laser light inside the terahertz generation material can also be done by making use of surface plasmon excitation. Surface plasmons are light waves bound to the interface between a metal and a dielectric. In our case, since the terahertz generation takes place at the interface between a metal and a semiconductor, surface plasmons can play a role in the process. As surface plasmons are bound to the interface, they can enhance the local intensity of the pump light at the interface where the generation of terahertz radiation takes place. Using this method, we demonstrate the enhancement of terahertz emission from a layer as thin as a single molecular layer of a nonlinear optical material called hemicyanine. We also go on to show that enhanced terahertz emission can be achieved from semiconductors deposited on a nanostructured metal surface, where again surface plasmons are excited. Concentration of the laser light intensity using plasmonics also leads to terahertz emission from the metal surfaces itself, i.e., without any semiconductor on top. In short, this thesis discusses the possibilities of terahertz generation from ultrathin semiconductor layers, metals and their interfaces, and on different optical techniques to enhance the terahertz emission. These techniques not only help in the study of the properties of ultrathin layers of materials, but can also help in miniaturizing terahertz sources for various applications.

Multi-fluid modeling of transient plasmas : a case study in the generation and guiding of light

Objectives: In this article, a new procedure for increasing the light penetration depth in a tissue is studied and simulated. It has been reported that the most important problem in biomedical optical imaging relates to the light penetration depth, and so this makes a dramatic restriction on its applications. In the optical imaging method, the detection of the backscattered photons from a deep tumor is rarely done or is done with a low efficiency; it is because of the high absorption and scattering losses. Methods: Unlike the common methods (using a high energy laser for deep penetration) by engineering the nanoparticles’ optical properties such as their anisotropy, absorption, and scattering efficiency, which are distributed into a tissue, the detected photons amplitude can be manipulated. In other words, by engineering the nanoparticle plasmon properties and their effect on the dye molecules’ quantum yield, fluorescence emission and more importantly influence on the scattering direction, the light penetration depth is dramatically increased. Results: The modeling results (Monte-Carlo statistical method) illustrate that the detected photons dramatically increased which is on order of 4 mm. So, this method can fix the light penetration problems in the optical imaging system. Conclusions: Finally, the original idea of this study attributes to the indirect and transient manipulation of the optical properties of the tissue through the nanoparticles plasmon properties engineering. Moreover, by engineering plasmonic nanoparticles, maybe, the penetration depth can be enhanced which means that we can easily send light into a soft tissue and get its back scattering.

The present work is devoted to the study of the dynamics of multi-photon processes in semiconductor heterostructures. A time-dependent description is important for understanding in detail the transient response of semiconductors excited by ultrashort optical pulses. In the first part of this thesis, we set up a phenomenological model based on rate equations, in order to investigate the possibility of measuring degenerate two-photon gain in a semiconductor microcavity. The amplification predicted by the model is fairly low ( 2%) and mainly limited by the intra-band relaxation of the carriers, which leads to rapid saturation. In the second part, we develop a general theory for the dynamics of multi-photon processes in semiconductors. It will give insight into complex effects related to the coherence between the bands, which are not included in usual absorption coefficients or susceptibilities. For this purpose, we derive effective multi-band Bloch equations that include resonant multi-photon processes induced by two linearly polarized electromagnetic pulses of frequency close to the band gap and close to the half of the band gap respectively. The benefit of the proposed approach is two-fold. First, the description of the dynamics is restricted to a reduced number of bands. However, the discarded bands are not neglected, but consistently taken into account in the higher order processes. Second, all quantities appearing in the effective multi-band Bloch equations vary on the same time scale, which makes the numerical integration much more efficient. The time-dependent polarization current, as well as some susceptibilities, are derived on a consistent level of approximation, and are discussed in detail. The propagation of the electromagnetic fields is neglected. Such a model is appropriate for the description of low-dimensional quantum confined systems (e.g. quantum wells or quantum wires) excited by two colinearly propagating pulses. It accounts for various linear and nonlinear optical processes, such as one- and two-photon absorption, second-harmonic generation, difference-frequency mixing, or coherent control of photocurrent. In this thesis, the general theory is applied to the study of three specific physical situations. First, we investigate the charge and spin current in a symmetric AlGaAs/GaAs quantum well, injected by interference between one- and two-photon inter-band transitions. We identify new coherent terms in the expression of the current, which contribute significantly to the terahertz emission. The effects of the Stark shifts and the inter-valence band two-photon transitions are also calculated and discussed. Second, we calculate the anisotropic two-photon absorption spectra of an AlGaAs/GaAs V-shaped quantum wire with realistic band structure. The Coulomb interaction is taken into account within the Hartree-Fock approximation. The various excitonic peaks are identified with respect to the involved subbands and to the symmetry properties. We also show that excitons that are dark for one-photon excitation may become bright for two-photon spectroscopy, when the light is polarized perpendicularly to the growth direction, but not along a symmetry axis of the wire. Finally, the last application focuses on the optical injection of current in the presence of excitonic effects. Concentrating on the same AlGaAs/GaAs V-shaped quantum wire, we show that the Coulomb interaction within the Hartree-Fock approximation induces terahertz oscillations in the injected charge current. The oscillation frequency corresponds to the energy spacing between the two lowest excitonic resonances, slightly below the band gap, excited respectively by the laser pulse with frequency close to the band gap, and the one with frequency close to the half of the band gap.

Singlet Exciton Fission and Photochemical Upconversion

While plasmons in noble metal nanostructures enable strong light-matter interactions on commensurate length scales, the overabundance of free electrons in these systems inhibits their tunability by weak external stimuli. Countering this limitation, the linear electronic dispersion in graphene endows the two-dimensional material with both an enhanced sensitivity to doping electron density, enabling active tunability of its highly confined plasmon resonances, and a very low electronic heat capacity that renders its thermo-optical response extraordinarily large. Here we show that these properties combined enables a substantial optical modulation in graphene nanostructures from the energy associated with just one of their supported plasmons. We base our analysis on realistic, complementary classical and quantum-mechanical simulations, which reveal that the energy of a single plasmon, absorbed in a small, moderately doped graphene nanoisland, can sufficiently modify its electronic temperature and chemical potential to produce unity-order modulation of the optical response within subpicosecond time scales, effectively shifting or damping the original plasmon absorption peak and thereby blockading subsequent excitation of a second plasmon. The proposed thermo-optical single-plasmon blockade consists in a viable ultralow power all-optical switching mechanism for doped graphene nanoislands, while their combination with quantum emitters could yield applications in biological sensing and quantum nano-optics.

Integration of microwave heating with continuously operated milli-reactors for fine chemical synthesis

Investigating DNA-Mediated Charge Transport by Time-Resolved Spectroscopy

Surface plasma waves arise from the collective oscillations of billions of electrons at the surface of a metal in unison. The simplest way to quantize these waves is by direct analogy to electromagnetic fields in free space, with the surface plasmon, the quantum of the surface plasma wave, playing the same role as the photon. It follows that surface plasmons should exhibit all of the same quantum phenomena that photons do, including quantum interference and entanglement. Unlike photons, however, surface plasmons suffer strong losses that arise from the scattering of free electrons from other electrons, phonons, and surfaces. Under some circumstances, these interactions might also cause “pure dephasing,” which entails a loss of coherence without absorption. Quantum descriptions of plasmons usually do not account for these effects explicitly, and sometimes ignore them altogether. In light of this extra microscopic complexity, it is necessary for experiments to test quantum models of surface plasmons. In this thesis, I describe two such tests that my collaborators and I performed. The first was a plasmonic version of the Hong-Ou-Mandel experiment, in which we observed two-particle quantum interference between plasmons with a visibility of 93 ± 1%. This measurement confirms that surface plasmons faithfully reproduce this effect with the same visibility and mutual coherence time, to within measurement error, as in the photonic case. The second experiment demonstrated path entanglement between surface plasmons with a visibility of 95 ± 2%, confirming that a path-entangled state can indeed survive without measurable decoherence. This measurement suggests that elastic scattering mechanisms of the type that might cause pure dephasing must have been weak enough not to significantly perturb the state of the metal under the experimental conditions we investigated. These two experiments add quantum interference and path entanglement to a growing list of quantum phenomena that surface plasmons appear to exhibit just as clearly as photons, confirming the predictions of the simplest quantum models.

Graphene is a monolayer of carbon atoms constructing a two-dimensional honeycomb structure, and it has an excellent carrier mobility and a very high thermal conductivity. Remarkably, it has been experimentally demonstrated that a monolayer graphene exhibits an exotic optical properties. To be specific, the plasmonic dispersion relation of a transverse magnetic graphene plasmon is electronically tunable by adjusting carrier density in graphene with external gate bias, and graphene plasmonic nano cavities have been utilized to modulate mid-infrared light. In this thesis, we present how to efficiently modulate mid-infrared light by combining graphene plasmonic ribbons with noble metal plasmonic structures. First, we propose and demonstrate electronically tunable resonant perfect absorption in graphene plasmonic metasurface enhanced by noble metal plasmonic effect, which results in modulating reflecting light. In this device, we improve coupling efficiency of free-space photons into graphene plasmons by reducing wavevector mismatching with a low permittivity substrate. In addition, the graphene plasmonic resonance is significantly enhanced by plasmonic light focusing effect of the coupled subwavelength metallic slit structure, which results in strongly fortifying resonance absorption in the graphene plasmonic metasurface. In the proposed device, theoretical calculation expects that perfect absorption in the graphene plasmonic metasurface is achievable with low graphene carrier mobility. We also present an analytical model based on surface admittance in order to fully understand how this enhancement occurs. In the second device, we propose and demonstrate a transmission type light modulator by combining graphene plasmonic ribbons with subwavelength metal slit arrays. In this device, extraordinary optical transmission resonance is coupled to graphene plasmonic ribbons to create electrostatic modulation of mid-infrared light. Absorption in graphene plasmonic ribbons situated inside metallic slits can efficiently block the coupling channel for resonant transmission, leading to a suppression of transmission. This phenomenon is also interpreted by anti-crossing between the graphene plasmonic resonance in the ribbons and the noble metal plasmonic resonance in the subwavelength metal slit arrays. Finally, we devise a platform to demonstrate graphene plasmonic resonance energy transport along graphene plasmonic ribbons. In this device, two metal-insulator-metal waveguides are connected by a subwavelength metal slit, and graphene plasmonic ribbons are located inside this slit. Due to the large impedance mismatch at the junction, light coupling efficiency across the junction is poor. If the graphene plasmonic ribbons are tuned to support strong graphene plasmonic resonances, the light energy can be transferred via graphene plasmons along the ribbons, and it leads to significant improvement in the light coupling efficiency across the junction. In addition to enhanced light coupling efficiency, we also present how to totally suppress the transmission by inducing a Fano resonance between a non-resonant propagation mode across the junction and a resonant graphene plasmonic transport mode, which can be utilized to efficiently modulate light in a noble metal plasmonic waveguide with the graphene plasmon resonance energy transfer.

Oxidative DNA Damage by Long-Range Charge Transport

The strong light-matter interaction in the atomically thin transition metal dichalcogenides (TMDCs) has allowed the use of optical spectroscopy to investigate these materials in great depth. It has been shown that optoelectronic properties of ultrathin TMDCs are remarkably different from their bulk counterparts. Among them, this dissertation focuses on ultrathin MoTe2 (molybdenum ditelluride) and ReS2 (rhenium disulfide). We first introduce the fundamental properties of the two material systems, MoTe2 and ReS2, investigated in this dissertation. Specific experimental methods for optical spectroscopy of 2D materials, 2D sample preparation, and related optics calculations are presented. Absorption and photoluminescence measurements are applied to demonstrate that semiconducting MoTe2, an indirect band gap bulk material, acquires a direct band gap in the monolayer limit. Furthermore, strain-tuned optical spectroscopy on MoTe2 shows that tensile strain can significantly redshift its optical gap and partially suppress the intervalley exciton-phonon scattering. This suppression results in a narrowing of the near-band excitonic transitions. We also discuss the effect of strain on the transport properties of MoTe2 due to this reduction in scattering. We investigate monolayer ReS2 as a TMDC system exhibiting strong in-plane anisotropy. These properties are explored by polarization-resolved spectroscopy. We show how the accessible optical properties vary with optical polarization. We find that the near-band excitons in ultrathin ReS2, absorb and emit light along specific polarizations. We also show that purely non-contact, optical techniques can determine the crystallographic orientation of ReS2.

In the present study, a simple, efficient and fast synthetic strategy was reported for the green biosynthesis of silver nanostructures (i.e. nanoroads and nanoparticles) by the extract of Suaeda Acuminata plant, without any catalyst, template or surfactant. Aqueous extracts were obtained by maceration and microwave assisted extraction (MAE) methods. In MAE procedure, the effect of microwave power on the extracted compounds was studied at 90, 270 and 450 W. Colloidal nano-scaled silver samples were synthesized by reacting aqueous silver nitrate with Suaeda Acuminata leaf extract at non-photomediated conditions. Comparative characterizations of the prepared Ag nanostructures were carried out by different techniques including UV-Vis absorption spectroscopy, scanning electron microscopy, X-ray diffraction, and FT-IR spectroscopy. The SEM images showed that it can be synthesized Ag nanoroads and nanoparticles with the average size of 132 nm and 73 nm, by maceration and MAE methods, respectively. According to UV-Vis absorption spectra, a broad absorption peak around 400 nm was observed for all prepared Ag nanostructure samples synthesized by maceration and MAE methods, at different reaction times and irradiation microwave powers. The band can be corresponded to the plasmon absorbance of nano-scaled silver samples.

With the increasing cognition of the importance of organic molecules, they are widely applied in printing, biological and pharmacological fields, because of their special capabilities of harvesting solar light, scavenging free radicals, and chelating metal ions. During the past decades, the unique photoelectronic and photochemical properties of organic molecules, such as phthalocyanine, cyanidin, and their relevant derivatives, attract tremendous attention, because they provide an excellent opportunity to solve the worldwide energy crisis by converting directly the solar light to electricity. The surface morphology and electronic interaction of these organic molecules with other molecules, surfaces or interfaces play a critical role in determining the performance of the electronic and optical devices based on organic molecules. In this thesis, we focus on the investigation of several selected organic molecules, and their interaction with molecules, inorganic semiconductors, and organic semi-conductors, by using first-principles electronic structure calculations based on density functional theory, and time-dependent density functional theory. Particular attention is paid to the atomic structure, electronic and optical properties of organic molecules and the corresponding interfaces. The focus of this thesis is on the following aspects of the organic molecules: (i) The complexation mechanism of flavonoids with metal ions. The most likely chelation site for Fe is the 3-hydroxyl-4-carbonyl group, followed by 4-carbonyl-5-hydroxyl group and the 3'-4' hydroxyl (if present) of quercetin. A complex of two quercetin molecules with a single Fe ion is energetically more stable, however, six orbitals of Fe in the three quercetin complex are saturated by three perpendicular molecules to form and octahedral configurations. Furthermore, the optical absorbance spectra serves as signatures to identify various complexes. (ii) The electronic coupling between a dye molecule (Cyanidin) and a TiO2 nanowire. Upon molecular adsorption on TiO2 [010]-wire, cyanidin will be deprotonated into the quinonoidal form. This results in its highest occupied molecular orbitals being located in the middle of TiO2 bandgap and its lowest occupied molecular orbitals being close to the TiO2 conduction band minimum, in turn enhancing visible light absorption. Moreover, the excited electrons are injected into TiO2 conduction band within a time scale of 50 fs with negligible electron-hole recombination. (iii) The atomic and electronic structure of copper (fluoro-)phthalocyanine and graphene. When adsorbed on graphene, F16CuPc molecules prefer to form a close-packed hexagonal lattice with two-ordered alternating α and β stripes, whereas CuPc would like to form a square lattice. In addition, phthalocyanine adsorption modifies the electronic structure of graphene introducing intensity smoothing at 2-3 eV below and a small peak at ∼0.4 eV above the Dirac point in the density of states. And finally, (iv) the electronic interaction between CuPc and fullerene. For CuPc/C60 molecular complex, CuPc prefers to lie flat on the C60 surface rather than taking the standing-up molecular orientation. The favorable adsorption site for CuPc is the bridge site of C60 with one N-Cu-N bond of CuPc being parallel to a C-C bond of C60. Based on the analysis of the molecular complex, we predict that CuPc/C60(001) thin film heterojunction adopting the lying-down molecular orientation should have a higher efficiency of charge transfer in comparison with the relevant CuPc/C60(111) heterojunction with the standing-up arrangement.

Based on the established theory of photoinduced exciton bleaching of the polymers with nondegenerated ground state,we calculated the transient mirrorless optical bistability of photoexcitation and relaxation processes in the polymers.The results show that the transient saturation absorption mirrorless optical bistabilities and the increasing absorption mirrorless optical bistabilities can be obtained from the ultrafast excitation and relaxation processes in the polymers with nondegenerate ground state,and the switching time of the optical bistabilities is of the order of fs or ps.With the increase of the delay time of the probe pulse relative to pump pulse,the nature of mirrorless optical bistability can change from the saturation absorption optical bistability to increasing absorption optical bistability,and the rate of change is proportional to the speed of relaxation of the photoinduced exciton.