Multiple Quantum MRS

In this thesis we report the studies of optical properties of ZnMnSe/ZnSeTe multiple quantum wells. Quite interesting results have been obtained from our studies which should be very useful for our understanding and application of these materials. Ⅰ. Novel properties of photoluminescence in type-II ZnMnSe/ZnSeTe multiple quantum wells The optical properties of type-II ZnMnSe/ZnSeTe multiple quantum wells were investigated by photoluminescence. From the photoluminescence (PL) spectra, a giant blueshift of the peak energy under moderate excitation power has been observed. It can be well interpreted in terms of the band-bending effect due to spatially separated photoexcited carriers in a type-II band alignment. In addition, a large optical anisotropy of ZnMnSe/ZnSeTe multiple quantum wells (MQWs) has been discovered. Based on the studies of the dependences of excitation intensity and temperature, we point out that the observed anisotropy is an inherent property of the type-II band alignment in ZnMnSe/ZnSeTe MQWs. Ⅱ. Thickness dependence of optical properties of ZnMnSe/ZnSeTe multiple quantum wells Photoluminescence spectra and Raman scattering were performed to study thickness dependence of optical properties of ZnMnSe/ZnSeTe multiple quantum wells. Strong photoluminescence associated with iso-electronic Te traps was observed. As the thickness of the ZnSeTe layer decreases to 1 nm the mechanism of luminescence changes. In Raman spectra, the LO and TO phonons of ZnSeTe layer were observed. A blueshift of optical phonon modes was observed as the thickness of the ZnSeTe decreases. This phenomenon can be well interpreted in terms of the compressive strain effect.

We review the occurrence of periodic or near periodic electric-field domains stimulated by bound-to-continuum transitions under the influence of infrared radiation in multiple uncoupled quantum wells. The conditions of formation of such domains and their stability are studied theoretically using the proposed phenomenological model.

III-nitride wide bandgap semiconductors have been at the center stage of the semiconductor R & D effort over the last decade. Photonic/electronic devices based on III-nitrides, including UV/blue/green/white LEDs, violet/blue LDs, UV detectors, and high power/temperature transistors, have reached high performance level. This talk will provide a brief overview on recent advances made by our group and others in the area of III-nitrides for energy and hydrogen generation. The presentation will concentrate on topics of material and device challenges of III-nitride solar cells and photoelectrochemical cells (PECs). The bandgap of InGaN expands from about 0.65 to 3.4 eV, which covers the entire solar spectrum. In principle, a multi-junction solar cell based on multiple layers of InGaN with different In-contents is capable of capturing the entire spectrum of the sunlight and can thus be highly efficient. To count for the relatively low materials quality for high In content InGaN alloys, InGaN/GaN multiple quantum wells (MQWs) have been adopted for the construction of nitride solar cells. We have grown, fabricated, and characterized InGaN based solar cells utilizing the InGaN/GaN MQWs p-i-n structures.1-3The fabricated solar cells exhibited open circuit voltage of about 2 V, a fill factor of about 60% and overall efficiency of about 3% under AM 1.5 irradiation. The performance of these nitride solar cells under concentrated sunlight has also been investigated. In additional to its bandgap match to the solar spectrum, the bandgap of InGaN alloys can also be engineered to optimally match with conditions necessary for direct hydrogen generation by water splitting using sunlight.4-6 The results on hydrogen gas generation via solar water splitting accomplished using n- and p-type InGaN epilayer as working electrodes will be discussed. Direct solar water splitting and hydrogen gas generation under zero bias has been realized using an InGaN/GaN multiple quantum well solar cell as a photoelectrochemical cell. Under the irradiation by a simulated sunlight (AM1.5G solar simulator), a 1.5% solar-to-fuel conversion efficiency has been achieved under zero bias, setting a new benchmark of employing III-nitrides as photoreactive electrodes for artificial photosynthesis. Time dependent hydrogen gas production photocurrent measured over a prolonged period revealed an excellent chemical stability of InGaN in aqueous solution. Our results indicate that InGaN is an excellent artificial photosynthesis material which is capable to provide usable clean fuel (hydrogen gas) with the sunlight being the only energy input. InGaN materials growth effort was supported by NSF (DMR-1206652) and the energy device effort was by DOE (FG02-09ER46552). Jiang & Lin are grateful to AT&T Foundation for the support of Ed & Linda Whitacre endowed chairs. R. Dahal, B. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, “InGaN/GaN multiple quantum well solar cells with long operating wavelengths,” Appl. Phys. Lett. 94, 063505 (2009). R. Dahal, J. Li, K. Aryal, J. Y. Lin, and H. X. Jiang, "InGaN/GaN multiple quantum well concentrator solar cells," Appl. Phys. Lett. 97, 073115 (2010). B N. Pantha, J. Y. Lin, and H. X. Jiang, “III-nitride nanostructures for energy generation,” Proc. SPIE 7608, 76081I (2010). J. Li, J. Y. Lin, and H. X. Jiang, “Direct hydrogen gas generation by using InGaN epilayers as working electrodes,” Appl. Phys. Lett. 93, 162107 (2008). K. Aryal, B. N. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, "Hydrogen generation by solar water splitting using p-InGaN photoelectrochemical cells," Appl. Phys. Lett. 96, 052110 (2010). R. Dahal, B. N. Pantha, J. Li, J. Y. Lin, and H. X. Jiang, “Realizing InGaN monolithic solar-photoelectrochemical cells for artificial photosynthesis,” Appl. Phys. Lett. 104, 143901 (2014)

We consider the multiple quantum (MQ) NMR dynamics of a gas of spin carrying molecules in nanocavities. MQ NMR dynamics is determined by the residual dipole-dipole interactions, which are not averaged completely due to the molecular diffusion in nanopores. Since the averaged nonsecular Hamiltonian describing MQ NMR dynamics depends on only one coupling constant, this Hamiltonian commutes with the square of the total spin angular momentum Î2. We use the basis of common eigenstates of Î2 and the projection of I on the external magnetic field for investigation of MQ NMR dynamics. This approach allows us to study MQ NMR dynamics in systems consisting of several hundreds of spins. The analytical approximation of the stationary profile of MQ coherences is obtained. The analytical expressions for MQ NMR coherence intensities of the five-spin system in a nanopore are found. Numerical investigations allow us to find the dependencies of intensities of MQ coherences on their orders (the profiles of MQ coherences) in systems consisting of 600 spins and even more. It is shown that the stationary MQ coherence profile in the considered system is an exponential one.

Investigation of defect structures in InGaN/GaN multiple quantum wells grown on Si(1 1 1) substrate by metalorganic vapor phase epitaxy

In this paper, we studied a method to reduce the compressive strain in the InGaN/GaN multiple quantum well (MQW) structures by inserting a strain relief layer between n-GaN and MQWs. The improvements in the interface quality and the optical properties were investigated by photoluminescence (PL) and high-resolution X-ray diffraction (HRXRD) analysis. The samples showed S-shaped emission energy peak and stronger band filling effect in the band tail model. It exhibited the higher activation energy and reduced localization effect. In addition, we have characterized the interface quality and optical properties by strain relief in InGaN/GaN multiple quantum wells based on the temperature-dependent PL analysis. The experimental results would be discussed with supporting models.

Dynamic piezo-phototronic effect in InGaN/GaN multiple quantum wells

With the aim of realizing a temperature sensor based on thermally induced optical bistability that can be operated by a conventional diode laser, we investigated the absorptive behaviour of GaAs/(AlGa)As multiple quantum well structures. Even at room temperature, these structures exhibit a steep excitonic absorption edge in the near infrared spectral region. With rising temperature, this absorption edge shifts strongly enough to the red to lead to a sharp increase in the absorption of suitable photon energies below the absorption edge of the quantum wells. This opens the possibility of observing thermally induced optical bistability in the sample with moderate pumping intensities. The bistability is observed by focusing an infrared laser beam on the multiple quantum well structure. The transmitted laser intensity shows the switching of the sample between the two possible absorptive states. The incident laser intensities at which this switching occurs are extremely sensitive to the temperature of the material surrounding the sample. Using this effect, an optical temperature sensor can be realized in a very practical design using an optical fiber to guide the incident and reflected laser beam that can possibly be provided by a diode laser.

A novel multiple quantum filtering method is proposed that uses a doubly selective pulse termed Delays Alternating with Nutations for Tailored Excitation (DANTE) for multiple quantum preparation. This method selectively prepares GABA-3 and GABA-4 into a multiple quantum state and suppresses all other resonances at 3.0 ppm in each single scan. Phantom tests demonstrated excellent GABA signal retention and complete suppression of overlapping metabolites. It is shown using numerical simulations that overlapping macromolecules are suppressed because the frequency of the first upfield 2pi rotation of the doubly selective DANTE pulse coincides with that of the macromolecules at 1.72 ppm. Excellent suppression of overlapping macromolecules was demonstrated in vivo. Using this method the concentration of GABA in the occipital lobe of healthy volunteers was measured to be 1.21 +/- 0.28 micromol/mL (mean +/-SD, N = 9).

A direct epitaxy of III-V nanowires with InGaAs/InP multiple quantum wells on v-shaped trenches patterned silicon on insulator (SOI) substrates was realized by combining the standard semiconductor fabrication process with the aspect ratio trapping growth technique. Silicon thickness as well as the width and gap of each nanowire were carefully designed to accommodate essential optical properties and appropriate growth conditions. The III-V element ingredient, crystalline quality, and surface topography of the grown nanowires were characterized by X-ray diffraction spectroscopy, photoluminescence, and scanning electron microscope. Geometrical details and chemical information of multiple quantum wells were revealed by transmission electron microscopy and energy dispersive spectroscopy. Numerical simulations confirmed that the optical guided mode supported by one single nanowire was able to propagate 50 μm with ∼30% optical loss. This proposed integration scheme opens up an alternative pathway for future photonic integrations of III-V devices on the SOI platform at nanoscale.

This paper represents the influences of temperature and hydrostatic pressure variations on GaAs/AlGaAs multiple quantum well slow light systems based on coherence population oscillations. An analytical model in non-integer dimension space is used to study the considerable effects of these parameters on optical properties of the slow light apparatus. Exciton oscillator strength and fractional dimension constants have special roles on the analytical model in fractional dimension. Hence, the impacts of hydrostatic pressure and temperature on exciton oscillator strength and fractional dimension quantity are investigated theoretically in this paper. Based on the achieved results, temperature and hydrostatic pressure play key roles on optical parameters of the slow light systems, such as the slow down factor and central energy of the device. It is found that the slope and value of the refractive index real part change with alterations of temperature and hydrostatic pressure in the range of 5-40 deg of Kelvin and 1 bar to 2 kbar, respectively. Thus, the peak value of the slow down factor can be adjusted by altering these parameters. Moreover, the central energy of the device shifts when the hydrostatic pressure is applied to the slow light device or temperature is varied. In comparison with previous reported experimental results, our simulations follow them successfully. It is shown that the maximum value of the slow down factor is estimated close to 5.5×104 with a fine adjustment of temperature and hydrostatic pressure. Meanwhile, the central energy shift of the slow light device rises up to 27 meV, which provides an appropriate basis for different optical devices in which multiple quantum well slow light is one of their essential subsections. This multiple quantum well slow light device has potential applications for use as a tunable optical buffer and pressure/temperature sensors.

The strong piezoelectric field in InGaN/GaN heterostructure quantum wells severely reduces the light emission efficiency of multiple quantum well (MQW) structures. To address this issue, a strain modulation interlayer is commonly used to mitigate the piezoelectric polarization field and improve the luminescence performance of the devices. To investigate the influence and mechanism of strain modulation in the InGaN/GaN superlattice (SL), epitaxial wafers with an n-type InGaN/GaN SL interlayer sample, and their corresponding control samples are prepared. The measured temperature-dependent photoluminescence (PL) spectra of the epitaxial wafers, show that the introduction of an SL interlayer leads to a shorter-wavelength emission and enhancement of internal quantum efficiency. As the temperature increases, a blue shift of the PL peak is observed. However, for the sample with an SL interlayer, the blue shift of the PL peak with temperature increasing is relatively small. Electroluminescence (EL) experiments indicate that the introduction of an SL interlayer significantly increases the integrated intensity of the EL peak and reduces its full width at half maximum. These phenomena collectively indicate that the incorporation of a superlattice interlayer can partly suppress the quantum-confined Stark effect (QCSE) that affects the light emission efficiency. Theoretical calculations show that the introduction of a superlattice strain layer before growing an active multiple quantum well can weaken the polarization-induced built-in electric field in the active quantum well, reduce the tilt of the energy band in the multiple quantum well active region, increase the overlap of electron and hole wave functions, enhance the emission probability, shorten the radiative recombination lifetime, and promote competition between radiative recombination and non-radiative recombination, thereby achieving higher recombination efficiency and improving light emission intensity. This study provides experimental and theoretical evidence that the strain modulation SL interlayer can effectively improve the device performance and offer guidance for optimizing the structural design of devices.

Monolithically integrated surface-emitting device consisting of an AlGaAs/GaAs multiple quantum well distributed feedback laser and a grating coupler is demonstrated for the first time. The grating coupler was made by utilizing the properties of low loss and easily saturable absorption in the multiple quantum well structure. No additional crystal growths or complicated fabrication processes were required for the integration. An output power of 2 mW and a narrow beam divergence of 0.22° were obtained under continuous wave operation.

We present a study of the optical properties of a series of InGaN/GaN multiple quantum well structures, with room temperature peak emission wavelengths of 540 nm. By adopting a two‐temperature growth method and designing structures to have short radiative lifetimes, we have obtained room temperature internal photoluminescence quantum efficiencies of 8%. Transmission electron microscopy images have revealed the presence of edge‐type threading misfit dislocations in the multiple quantum well stack, which may be preventing the efficiency from reaching the very high values obtained in blue emitting InGaN/GaN multiple quantum well structures. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

This thesis describes several studies in which nuclear magnetic resonance (nmr) spectroscopy has been used to probe the structure, orientation and dynamics of liquid crystal mesogens and molecules dissolved in liquid crystalline phases. In addition, a modern high field nmr spectrometer is described which has been used to perform such nmr studies. Chapter 1 introduces the quantum mechanical formalisms used throughout this thesis and briefly reviews the fundamentals of nuclear spin physics and pulsed nmr spectroscopy. First the density operator is described and a specific form for the canonical ensemble is derived. Then Clebsch-Gordon coefficients, Wigner rotation matrices, and irreducible tensor operators are reviewed. An expression for the equilibrium (Curie) magnetization is obtained and the linear response of a spin system to a strong pulsed r.f. irradiation is described. Finally, the spin interaction Hamiltonians relevant to this work are reviewed together with their truncated forms. Chapter 2 is a deuterium magnetic resonance study of two 'nom' liquid crystals which possess several low temperature mesomorphic phases. Specifically, deuterium quadrupolar echo spectroscopy is used to determine the orientation of the liquid crystal molecules in smectic phases, the changes in molecular orientation and motion that occur at smectic-smectic phase transitions, and the order of the phase transitions. For both compounds, the phase sequence is determined to be isotropic, nematic, smectic A, smectic C, smectic BA, smectic BC, and crystalline. The structure of the smectic A phase is found to be consistent with the well-known model of a two dimensional liquid in which molecules are rapidly rotating about their long axes and oriented at right angles to the plane of the layers. Molecules in the smectic C phase are found to have their long axes tilted with respect to the layer normal, and the tilt angle is temperature dependent, increasing from zero at the smectic A - smectic C transition and reaching a maximum at 9° at the smectic C - smectic BA transition. This finding contradicts the results of X-ray diffraction studies which indicate that the tilt angle is 18° and temperature independent. The smectic BA - smectic BC phase transition is observed for the first time, and is found to be first order, a result that contradicts the prediction of a mean theory by McMillian. Chapter 3 is a multiple quantum nmr study of n-hexane oriented in a nematic liquid crystal solvent. The basic three pulse multiple quantum experiment is discussed which enables the observation of transitions for which |Δm|>1, and then the technique of the separation of multiple quantum orders by phase incrementation in the multiple quantum evolution period is reviewed (TPPI). An explicit example of multiple quantum nmr is given by the calculation of the multiple quantum spectrum of an oriented methyl group.