Characterisation and processing of amorphous binary mixtures with low glass transition temperature

The number of drug substances in formulation development in the pharmaceutical industry is increasing. Some of these are amorphous drugs and have glass transition below ambient temperature, and thus they are usually difficult to formulate and handle. One reason for this is the reduced viscosity, related to the stickiness of the drug, that makes them complicated to handle in unit operations. Thus, the aim in this thesis was to develop a new processing method for a sticky amorphous model material. Furthermore, model materials were characterised before and after formulation, using several characterisation methods, to understand more precisely the prerequisites for physical stability of amorphous state against crystallisation. The model materials used were monoclinic paracetamol and citric acid anhydrate. Amorphous materials were prepared by melt quenching or by ethanol evaporation methods. The melt blends were found to have slightly higher viscosity than the ethanol evaporated materials. However, melt produced materials crystallised more easily upon consecutive shearing than ethanol evaporated materials. The only material that did not crystallise during shearing was a 50/50 (w/w, %) blend regardless of the preparation method and it was physically stable at least two years in dry conditions. Shearing at varying temperatures was established to measure the physical stability of amorphous materials in processing and storage conditions. The actual physical stability of the blends was better than the pure amorphous materials at ambient temperature. Molecular mobility was not related to the physical stability of the amorphous blends, observed as crystallisation. Molecular mobility of the 50/50 blend derived from a spectral linewidth as a function of temperature using solid state NMR correlated better with the molecular mobility derived from a rheometer than that of differential scanning calorimetry data. Based on the results obtained, the effect of molecular interactions, thermodynamic driving force and miscibility of the blends are discussed as the key factors to stabilise the blends. The stickiness was found to be affected glass transition and viscosity. Ultrasound extrusion and cutting were successfully tested to increase the processability of sticky material. Furthermore, it was found to be possible to process the physically stable 50/50 blend in a supercooled liquid state instead of a glassy state. The method was not found to accelerate the crystallisation. This may open up new possibilities to process amorphous materials that are otherwise impossible to manufacture into solid dosage forms.

High-coherence hybrid Si/III-V semiconductor lasers

The relentlessly increasing demand for network bandwidth, driven primarily by Internet-based services such as mobile computing, cloud storage and video-on-demand, calls for more efficient utilization of the available communication spectrum, as that afforded by the resurging DSP-powered coherent optical communications. Encoding information in the phase of the optical carrier, using multilevel phase modulationformats, and employing coherent detection at the receiver allows for enhanced spectral efficiency and thus enables increased network capacity. The distributed feedback semiconductor laser (DFB) has served as the near exclusive light source powering the fiber optic, long-haul network for over 30 years. The transition to coherent communication systems is pushing the DFB laser to the limits of its abilities. This is due to its limited temporal coherence that directly translates into the number of different phases that can be imparted to a single optical pulse and thus to the data capacity. Temporal coherence, most commonly quantified in the spectral linewidth Δν, is limited by phase noise, result of quantum-mandated spontaneous emission of photons due to random recombination of carriers in the active region of the laser.

In this work we develop a generically new type of semiconductor laser with the requisite coherence properties. We demonstrate electrically driven lasers characterized by a quantum noise-limited spectral linewidth as low as 18 kHz. This narrow linewidth is result of a fundamentally new laser design philosophy that separates the functions of photon generation and storage and is enabled by a hybrid Si/III-V integration platform. Photons generated in the active region of the III-V material are readily stored away in the low loss Si that hosts the bulk of the laser field, thereby enabling high-Q photon storage. The storage of a large number of coherent quanta acts as an optical flywheel, which by its inertia reduces the effect of the spontaneous emission-mandated phase perturbations on the laser field, while the enhanced photon lifetime effectively reduces the emission rate of incoherent quanta into the lasing mode. Narrow linewidths are obtained over a wavelength bandwidth spanning the entire optical communication C-band (1530-1575nm) at only a fraction of the input power required by conventional DFB lasers. The results presented in this thesis hold great promise for the large scale integration of lithographically tuned, high-coherence laser arrays for use in coherent communications, that will enable Tb/s-scale data capacities.

Magnetic field and pressure effects in a saturated gas laser amplifier

Theoretical and experimental studies of a gas laser amplifier are presented, assuming the amplifier is operating with a saturating optical frequency signal. The analysis is primarily concerned with the effects of the gas pressure and the presence of an axial magnetic field on the characteristics of the amplifying medium. Semiclassical radiation theory is used, along with a density matrix description of the atomic medium which relates the motion of single atoms to the macroscopic observables. A two-level description of the atom, using phenomenological source rates and decay rates, forms the basis of our analysis of the gas laser medium. Pressure effects are taken into account to a large extent through suitable choices of decay rate parameters.

Two methods for calculating the induced polarization of the atomic medium are used. The first method utilizes a perturbation expansion which is valid for signal intensities which barely reach saturation strength, and it is quite general in applicability. The second method is valid for arbitrarily strong signals, but it yields tractable solutions only for zero magnetic field or for axial magnetic fields large enough such that the Zeeman splitting is much larger than the power broadened homogeneous linewidth of the laser transition. The effects of pressure broadening of the homogeneous spectral linewidth are included in both the weak-signal and strong-signal theories; however the effects of Zeeman sublevel-mixing collisions are taken into account only in the weak-signal theory.

The behavior of a He-Ne gas laser amplifier in the presence of an axial magnetic field has been studied experimentally by measuring gain and Faraday rotation of linearly polarized resonant laser signals for various values of input signal intensity, and by measuring nonlinearity - induced anisotropy for elliptically polarized resonant laser signals of various input intensities. Two high-gain transitions in the 3.39-μ region were used for study: a J = 1 to J = 2 (3s₂ → 3p₄) transition and a J = 1 to J = 1 (3s₂ → 3p₂) transition. The input signals were tuned to the centers of their respective resonant gain lines.

The experimental results agree quite well with corresponding theoretical expressions which have been developed to include the nonlinear effects of saturation strength signals. The experimental results clearly show saturation of Faraday rotation, and for the J = 1 t o J = 1 transition a Faraday rotation reversal and a traveling wave gain dip are seen for small values of axial magnetic field. The nonlinearity induced anisotropy shows a marked dependence on the gas pressure in the amplifier tube for the J = 1 to J = 2 transition; this dependence agrees with the predictions of the general perturbational or weak signal theory when allowances are made for the effects of Zeeman sublevel-mixing collisions. The results provide a method for measuring the upper (neon 3s₂) level quadrupole moment decay rate, the dipole moment decay rates for the 3s₂ → 3p₄ and 3s₂ → 3p₂ transitions, and the effects of various types of collision processes on these decay rates.

TAILORED DFB LASER PROPERTIES BY INDIVIDUALLY CHIRPED GRATINGS USING BENT WAVE-GUIDES

DFB lasers with continuously and arbitrarily chirped gratings of ultrahigh spatial precision are implemented by a method we proposed recently, using bent waveguides on homogeneous grating fields. Choosing individual bending functions we generate special chirping functions and obtain additional degrees of freedom to tailor and improve specific device performances, We present two applications for lasers showing several improved device properties and the effectiveness of our method, First, we implement continuously distributed phase-shifted lasers, revealing a considerably reduced photon pile-up, higher single-longitudinal mode stability, higher output power, lower linewidth, and higher yield than conventional abruptly phase-shifted lasers, Second, a novel tuning principle is applied in chirped multiple-section DFB lasers, showing 5.5-nm wavelength tuning, without any gaps, maintaining high side-mode suppression.

CONTINUOUSLY CHIRPED DFB GRATINGS BY SPECIALLY BENT WAVE-GUIDES FOR TUNABLE LASERS

We have implemented and studied a new type of tunable multiple-section semiconductor distributed feedback (DFB) laser using tailored chirped DFB gratings. Arbitrarily and continuously chirped DFB gratings are defined by bent waveguides on homogeneous grating fields with ultrahigh spatial precision, The mathematical bending functions are optimized in this case to provide enlarged wavelength tuning ranges. We present the results of model calculations, the technological device realization and experimental results of the DFB laser characterization e.g. a tuning range of 5.5 mm without wavelength gaps and high side mode suppression ratio.

Plasmons as sensors

“Plasmon” is a synonym for collective oscillations of the conduction electrons in a metal nanoparticle (excited by an incoming light wave), which cause strong optical responses like efficient light scattering. The scattering cross-section with respect to the light wavelength depends not only on material, size and shape of the nanoparticle, but also on the refractive index of the embedding medium. For this reason, plasmonic nanoparticles are interesting candidates for sensing applications. Here, two novel setups for rapid spectral investigations of single nanoparticles and different sensing experiments are presented.rnrnPrecisely, the novel setups are based on an optical microscope operated in darkfield modus. For the fast single particle spectroscopy (fastSPS) setup, the entrance pinhole of a coupled spectrometer is replaced by a liquid crystal device (LCD) acting as spatially addressable electronic shutter. This improvement allows the automatic and continuous investigation of several particles in parallel for the first time. The second novel setup (RotPOL) usesrna rotating wedge-shaped polarizer and encodes the full polarization information of each particle within one image, which reveals the symmetry of the particles and their plasmon modes. Both setups are used to observe nanoparticle growth in situ on a single-particle level to extract quantitative data on nanoparticle growth.rnrnUsing the fastSPS setup, I investigate the membrane coating of gold nanorods in aqueous solution and show unequivocally the subsequent detection of protein binding to the membrane. This binding process leads to a spectral shift of the particles resonance due to the higher refractive index of the protein compared to water. Hence, the nanosized addressable sensor platform allows for local analysis of protein interactions with biological membranes as a function of the lateral composition of phase separated membranes.rnrnThe sensitivity on changes in the environmental refractive index depends on the particles’ aspect ratio. On the basis of simulations and experiments, I could present the existence of an optimal aspect ratio range between 3 and 4 for gold nanorods for sensing applications. A further sensitivity increase can only be reached by chemical modifications of the gold nanorods. This can be achieved by synthesizing an additional porous gold cage around the nanorods, resulting in a plasmon sensitivity raise of up to 50 % for those “nanorattles” compared to gold nanorods with the same resonance wavelength. Another possibility isrnto coat the gold nanorods with a thin silver shell. This reduces the single particle’s resonance spectral linewidth about 30 %, which enlarges the resolution of the observable shift. rnrnThis silver coating evokes the interesting effect of reducing the ensemble plasmon linewidth by changing the relation connecting particle shape and plasmon resonance wavelength. This change, I term plasmonic focusing, leads to less variation of resonance wavelengths for the same particle size distribution, which I show experimentally and theoretically.rnrnIn a system of two coupled nanoparticles, the plasmon modes of the transversal and longitudinal axis depend on the refractive index of the environmental solution, but only the latter one is influenced by the interparticle distance. I show that monitoring both modes provides a self-calibrating system, where interparticle distance variations and changes of the environmental refractive index can be determined with high precision.

Measurement of the intrinsic damping constant in individual nanodisks of Y3Fe5O12 and Y3Fe5O12|Pt

We report on an experimental study on the spin-waves relaxation rate in two series of nanodisks of diameter ϕ=300 , 500, and 700 nm, patterned out of two systems: a 20 nm thick yttrium iron garnet (YIG) film grown by pulsed laser deposition either bare or covered by 13 nm of Pt. Using a magnetic resonance force microscope, we measure precisely the ferromagnetic resonance linewidth of each individual YIG and YIG|Pt nanodisks. We find that the linewidth in the nanostructure is sensibly smaller than the one measured in the extended film. Analysis of the frequency dependence of the spectral linewidth indicates that the improvement is principally due to the suppression of the inhomogeneous part of the broadening due to geometrical confinement, suggesting that only the homogeneous broadening contributes to the linewidth of the nanostructure. For the bare YIG nano-disks, the broadening is associated to a damping constant α=4 × 10−4 . A threefold increase of the linewidth is observed for the series with Pt cap layer, attributed to the spin pumping effect. The measured enhancement allows to extract the spin mixing conductance found to be G↑↓=1.55 × 1014 Ω−1 m−2 for our YIG(20nm)|Pt interface, thus opening large opportunities for the design of YIG based nanostructures with optimized magnetic losses.

Self-seeded gain-switched operation of an InGaN MQW laser diode

Self-seeded, gain-switched operation of an InGaN multi-quantum-well laser diode has been demonstrated for the first time. An external cavity comprising Littrow geometry was implemented for spectral control of pulsed operation. The feedback was optimized by adjusting the external cavity length and the driving frequency of the laser. The generated pulses had a peak power in excess of 400mW, a pulse duration of 60ps, a spectral linewidth of 0.14nm and maximum side band suppression ratio of 20dB. It was tunable within the range of 3.6nm centered at a wavelength of 403nm.

Part 1: Longitudinal static and dynamic effects in semiconductor lasers. Part II: Spectral characteristics of passively mode-locked quantum well lasers

In the first part of this thesis a study of the effect of the longitudinal distribution of optical intensity and electron density on the static and dynamic behavior of semiconductor lasers is performed. A static model for above threshold operation of a single mode laser, consisting of multiple active and passive sections, is developed by calculating the longitudinal optical intensity distribution and electron density distribution in a self-consistent manner. Feedback from an index and gain Bragg grating is included, as well as feedback from discrete reflections at interfaces and facets. Longitudinal spatial holeburning is analyzed by including the dependence of the gain and the refractive index on the electron density. The mechanisms of spatial holeburning in quarter wave shifted DFB lasers are analyzed. A new laser structure with a uniform optical intensity distribution is introduced and an implementation is simulated, resulting in a large reduction of the longitudinal spatial holeburning effect.

A dynamic small-signal model is then developed by including the optical intensity and electron density distribution, as well as the dependence of the grating coupling coefficients on the electron density. Expressions are derived for the intensity and frequency noise spectrum, the spontaneous emission rate into the lasing mode, the linewidth enhancement factor, and the AM and FM modulation response. Different chirp components are identified in the FM response, and a new adiabatic chirp component is discovered. This new adiabatic chirp component is caused by the nonuniform longitudinal distributions, and is found to dominate at low frequencies. Distributed feedback lasers with partial gain coupling are analyzed, and it is shown how the dependence of the grating coupling coefficients on the electron density can result in an enhancement of the differential gain with an associated enhancement in modulation bandwidth and a reduction in chirp.

In the second part, spectral characteristics of passively mode-locked two-section multiple quantum well laser coupled to an external cavity are studied. Broad-band wavelength tuning using an external grating is demonstrated for the first time in passively mode-locked semiconductor lasers. A record tuning range of 26 nm is measured, with pulse widths of typically a few picosecond and time-bandwidth products of more than 10 times the transform limit. It is then demonstrated that these large time-bandwidth products are due to a strong linear upchirp, by performing pulse compression by a factor of 15 to a record pulse widths as low 320 fs.

A model for pulse propagation through a saturable medium with self-phase-modulation, due to the a-parameter, is developed for quantum well material, including the frequency dependence of the gain medium. This model is used to simulate two-section devices coupled to an external cavity. When no self-phase-modulation is present, it is found that the pulses are asymmetric with a sharper rising edge, that the pulse tails have an exponential behavior, and that the transform limit is 0.3. Inclusion of self-phase-modulation results in a linear upchirp imprinted on the pulse after each round-trip. This linear upchirp is due to a combination of self-phase-modulation in a gain section and absorption of the leading edge of the pulse in the saturable absorber.

Spectral hole-burning of Eu3+: Y2SiO5 crystal at 579.62 mm

By using an Ar+ ion laser, a tunable Rh 6G dye laser(Linewidth : 0.5 cm(-1)) and a Coherent 899-21 dye laser as light sources and using a monochromator and a phase-locking amplifier, the optical properties of Eu3+ : Y2SiO5 crystal were detected. Persistent spectral hole burning (PSHB) were also observed in (5)Do-(7)Fo transition in the crystal at the temperature of 16 K. For 15 mW dye laser (Wavelength : 579.62 nm) burning the crystal for 0.1 s a spectral hole with about 80 MHz hole width were detected and the hole can been keep for longer than 10 h.

Hyperfine spectral structure of semiconductor lasers

In this paper, we propose an interference technique that can provide a quantitative and ultrafine-resolution spectral analysis because the optical heterodyning is performed at nonzero frequency and interfering waves propagate in optical fiber. The spectrum of a laser consists of a large number of wave trains. Our study is focused on the features of wave trains. We demonstrate that wave trains emitting simultaneously have random frequency spacings, and the probability of occurrence of two or more joint wave trains with the same frequency is high. The estimated linewidth of the wave train is narrower than 1 mHz, corresponding to a wavelength range of 10(-23) m.

Linewidth reduction in a large-smile laser diode array

We present theory and simulations for a spectral narrowing scheme for laser diode arrays (LDAs) that employs optical feedback from a diffraction grating. We calculate the effect of the so-called smile of the LDA and show that it is possible to reduce the effect by using a cylindrical lens set at an angle to the beam. The scheme is implemented on a 19-element LDA with smile of 7.6 mu m and yields frequency narrowing from a free-running width of 2 to 0.15 nm. The experimental results are in good agreement with the theory. (c) 2005 Optical Society of America.

Photonic processing for wideband cancellation and spectral discrimination of RF signals

Photonic signal processing is used to implement common mode signal cancellation across a very wide bandwidth utilising phase modulation of radio frequency (RF) signals onto a narrow linewidth laser carrier. RF spectra were observed using narrow-band, tunable optical filtering using a scanning Fabry Perot etalon. Thus functions conventionally performed using digital signal processing techniques in the electronic domain have been replaced by analog techniques in the photonic domain. This technique was able to observe simultaneous cancellation of signals across a bandwidth of 1400 MHz, limited only by the free spectral range of the etalon. © 2013 David M. Benton.

Multimodal spectral control of a quantum-dot diode laser for THz difference frequency generation

Generation of stable dual and/or multiple longitudinal modes emitted from a single quantum dot (QD) laser diode (LD) over a broad wavelength range by using volume Bragg gratings (VBG's) in an external cavity setup is reported. The LD operates in both the ground and excited states and the gratings give a dual-mode separation around each emission peak of 5 nm, which is suitable as a continuous wave (CW) optical pump signal for a terahertz (THz) photomixer device. The setup also generates dual modes around both 1180m and 1260 nm simultaneously, giving four simultaneous narrow linewidth modes comprising two simultaneous difference frequency pump signals. (C) 2011 American Institute of Physics.