Nd:YAG based laser sources for targeting applications

The aim of the research is to improve laser system products manufactured at Selex ES which are used primarily for airborne targeting applications. This is achieved by developments to the design that prevent failures during manufacture or improve beam parameters such as divergence. A Q-switched diode-side-pumped Nd:YAG zig-zag geometry slab laser within a cross Porro prism resonator is investigated. This perturbation insensitive resonator design is used in laser systems operating over the full military environment of vibration and temperature. A number of aspects of the design are computer modelled with experimental verification, such as the effects of thermal lensing in the Nd:YAG slab, and the polarisation states in the resonator. These were used to analyse a number of issues encountered during manufacture, such as the lack of control over the polarisation state for output coupling, pre-lase causing damage to optical elements, and thermal lensing producing variations in beam quality. A number of design changes were made and, after experiments to verify improved performance, they were successfully integrated into a number of laser production programmes. The beam quality of laser systems was found to be affected by thermal lensing. A number of novel solutions were tested experimentally, which affected the thermal lens. Results of the alteration of the pump distribution in the Nd:YAG slab and the profile of conduction cooling are presented. 885 nm pumping instead of the traditional 808 nm pumping produced a reduction of the thermal lens by a factor of two from -0.1 D to -0.05 D, producing an improvement in the laser beam quality from M2 6.5 to 3.5. An enhancement in brightness of 2.2 was demonstrated using a laser resonator incorporating a deformable bimorph mirror. A new concept for a targeting laser source, which incorporated an eye-safe wavelength, was demonstrated using a common resonator intracavity OPO design. A conversion efficiency of 40% was achieved for 36 mJ output of 1573 nm eye-safe light from a 90 mJ laser at 1064 nm. The relative pointing directions of the two wavelength beams was measured to be within 250 μRad angular separation, which will be unaffected by ambient temperature variation. This level of performance is challenging to achieve in the current laser system design incorporating an extracavity OPO

High Resolution Scanning Probes for Ferroelectric Thin Films

Advances in materials growth techniques enable precise control over the growth of novel functional materials such as ferroelectric thin films, which are interesting from both a physics and applications perspective. Physical properties of ferroelectric thin films differ a lot from their bulk counterparts, mainly due to the lattice mismatch at the film-substrate interface and differential thermal contraction experienced during growth. Those property anomalies are confined to a narrow range usually thinner than 1000 nm. High-resolution probes are important for understanding the spatial and temporal properties of these systems. We have developed mechanical and optical scanning probe techniques and used them to investigate various strain-engineered ferroelectric thin films. These optical and scanning probe techniques are designed to detect ferroelectric domain dynamics. Our experimental results either give direct evidence to verify material functionality, or reveal the relation between nano-scale dynamics to their macroscopic properties

Ultrafast electro-optic Time-Frequency Fractional Fourier Imaging at the Single-Photon Level

The Fractional Fourier Transform (FRT) corresponds to an arbitrary-angle rotation in the phase space, e.g. the time-frequency (TF) space, and generalizes the fundamentally important Fourier Transform. FRT applications range from classical signal processing (e.g. time-correlated noise optimal filtering) to emerging quantum technologies (e.g. super-resolution TF imaging) which rely on or benefit from coherent low-noise TF operations. Here a versatile low-noise single-photon-compatible implementation of the FRT is presented. Optical TF FRT can be synthesized as a series of a spectral disperser, a time-lens, and another spectral disperser. Relying on the state-of-the-art electro-optic modulators (EOM) for the time-lens, our method avoids added noise inherent to the alternatives based on non-linear interactions (such as wave-mixing, cross-phase modulation, or parametric processes). Precise control of the EOM-driving radio-frequency signal enables fast all-electronic control of the FRT angle. In the experiment, we demonstrate FRT angles of up to 1.63 rad for pairs of coherent temporally separated 11.5 ps-wide pulses in the near-infrared (800 nm). We observe a good agreement between the simulated and measured output spectra in the bright-light and single-photon-level regimes, and for a range of pulse separations (20 ps to 26.67 ps). Furthermore, a tradeoff is established between the maximal FRT angle and bandwidth, with the current setup accommodating up to 248 GHz of bandwidth. With the ongoing progress in EOM on-chip integration, we envisage excellent scalability and vast applications in all-optical TF processing both in the classical and quantum regimes.Comment: 15 pages, 6 figure

Development and Applications of Terahertz Near-Field Microscopes for Surface Plasmon Imaging

The confined nature of surface plasmons (SPs) often imposes challenges on their experimental detection and makes specific near-field probes necessary. While various SP detection methods have been developed in the optical domain, only a few examples of SP imaging have been reported in the terahertz range. In this thesis, specific problems of current terahertz near-field detection systems have been addressed which has led to the development of two new SP imaging methods. In the first method, SP imaging is demonstrated using the integrated subwavelength aperture near-field probe. The photoconductive antenna inside the probe is sensitive to the SP electric-field despite the orthogonal spatial orientation between the antenna and the SP polarisation. This enables SP imaging directly on a metallic surface employing a photoconductive antenna. This unexpected sensitivity has been applied to SP imaging in two examples: first, the SP propagation has been imaged on a resonant THz bow-tie antenna and second, the SP excitation by a strongly focused terahertz beam directly on the metallic probe surface has been investigated. The second method presents an electro-optic micro-resonator for SP imaging. A micro-resonator structure has the potential to provide a better sensitivity and spatial resolution, as well as a lower level of invasiveness compared to bulk crystals, which are commonly used in terahertz near-field systems. The micro-resonator design is explained in detail and the impact of the micro-resonator geometry on the probe performance is discussed. This micro-resonator has then been fabricated and embedded into an electro-optic detection system. This detection system has been fully characterised with the focus on two functional units which are essential for its performance: a tapered parallel plate waveguide for broadband terahertz transmission and the balanced detector for noise reduction. The overall performance of the detection system has been evaluated for its use as a terahertz near-field microscope

Polarization manipulation in femtosecond laser direct written waveguides in fused silica

Using ultrashort laser pulses to create refractive index modifications in transparent materials has become a popular method for fabricating integrated photonic circuits, since it allows three-dimensional structuring and rapid prototyping. While the precise control of the light propagation path in such laser direct written photonic circuits has been subject of numerous investigations, the control of the lights polarization has rarely been examined. At the same time, achieving full control over the lights polarization in a photonic circuit is crucial for using this degree of freedom to encode information. The goal of this theses is to demonstrate ways of achieving this polarization control in femtosecond laser direct written circuits in fused silica. To ensure that a waveguide does not alter the polarization state (and thereby the information content) of transmitted light, the waveguide should exhibit minimal birefringence and minimal losses. For this purpose, the influence of different beam shaping methods for the inscription laser on the waveguides optical properties are investigated. Waveguides with lowest losses at a low birefringence were achieved using an anamorphic zoom system. Furthermore, two concepts for embedding compact polarization manipulating elements into the waveguides are presented. Firstly, it is shown that the self-ordered material modification nanograting can be used to create embedded waveplates, where the optical properties are determined by the laser inscription parameters during the fabrication process. The obtained structures can be used not only for classical applications, but also as single qubit quantum gates, which is demonstrated using single photon experiments. The second concept for embedding polarization manipulating elements into waveguides is based on liquid crystals, which allow active reconfiguration of the optical circuit after inscription and a dynamic change of the polarization state in a waveguide

Construction of a THz-STM

The combination of high spatial and temporal resolution is still a limiting factor in the field of surface science. Only very recently, a novel experimental approach has been presented that allows versatile access to this regime by combining ultrafast Terahertz (THz) pulses with the extraordinary spatial resolution of scanning tunneling microscopy (STM): The so-called THz-STM. In this device, the THz pulses act as ultrafast transient bias voltage and hence allow to unravel the ultrafast physics of highly localized electronic states via pump-probe experiments. In this thesis, we describe the process of establishing this novel technique in our group. We firstly discuss the new configuration of the designated lab, the design and alignment of our THz-source as well as its characterization using electro-optic sampling. Subsequently we describe the modifications performed on the preexisting STM to facilitate the coupling of THz radiation. The THz-STM was thoroughly tested in different scenarios showcasing its capabilities. We initially present data originating from optical-pump, THz-probe measurements based on photoelectron emission, confirming the successful coupling of ultrashort THz pulses into the STM. Subsequent measurements using field emission resonances show the stability of our system under cryo conditions and using junctions at tunneling distances. Experiments involving Kondo resonances of point defects in MoS2 prove the sensitivity of our THz-STM when used with small nonlinearities and are used to perform exemplary spatially resolved measurements. Finally we present preliminary time-resolved data as part of an outlook

High Power THz Generation in a GaP waveguide and the THz Carrier Dynamics in Epitaxial Graphene.

The generation and detection of ultrafast time domain (TD) THz pulse trains is an active area of research, with recent developments pushing sources to higher power and greater bandwidth. This thesis presents research in two frontiers of the science and technology of THz radiation; the generation of high power TD-THz pulses and the dynamic THz spectroscopy of an emerging new material, epitaxial graphene. To increase the SNR of conventional time domain (TD) THz sources, a novel method is proposed for high average power, high repetition rate, TD-THz generation based on an ultrafast fiber laser and optical rectification inside a GaP waveguide. A model for the THz generation is developed by combining a finite-difference frequency-domain mode solver with the 1D generation equation. The measured 150-µW average power and 3 THz bandwidth represent nearly a two order of magnitude increase over conventional TD-THz systems, and are in good agreement with the theoretical model. Since the demonstration of the isolation of single atomic sheets of graphite, graphene has received tremendous attention due to its unique mechanical and electrical properties. These unique properties indicate graphene is a highly promising material for high-speed (THz-bandwidth) electronic devices. This thesis presents TD-THz spectroscopy of multilayer epitaxial graphene samples, with the goals of identifying the presence of a possible bandgap opening at low energies and of measuring the hot carrier recovery dynamics on picosecond timescales. The graphene transmission spectrum is shown to be remarkably flat and is used to verify the absence of a bandgap at meV energies. Optical pump – THz probe measurements of the temperature-dependent recovery dynamics show a biexponential recovery with which is compared with theoretical predictions. Lastly, THz detection of coherent controlled photocurrents is demonstrated for the first time in epitaxial graphene. Optical coherent control provides a method for contactless injection of ultrafast current bursts into semiconductor materials. The associated radiated THz pulse is used to verify the unique polarization independence and power scaling with theoretical predictions. The effect of background hot carriers on the coherent generation process is explored and the dephasing of the coherent current injection is observed for the first time.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/64643/1/cdivin_1.pd

Injection-locked Semiconductor Lasers For Realization Of Novel Rf Photonics Components

This dissertation details the work has been done on a novel resonant cavity linear interferometric modulator and a direct phase detector with channel filtering capability using injection-locked semiconductor lasers for applications in RF photonics. First, examples of optical systems whose performance can be greatly enhanced by using a linear intensity modulator are presented and existing linearized modulator designs are reviewed. The novel linear interferometric optical intensity modulator based on an injection-locked laser as an arcsine phase modulator is introduced and followed by numerical simulations of the phase and amplitude response of an injection-locked semiconductor laser. The numerical model is then extended to study the effects of the injection ratio, nonlinear cavity response, depth of phase and amplitude modulation on the spur-free dynamic range of a semiconductor resonant cavity linear modulator. Experimental results of the performance of the linear modulator implemented with a multi-mode Fabry-Perot semiconductor laser as the resonant cavity are shown and compared with the theoretical model. The modulator performance using a vertical cavity surface emitting laser as the resonant cavity is investigated as well. Very low Vπ in the order of 1 mV, multi-gigahertz bandwidth (-10 dB bandwidth of 5 GHz) and a spur-free dynamic range of 120 dB.Hz2/3 were measured directly after the modulator. The performance of the modulator in an analog link is experimentally investigated and the results show no degradation of the modulator linearity after a 1 km of SMF. The focus of the work then shifts to applications of an injection-locked semiconductor laser as a direct phase detector and channel filter. This phase detection technique does not iv require a local oscillator. Experimental results showing the detection and channel filtering capability of an injection-locked semiconductor diode laser in a three channel system are shown. The detected electrical signal has a signal-to-noise ratio better than 60 dB/Hz. In chapter 4, the phase noise added by an injection-locked vertical cavity surface emitting laser is studied using a self-heterodyne technique. The results show the dependency of the added phase noise on the injection ratio and detuning frequency. The final chapter outlines the future works on the linear interferometric intensity modulator including integration of the modulator on a semiconductor chip and the design of the modulator for input pulsed light

Method to Sense Changes in Network Parameters with High-Speed, Nonlinear Dynamical Nodes

<p>The study of dynamics on networks has been a major focus of nonlinear science over the past decade. Inferring network properties from the nodal dynamics is both a challenging task and of growing importance for applied network science. A subset of this broad question is: How can one determine changes to the coupling strength between elements in a small network of chaotic oscillators just by measuring the dynamics of one of the elements (nodes) in the network? In this dissertation, I propose and report on an implementation of a method to simultaneously determine: (1) which link is affected and (2) by how much it is attenuated when the coupling strength along one of the links in a small network of dynamical nodes is changed. After proper calibration, realizing this method involves only measurements of the dynamical features of a single node. </p><p>Previous attempts to solve this problem focus mainly on synchronization-based approaches implemented in low-speed, homogeneous experimental systems. In contrast, the experimental apparatus I use to implement my method comprises two high-speed (ps-timescale), heterogeneous optoelectronic oscillators (OEOs). Each OEO constitutes a node, and a network is formed by mutually coupling two nodes. I find that the correlation properties of the chaotic dynamics generated by the nodes, which are heavily influenced by the propagation time delays in the network, change in a quantifiable way when the coupling strength along either the input or output link is attenuated. By monitoring multiple aspects of the correlation properties, which I call ``time delay signatures'' (TDSs), I find that the affected link can be determined for changes in coupling strength greater than 20% &plusmn; 10%. Due to the sensitivity with which the TDSs change, it is also feasible to determine approximately the time-varying coupling strength for large enough attenuations.</p><p>I also verify that the TDSs' sensitivity to changes in coupling strength are captured by a simple deterministic model that takes into account each OEO's nonlinearities, bandpass filtering, and time delays. I find qualitative agreement between my experimental observations and numerical simulations of the model and also use the model to explore the dependence of the TDS signature on the OEO heterogeneity. I find that making the time delays identical leads to larger changes in TDSs, which improves the precision with which the coupling strength can be determined. This also leads, however, to a decrease in the ability to determine which link has been attenuated, indicating that a balance must be struck between optimizing the network's ability to discern the new coupling strength and the affected link. To investigate the role of the nonlinearity, I again test my method numerically using the same delay-coupled topology, but with dynamics generated by a linear stochastic process. I find that sensing can be achieved in the absence of nonlinear effects, but that, with regards to determining which link is affected, the performance is optimized differently in the linear and nonlinear cases.</p><p>This method could be extended to design a low-profile intrusion detection system, where several OEOs are spread around a scene and wirelessly coupled via antennas. The ultra-wide-band signals emitted by the nodes (OEOs) can pass through building materials with little attenuation, but would be strongly attenuated by a person who enters the path between two nodes. Beyond practical applications, it also remains to be seen if TDSs could prove to be a simple way to analyze information flow in networks with chaotic dynamics and propagation delays between the nodes.</p>Dissertatio