Novel Devices for Terahertz Wave Imaging, Wave-guiding and Sensing

Several novel optical devices, which were designed to manipulate terahertz waves for broadband near-field imaging, wave-guiding (invisible space), and sensing (resonator), are presented in this thesis. We developed the original working concepts of each device, and demonstrated the prototype experimentally in our lab. The working concepts of physics were investigated in experiment, in simulation and in theoretical analysis. We exploited a tapered parallel-plate waveguide (PPWG) as a novel probe for broadband near-field imaging. This imaging probe consists of two metal plates with the plate spacing gradually tapered from one end to the other. We proved that the space tapering enables this probe to propagate the broadband THz waves efficiently (with low-loss, no cut-off and nearly no dispersion) from the input end of large spacing into the narrow end of sub-wavelength spacing. Working in a reflection mode, this imaging probe is proved to be able to differentiate the dielectric features as well as topographic information on the sample. Combined with the methodology of filtered back projection, we reconstructed a two-dimensional image of a gold pattern on a GaAs chip by using this tapered PPWG probe. The smallest feature of ~100 µm is resolved by using the waves with average wavelength of 1.5 mm. We studied the phenomenon of surface plasmon-polariton in THz range on the platform of a parallel-plate waveguide (PPWG). In this thesis, we show the characterization of the waveguide mode of a finite-width parallel plate waveguide by using an improved scattering-probe technique. An abrupt waveguide mode transition was observed at a very narrow frequency range. We demonstrated that this transition frequency is determined by the material properties of the waveguide, the frequencies of the electromagnetic waves as well as the geometry of the waveguide. This result provides a good guidance for the waveguide design for THz transmission. We also exploited the capability of using the spoof surface plasmon to enhance the reflectivity of an interface between free space and a PPWG. We demonstrated that the reflection coefficient of this interface can be enhanced up to ~100 % at a designed frequency, by cutting a designed pattern of periodic rectangular groove on the output facet of the PPWG. A lateral shift and a phase shift of the reflected beam is observed in the experiment, which is a strong reminiscent of Goos-Hanchen shift. We carried out the experimental, simulation and theoretical characterizations of the lateral and phase shift. As an application, we designed and demonstrated a prototype of a band-pass THz resonator. We introduced the concept of a waveguide-based two-dimensional inhomogeneous artificial dielectric into THz range. This artificial dielectric is the space between the two metal plates of a PPWG working in TE1 mode. We designed a THz mirage device (or an invisible space device) by using ray-tracing and full-wave simulations, which contributed to the first experimental demonstration of such a device. A metal coin of size several times larger than the working wavelength can be hidden in the device without casting any shadow. This work is in collaboration with Dr. Rajind Mendis and the author of this thesis contributed to the design and characterization of the device in simulations

Doctor of Philosophy

dissertationThis dissertation describes our work on design, fabrication and characterization of plasmonic metamaterials and tapered structures, with primary focus on their applications at terahertz (THz) frequencies. The phenomena associated with these structures rely on surface plasmon polaritons (SPPs), which may allow for high field enhancement and tight field confinement. We have investigated the underlying mechanisms of these structures and used that knowledge to develop unique and practical applications. We first studied two-dimensional periodic and random lattices based on aperture arrays, and modified the model to describe the effective dielectric response of the perforated metallic medium. Using two layers of the perforated stainless steel films, we demonstrated the emergence of an additional resonance and reproduced the transmission spectra using the effective dielectric model of the single-layer medium. Also, we improved the filtering performance of the multilayer periodic aperture arrays by adjusting the relative distance and angle between the layers, and demonstrated its application as a high quality bandpass filter. Then, we examined the transmission properties of graphite and carbon nanotube (CNT) films, and then the same films perforated with periodically distributed aperture arrays. The extracted dielectric constants of the graphite and CNT films demonstrate their availability for THz surface plasmonic devices. Moreover, we developed a narrow band/multiband THz detector in which the photoconductive antenna was surrounded by periodically corrugated gratings. This detector not only enhanced the sensitivity of detection at the specific frequencies, but also efficiently collected the radiation within the structure area, which obviated the need for a substrate lens. Finally, we improved the concentration properties of conically tapered apertures. Based on the optimal taper angle we determined, we introduced various modifications to the individual tapered aperture, e.g., to form an array and insert a gap spacing, and further enhanced the concentration capabilities and realized complete broadband transmission. Based on these studies and results, we are currently extending our work towards development of more reconfigurable and active devices that could enrich the available pool of THz and optical devices. Furthermore, such THz devices have great promise for the development of THz systems level applications and even a THz-based world in the future

Plasmonic and Photonic Designs for Light Trapping in Thin Film Solar Cells

Thin film solar cells are promising to realize cheap solar energy. Compared to conventional wafer cells, they can reduce the use of semiconductor material by 90%. The efficiency of thin film solar cells, however, is limited due to insufficient light absorption. Sufficient light absorption at the bandgap of semiconductor requires a light path more than 10x the thickness of the semiconductor. Advanced designs for light trapping are necessary for solar cells to absorb sufficient light within a limited volume of semiconductor. The goal is to convert the incident light into a trapped mode in the semiconductor layer. In this dissertation, a critical review of currently used methods for light trapping in solar cells is presented. The disadvantage of each design is pointed out including insufficient enhancement, undesired optical loss and undesired loss in carrier transport. The focus of the dissertation is light trapping by plasmonic and photonic structures in thin film Si solar cells. The performance of light trapping by plasmonic structures is dependent on the efficiency of photon radiation from plasmonic structures. The theory of antenna radiation is used to study the radiation by plasmonic structures. In order to achieve efficient photon radiation at a plasmonic resonance, a proper distribution of surface charges is necessary. The planar fishnet structure is proposed as a substitution for plasmonic particles. Large particles are required in order to resonate at the bandgap of semiconductor material. Hence, the resulting overall thickness of solar cells with large particles is large. Instead, the resonance of fishnet structure can be tuned without affecting the overall cell thickness. Numerical simulation shows that the enhancement of light absorption in the active layer is over 10x compared to the same cell without fishnet. Photons radiated from the resonating fishnet structure travel in multiple directions within the semiconductor layer. There is enhanced field localization due to interference. The short circuit current was enhanced by 13.29%. Photonic structures such as nanodomes and gratings are studied. Compared to existing designs, photonic structures studied in this dissertation exhibited further improvements in light absorption and carrier transport. The nanodome geometry was combined with conductive charge collectors in order to perform simultaneous enhancement in optics and carrier transport. Despite the increased volume of semiconductor material, the collection length for carriers is less than the diffusion length for minority carriers. The nanodome geometry can be used in the back end and the front end of solar cells. A blazed grating structure made of transparent conductive oxide serves as the back passivation layer while enhancing light absorption. The surface area of the absorber is increased by only 15%, indicating a limited increase in surface recombination. The resulting short circuit current is enhanced by over 20%. The designs presented in the dissertation have demonstrated enhancement in Si thin film solar cells. The enhancement is achieved without hurting carrier transport in solar cells. As a result, the enhancement in light absorption can efficiently convert to the enhancement in cell efficiency. The fabrication of the proposed designs in this dissertation involves expensive process such as electron beam lithography. Future work is focused on optical designs that are feasible for cheap fabrication process. The designs studied in this dissertation can serve as prototype designs for future work

Large-aperture Wide-bandwidth Antireflection-coated Silicon Lenses for Millimeter Wavelengths

The increasing scale of cryogenic detector arrays for submillimeter and millimeter wavelength astrophysics has led to the need for large aperture, high index of refraction, low loss, cryogenic refracting optics. Silicon with n 3.4, low loss, and high thermal conductivity is a nearly optimal material for these purposes but requires an antireflection (AR) coating with broad bandwidth, low loss, low reflectance, and a matched coefficient of thermal expansion. We present an AR coating for curved silicon optics comprised of subwavelength features cut into the lens surface with a custom three-axis silicon dicing saw. These features constitute a metamaterial that behaves as a simple dielectric coating.We have fabricated silicon lenses as large as 33.4 cm in diameter with micromachined layers optimized for use between 125 and 165 GHz. Our design reduces average reflections to a few tenths of a percent for angles of incidence up to 30deg with low cross polarization.We describe the design, tolerance, manufacture, and measurements of these coatings and present measurements of the optical properties of silicon at millimeter wavelengths at cryogenic and room temperatures. This coating and lens fabrication approach is applicable from centimeter to submillimeter wavelengths and can be used to fabricate coatings with greater than octave bandwidth

Large-Aperture Wide-Bandwidth Anti-Reflection-Coated Silicon Lenses for Millimeter Wavelengths

The increasing scale of cryogenic detector arrays for sub-millimeter and millimeter wavelength astrophysics has led to the need for large aperture, high index of refraction, low loss, cryogenic refracting optics. Silicon with n = 3.4, low loss, and relatively high thermal conductivity is a nearly optimal material for these purposes, but requires an antireflection (AR) coating with broad bandwidth, low loss, low reflectance, and a matched coffecient of thermal expansion. We present an AR coating for curved silicon optics comprised of subwavelength features cut into the lens surface with a custom three axis silicon dicing saw. These features constitute a metamaterial that behaves as a simple dielectric coating. We have fabricated and coated silicon lenses as large as 33.4 cm in diameter with coatings optimized for use between 125-165 GHz. Our design reduces average reflections to a few tenths of a percent for angles of incidence up to 30 deg. with low cross-polarization. We describe the design, tolerance, manufacture, and measurements of these coatings and present measurements of the optical properties of silicon at millimeter wavelengths at cryogenic and room temperatures. This coating and lens fabrication approach is applicable from centimeter to sub-millimeter wavelengths and can be used to fabricate coatings with greater than octave bandwidth

The Lateral Confinement of Microwave Surface Waves

Surface waves and their applications have been extensively studied by the photonics and radio engineering communities throughout the whole of the twentieth century. This thesis details briefly the history of both approaches and highlights their signi cance with regard to the subject of this thesis; laterally confining a surface wave in the microwave regime. Detailed within are the experimental, analytical and numerical methods used to ascertain what, if any, effect a change in the dimension of a guiding structure has on the dispersion of a mode supported by a metamaterial. The method of experimentally determining the dispersion of a microwave surface wave is discussed. The insensitivity of a mode supported on a one-dimensional corrugated array to the lateral width of the supporting array, even when the width is much less than the wavelength of radiation incident upon it, is investigated. Spatial dependent reduction of group velocity associated with a microwave surface wave is also detailed. Local electric-field and phase measurements are used to probe this condition. In particular, the measurement of phase associated with the supported microwave surface wave is shown to indicate the trapping location of a surface wave more accurately when compared to local electric-field measurement. The channelling of surface waves via the addition of dielectric overlayers to a metamaterial surface is investigated. By progressively narrowing the width of the channel, the interaction of the electric fields associated with the mode supported in the channel with the bordering dielectric overlayer increases. This investigation leads to a discussion of the electric field overlap between two regions of differing surface impedance.EPSR

Computational nanooptics in hyperbolic metamaterials and plasmonic structures

This dissertation concerns several problems in the fields of light interaction with nanostructured media, metamaterials, and plasmonics. We present a technique capable of extending operational bandwidth of hyperbolic metamaterials based on interleaved highly-doped InGaAs and undoped AlInAs multilayer stacks. The experimental results confirm theoretical predictions, exhibiting broadband negative refraction response in mid-infrared frequency. We propose a new class of nanofocusing structures, named hypergrating, combining hyperbolic metamaterials with Fresnel optics, able to achieve extremely subwavelength focal spots (up to 50 times smaller than free-space wavelength) in the far field of the input interface. Several experimental realizations of hypergratings for visible and infrared frequencies are presented. We further develop a new technique capable of imaging subwavelength objects with far-field measurements. The approach utilizes a diffraction grating, placed at the object plane, to convert subwavelength information of objects into propagating waves and project this information into far-field. The set of far-field measurements is used to deconvolute the images. The resolution of the proposed method can surpass 1/20-th of the free-space limit, strongly overperforming other subwavelength imaging technology. We develop a new mode matching approach for analysis of scattering and propagation of surface and volume modes in multiple multilayered-stack structures. Our theory relies on the complete spectrum of free-space and guided electromagnetic modes to solve Maxwell's equations in the extended systems that have relatively few interfaces. We demonstrate the convergence of this technique on a number of plasmonic and metamaterial structures. Finally, we consider the problem of plasmonic beam-steering structures consisting of a single slit flanked by a periodic set of metallic corrugations. We show that the light emitted by the structures forms a prolonged focal range that may extend for hundreds of wavelength from the plasmonic interface and eventually splits into two plasmonic beams. We develop a quantitative theory to physically describe the beam formations and evolution of field pattern. The numerical and analytical results presented here can be applied to several nanooptics applications including deep-subwavelength imaging, nanolithography, on-chip communications, high-density energy focusing, and beaming devices, and can be used for metamaterial and plasmonic composites operating across ultraviolet, visible, infrared, or terahertz spectra

Controlling Terahertz Radiation - Novel Fabrication Methods and Materials for Terahertz Components

The interaction between light and matter has been a field of research for centuries, from the days of Sir Isaac Newton in the 17th century up to today, where new effects, such as plasmonics open up new applications or the extension of the accessible electromagnetic spectrum, are still engaging scientists and engineers in this field of research. The understanding of the interaction between light, or more general: electromagnetic radiation and matter is a crucial step in the development of components which give the necessary control to gain access to the desired part of the electromagnetic spectrum. One of the less developed parts of the electromagnetic spectrum is terahertz (THz) radiation. THz radiation promises many applications, from spectroscopy for material and medical applications to communication technology. But, so far, most applications have not managed to overcome the experimental status, mostly because of missing materials and manufacturing methods suitable for the required length scales and material properties in the terahertz regime. This thesis focuses on structures for the control of THz radiation. To do so, and to overcome the natural limitations of many materials in the THz region, new materials and modern fabrication techniques are used to find new ways to overcome the shortage of readily available components for this part of the electromagnetic spectrum. As such, ceramics and polymers are used for various components, from lenses to spoof plasmonic waveguides, fabricated with a variety of techniques, including 3D printing and micro-milling. Finite-Difference Time-Domain simulations are used for the design of all structures. The ultimate goal is to demonstrate low-cost methods to produce THz components for future industrial implementation

The Development of Quasi-Optical Techniques for Log Wavelength Imaging

This thesis concerns the development of quasi-optical techniques for long-wavelength imaging, which was conducted through a combination of experimentation and computer simulation. This work was conducted as part of a SFI-funded research program undertaken by the THz Optics group of the Department of Experimental Physics at NUI Maynooth, the aim of which was to extend existing quasi-optical techniques through experimental measurements and the development of simulation tools necessary for efficient design and analysis of long-wavelength optical systems. Description of the upgrading of the 100 GHz test measurement facilities at NUIM and the results obtained from transmission- and reflection-mode active imaging experiments are presented. Numerical simulation of quasi-optical components and systems using scalar wave diffraction techniques was performed. In particular, Gaussian Beam Mode Analysis (GBMA) was applied to the design and analysis of discrete and continuous phase modulating optical multiplexers (phase gratings) for use at 100 GHz. The use of GBMA for iterative phase retrieval was investigated for application to phase grating design. Several phase unwrapping techniques were also investigated in order to simplify manufacture of phase gratings with difficult-to-fabricate profiles. Results of experimental measurements from a number of test gratings are presented and verified using the Maynooth Optical Design and Analysis Laboratory (MODAL) software package. Further improvements to phase grating design are also presented

The Development of Microfluidic and Plasmonic Devices for Terahertz Frequencies

The wealth of opportunities associated with the terahertz (THz) region of the electromagnetic spectrum have only recently, thanks to advances in technology, begun to be fully recognised and exploited. The advent of terahertz time-domain spectroscopy (THz-TDS) has led to a wide spectrum of research, spanning chemical, biological and physical systems. However, the relative immaturity of THz techniques results in a variety of inherent problems which limit the potential applications. With an equality existing between the wavelength of THz radiation, and the length scales associated with modern microfabrication techniques, such technology can be exploited to facilitate in finding solutions to these problems. This thesis seeks to address one of these problems, namely the strong absorptions associated with liquid water in the THz region. A simple design idea, that if the optical path length through a fluidic sample were reduced, strong signals could be detected after direct transmission, resulted in a micromachined fluidic cell being devised. The design, fabrication and testing of a microfluidic device inherently transparent to THz radiation, and designed for use in a standard THz-TDS arrangement, is presented. A range of samples, including primary alcohol-water mixtures, commercial whiskies and organic materials are analysed, which, when used in conjunction with data extraction algorithms, allows for accurate dielectric information to be yielded. Further exploitation of micromachining techniques are presented, where a variety of structures, seeking to initiate and utilise a class of surface electromagnetic wave known as surface plasmon polaritons (SPPs), are realised. By flanking a single sub-wavelength aperture with sub-wavelength periodic corrugations, extraordinary optical transmission (EOT) can be observed. This technique allows smaller apertures to be used for THz near-field imaging applications, with a view to increase spatial resolution. The first demonstration of THz near-field imaging using sub-wavelength plasmonic apertures in conjunction with a THz quantum cascade laser source, is presented. Detailed investigations into EOT for the case of two-dimensional, sub-wavelength aperture arrays are documented. A qualitative time-of-flight model describing the transmission properties of these structures is presented, resulting from systematic investigations into a variety of geometrical effects. This model has allowed sharp resonances to be engineered in the frequency domain. A hybrid device featuring a combination of sub-wavelength periodic apertures and corrugations is also investigated. Such a structure is not known to have been described previously in the literature, either in the optical or THz domains. The device demonstrates unparallelled transmission efficiencies, termed `super' EOT. Finally, a device combining the microfluidic technology with the highly resonant SPP structures is presented. This device seeks to exploit the innate dependence of SPPs to a metal-dielectric interface, for use as a sensor. By introducing a range of fluids into the device, the change in the metal-dielectric interface induced a change in the frequency response of the resonant structure. The magnitude of the observed frequency shift can be related back to the dielectric properties of the fluid. This result displays how microfabrication techniques can be successfully exploited to create devices for THz applications, seeking to provide solutions to the inherent problems associated with this part of the electromagnetic spectrum