Advancing THz nanoscopy of 2D materials using compact sources and optimized nanoprobes

(English) The goal of this thesis is to probe the infrared optical response of twisted bilayer graphene (TBG) using Fourier transform infrared spectroscopy (FTIR). First, I used a commercial FTIR to measure the TBG in the mid-infrared range at room temperature. I improved the device fabrication technique and fabricated the TBG devices with a large area and simultaneously a low inhomogeneity. I observe that the TBG has abundant optical absorption features originating from the interband transitions that are uniquely determined by the twist angle. Then, I want to probe the interband transition of the TBG that lies in the terahertz range, which evolves the flat band of the TBG that hosts strongly correlated effects. I built a homemade FTIR that works in both the mid-infrared and terahertz range. I wired the cryostat carefully and achieved an electrical noise level approaching the Johnson noise limit. By guiding the light from the FITR into the cryostat, I successfully measured the exciton states in the Bernal bilayer graphene device over a broad spectral range, demonstrating that the system is ready for future experimental study of TBG. (Català) L'objectiu d'aquesta tesi és sondar la resposta òptica infraroja del grafè de bicapa retorçada (TBG) mitjançant l'espectroscòpia infraroja de transformada de Fourier (FTIR). Primer, vaig utilitzar un FTIR comercial per mesurar el TBG en el rang d'infraroig mitjà a temperatura ambient. Vaig millorar la tècnica de fabricació del dispositiu i vaig fabricar els dispositius TBG amb una gran àrea i alhora una baixa deshomogeneïtat. Observo que el TBG té abundants característiques d'absorció òptica originades a partir de les transicions entre bandes que es determinen únicament per l'angle de gir. Aleshores, vull investigar la transició entre bandes del TBG que es troba en el rang de terahertzs, que evoluciona la banda plana del TBG que allotja efectes fortament correlacionats. Vaig construir un FTIR casolà que funciona tant en l'infraroig mitjà com en el rang de terahertz. Vaig connectar el criostat amb cura i vaig aconseguir un nivell de soroll elèctric que s'acostava al límit de soroll de Johnson. En guiar la llum del FITR al criostat, vaig mesurar amb èxit els estats d'excitons al dispositiu de grafè de bicapa Bernal en un ampli rang espectral, demostrant que el sistema està preparat per a futurs estudis experimentals de TBG. (Español) El objetivo de esta tesis es investigar la respuesta óptica infrarroja del grafeno bicapa retorcido (TBG) mediante espectroscopia infrarroja por transformada de Fourier (FTIR). Primero, utilicé un FTIR comercial para medir el TBG en el rango del infrarrojo medio a temperatura ambiente. Mejoré la técnica de fabricación del dispositivo y fabriqué los dispositivos TBG con un área grande y, al mismo tiempo, una baja inhomogeneidad. Observé que el TBG tiene abundantes características de absorción óptica que se originan a partir de las transiciones entre bandas que están determinadas únicamente por el ángulo de torsión. Luego, quiero investigar la transición entre bandas del TBG que se encuentra en el rango de terahercios, que desarrolla la banda plana del TBG que alberga efectos fuertemente correlacionados. Construí un FTIR casero que funciona tanto en el rango del infrarrojo medio como en el de los terahercios. Conecté el criostato con cuidado y logré un nivel de ruido eléctrico que se acerca al límite de ruido de Johnson. Al guiar la luz desde el FITR hacia el criostato, medí con éxito los estados de excitón en el dispositivo de grafeno bicapa de Bernal en un amplio rango espectral, lo que demuestra que el sistema está listo para futuros estudios experimentales de TBG.

The generating of vortex beams in the terahertz (THz) band attracts significant attention due to their applications in high-speed communication and high-resolution imaging. In this article, a novel reflective metasurface working in the THz band is designed to generate four vortex beams with different topological charges in different directions. The unit cell is designed based on the geometric phase, and it consists of two metallic (gold) layers and one dielectric layer in between. The top layer of the unit cell includes an elliptic patch and a circular ring, and the bottom layer of the unit cell is a metallic ground. The reflection efficiency of the unit cell is very high due to the presence of metallic ground. To break through the limitations of traditional methods, the metasurface is a good choice to generate beams that carry orbital angular momentum (OAM). Using the concept of geometric phase, the reflection phase of reflective circular polarization (CP) electromagnetic waves can be controlled in an ingenious way. Owing to the property of the geometric phase, inverse phase shift can be achieved for left-handed circular polarization and right-handed circular polarization waves. By utilizing this trait of geometric phase, one can decompose a linear polarization wave into two orthogonal circular polarization waves and control their properties respectively. By rotating the top layer of the unit cell, 360-degree phase shift and the phase distribution satisfying the requirement for generating the multi-beam multi-mode vortex beam can be achieved. In order to control the direction and the topological charge of each beam, based on the geometric phase, the theory of reflectarray and the phase composition principle, the phase distribution of the reflective metasurface is calculated to provide the phase compensation to make the vortex beams point to certain directions. It is worthwhile to point out that the method presented in this paper provides a way to generate complex multi-mode vortex beams in the THz band. The simulations and measurements show that the metasurface can generate four vortex beams with topological charges <i>l</i> = ± 1 and ± 2 in different directions in the THz band. These results also indicate that our design has great potential applications in wireless communication and high-resolution imaging.

The quantum cascade laser (QCL) is an important laser source in the mid-infrared and terahertz frequency range. The past twenty years have witnessed its tremendous development in power, wall plug efficiency, frequency coverage and tunability, beam quality, as well as various applications based on QCL technology. Nowadays, QCLs can deliver high continuous wave power output up to 5.1 W at room temperature, and cover a wide frequency range from 3 to 300 μm by simply varying the material components. Broadband heterogeneous QCLs with a broad spectral range from 3 to 12 μm, wavelength agile QCLs based on monolithic sampled grating design, and on-chip beam QCL combiner are being developed for the next generation tunable mid-infrared source for spectroscopy and sensing. Terahertz sources based on nonlinear generation in QCLs further extend the accessible wavelength into the terahertz range. Room temperature continuous wave operation, high terahertz power up to 1.9 mW, and wide frequency tunability form 1 to 5 THz makes this type of device suitable for many applications in terahertz spectroscopy, imaging, and communication.

In last couple of decades, the THz science and technology has made huge leap of progress and THz sources and detectors in limited field strength and frequency bandwidth are being made available commercially for applications in spectroscopy, sensing, imaging and medical diagnostics. This is primarily because of the mature NIR femtosecond laser technology and the coherent nature of the THz radiation allowing researchers to create simple table top experimental tools for their THz studies. However, there is a growing demand for intense THz sources which can allow perform nonlinear optics and spectroscopy and develop new methodologies. For nonlinear THz radiation and matter interaction, one needs typically THz-fields comparable to the atomic fields $(\sim 10\wedge 6\mathrm{V}/$cm) such as those provided by lasers in other regions of the electromagnetic spectrum. Photoconductive antennas based on narrow band-gap materials such as InGaAs have seen a great improvement in their performances in terms of optical-to-THz efficiencies and maximum output pulsed THz radiation powers at frequencies above 1 THz due to the availability of high power laser sources operating at the optical telecommunication wavelengths. Even more powerful THz emitters have generally been developed by using nonlinear optical crystals where optical rectification of intense near infrared femtosecond laser pulses produces broadband THz radiation of field strengths reaching few hundreds of kV/cm to even few thousands kV/cm. Zinc telluride and lithium niobate crystals are very popular in this case [1]. Another popular mechanism for generating strong THz fields is by creating plasma in air and phase mismatch between the fundamental and the second harmonic fields of NIR femtosecond pulses [2]. Hetero-structures of layers of ferromagnetic materials and nonferromagnetic films have recently been used to produce THz radiation by excitation with femtosecond laser pulses [3].

Terahertz (THz) wave science and technology have been found countless applications in biomedical imaging, security screening, and non-destructive testing as they approach maturity. However, due to the challenge of high ambient moisture absorption, the development of remote open-air broadband THz spectroscopy technology is lagging behind the compelling need that exists in homeland security, astronomy and environmental monitoring. Furthermore, the underlying physical mechanisms behind the interaction between the THz wave and laserinduced plasma which responds strongly to electromagnetic waves have not been fully understood. This review aims to explain the light-plasma interaction at THz frequencies within a semiclassical framework along with experimental study of the femtosecond-laserinduced nitrogen plasma fluorescence under the illumination of single-cycle THz pulses. The results indicate that THz-radiation-enhanced-emission-of-fluorescence (THz-REEF) is dominated by electron kinetics in the THz field and the electron-impact excitation of gas molecules/ions. The information of the time-dependent THz field can be recovered from the measured time-resolved THz-REEF from single-color laser induced plasma with the help of the bias as local oscillator. The calculations and experimental verification lead to complete understanding of the science behind these effects and push forward to extend their capabilities in related applications such as remote THz sensing, plasma diagnostics and ultrafast photoluminescence modulation. Systematic studies in selected gases including neon, argon, krypton, xenon, methane (CH4), ethane (C2H6), propane (C3H8), and n-butane (C4H10) gases were performed to obtain an improved understanding of the THz-REEF. The dependences of the enhanced fluorescence on the THz field, laser excitation intensity, gas pressure, and intrinsic atomic properties were experimentally characterized. Both narrow line emission and broad continuum emission of the gas plasma were enhanced by the THz field. Their fluorescence enhancement ratios and time-resolved enhanced fluorescence were largely dependent on the scattering cross section and ionization potential of atoms. For the first time, we demonstrated a novel ‘all-optical’ technique of broadband THz wave remote sensing by coherently manipulating the fluorescence emission from asymmetrically ionized gas plasma that interacted with THz waves. By studying the ultrafast electron dynamics under the single cycle THz radiation, we found that the fluorescence emission from laser-induced air plasma was highly dependent on the THz electric field and the symmetry of the electron drift velocity distribution created by two-color laser fields. The time-resolved THz-REEF can be tailored by switching the relative two-color phase and laser polarizations. Owing to the high atmospheric transparency and omni-directional emission pattern of fluorescence, this technique can be used to measure THz pulses at standoff distances with minimal water vapor absorption and unlimited directionality for optical signal collection. The coherent THz wave detection at a distance of 10 m had been demonstrated. The combination of this method and previously demonstrated remote THz generation would eventually make remote THz spectroscopy available. We also introduced a unique plasma diagnostic method utilizing the THz-wave-enhanced fluorescence emission from the excited atoms or molecules. The electron relaxation time and plasma density were deduced through applying the electron impact excitation/ionization and electron-ion recombination processes to the measured time-delay-dependent enhanced fluorescence. The electron collision dynamics of nitrogen plasma excited at different gas pressures and laser pulse energies were systematically investigated. This plasma diagnostic method offers picosecond temporal resolution and is capable of omnidirectional optical signal collection. The ultrafast quenching dynamics of laser-pulse-induced photoluminescence in semiconductors under the radiation of single-cycle THz pulses was studied. It was found that the quenching in both cadmium telluride (CdTe) and gallium arsenide (GaAs) was linearly proportional to the intensity of incident THz waves and reaches up to 17% and 4% respectively at the peak intensity of 13 MW/cm2. The THz-wave-induced heating of the carriers and lattice and the subsequent decreased efficiency of photocarrier generation and recombination were most likely to be responsible for the quenching. This is potentially useful for the applications of a non-invasive ultrafast light modulator for photoluminescence devices with picoseconds switching time in the fields of the light-emitting devices and optical communication.

We use plasmon enhancement to achieve terahertz (THz) photoconductive switches that combine the benefits of low-temperature grown GaAs with mature 1.5 μm femtosecond lasers operating below the bandgap. These below bandgap plasmon-enhanced photoconductive receivers and sources significantly outperform commercial devices based on InGaAs, both in terms of bandwidth and power, even though they operate well below saturation. This paves the way for high-performance low-cost portable systems to enable emerging THz applications in spectroscopy, security, medical imaging, and communication.

Novel approach to controlling polarization of THz radiation is introduced. Polarization plane rotation of transmitted wave by a monolayer of precise microhelices was demonstrated by experiments on the Novosibirsk free electron laser. It had strongly resonant character. Modulation of polarization opens up new possibilities in communication, biomedical, chemistry, security, imaging and spectroscopy applications.

Terahertz (THz) radiation is gaining momentum in biology, medicine, communication, security, chemistry, and spectroscopy applications. To expand the usability of terahertz radiation the man-made metal-dielectric composite metamaterials are typically considered owing to their ability to effectively manipulate electromagnetic waves. The possibilities of light manipulation can be extended even more by involving new active materials as a structural component – such as, for example, graphene. Its prominent conductivity tunability through the electrochemical potential change allows converting a multilayer graphene/dielectric structure into an artificial medium with widely varying properties – transparent or opaque, plasmonic, low-index or high-index dielectric – in a certain THz or infra-red frequency range. This chapter presents several examples of effective THz components like absorbers, modulators, and filters based on graphene-dielectric multilayers.

Twisted bilayer graphene (TBG) generates significant attention in the fundamental research of 2D materials due to its distinct twist-angle-dependent properties. Exploring the efficient production of TBG with a wide range of twist angles stands as one of the major frontiers in moiré materials. Here, the local space-confined chemical vapor deposition growth technique for high-quality single-crystal TBG with twist angles ranging from 0° to 30° on liquid copper substrates is reported. The clean surface, pristine interface, high crystallinity, and thermal stability of TBG are verified by using comprehensive characterization techniques including optical microscopy, electron microscopy, and secondary-ion mass spectrometry. The proportion of TBG in bilayer graphene reaches as high as 89%. In addition, the stacking structure and growth mechanism of TBG are investigated, revealing that the second graphene layer develops beneath the first one. A series of comparative experiments illustrates that the liquid copper surface, with its excellent fluidity, promotes the growth of TBG. Electrical measurements show the twist-angle-dependent electronic properties of as-grown TBG, achieving a room-temperature carrier mobility of 26640 cm2 V-1 s-1 . This work provides an approach for the in situ preparation of 2D twisted materials and facilitates the application of TBG in the fields of electronics.

The optical gain spectrum has been investigated theoretically for various designs of active region based on InAs/GaInSb quantum wells—i.e., a type II material system employable in interband cascade lasers (ICLs) or optical amplifiers operating in the mid-infrared spectral range. The electronic properties and optical responses have been calculated using the eight-band k·p theory, including strain and external electric fields, to simulate the realistic conditions occurring in operational devices. The results show that intentionally introducing a slight nonuniformity between two subsequent stages of a cascaded device via the properly engineered modification of the type II quantum wells of the active area offers the possibility to significantly broaden the gain function. A −3 dB gain width of 1 µm can be reached in the 3–5 µm range, which is almost an order of magnitude larger than that of any previously reported ICLs. This is a property strongly demanded in many gas-sensing or free-space communication applications, and it opens a way for a new generation of devices in the mid-infrared range, such as broadly tunable single-mode lasers, mode-locked lasers for laser-based spectrometers, and optical amplifiers or superluminescent diodes which do not exist beyond 3 µm yet.

Stable ultrashort light pulses and frequency combs generated by mode-locked lasers have many important applications including high-resolution spectroscopy, fast chemical detection and identification, studies of ultrafast processes, and laser metrology. While compact mode-locked lasers emitting in the visible and near infrared range have revolutionized photonic technologies, the systems operating in the mid-infrared range where most gases have their strong absorption lines, are bulky and expensive and rely on nonlinear frequency down-conversion. Quantum cascade lasers are the most powerful and versatile compact light sources in the mid-infrared range, yet achieving their mode-locked operation remains a challenge, despite dedicated effort. Here we report the demonstration of active mode locking of an external-cavity quantum cascade laser. The laser operates in the mode-locked regime at room temperature and over the full dynamic range of injection currents.

In recent years, the field of Terahertz (THz) science and technology has entered a completely new phase of unprecedented expansion that is generating ever-growing levels of broadbased international attention. Indeed, the plethora of activities that have arisen recently in both the technology and scientific arenas associated with the THz frequency domain i.e., between 1 millimeter (300 GHz) and 100 micrometers (3 THz), suggest that the field might be attempting to undergo a dramatic transition that could lead to long-awaited payoffs in a number of application areas. The inherent advantages and potential payoffs of the THz regime for military & security as well as industry relevant applications have long stood as an important driver of interest in this science and technology area. This extremely expansive and spectrally unique portion of the EM spectrum had initial application in space-based communications, upper atmospheric sensing and potentially for short-range terrestrial communications and non-intrusive package screening. However, the very rapid growth in more recent years is arguably most closely linked to the potential payoffs of THz sensing and imaging for an array of military, security and industrial applications. These applications include the spectroscopic-based detection identification and characterization of chemical and biological agents and materials, remote and standoff early-warning for chemicalbiological warfare threats, and imaging of concealed weapons and explosives, just to name a few. In addition, THz-regime finds its application possibilities in industry and private-sector areas as food-industry process control, pharmaceutical industry, biological science, medical diagnostics and security screening. Systems for rapidly emerging applications at THz frequencies thus require reliable highpower sources. In the last few years, the development of suitable sources for this frequency regime is being extensively explored worldwide. There are broadly two technology roadmaps for THz semiconductor devices. Approaching from the lower frequency range in the THz regime, electronic devices such as, Gunn diode, Resonant Tunneling diode (RTD) and nanometer Field Effect Transistors (FET) based on plasma wave have been widely investigated for THz frequency generation. From higher portion of the THz frequency spectrum, the photonics-based device Quantum Cascade Laser (QCL) extends the emission 7

Summary form only given. The terahertz (THz) band of the electromagnetic spectrum has long held interest in the science and engineering community for its unique applications in imaging and spectroscopy. However, due to the difficulty in generating and receiving THz radiation, only very recently practical and efficient THz systems become available. Due to recent advances in the development and manufacturing of commercially available THz spectroscopy and imaging system many of these applications are coming to fruition. As such, the practical implementation of THz in applications needs to be investigated in order to determine the advantages of THz technology. In this work, THz spectroscopy of photovoltaic semiconductor devices will be performed throughout various critical stages in the fabrication process. Due to the few millimeter penetrating and none-destructive properties of THz radiation it will be possible to monitor semiconductor wafers using THz spectroscopy. This technology has a potential to provide insight into the development of wafer defects earlier in the fabrication process. Using a pulsed Terahertz imaging and spectroscopy system at the University of Arkansas, our group will investigate the electrical properties of semiconductor photovoltaics and will conduct THz imaging of defected ones. Testing the wafers throughout various key steps in fabrication is a process known to produce a functional device. This will then be repeated for the same process, only now with a critical fatal defect such as a buried crack, surface crack or a defect in the doping level. These defects will be intentionally inserted at an early stage to determine if THz spectroscopy can be used to detect them early on. THz characterization of the semiconductors for the impurity doping levels and crystal orientation to be studied will be performed to determine the electrical properties in this frequency range. This will allow for accurate numerical simulation to compare with the experimental results.

Terahertz (THz) waves have exhibited promising applications in imaging, sensing, and communications, especially for the next-generation wireless communications due to the large bandwidth and abundant spectral resources. Modulators and waveguides to manipulate THz waves are becoming key components to develop the relevant technologies where metamaterials have exhibited extraordinary performance to control free-space and on-chip propagation, respectively. In this review, we will give a brief overview of the current progress in active metadevices and topological photonic crystals, for applications of terahertz free-space modulators and on-chip waveguides. In the first part, the most recent research progress of active terahertz metadevices will be discussed by combining metamaterials with various active media. In the second part, fundamentals of photonic topological insulations will be introduced where the topological photonic crystals are an emerging research area that would boost the development of on-chip terahertz communications. It is envisioned that the combination of them would find great potential in more advanced terahertz applications, such as reconfigurable topological waveguides and topologically-protected metadevices.

High-efficiency terahertz (THz) emission and detection are of great interest because of their promising applications in high-speed communications, biomedicine, and imaging. A previous study has achieved efficient room-temperature THz emission at ∼360 GHz by green-light exciting the lattice symmetric stretching vibrations of ZnO nanoplates self-assembled into ZnO microspheres (MSs). Herein, we explore resonant THz radiation of this kind of ZnO MSs under around 360 GHz excitation. A Fabry–Perot resonant cavity is designed and used to obtain the resonant THz signal. Compared to the case without the ZnO MSs, the THz output powers are enhanced by 1.5 and 3.2 times under two excitations of 356.1 and 375.8 GHz with an input power of 6.5 mW, respectively. Furthermore, it is shown that when a wide frequency THz wave irradiates on the ZnO MSs in the cavity, the output THz signal strength shows an obvious variation with frequency and can thus be utilized to detect the presence of some THz waves with specific frequencies. This work indicates that such self-assembled MSs can not only radiate the enhanced THz waves via a resonator, but also effectively apperceive some specific THz signals as a detector.