R-3 Repository :: Browsing by Author "Mittleman, Daniel M."

Browsing by Author "Mittleman, Daniel M."

Now showing 1 - 20 of 54

A frequency-agile retrodirective tag for large-scale sub-terahertz data backscattering
(Springer Nature, 2024) Kludze, Atsutse; Kono, Junichiro; Mittleman, Daniel M.; Ghasempour, Yasaman
Backscattering is a promising power-efficient communication technique providing sustainable wireless links with a low carbon footprint. This approach is a critical enabler for dense IoT networks, which are forecast to grow to 41 billion by 2025. However, existing backscatter designs are limited to the sub-6 GHz bands or narrowband operation in the millimeter-wave regime; therefore, they fail to concurrently support many interference-free low-power users. Enabling a frequency-agile wideband backscatter design in the sub-terahertz offers a two-pronged advantage for densely deployed backscatter networks: spatial reuse enabled by directionality and frequency multiplexing enabled by the large available bandwidth. We present the first sub-THz backscatter architecture that operates above 100 GHz. Our design relies on a detailed understanding of reciprocity in leaky-wave devices and offers a realistic joint localization and communication protocol for sub-THz backscatter networks.
A study of background signals in terahertz apertureless near-field microscopy and their use for scattering-probe imaging
(2009) Astley, Victoria; Mittleman, Daniel M.
Apertureless near-field microscopy is an imaging technique in which a small metal tip is held close to a surface, converting evanescent waves to propagating waves and permitting sub-wavelength spatial resolution. In the terahertz region of the spectrum, the interpretation of measured signals and the suppression of background scattering can be complicated by the broad bandwidth of the THz source and by the phase-sensitive detection of the scattered radiation. We have analyzed the use of tip-sample distance modulation for the removal of background signals. We find that significant background signals, originating from scattering off the probe tip, can be observed even after modulation. These background signals result from path-length difference modulation, and thus depend on phase-sensitive detection. We use a dipole antenna model to explain the spatial variation of this signal. Since it originates from the tip only, it can be used to characterize free-space terahertz wave fronts with sub-wavelength resolution.
A terahertz band-pass resonator based on enhanced reflectivity using spoof surface plasmons
(IOP Publishing, 2013) Liu, Jingbo; Mendis, Rajind; Mittleman, Daniel M.
We demonstrate a band-pass resonator in the terahertz (THz) range, based on a frequency-selective designer reflector. The resonator consists of a parallel-plate waveguide, a designed groove pattern cut into the output facet of each plate, and a reflecting mirror. The patterned facet supports a spoof surface plasmon mode, which modifies the reflectivity at the waveguide output facet by interacting with the waveguide mode. By tuning the geometrical parameters of the groove pattern, the reflectivity at the patterned output facet can be increased up to ~100% for a selected frequency. Broadband THz waves are quasi-optically coupled into this resonator and reflected multiple times from the patterned facet. This leads to a narrowing of the spectrum at the selected frequency. The Q value of the resonator increases as the number of reflections on the patterned facet increases, reaching ~25 when the THz wave has experienced 12 reflections.
Applications of surface plasmon polaritons in terahertz spectral regime
(2010) Zhan, Hui; Mittleman, Daniel M.
This thesis presents the experimental work on the applications of surface plasmon polariton (SPP) in terahertz (THz) spectral range. Apertureless near-field optical microscopy (ANSOM) has been widely used to study the localized SPP on various material surfaces. THz ANSOM technique was recently developed to combine the THz time-domain spectroscopy and the ANSOM technique to provide a near-field detection on the localized THz surface waves with improved spatial resolution and signal-noise ratio. We have studied the metal-insulator transition in vanadium dioxide (VO2) thin film using THz ANSOM. We observe a variation of the terahertz amplitude due to the phase transition induced by an applied voltage across the sample. The change of the terahertz signal is related to the abrupt change of the conductivity of the VO2 film at the metal-insulator transition. The subwavelength spatial resolution of this near-field microscopy makes it possible to detect signatures of metallic domains, which exist in the VO2 thin films in the vicinity of the phase transition. We experimentally investigate the propagation of guided waves in finite-width parallel-plate waveguides (PPWGs) in the terahertz spectral range. We observe the propagation of SPPs in this guiding structure, instead of the fundamental transverse electromagnetic (TEM) mode. We find that the two-dimensional (2-D) energy confinement within the finite-width PPWG increases exponentially as the plate separation is reduced. We speculate that edge plasmons play an important role in the energy confinement in this open-structure waveguide. For comparison, the infinite-width PPWGs, the plates of which are much wider than the THz beam size, are also studied with several plate separations. The free-space beam diffraction produces a Gaussian profile along the unconfined direction. The unusual electric field profiles along the vertical direction, perpendicular to the plate are observed. The field enhancement near the metal surfaces are also explained by the SPPs coupled to the metal surfaces. Based on the 2-D energy confinement in the finite-width PPWGs, we design the tapered slot waveguide by slowly tapering the plate width and slot gap. We first study the transverse component of the THz electric field, where a subwavelength 2-D energy confinement is observed. The output spot size strongly depends on the output facet size, where the slot gap and the tip width are in the same scale range. Subwavelength confinement is obtained, corresponding to lambda/4. Further confinement is limited by the spatial resolution of the detecting technique. To overcome this problem, we adapt the THz ASNOM setup to scattering-probe imaging technique, which has been proven to obtain deep subwavelength spatial resolution and great signal-noise ratio. Scattering-probe imaging setup measures the longitudinal component of the electric field of SPPs in the tapered slot waveguides. By slowly tapering the tip width and the slot gap, we squeeze a single-cycle THz pulse down to a size of 10 mum (lambda/260) by 18 mum (lambda/145), a mode area of only 2.6 x 10-5lambda2. We also observe a polarity reversal for the electric field between the guiding region near the upper and lower plates of the waveguide. This polarity flip is similar to that associated with the symmetric plasmon mode of slot waveguides.
Applications of Terahertz Imaging
(1998-08-01) Mittleman, Daniel M.; Neelamani, Ramesh; Baraniuk, Richard G.; Nuss, Martin C.; Digital Signal Processing (http://dsp.rice.edu/)
The recent advances involving imaging with sub-picosecond terahertz pulses have opened up a wide range of possibilities in the applications of far-infrared technology. For the first time, a commercially viable terahertz imaging spectrometer seems a realizable prospect. However, several substantial engineering research challenges remain to be overcome before this goal can be achieved. One of these involves the necessity for a femtosecond laser system, required for gating the emitter and receiver antennas used in the THz-TDS system. The demonstration experiments performed to date have employed rather crude signal processing algorithms. The shortcomings of these are evident in some of the results presented here, highlighting the need for a more sophisticated treatment.
Audio misinformation encoding via an on-phone sub-terahertz metasurface
(Optica Publishing Group, 2024) Shaikhanov, Zhambyl; Al-Madi, Mahmoud; Chen, Hou-Tong; Chang, Chun-Chieh; Addamane, Sadhvikas; Mittleman, Daniel M.; Knightly, Edward W.
We demonstrate a wireless security application to protect the weakest link in phone-to-phone communication, using a terahertz metasurface. To our knowledge, this is the first example of an eavesdropping countermeasure in which the attacker is actively misled.
Charge Transport and Transfer at the Nanoscale Between Metals and Novel Conjugated Materials
(2012-09-05) Worne, Jeffrey; Natelson, Douglas; Kelly, Kevin F.; Mittleman, Daniel M.
Abstract Organic semiconductors (OSCs) and graphene are two classes of conjugated materials that hold promise to create flexible electronic displays, high speed transistors, and low-cost solar cells. Crucial to understanding the behavior of these materials is understanding the effects metallic contacts have on the local charge environment. Additionally, characterizing the charge carrier transport behavior within these materials sheds light on the physical mechanisms behind transport. The first part of this thesis examines the origin of the low-temperature, high electric field transport behavior of OSCs. Two chemically distinct OSCs are used, poly-3(hexylthiophene) (P3HT) and 6,13- bis(triisopropyl-silylethynyl) (TIPS) pentacene. Several models explaining the low-temperature behavior are presented, with one using the Tomonaga-Luttinger liquid (TLL) insulator-to-metal transition model and one using a field-emission hopping model. While the TLL model is only valid for 1-dimensional systems, it is shown to work for both P3HT (1D) and TIPS-pentacene (2D), suggesting the TLL model is not an appropriate description of these systems. Instead, a cross-over from thermally-activated hopping to field-emission hopping is shown to explain the data well. The second part of this thesis focuses on the interaction between gold and platinum contacts and graphene using suspended graphene over sub-100 nanometer channels. Contacts to graphene can strongly dominate charge transport and mobility as well as significantly modify the charge environment local to the contacts. Platinum electrodes are discovered to be strong dopants to graphene at short length scales while gold electrodes do not have the same effect. By increasing the separation distance between the electrodes, this discrepancy is shown to disappear, suggesting an upper limit on charge diffusion from the contacts. Finally, this thesis will discuss a novel technique to observe the high-frequency behavior in OSCs using two microwave sources and an organic transistor as a mixer. A theoretical model motivating this technique is presented which suggests the possibility of retrieving gigahertz charge transport phenomena at kilohertz detection frequencies. The current state of the project is presented and discrepancies between devices made with gold and platinum electrodes measured in the GHz regime are discussed.
Development of a Terahertz Leaky-Wave Antenna using the Parallel- Plate Waveguide
(2014-07-23) McKinney, Robert Warren; Mittleman, Daniel M.; Kelly, Kevin F; Natelson, Douglas
Because of a growing bandwidth problem within wireless communications, the terahertz (THz) spectrum is being investigated as a possible technology for short-range, high-bandwidth communications. For this reason, it is worth implementing known communications technologies within the radio frequency (RF) and microwave bands, such as antennas, in the terahertz band. One such technology is the leaky-wave antenna. Leaky-wave antennas have been in use within the RF and microwave bands since the 1940’s. The leaky-wave antenna is a travelling wave antenna in which a fast wave with a phase velocity greater than the speed of light, c, propagates through a waveguide. This fast wave is allowed to leak out of the waveguide via an opening along the length of the waveguide. A THz leaky-wave antenna is implemented using the TE1 mode of a parallel-plate waveguide (PPWG). Various plate separations are used during this project in order to show the leaky-wave effect for different dispersion relations. Using a commercial THz time domain spectroscopy (THz-TDS) system, the input of the waveguide is a broadband THz signal. The expected output from such an input would be dispersed in the frequency domain. This is particularly interesting because it would allow the leaky-wave antenna to act as a THz demultiplexer by separating a broadband signal into individual frequency components that vary with angle. Our measured experimental results show that the waveguide indeed produces a dispersed output matching the analytical result. The propagation angle of lower frequencies is closer to perpendicular to the waveguide, with the cutoff frequency of the PPWG at the normal. Higher frequencies are transmitted closer to the axis of the waveguide. Since the phase-matching condition for a leaky-wave antenna can work in either direction, this THz leaky-wave antenna can also receive radiation. Our results show that when operating in this orientation, the receiving angle matches the angle of transmission from the transmitter setup for each frequency. This again shows agreement with the analytical result froe leaky-wave antennas. Using the leaky-wave antenna in this manner, we see the potential for THz frequency domain multiplexing. Varying the plate separation of a PPWG changes the dispersion relation. Since the angle of leaky-wave propagation depends on the dispersion, by varying the plate separation, one can vary the angle of the leaky-wave along the length of the waveguide. We implement such a waveguide in order to focus a chosen frequency to a point. Simulations of the field intensity show that this is possible. By mapping out the field intensity for each design frequency, our results validate this concept by showing that the field focuses within the plane of propagation. To the best of our knowledge, this work shows for the first time that these types of antennas can be implemented within the THz spectrum in order to transmit and receive THz signals.
Device and method for modulating transmission of terahertz waves
(2015-08-11) Xu, Quinfan; Shu, Jie; Mittleman, Daniel M.; Qiu, Ciyuan; Rice University; United States Patent and Trademark Office
A device for modulating terahertz waves includes a metal layer (703) including a continuous metal portion (705) and island metal portions (707). The metal portions (705, 707) are separated by apertures (709). The device further includes a semiconductor layer (715) affixed to a bottom surface of the metal layer (703). The semiconductor layer (715) includes carrier regions (717) located below the apertures (709). The transmission of terahertz waves through the apertures (709) is modulated by changing a voltage applied across the aperture via voltage source (715). By injecting free carriers into carrier regions (717) due to a change of the voltage an extraordinary terahertz transmission effect of the metal layer (703) can be switched off. A small increase in the free-carrier absorption is significantly enhanced by the Fabry-Perot resonance, resulting in a substantial decrease in transmission. The disclosed ring aperture terahertz modulator allows for electrical control of the carrier density only in the area underneath the aperture. This design minimizes the power consumption and maximizes the operation speed.
Electrically Driven Terahertz Metamaterial Diffractive Modulator
(2014-09-19) Karl, Nicholas; Mittleman, Daniel M.; Natelson, Doug; Kono, Jun
This thesis describes a method for terahertz (THz) modulation using active metamaterials. At optical and telecom frequencies, modulation of freely propagating and guided signals is an important concept that is fundamental to key technologies and applications such as communications and imaging. At THz frequencies, the ability to modulate these signals at the capacity necessary to realize these applications effectively does not exist. In order to remedy this, we investigate the use of metamaterials to modulate free space propagating waves at THz frequencies. Metamaterials are an ideal modulation platform for use in the THz frequency range because they avoid many issues that have challenged traditional THz modulators. Furthermore, the ability of metamaterials to function as artificial media holds considerable potential in the THz frequency range because it allows for a wide range of tunable material parameters extending beyond natural THz material responses. We develop and demonstrate a switchable diffractive modulator using an electrically driven metamaterial to tune transmission in real-time via voltage application. The metamaterial elements are grouped giving rise to the grating structure. We observe that the device operates as a relatively high-speed, wide-bandwidth, high-contrast modulator, with more than 20 dB of dynamic range.
Evanescent Wave Coupling in Terahertz Waveguide Arrays
(2013-06-17) Reichel, Kimberly; Mittleman, Daniel M.; Xu, Qianfan; Natelson, Douglas
At optical frequencies, evanescent wave coupling in waveguides is an important concept underlying key technologies such as optical fiber splitters and combiners. At terahertz (THz) frequencies, there is a lack of such devices. In order to fill this gap, we investigate evanescent wave coupling at THz frequencies in an array of narrow-width parallel-plate waveguides (PPWGs). Although researchers have studied THz wave coupling between two adjacent wire waveguides, evanescent coupling in an array of PPWGs has not previously been considered. Metal PPWGs are ideal THz waveguide platforms since they offer low losses and negligible dispersion in the TEM waveguide mode. Additionally, PPWGs can exhibit energy leakage when the plates are narrow and the plate separation is large, indicating that an array of narrow-width PPWGs is a convenient platform for studying THz energy coupling between waveguides. By using the presented design of an array of identical narrow-width PPWGs in close proximity with their unconfined sides facing each other, we have demonstrated evidence of evanescent wave coupling in THz PPWG arrays. Thereby, we observed stronger coupling with larger waveguide plate separations and longer propagation paths. We confirmed these results through THz time-domain spectroscopy (THz-TDS) experiments and finite-element method (FEM) simulations. Based on evanescent wave coupling, this work establishes a platform to investigate new opportunities for THz waveguide devices and components such as splitters and power combiners.
Gas Sensing using Terahertz Time-Domain Spectroscopy
(1998-01-15) Mittleman, Daniel M.; Jacobsen, R.H.; Neelamani, Ramesh; Baraniuk, Richard G.; Nuss, Martin C.; Center for Multimedia Communications (http://cmc.rice.edu/); Digital Signal Processing (http://dsp.rice.edu/)
A method for detection and identification of polar gases and gas mixtures based on the technique of terahertz time-domain spectroscopy is presented. This relatively new technology promises to be the first portable far-infared spectrometer, providing a means for real-time spectroscopic measurements over a broad bandwidth up to several THz. The measured time-domain waveforms can be efficiently parameterized using standard tools from signal processing, including procedures developed for speech recognition applications. These are generally more efficient than conventional methods based on Fourier analysis, and are easier to implement in a real-time sensing system. Preliminary results of real-time gas mixtures analysis using a linear predictive coding algorithm are presented. A number of possible avenues for improved signal processing schemes are discussed. In particular, the utility of a wavelet-based signal analysis for tasks such as denoising is demonstrated.
Graphene Photonic Devices for Terahertz and Mid-Infrared
(2013-11-08) Gao, Weilu; Xu, Qianfan; Kono, Junichiro; Mittleman, Daniel M.
Graphene and other strictly two-dimensional materials are the rising stars on the horizon of material science, condensed matter physics and engineering. The richness of optical and electronic properties of graphene attract much interest due to the exceptional high crystal and electronic quality resulting in large carrier mobility at room temperature and easily electrical control of carrier density, which find its true potential in photonics and optoelectronics. Novel graphene based broadband modulators, polarizer, active plasmonic resonators, ultra-fast lasers and etc are proposed and implemented in many literatures. Despite ample demonstrations of the true potential of graphene in optoelectronic devices, there is still unexplored region. In this thesis, we investigate the graphene photonic properties and optoelectronic devices in different regions ranging from longer wavelength terahertz frequency (THz) to shorter wavelength telecommunication frequency to reveal the whole picture of graphene. The Drude-like intraband absorptoion (i.e. free carrier effect) in graphene plays an important role in THz region. However, the extinction ratio that can be obtained when THz waves passing through a single layer graphene is limited due to its one-atomic-layer thickness and the non-resonant nature of the intraband absorption. By incorporating resonate structures with graphene, the high extinction ratio of THz wave transmission will be achieved utilizing the high localized electric field near the graphene layer. Combining the electrically controlled carrier density in graphene, a graphene-based THz modulators with high modulation depth, fast speed can be built. High carrier mobility of graphene at room temperature makes it a new platform for plasmonics with strong light-matter interactions, which has been theoretically proved to be able to support surface plasmon polarions (SPPs) with lower loss and higher mode confinement compared with metals. Furthermore, the electrically controlled carrier density of graphene renders it new possibility to build active plasmonic devices. Although many efforts have been done by either shaping the graphene to excite localized plamons or using near-field method to excite SPPs in continuous graphene layer in spite of low efficiency, we theoretically propose and experimentally demonstrate to utilize silicon diffractive gratings underneath the graphene to excite graphene SPPs, which can be actively controlled via back-gating structure. The ac carrier dynamics of graphene have different contribution at different frequency range, which are investigated by incorporating graphene with resonators operating at different frequencies. In mid-infrared region the same structure as that in THz region is integrated with graphene that proves the almost complete transparency of graphene in mid-infrared region for intrinsically doping graphene while in the shorter wavelength of telecommnucations frequency, graphene is also integrated with silicon ring resonators, which shows large absorption. However, this large absorption is resulted from interband absorption that is quite different from what we have observed in THz region that is from intraband absorption. So in summary, the intraband and interband carrier dynamics of graphene will have different contributions in devices operating at different frequency region, which makes the various applications available.
Imaging with Terahertz Pulses
(2000-07-01) Dorney, Timothy D.; Johnson, Jon L.; Mittleman, Daniel M.; Baraniuk, Richard G.; Center for Multimedia Communications (http://cmc.rice.edu/); Digital Signal Processing (http://dsp.rice.edu/)
Recently, a real-time imaging system based on terahertz (THz) time-domain spectroscopy has been developed. This technique offers a range of unique imaging modalities due to the broad bandwidth, sub-picosecond duration, and phase-sensitive detection of the THz pulses. This paper provides a brief introduction of the state-of-the art in THz imaging. It also focuses on expanding the potential of this new and exciting field through two major efforts. The first concentrates on improving the experimental sensitivity of the system. We are exploring an interferometric arrangement to provide a background-free reflection imaging geometry. The second applies novel digital signal processing algorithms to extract useful information from the THz pulses. The possibility exists to combine spectroscopic characterization and/or identification with pixel-by-pixel imaging. We describe a new parameterization algorithm for both high and low refractive index materials.
Imaging with THz Pulses
(2000-09-01) Dorney, Timothy D.; Johnson, Jon L.; Mittleman, Daniel M.; Baraniuk, Richard G.; Digital Signal Processing (http://dsp.rice.edu/)
A real-time imaging system based on terahertz (THz) time-domain spectroscopy has been demonstrated. This technique offers a range of unique imaging modalities due to the broad bandwidth, sub-picosecond duration, and phase-sensitive detection of the THz pulses. This paper provides an introduction of the state-of-the art in THz imaging. It also focuses on expanding the potential of this new and exciting field through two major efforts. The first concentrates on improving the experimental sensitivity of the system. We are exploring an interferometric arrangement to provide a background-free reflection imaging geometry. The second applies novel digital signal processing algorithms to extract useful information from the THz pulses. The possibility exists to combine spectroscopic characterization and/or identification with pixel-by-pixel imaging.
Infrared Spectroscopy of Graphene in Ultrahigh Magnetic Fields
(2012-09-05) Booshehri, Layla; Kono, Junichiro; Morosan, Emilia; Mittleman, Daniel M.; Mielke, Charles H.
Graphene – a two-dimensional honeycomb lattice of sp2-bonded carbon atoms – possesses unusual zero-gap band structure with linear band dispersions, accommodating photon-like, massless electrons that have exhibited a variety of surprising phenomena, primarily in DC transport, in the last several years. In this thesis dissertation, we investigate graphene’s AC or infrared properties in the presence of an ultrahigh magnetic field, produced by a destructive pulsed method. The linear dispersions of graphene lead to unequally spaced Landau levels in a magnetic field, which we probe through cyclotron resonance (CR) spectroscopy in the magnetic quantum limit. Specifically, using magnetic fields up to 170 T and polarized midinfrared radiation with tunable wavelengths from 9.22 to 10.67 μm, we experimentally investigated CR in large-area graphene grown by chemical vapor deposition. Circular-polarization-dependent studies revealed strong p-type doping for as-grown graphene, and the dependence of the CR fields on the radiation wavelength allowed for an accurate determination of the Fermi energy. Upon annealing the sample to remove physisorbed molecules, which shifts the Fermi energy closer to the Dirac point, we made the unusual observation that hole and electron CR emerges in the magnetic quantum limit, even though the sample is still p-type. We theoretically show that this non-intuitive phenomenon is a direct consequence of the unusual Landau level structure of graphene. Namely, if the Fermi energy lies in the n = 0 Landau level, then CR is present for both electron-active and hole-active circular polarizations. Furthermore, if the Fermi level lies in the n = 0 Landau level, the ratio of CR absorption between the electron-active and hole-active peaks allows one to accurately determine the Fermi level and carrier density. Hence, high-field CR studies allow not only for fundamental studies but also for characterization of large-area, low-mobility graphene samples.
Interferometric imaging system and method
(2003-12-16) Mittleman, Daniel M.; Johnson, Jon L.; Rice University; United States Patent and Trademark Office
A broadband imaging system is disclosed that provides greatly enhanced depth resolution through the use of phase shift interferometry. The system may comprise a transmitter, a splitter, a phase inverter, and a receiver. The transmitter transmits a signal pulse that is split into a measurement pulse and a reference pulse. The measurement pulse is applied to a sample, and a relative phase shift of approximately π radians is introduced between the measurement pulse and the reference pulse by the phase inverter. The measurement and reference pulses are then recombined to form a combined pulse that is detected by the receiver. The phase inverter may be a simple lens that introduces a Gouy phase shift by passing the measurement or reference pulse through a focal point. In this manner, a background-free measurement is provided, which provides a greatly enhanced sensitivity to small changes in the measurement waveform, regardless of origin.
Interferometry in terahertz imaging
(2001) Johnson, Jon Lars; Mittleman, Daniel M.
Terahertz Time-Domain Spectroscopy (THz-TDS) techniques have been shown over the last decade to be useful in many diverse applications. This thesis describes the implementation of interferometry in imaging with few-cycle terahertz pulses for the purpose of enhancing depth resolution. By configuring terahertz imaging optics in a Michelson interferometric arrangement, a phase shift of approximately Pi radians can be introduced between the interferometer's two arms via the Gouy effect. The resulting destructive interference provides a nearly background-free measurement and a dramatic enhancement in imaging sub-coherence length features. It is possible to image features thinner than 4% of the coherence length of the radiation. This technique could have applications in THz imaging and other THz-TDS systems, as well as in other low-coherence optical tomographic measurements.
Line-of-sight and non-line-of-sight links for dispersive terahertz wireless networks
(AIP Publishing LLC, 2021) Ghasempour, Yasaman; Amarasinghe, Yasith; Yeh, Chia-Yi; Knightly, Edward; Mittleman, Daniel M.
Despite the rapidly growing interest in exploiting millimeter and terahertz waves for wireless data transfer, the role of reflected non-line-of-sight (NLOS) paths in wireless networking is one of the least explored questions. In this paper, we investigate the idea of harnessing these specular NLOS paths for communication in directional networks at frequencies above 100 GHz. We explore several illustrative transmitter architectures, namely, a conventional substrate-lens dipole antenna and a leaky-wave antenna. We investigate how these high-gain directional antennas offer both new challenges and new opportunities for exploiting NLOS paths. Our results demonstrate the sensitivity to antenna alignment, power spectrum variations, and the disparity in supported bandwidth of various line-of-sight (LOS) and reflected path configurations. We show that NLOS paths can, under certain circumstances, offer even higher data rates than the conventional LOS path. This result illustrates the unique opportunities that distinguish THz wireless systems from those that operate at lower frequencies.
Low-Energy Charge and Spin Dynamics in Quantum Confined Systems
(2012-09-05) Rice, William; Kono, Junichiro; Mittleman, Daniel M.; Si, Qimiao
Condensed matter systems exhibit a variety of dynamical phenomena at low energy scales, from gigahertz (GHz) to terahertz (THz) frequencies in particular, arising from complex interplay between charge, spin, and lattice. A large number of collective and elementary excitations in solids occur in this frequency range, which are further modified and enriched by scattering, interactions, and disorder. Recent advancements in spectroscopic methods for probing low-energy dynamics allow us to investigate novel aspects of charge and spin dynamics in solids. In this dissertation work, we used direct current (DC) conductivity, GHz, THz, and mid-infrared (MIR) techniques to provide significant new insights into interaction and disorder effects in low-dimensional systems. Specifically, we have studied temperature-dependent magnetoresistance (MR) and electron spin resonance (ESR) in single-wall carbon nanotubes (SWCNTs), intra-exciton scattering in InGaAs quantum wells, and high-field MIR-induced band gaps in graphene. Temperature-dependent resistance and MR were measured in an ensemble of SWCNTs from 0.3 to 350 K. The resistance temperature behavior followed a 3D variable range hopping (VRH) behavior from 0.3 to ~100 K. A positive MR was observed at temperatures above 25 K and could be fit with a spin-dependent VRH model; negative MR was seen at low temperatures. In the GHz regime, the ESR linewidth for SWCNTs was observed to narrow by as much as ~50% as the temperature was increased from 3 to 300 K, a phenomenon known as motional narrowing, suggesting that we are detecting the ESR of hopping spins. From the linewidth change versus temperature, we find the hopping frequency to be 285 GHz. For excitons in InGaAs quantum wells, we demonstrate the manipulation of intra-excitonic populations using intense, narrow-band THz pulses. The THz radiation temporarily quenches the 1s emission, which is then followed by an enhancement and subsequent decay of 2s emission. After the quenching, the 1s emission recovers and then eventually becomes enhanced, a demonstration of energy storage in intra-exciton states known as excitonic shelving. We show that the diffusive Coulomb scattering between the 2p and 2s states produces a symmetry breaking, leading to a THz-field-induced 1s-to-2s exciton population transfer.

Browsing by Author "Mittleman, Daniel M."

Results Per Page

Sort Options