R-3 Repository :: Browsing by Author "Xu, Qianfan"

Browsing by Author "Xu, Qianfan"

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Active dielectric antenna on chip for spatial light modulation
(Nature Publishing Group, 2012-11-14) Qiu, Ciyuan; Chen, Jianbo; Xia, Yang; Xu, Qianfan
Integrated photonic resonators are widely used to manipulate light propagation in an evanescently-coupled waveguide. While the evanescent coupling scheme works well for planar optical systems that are naturally waveguide based, many optical applications are free-space based, such as imaging, display, holographics, metrology and remote sensing. Here we demonstrate an active dielectric antenna as the interface device that allows the large-scale integration capability of silicon photonics to serve the free-space applications. We show a novel perturbation-base diffractive coupling scheme that allows a high-Q planer resonator to directly interact with and manipulate free-space waves. Using a silicon-based photonic crystal cavity whose resonance can be rapidly tuned with a p-i-n junction, a compact spatial light modulator with an extinction ratio of 9.5 dB and a modulation speed of 150 MHz is demonstrated. Method to improve the modulation speed is discussed.
Curvature Effects on the Optical Transitions of Single-Wall Carbon Nanotubes
(2013-07-24) Haroz, Erik; Tittel, Frank K.; Xu, Qianfan; Hauge, Robert H.
Optical transition energies are widely used for providing experimental insight into the electronic band structure of single-wall carbon nanotubes (SWCNTs). While the first and second optical transitions in semiconducting carbon nanotubes have already been heavily studied, due to experimental difficulties in accessing the relevant excitation energy region, little is known about higher lying transitions. Here, I present measurements of the third and fourth optical transitions of small-diameter (0.7-1.2 nm), semiconducting single-wall carbon nanotubes via resonant Raman spectroscopy in the visible deep blue region (415-465 nm) and photoluminescence excitation spectroscopy in the ultraviolet and visible blue optical regions (280-488 nm). Diameter-dependent Raman radial breathing mode features, as well as resonant energy excitation maxima determined by Raman and photoluminescence measurements, are assigned to specific (n,m) nanotube species. The Raman intensity within a given 2n+m branch is found to increase with decreasing chiral angle, consistent with similar measurements for lower order optical states. Additionally, increased excitation line widths and weaker Raman intensities are observed as higher lying transitions are accessed for a given nanotube, in agreement with previous Raman measurements. Chiefly, a scaling law analysis that removes the chiral-angle-dependent contribution to the optical transition energy indicates that the third and fourth transition energies exhibit a significant deviation from the energy trend line observed for the first and second optical transitions, when the transition energies are plotted as a function of nanotube diameter. This deviation can be understood in the context of a change in the competition between exchange and excitonic correction terms. Furthermore, for semiconducting SWCNTs with diameters less than 0.9 nm, an additional deviation is observed that is interpreted as the first observation of crossing-over of the third and fourth transition energy trend lines for a given 2n+m branch and a chirality dependence in the many-body excitonic effects that becomes significant at high nanotube curvatures.
Dual-ring silicon electro-optic modulator
(2013-11-12) Xu, Qianfan; Rice University; United States Patent and Trademark Office
A device, system, and method for the electro-optic modulation of light. The device includes a substrate having a first ring waveguide and a second ring waveguide on the surface. The device includes a first p-doped region inside the first ring waveguide and a second p-doped region inside the second ring waveguide. The device includes a first n-doped region interposed between the first ring waveguide and the second ring waveguide, a second n-doped region outside the first ring waveguide, and a third n-doped region out the second ring waveguide. The device includes a first linear waveguide located on the surface adjacent to the first ring waveguide and the second ring waveguide. The device includes a fourth n-doped region on the surface located adjacent to the first linear waveguide. The device includes a control circuit configured to modulate light in the first linear waveguide using a voltage source and electrical connections.
Efficient Modulation of 1.55 μm Radiation with Gated Graphene on a Silicon Microring Resonator
(American Chemical Society, 2014) Qiu, Ciyuan; Gao, Weilu; Vajtai, Robert; Ajayan, Pulickel M.; Kono, Junichiro; Xu, Qianfan
The gate-controllability of the Fermi-edge onset of interband absorption in graphene can be utilized to modulate near-infrared radiation in the telecommunication band. However, a high modulation efficiency has not been demonstrated to date, because of the small amount of light absorption in graphene. Here, we demonstrate a ~40% amplitude modulation of 1.55 μm radiation with gated single-layer graphene that is coupled with a silicon microring resonator. Both the quality factor and resonance wavelength of the silicon microring resonator were strongly modulated through gate tuning of the Fermi level in graphene. These results promise an efficient electro-optic modulator, ideal for applications in large-scale on-chip optical interconnects that are compatible with complementary metal-oxide-semiconductor technology
Enrichment and Fundamental Optical Processes of Armchair Carbon Nanotubes
(2013-09-16) Haroz, Erik; Tittel, Frank K.; Xu, Qianfan; Hauge, Robert H.
The armchair variety of single-wall carbon nanotubes (SWCNTs) is the only nanotube species that behaves as a metal with no electronic band gap and massless carriers, making them ideally suited to probe fundamental questions of many-body physics of one-dimensional conductors as well as to serve in applications such as high-current power transmission cables. However, current methods of nanotube synthesis produce bulk material comprising of a mixture of nanotube lengths, diameters, wrapping angles, and electronic types due to the inability to control the growth process at the nanometer level. As a result, measurements of as-grown SWCNTs produce a superposition of electrical and optical responses from multiple SWCNT species. This thesis demonstrates production of aqueous suspensions composed almost entirely of armchair SWCNTs using a post-synthesis separation method employing density gradient ultracentrifugation (DGU) to separate different SWCNT types based on their mass density and surfactant-specific interactions. Resonant Raman spectroscopy determines the relative abundances of each nanotube species, before and after DGU, by measuring the integrated intensity of the radial breathing mode, the diameter-dependent radial vibration of the SWCNT perpendicular to its main axis, and quantifies the degree of enrichment of bulk nanotube samples to exclusively armchair tubes. Raman spectroscopy of armchair-enriched samples of the G-band mode, which is composed of longitudinal (G-) and circumferential (G+) vibrations oscillating parallel and perpendicular to the tube axis, shows that the G- peak, long-held to be an indicator for the presence of metallic SWCNTs, appears only when electronic resonance with narrow-gap semiconducting SWCNTs occurs and shows only the G+ component in spectra containing only armchair species. Finally, by combining optical absorption measurements with nanotube composition as determined earlier via Raman scattering, peak fitting of absorption spectra indicates that interband transitions of armchair SWCNTs are strongly excitonic as shown by the highly symmetric peak lineshapes, a property normally attributed to semiconductors. Such lineshapes allow classification of armchair SWCNTs as a unique hybrid class of optical nanomaterial. Combining absorption and Raman scattering measurements establishes a distinct optical signature that describes the fundamental optical processes within armchair SWCNTs and lays the foundation for future studies of many-body photophysics and electrical applications.
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.
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.
Nanomechanical and Electro-mechanical Characterization of Materials for Flexible Electrodes Applications
(2013-09-16) Peng, Cheng; Lou, Jun; Dick, Andrew J.; Xu, Qianfan
Flexible electronics attract research and commercial interests in last 2 decades for its flexibility, low cost, light weight and etc. To develop and improve the electro-mechanical properties of flexible electrodes is the most critical and important step. In this work, we have performed nanomechanical and electro-mechanical characterization of materials for flexible electrode applications, including metallic nanowires (NWs), indium tin oxide (ITO)-based and carbon nanotube (CNT)-based electrodes. First, we designed and developed four different testing platforms for nanomechanical and electro-mechanical characterization purpose. For the nano/sub-micro size samples, the micro mechanical devices can be used for uni-axial and bi-axial loading tests. For the macro size samples, the micro tester will be used for in situ monotonic tensile test, while the fatigue tester can be used for in situ cyclic tensile or bending testing purpose. Secondly, we have investigated mechanical behaviors of single crystalline Ni nanowires and single crystalline Cu nanowires under uni-axial tensile loading inside a scanning electron microscope (SEM) chamber. We demonstrated both size and strain-rate dependence on yield stress of single-crystalline Ni NWs with varying diameters (from 100 nm to 300 nm), and themolecular dynamics (MD) simulation helped to confirm and understand the experimental phenomena. Also, two different fracture modes, namely ductile and brittle-like fractures, were found in the same batch of Cu nanowire samples. Finally, we studied the electro-mechanical behaviors of flexible electrodes in macro scale. We reported a coherent study integrating in situ electro-mechanical experiments and mechanics modeling to decipher the failure mechanics of ITO-based and CNT-based electrodes under tension. It is believed that our combined experimental and simulation results provide some further insights into the important yet complicated deformation mechanisms for nanoscale metals and fracture mechanism for flexible electrodes applications.
Silicon Photonic Devices for Optical Computing
(2013-11-26) Qiu, Ciyuan; Xu, Qianfan; Tittel, Frank K.; Lou, Jun
The requirement for high performance computer will be significantly increased by the fast development of the internet. However, traditional CMOS computer will meet its bottleneck due to the miniaturization problem. Optical computer comes to be the leading candidate to solve this issue. Silicon photonic technology has tremendous developments and thus it becomes an ideal platform to implement optical computing system. In Chapter 1, I will first show the development of the optical computing and silicon photonic technology. I will also discuss some key nonlinear optical effects of silicon photonic devices. Based on the current silicon photonic technology, I will then make a brief introduction on the optical direct logic for the 2D optical computing and spatial light modulator for the 3D optical computing, both of which will be discussed in detail in the followed chapters. In Chapter 2, I will discuss micro-ring resonator which is the key element of optical directed logic circuit discussed in Chapter 3. I will give the analytical model based on photonic circuit to explain the performance of the micro-ring resonator. The group delay and the loss of the micro-ring resonator will be analyzed. And I will also show the active tuning of the transmission spectrum by using the nonlinear effect of silicon. In Chapter 3, I will show a revised optical direct-logic (DL) circuit for 2D optical computer that is well suited for complementary metal–oxide–semiconductor (CMOS)-compatible silicon photonics. It can significantly reduce the latency compared with traditional CMOS computers. For proof of concept, I demonstrated a scalable and reconfigurable optical directed-logic architecture consisting of a regular array of micro-ring resonator based optical on-off switches. The switches are controlled by electrical input logic signals through embedded p-i-n junctions. The circuit can be reconfigured to perform any 2×2 combinational logic operations by thermally tuning the operation modes of the switches. In Chapter 4, I will present a diffraction-based coupling scheme that for the first time allows silicon micro-resonator to directly manipulate a free-space optical beam at normal incidence. A high-Q micro-gear resonator with a 1.57-um radius is demonstrated whose vertical transmission and reflection change 40% over a wavelength range of only 0.3 nm. A dense 2D array of such resonators can also be integrated on a chip for spatial light modulation and parallel bio-sensing. In Chapter 5, I will demonstrate a spatial light modulator based on 1D photonics crystal cavity for 3D optical computing. It can control the free-space optical beam through perturbation coupling scheme as shown in Chapter 4. Compared with micro-gear resonator, it has much higher extinction ratio(ER) and quality factor. As the resonance of the silicon-based photonic crystal cavity can be rapidly tuned with an embedded p-i-n junction, a compact spatial light modulator is demonstrated with speed up to 150 MHz and extinction ratio of 9.5 dB. In Chapter 6, I will make a summary of my work and then talk about the future trend in our field.
Sub-wavelength Metallic Ring Apertures for Communications, Sensing and Nonlinear Optics
(2013-11-06) Shu, Jie; Xu, Qianfan; Mittleman, Daniel M.; Nordlander, Peter J.
In this thesis, we demonstrate sub-wavelength ring aperture arrays in a metal film which can be used for communications, material sensing, and nonlinear optics. We show in simulation and experiment extraordinary optical transmission through ring apertures on a metal film both in terahertz (THz) and mid-infrared (MIR) regions. For THz metallic ring aperture arrays, transmission of 60% is obtained with an aperture-to-area ratio of only 1.4%. We show that the high transmission can be suppressed by over 18 dB with a thin layer of free carriers in the silicon substrate underneath the metal film. We also experimentally demonstrate graphene-based active electro-optic modulation of THz waves. The metallic nanostructure provides ~4 times absorption enhancement and ~50% modulation depth is obtained with monolayer graphene. These results suggest that CMOS-compatible terahertz modulators can be built by controlling the carrier density near the aperture. We also demonstrate extraordinary optical transmission in MIR metallic ring aperture arrays. We observe enhanced field-matter interaction with the MIR ring apertures due to enhanced near field, and we present its applications in sensing and nonlinear optical effects. We demonstrate using the devices for enhancing the absorption of PMMA by ~8 times. We also show in simulation the enhanced sensing of monolayer graphene with different Fermi level. We obtain a 60 cm-1 shift of resonance frequency per 0.1 eV change of Fermi level in graphene, and a 6% change in transmission peak intensity. Next we experimentally demonstrate enhanced 2D IR spectrum by using the MIR concentric apertures. At last, we experimentally show polarization-independent Fano resonance in concentric metallic ring apertures both in THz and MIR regions. A high-Q and intensive dark mode is indirectly excited by coupling with a low-Q bright mode. The coupling is enabled by the intrinsic asymmetry between the two concentric rings. A coupled optical resonator model is used to analyze the coupling process between the bright and dark modes. We find the Q of the dark mode is 3~6 times higher than that of the bright mode. We show that the dark mode can be selectively disabled without affecting the bright mode due to its unique current flow pattern. We also observe enhanced field-matter interaction and the absorption-induced transparency effect caused by the intense E-field in the aperture when a material absorption line is aligned with the dark mode.
Suspended Si Ring Resonator for Mid-IR Application
(2013-11-22) Xia, Yang; Xu, Qianfan; Mittleman, Daniel M.; Kono, Junichiro
Suspended ring resonators formed by both single-mode waveguide (SMW) and multi-mode waveguide (MMW) are designed, fabricated and characterized near 3.4 μm by thermal tuning and near 4.5 μm and 5.2 μm by tunable quantum cascade lasers. The dispersion property is analyzed by simulation in regards to frequency comb generation. The taper width is optimized for maximum coupling. Measurement setup is built up and described. For the SMW ring resonator, the intrinsic quality factor is fitted to be 6,800 and 16,000 near 5.2 μm and 4.5 μm, respectively. For the MMW ring resonator, it rises to 35,000 near 4.5 μm. Transmission spectrum distortion is observed at high input power, and is modeled as heat effect. Thermal tuning rate is experimentally confirmed at 0.21 nm/°C. Based on the measured distortion and heat simulation, absorption loss is estimated. All-optical modulation is conducted to estimate the response time of this process. It can be shown that main loss is from surface thus is reducible by improving surface quality. On-chip electrical heater is designed and preliminary experiment indicate the feasibility to pattern it with our Electron Beam Lithography system.

Browsing by Author "Xu, Qianfan"

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