Showing papers by "Richard D. Averitt published in 2015"

A review of non-linear terahertz spectroscopy with ultrashort tabletop-laser pulses

Abstract: Over the past decade, breakthroughs in the generation and control of ultrafast high-field terahertz (THz) radiation have led to new spectroscopic methodologies for the study of light-matter interactions in the strong-field limit. In this review, we will outline recent experimental demonstrations of non-linear THz material responses in materials ranging from molecular gases, to liquids, to varieties of solids – including semiconductors, nanocarbon, and correlated electron materials. New insights into how strong THz fields interact with matter will be discussed in which a THz field can act as either a non-resonant electric field or a broad bandwidth pulse driving specific resonances within it. As an emerging field, non-linear THz spectroscopy shows promise for elucidating dynamic problems associated with next generation electronics and optoelectronics, as well as for demonstrating control over collective material degrees of freedom.

Cooperative photoinduced metastable phase control in strained manganite films

Abstract: A major challenge in condensed matter physics is active control of quantum phases. Dynamic control with pulsed electromagnetic fields can overcome energetic barriers enabling access to transient or metastable states that are not thermally accessible. Here we demonstrate strain-engineered tuning of La2/3Ca1/3MnO3 into an emergent charge-ordered insulating phase with extreme photo-susceptibility where even a single optical pulse can initiate a transition to a long-lived metastable hidden metallic phase. Comprehensive single-shot pulsed excitation measurements demonstrate that the transition is cooperative and ultrafast, requiring a critical absorbed photon density to activate local charge excitations that mediate magnetic-lattice coupling that, in turn, stabilize the metallic phase. These results reveal that strain engineering can tune emergent functionality towards proximal macroscopic states to enable dynamic ultrafast optical phase switching and control.

Phase transition in bulk single crystals and thin films of VO2 by nanoscale infrared spectroscopy and imaging

Abstract: Infrared spectroscopy of VO${}_{2}$ thin films at high spatial resolution shows new electronic and lattice states due to epitaxial strain that differ fundamentally from those in the bulk. This is the first ultra-broadband infrared near-field study of a correlated electron material, made possible by the newly developed synchrotron infrared near-field spectroscopy method (SINS) on the Advanced Light Source at Lawrence Berkeley National Laboratory.

Nonlinear terahertz devices utilizing semiconducting plasmonic metamaterials

Abstract: The development of responsive metamaterials has enabled the realization of compact tunable photonic devices capable of manipulating the amplitude, polarization, wave vector, and frequency of light. Integration of semiconductors into the active regions of metallic resonators is a proven approach for creating nonlinear metamaterials through optoelectronic control of the semiconductor carrier density. Metal-free subwavelength resonant semiconductor structures offer an alternative approach to create dynamic metamaterials. We present InAs plasmonic disk arrays as a viable resonant metamaterial at terahertz frequencies. Importantly, InAs plasmonic disks exhibit a strong nonlinear response arising from electric field induced intervalley scattering resulting in a reduced carrier mobility thereby damping the plasmonic response. We demonstrate nonlinear perfect absorbers configured as either optical limiters or saturable absorbers, including flexible nonlinear absorbers achieved by transferring the disks to polyimide films. Nonlinear plasmonic metamaterials show potential for use in ultrafast THz optics and for passive protection of sensitive electromagnetic devices.

Optically tunable metamaterial perfect absorber on highly flexible substrate

Abstract: We present our recent progress on a highly flexible tunable perfect absorber at terahertz frequencies. Metamaterial unit cells were patterned on thin GaAs patches, which were fashioned in an array on a 10 μm thick polyimide substrate via semiconductor transfer technique, and the backside of the substrate was coated with gold film as a ground plane. Optical-pump THz-probe reflection measurements show that the absorptivity can be tuned up to 25% at 0.78 THz and 40% at 1.75 THz through photo-excitation of free carriers in GaAs layers in presence of 800 nm pump beam. Our flexible tunable metamaterial perfect absorber has potential applications in energy harvesting, THz modulation and even camouflages coating.

Dynamic conductivity scaling in photoexcited V 2 O 3 thin films

Abstract: Optical-pump terahertz-probe spectroscopy is used to investigate ultrafast far-infrared conductivity dynamics during the insulator-to-metal transition in vanadium sesquioxide $({\mathrm{V}}_{2}{\mathrm{O}}_{3})$. The resultant conductivity increase occurs on a tens of picosecond time scale, exhibiting a strong dependence on the initial temperature and fluence. We have identified a scaling of the conductivity dynamics upon renormalizing the time axis with a simple power law $(\ensuremath{\alpha}\ensuremath{\simeq}1/2)$ that depends solely on the initial, final, and conductivity onset temperatures. Qualitative and quantitative considerations indicate that the dynamics arise from nucleation and growth of the metallic phase which can be described by the Avrami model. We show that the temporal scaling arises from spatial scaling of the growth of the metallic volume fraction, highlighting the self-similar nature of the dynamics. Our results illustrate the important role played by mesoscopic effects in phase transition dynamics.

Voltage switching of a VO$_2$ memory metasurface using ionic gel

Abstract: We demonstrate an electrolyte-based voltage tunable vanadium dioxide (VO2) memory metasurface. Large spatial scale, low voltage, non-volatile switching of terahertz (THz) metasurface resonances is achieved through voltage application using an ionic gel to drive the insulator-to-metal transition in an underlying VO2 layer. Positive and negative voltage application can selectively tune the metasurface resonance into the “off” or “on” state by pushing the VO2 into a more conductive or insulating regime respectively. Compared to graphene based control devices, the relatively long saturation time of resonance modification in VO2 based devices suggests that this voltage-induced switching originates primarily from electrochemical effects related to oxygen migration across the electrolyte–VO2 interface.

Terahertz radiation-induced sub-cycle field electron emission across a split-gap dipole antenna

Abstract: We use intense terahertz pulses to excite the resonant mode (0.6 THz) of a micro-fabricated dipole antenna with a vacuum gap. The dipole antenna structure enhances the peak amplitude of the in-gap THz electric field by a factor of ∼170. Above an in-gap E-field threshold amplitude of ∼10 MV/cm−1, THz-induced field electron emission is observed as indicated by the field-induced electric current across the dipole antenna gap. Field emission occurs within a fraction of the driving THz period. Our analysis of the current (I) and incident electric field (E) is in agreement with a Millikan-Lauritsen analysis where log (I) exhibits a linear dependence on 1/E. Numerical estimates indicate that the electrons are accelerated to a value of approximately one tenth of the speed of light.

Phase transition in bulk single crystals and thin films of VO2 by nano-infrared spectroscopy and imaging

Abstract: We have systematically studied a variety of vanadium dioxide (VO2) crystalline forms, including bulk single crystals and oriented thin films, using infrared (IR) near-field spectroscopic imaging techniques. By measuring the IR spectroscopic responses of electrons and phonons in VO2 with sub-grain-size spatial resolution (~20 nm), we show that epitaxial strain in VO2 thin films not only triggers spontaneous local phase separations but also leads to intermediate electronic and lattice states that are intrinsically different from those found in bulk. Generalized rules of strain and symmetry dependent mesoscopic phase inhomogeneity are also discussed. These results set the stage for a comprehensive understanding of complex energy landscapes that may not be readily determined by macroscopic approaches.

Spin-dependent polaron formation dynamics in Eu 0.75 Y 0.25 MnO 3 probed by femtosecond pump-probe spectroscopy

Abstract: We present a femtosecond optical pump-probe study of the multiferroic manganite ${\text{Eu}}_{0.75}{\text{Y}}_{0.25}{\text{MnO}}_{3}$. The optical response of the material at pump energies of 1.55 and 3.1 eV is dominated by the $d\text{\ensuremath{-}}d$ and $p\text{\ensuremath{-}}d$ transitions of the ${\mathrm{Mn}}^{3+}$ ions. The relaxation of photoexcited electrons includes the relaxation of the Jahn-Teller distortion and polaron trapping at ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ sites. Ultrafast switching of superexchange interactions due to modulated ${e}_{g}$ orbital occupancy creates a localized spin excitation, which then decays on a time scale of tens of picoseconds at low temperatures. The localized spin state decay appears as a tremendous increase in the amplitude of the photoinduced reflectance, due to the strong coupling of optical transitions to the spin-spin correlations in the crystalline $a\text{\ensuremath{-}}b$ plane.

A real-time tunable terahertz metamaterial based on broadside-coupled split ring resonators

Abstract: This paper reports a real-time tunable metamaterial based on broadside-coupled split ring resonators (BC-SRRs). The device is composed of two layers of SRRs stacked together with an air gap spacer, forming the BC-SRR configuration. One of the layers is fixed, while the other can be driven by an electrostatic comb-drive actuator. The lateral displacement between the two layers of SRRs can be changed by the actuator, resulting in the tuning of the resonance frequencies of the BC-SRRs and the transmission spectrum of the device. The preliminary results show that the resonance frequency can be tuned up to 100GHz corresponding to an 18µm lateral displacement. Our tunable metamaterials have promising applications as THz modulators, filters and sensors.

Visualization of guided and leaky wave behaviors in an indium tin oxide metallic slab waveguide.

Abstract: We explored the use of the optically transparent semiconductor indium tin oxide (ITO) as an alternative to optically opaque metals for the fabrication of photonic structures in terahertz (THz) near-field studies. Using the polaritonics platform, we confirmed the ability to clearly image both bound and leaky electric fields underneath an ITO layer. We observed good agreement between measured waveguide dispersion and analytical theory of an asymmetric metal-clad planar waveguide with TE and TM polarizations. Further characterization of the ITO revealed that even moderately conductive samples provided sufficiently high quality factors for studying guided and leaky wave behaviors in individual transparent THz resonant structures such as antennas or split ring resonators. However, without higher conductive ITO, the limited reflection efficiency and high radiation damping measured here both diminish the applicability of ITO for high-reflecting, arrayed, or long path-length elements.

Terahertz field detector based on electron emission

Abstract: This paper reports the electric field induced electron emission across the capacitive gap in a metamaterial structure under intense terahertz (THz) pulses. The gold metamaterial fabricated on a silicon nitride thin film can enhance the electric field greatly around the resonance frequency, resulting in THz field induced electron emission across the capacitive gap in the metamaterial structure when the incident electric field is > 60kV/cm. The emission current is measured experimentally and shows dependency on the strength of the incident field strength, which can be used as a THz detector.

Infrared Pump-Probe Spectroscopy of Plasmons in Graphene and Semiconductors

Abstract: 1. University of California San Diego, Department of Physics, La Jolla, California 92093. 2. The University of Texas at Austin, Microelectronics Research Center, Austin, TX 78758. 3. University of Maryland, Materials Research Science and Engineering Center, College Park, Maryland 20742. 4. University of California, Department of Physics and Astronomy, Riverside, California 92521. 5. California State University, Department of Physics, San Marcos, San Marcos, California 92096. 6. University of California San Diego, Department of Chemistry and Biochemistry, La Jolla, California 92093. 7. Graphene Research Centre and Department of Physics, National University of Singapore, 117542, Singapore. 8. Neaspec GmbH, Bunsenstr. 5, 82152 Martinsried, München, Germany. 9. Ludwig-Maximilians-Universität and Center for Nanoscience, 80539 München, Germany.

THz materials discovery and integration: The search for novel functionality

Abstract: The functionality of terahertz metamaterials can be dramatically increased through judicious materials integration. In addition to semiconductors, materials ranging from graphene to superconductors can enhance or enable new functionality. Following a brief review, we present recent results creating nonlinear metamaterials using InAs and YBa 2 Cu 3 O 7 , and discuss the broad range of possibilities to explore using transition metal oxides in the design of metamaterials.