A field trial was performed to estimate the damage to Japonica (Wuyugeng 3) by the second-generation population of the brown planthopper, Nilaparavala lugen (Stal.), and the economic threshold for the pest. Infestation was established in caged rice field plots(10 hills per plot)at the densities of 100，250，500,and 1000 nymphs (1st or 2nd instar)per 100 hills with 3 replications. Three plots without infestation were sprayed with the insecticide buprofecin and used as control treatment. The results showed that the increase of the population differed among the treatments with varying levels of initial infestation. Higher reproductive potential was observed in the treatments with lower levels of infestation though the density was eventually greater in the plots infested at higher densities. The yield loss was significantly correlated with the population density: (Y=-16.3732+9.937 lg (x), r=0.9557). The economic threshold was thus estimated to be 150-200 planthoppers per 100 hills. This suggests that spray is needed when the second-generation population infests the crop earlier at a medium level.
Author(s): Zhang, Gufeng | Advisor(s): Averitt, Richard D | Abstract: The development of ultrafast spectroscopy gives us access to the time dimension to study the dynamics of condensed matter systems. Different from conventional condensed matter experimental techniques, the pump-probe technique allows us not only to study the relaxation of nonequilibrium to equilibrium states, but also the possibility to selectively control the properties of a material using ultrafast light. In my thesis, I will introduce the fundamental of optics in the first chapter. Second chapter consists of the experimental set-ups I developed and built during my Ph.D.: Terahertz time-domain spectroscopy (THz-TDS), broadband THz source from two-color laser-induced gas plasma, intensive mid-infrared pump Kerr rotation probe and data acquisition (DAQ) system. The third chapter is the background of spin-orbit coupled Mott insulator Sr2IrO4,and the last chapter includes my work of ultrafast magnetic control in Sr2IrO4: we study the magnetic dynamics of the Jeff = 1/2 Mott state using strong mid-Infrared 9 µm (below the charge gap), and near-infrared 1.3 µm (above the charge gap) circularly polarized excitations, and monitor the pump induced Kerr signal. For both pump wavelength, the 2D in-plane B2g coherent magnon oscillation of frequency ~ 0.5 THz was observed in the pump-induced Kerr rotation signal. The circularly polarized 9 µm pumps of opposite helicities excite oscillations of opposite phase, while 1.3 µm pumps excite oscillations of same phase. The quadratically scaling of the fluence dependent magnon amplitude for the 9 µm pump indicates a novel photon-two-magnon coupling mechanism for the magnon generation. The directly excitation (9 µm) of the spin spectrum without photodoping electrons permits extremely efficient magnon generation, almost ten times better than the resonant 1.3 µm pump.
Polyamines are nitrogenous compounds that alter the odour sensitivity of insects.The objective of this study was to determine the effects of polyamines on the expression levels of olfactory-related genes in male/female adult diamondback moths, Plutella xylostella.Results indicate that treatment with polyamines or α-difluoromethyl-ornithine (DFMO), a polyamine synthesis inhibitor, significantly affected the expression of various olfactory related genes.Although DFMO had no significant effect on the expression of PxylPBP1, it did significantly decrease the expression of other olfactory related genes (PxylGOBP1, PxylGOBP2 and PxylCSP1) in a gender-related fashion that correlated with the mating status of the adults.Polyamines [spermine (spm), putrescine (put) and spermidine (spd)] decreased the expression of PxylGOBP2 in naive male moths, but increased the expression of the same gene in virgin female moths.In addition, put treatment significantly increased the expression of PxylGOBP1 in virgin females and unmated male moths, but significantly decreased its expression in mated female and male moths.Based on these results we conclude that polyamines have a regulatory role in the expression of olfactory related genes and that these effects depend on the gene, gender and mating status of diamondback moths.These results indicate that polyamines affect the expression of the genes in insects that determine their olfactory ability.
Antiferromagnets exhibit rapid spin dynamics in a net zero magnetic background which enables novel spintronic applications and interrogation of many-body quantum phenomena. The layered antiferromagnet Sr$_2$IrO$_4$ hosts an exotic spin one-half Mott insulating state with an electronic gap arising from on-site Coulomb repulsion and strong spin-orbit coupling. This makes Sr$_2$IrO$_4$ an interesting candidate to interrogate dynamical attributes of the magnetic order using ultrafast laser pulses. We investigate the magnetization dynamics of Sr$_2$IrO$_4$ following circularly-polarized photoexcitation with below-gap mid-infrared (mid-IR -- 9 $\mu m$) and above-gap near-infrared (near-IR -- 1.3 $\mu m$) pulses. In both cases, we observe excitation of a zone-center coherent magnon mode featuring a 0.5 THz oscillation in the pump-induced Kerr-rotation signal. However, only below-gap excitation exhibits a helicity dependent response and linear (quadratic) scaling of the coherent magnon amplitude with excitation fluence (electric field). Moreover, below-gap excitation has a magnon generation efficiency that is at least two orders of magnitude greater in comparison to above-gap excitation. Our analysis indicates that the helicity dependence and enhanced generation efficiency arises from a unique one-photon two-magnon coupling mechanism for magnon generation. Thus, preferential spin-photon coupling without photoexcitation of electrons permits extremely efficient magnon generation. Our results reveal new possibilities for ultrafast control of antiferromagnets.
Topological insulators are bulk insulators that possess robust chiral conducting states along their interfaces with normal insulators. A tremendous research effort has recently been devoted to topological insulator-based heterostructures, in which conventional proximity effects give rise to a series of exotic physical phenomena. Here we establish the potential existence of topological proximity effects at the interface between a topological insulator and a normal insulator, using graphene-based heterostructures as prototypical systems. Unlike conventional proximity effects in topological insulator based heterostructures, which refer to various phase transitions associated with the symmetry breaking of specific local order parameters, topological proximity effects describe the rich variety of quantum phase transitions associated with the global properties of the system measured by the location of the topological edge states. Specifically, we show that the location of the topological edge states exhibits a versatile tunability as a function of the interface orientation, the strength of the interface tunnel coupling between a topological graphene nanoribbon and a normal graphene nanoribbon, the spin–orbit coupling strength in the normal graphene nanoribbon, and the width of the system. For zigzag and bearded graphene nanoribbons, the topological edge states can be tuned to be either at the interface or outer edge of the normal ribbon. For armchair graphene nanoribbons, the potential location of the topological edge state can be further shifted to the edge of or within the normal ribbon, to the interface, or diving into the topological graphene nanoribbon. We further show that the topological phase diagram established for the prototypical graphene heterostructures can also explain the intriguing quantum phase transition reported recently in other topological-insulator heterostructures. We also discuss potential experimental realizations of the predicted topological proximity effects, which may pave the way for integrating the salient functionality of topological insulators and graphene in future device applications.
Conventional dielectric metasurfaces achieve their properties through geometrical tuning and consequently are static. Although some unique properties are demonstrated, the usefulness for realistic applications is thus inherently limited. Here, control of the resonant eigenmodes supported by Huygens' metasurface (HMS) absorbers through optical excitation is proposed and demonstrated. An intensity transmission modulation depth of 99.93% is demonstrated at 1.03 THz, with an associated phase change of greater than π/2 rad. Coupled mode theory and S-parameter simulations are used to elucidate the mechanism underlying the dynamics of the metasurface and it is found that the tuning is primarily governed by modification of the magnetic dipole-like odd eigenmode, which both lifts the degeneracy, and eliminates critical coupling. The dynamic HMS demonstrates wide tunability and versatility which is not limited to the spectral range demonstrated, offering a new path for reconfigurable metasurface applications.
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