Ultra-strong nonlinear optical processes and trigonal warping in MoS2 layers

Nonlinear optical processes, such as harmonic generation, are of great interest for various applications, e.g., microscopy, therapy, and frequency conversion. However, high-order harmonic conversion is typically much less efficient than low-order, due to the weak intrinsic response of the higher-order nonlinear processes. Here we report ultra-strong optical nonlinearities in monolayer MoS2 (1L-MoS2): the third harmonic is 30 times stronger than the second, and the fourth is comparable to the second. The third harmonic generation efficiency for 1L-MoS2 is approximately three times higher than that for graphene, which was reported to have a large χ (3). We explain this by calculating the nonlinear response functions of 1L-MoS2 with a continuum-model Hamiltonian and quantum mechanical diagrammatic perturbation theory, highlighting the role of trigonal warping. A similar effect is expected in all other transition-metal dichalcogenides. Our results pave the way for efficient harmonic generation based on layered materials for applications such as microscopy and imaging.We acknowledge funding from the Academy of Finland (Nos: 276376, 284548, 295777, 298297, and 304666), TEKES (NP-Nano, OPEC), Royal Academy of Engineering (RAEng) Research Fellowships, Fondazione Istituto Italiano di Tecnologia, the Graphene Flagship, ERC grants Hetero2D, Nokia Foundation, EPSRC Grants EP/K01711X/1, EP/K017144/1, EP/L016087/1, AFOSR COMAS MURI (FA9550-10-1-0558), ONR NECom MURI, CIAN NSF ERC under Grant EEC-0812072, and TRIF Photonics funding from the state of Arizona and the Micronova, Nanofabrication Centre of Aalto University

Synchrotron X-ray topography and electrical characterization of epitaxial GaAs p-i-n structures

In this thesis, dislocations and other crystal defects of compound semiconductor materials were characterized by synchrotron radiation X-ray topography (SR-XRT) and X-ray diffraction (XRD).Crystal defects in GaAs structures grown by hydride vapor phase epitaxy were characterized. GaAs diode components were processed of epitaxial p-i-n structures. The response of the diode components was measured with fluorescent X-ray photons. Results of SR-XRT measurements were compared to the electrical characteristics of GaAs detector diodes. Medipix2 read-out circuit compatible GaAs sensor was processed of epitaxial GaAs wafer. The sensor was bump bonded to the Medipix2 read-out circuit. The response of the manufactured radiation detector was measured with fluorescent photons. GaAs/Ge detector diodes were processed of epitaxial GaAs/Ge structures. Electrical characterization was performed for the GaAs/Ge structures. Crystal defects of GaAs layers grown on high purity germanium substrate were characterized. It was found that the crystal defect density was low in best samples, however, it was observed that arsenic diffusion in germanium substrate dominates the electrical characteristics of the structure and thus the structure can't be used for radiation detector purposes. Crystal defects of TlBr crystals were measured with SR-XRT. Radiation detector components were processed of the TlBr crystals and current-voltage characteristics were measured. The information about crystal defects, which was acquired in this study, can be used to better understand electrical characteristics and performance of various compound semiconductor detector structures.Tässä työssä tutkittiin röntgenilmaisinsovelluksiin sopivien yhdistepuolijohdemateriaalien rakennevirheitä ja sähköisiä ominaisuuksia. Rakennevirheitä tutkittiin käyttämällä synkrotronitopografiaa ja röntgendiffraktiota. Sähköisiä ominaisuuksia tutkittiin mittaamalla virta-jännitekäyriä valmistetuista tasasuuntaavista puolijohdekomponenteista. Synkrotronitopografia osoittautui erittäin hyväksi menetelmäksi mitata rakennevirheitä tutkituista materiaaleista niiden suhteellisen vähäisen virhetiheyden vuoksi. Työssä valmistettiin myös säteilynilmaisimia ja tutkittiin niiden vastetta röntgensäteilyyn. Hydridikaasufaasiepitaksialla valmistetuista GaAs p-i-n kerrosrakenteista tutkittiin kidevirheitä käyttämällä synkrotronitopografiaa. GaAs-kerrosrakenteista valmistettiin tasasuuntaavia puolijohdekomponentteja, joita voidaan käyttää säteilynilmaisimina. Valmistetuista puolijohdekomponenteista mitattiin virtajännitekäyrästö ja havaittiin selvä yhteys kidevirhetiheyden ja pimeävirran välillä. Epitaktisesti kasvatetusta GaAs-kerrosrakenteesta valmistettiin Medipix2-lukupiirin kanssa yhteensopiva säteilynilmaisinkomponentti. Komponetti liitettiin Medipix2-lukupiiriin ja valmistetulla lukupiiri-ilmaisinyhdistelmän ominaisuuksia säteilynilmaisimena tutkittiin. Germaniumin päälle kasvatettujen GaAs-kerroksien kidevirheitä tutkittiin synkrotronitopografialla. Tällaisissa rakenteissa havaittiin parhaimmillaan pieni kidevirhetiheys mutta havaittiin, että rakenteen sähköisiä ominaisuuksia dominoi arseenin diffuusio germaniumiin. Tämän vuoksi rakenne todettiin soveltumattomaksi säteilynilmaisimeksi. Thalliumbromidikiteitä tutkittiin synkrotronitopografialla ja röntgendiffraktiolla. TlBr-kiteistä valmistettiin säteilynilmaisimiksi sopivia komponentteja ja niiden sähköisiä ominaisuuksia tutkittiin. Tässä työssä löydettiin uutta tietoa säteilynilmaisimiin sopivien yhdistepuolijohteiden virherakenteista ja niiden yhteydestä säteilynilmaisinkomponenttien sähköisiin ominaisuuksiin. Työn tuloksien avulla voidaan ymmärtää paremmin kidevirheiden vaikutusta säteilynilmaisinkomponettien toimintaan

Ultra-strong nonlinear optical processes and trigonal warping in MoS2 layers

Nonlinear optical processes, such as harmonic generation, are of great interest for various applications, e.g., microscopy, therapy, and frequency conversion. However, high-order harmonic conversion is typically much less efficient than low-order, due to the weak intrinsic response of the higher-order nonlinear processes. Here we report ultra-strong optical nonlinearities in monolayer MoS2 (1L-MoS2): the third harmonic is 30 times stronger than the second, and the fourth is comparable to the second. The third harmonic generation efficiency for 1L-MoS2 is approximately three times higher than that for graphene, which was reported to have a large χ (3). We explain this by calculating the nonlinear response functions of 1L-MoS2 with a continuum-model Hamiltonian and quantum mechanical diagrammatic perturbation theory, highlighting the role of trigonal warping. A similar effect is expected in all other transition-metal dichalcogenides. Our results pave the way for efficient harmonic generation based on layered materials for applications such as microscopy and imaging