Collective States in Molecular Monolayers on 2D Materials

Collective excited states form in organic two-dimensional layers through the Coulomb coupling of the molecular transition dipole moments. They manifest as characteristic strong and narrow peaks in the excitation and emission spectra that are shifted to lower energies compared to the monomer transition. We study experimentally and theoretically how robust the collective states are against homogeneous and inhomogeneous broadening as well as spatial disorder that occur in real molecular monolayers. Using a microscopic model for a two-dimensional dipole lattice in real space we calculate the properties of collective states and their extinction spectra. We find that the collective states persist even for 1-10% random variation in the molecular position and in the transition frequency, with similar peak position and integrated intensity as for the perfectly ordered system. We measure the optical response of a monolayer of the perylene-derivative MePTCDI on two-dimensional materials. On the wide band-gap insulator hexagonal boron nitride it shows strong emission from the collective state with a line width that is dominated by the inhomogeneous broadening of the molecular state. When using the semimetal graphene as a substrate, however, the luminescence is completely quenched. By combining optical absorption, luminescence, and multi-wavelength Raman scattering we verify that the MePTCDI molecules form very similar collective monolayer states on hexagonal boron nitride and graphene substrates, but on graphene the line width is dominated by non-radiative excitation transfer from the molecules to the substrate. Our study highlights the transition from the localized molecular state of the monomer to a delocalized collective state in the two-dimensional molecular lattice that is entirely based on Coulomb coupling between optically active excitations of the electrons or molecular vibrations

Far-Infrared Laser Emissions from Optically Pumped Methanol

The invention of the LASER (an acronym for Light Amplification by Stimulated Emission of Radiation) in 1960 came with no specific application in mind. Initially, some critics dubbed it “the solution in search of a problem.” It was only after the laser was invented that scientists and entrepreneurs found the laser’s real potential and created enormous amounts of applications spanning from technology used in everyday life to medical and defensive applications. At Central Washington University, an optically pumped molecular laser system is used to search for new sources of far-infrared radiation. The far-infrared region is loosely defined as light having wavelengths ranging from about 0.030 to 2.000 mm. With this experimental system, 71 far-infrared laser emissions were discovered using the methanol isotopologues 13CHD2OH, CH318OH, CHD2OH, and CH2DOH as the lasing medium. Additionally, several of these newly discovered laser emissions have been used to support the spectroscopic assignments of laser transitions previously proposed by other researchers. This presentation will outline the experimental system and method used in the search for new sources of far-infrared laser radiation along with a brief discussion of the experimental results and their role in performing spectroscopic assignments of molecular transitions. For his work on this project, Mark McKnight was nominated for the SOURCE 2014 Scholar of the Year Award. The presentation also received a College of the Sciences Best Oral Presentation Award for 2014

New Far-Infrared Laser Emissions from Optically Pumped CH2DOH, CHD2OH, and (CH3OH)-O-18

An optically pumped molecular laser system utilizing a transverse or zig-zag pumping geometry of the far-infrared (FIR) laser medium has enabled the reinvestigation of the CH 2 DOH, CHD 2 OH, and CH 3 18 OH isotopic forms of methanol for wavelengths \u3e 100 μm. With this system, 28 FIR laser emissions have been discovered with wavelengths ranging from 117.2 to 744.7 μm. Along with the wavelength, each laser emission is reported with its optimal operating pressure, polarization with respect to the CO 2 pump laser, and relative intensity. Three of the laser emissions generated by CH 3 18 OH support the spectroscopic assignments of FIR laser transitions originally proposed through combination-difference loops. A fourth CH 3 18 OH laser emission should contribute to another, incomplete FIR laser scheme