Plasmonic lasing using dye molecules and 2D materials

Plasmonic systems are widely used to enhance light emission from active materials. [220–224] By using periodic plasmonic resonances, the stimulated emission of the dye molecules can be excited. As shown in Figure 97A, we implemented an inverse microscope for the detection of lasing. The dye molecule LDS821 as the gain material, which is dissolved in dimethylsulfoxide (DMSO) solution as the device under test (DUT). The period of the periodic metallic structure is 530 nm. PL measurements were performed using a florescent microscope equipped with an excitation laser operated at a wavelength of 532 nm as shown in Figure 97B. A laser power of 30uW with a focus spot of 10um (focused by a 40X objective lens) was used to illuminate on the sample. A spectrometer (Andor 550i) were used to detect the PL signal with integration time of 20s. Two-dimensional (2D) materials, such as groups of transition metal dichalcogenides (TMDCs) are crystalline materials comprised of multiple monolayers of atoms. They are direct bandgap two- dimensional (2D) material semiconductors and have properties dramatically different from the

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bulk materials, making them promising light active materials for optoelectronic applications [225]. Recently, TMDCs have shown great potential in ultrafast and ultrasensitive photodetectors as ultrathin light absorbers and emitters [226,227]. However, their application in photonic devices is limited by their low absolute PL due to low quantum efficiency and weak absorption. A lot of efforts have been dedicated towards obtaining enhanced light emission from TMDCs. [228–231] However, several intrinsic issues are still present when integrating 2D materials with plasmonic nanostructures. The most severe one among them is the unavoidable damage with the direct deposition of metallic nanostructures on 2D materials [232].

Figure 97 Optical setup of plasmonic laser.

(A) Inverse microscope setup for device under test. Dye molecules are dissolved in DMSO and covered by a glass slide. (A1) The SEM image of the periodic metal hole array. (B) Optical path for laser pumping and detection. (C) Measured PL spectra of exfoliated monolayer WSe2 flakes

on SiO2/Au/Glass DMD structures with the SiO2 spacer layer range from 5 nm to 25 nm. The

spectra are normalized to the peak value of the case of WSe2 directly on PDMS. (C1) Schematic

of enhanced signal observed of WSe2.

We also performed the PL enhancement for a 2D semiconductor tungsten diselenide (WSe2). Our

planar dielectrics/metal/dielectrics (DMD) structure consists of a glass substrate, 40nm Au film and one SiO2 spacer (5nm-25nm) layer separating the metal film and the WSe2 monolayer as in

shown in Figure 97C1. In this test, all the conditions were kept the same for all those samples in order to investigate the role of the spacer thickness. Figure 97C shows the measured PL spectra of

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WSe2 monolayers on SiO2/Au/Glass DMD structures with the SiO2 spacer layers ranging from 5

nm to 25 nm. The dependence of the separation between the metal and WSe2 are apparent from

the plot, as either enhancement or inhibition can be achieved when the 2D materials interact with the DMD structures. Considerable shifts of the spectra peak were also observed. Both the enhancement and shift show highly sensitive dependence on the thickness of the spacer layer for changes as small as only 5 nm. Investigation of the underlying physical processes such as non- radiative exciton-plasmon energy transfer and group coherency can be conducted. [233,234] This plasmonic laser setup not only provides a general method to enhance light emission from active materials but also offers a good platform to study the fundamental physics of plasmon interacted exciton dynamics.

5.5 Summary

In this chapter, several applications of plasmonic materials are discussed in simulations and experiments. A Si nanoparticle is placed on top of the HMM, which can be treated as a localized dipolar emitter. Depending on the polarization of the incident light, the scattering of the particle can be unidirectional when the dipole is circularly polarized. When a plane wave is incident onto the HMM, the reflected light acquires a large phase change and forms interference patterns. This phenomenon can be utilized for a lithography system, and the detection of polarization states. It provides a new platform to passively generate spin particles, inspire potential applications of the unidirectional scattering and study of chiral molecules and structures. In addition, a Lidar design based on dielectric particles coated with metallic caps is proposed for object detection suitable for self-driving cars. For both normal and angled incidences, the particles provide a strong backscattering near a wavelength of 900 nm to detect obstacles in IR range. The strong resonance is based on intrinsic hybrid MD resonance induced by the core, while the metal caps improves the

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confinement of light. Those particles also produce strong forward scattering in the visible range, which does not comprise the transparency of the windshield. Both SiO2 and TiO2 particles with

different dimensions and capping materials are studied and they showed comparable behaviors, which proves the flexibility and robustness of the Lidar design. In addition, Si NW forests were fabricated by metal assisted chemical etching. The size, density and height of the NW array can be controlled, to generate different colors. Due to the plasmonic properties of Si in IR range, the NW arrays can also find their application as IR sensors to enhance the surface absorption. Other progresses towards plasmonic functional devices including thermal-light converters and plasmonic lasers are studied as well.

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