Corey Janisch

Extraordinary Second Harmonic Generation in Tungsten Disulfide Monolayers

We investigate Second Harmonic Generation (SHG) in monolayer WS2 both deposited on a SiO2/Si substrate or suspended using transmission electron microscopy grids. We find unusually large second order nonlinear susceptibility, with an estimated value of deff ~ 4.5 nm/V nearly three orders of magnitude larger than other common nonlinear crystals. In order to quantitatively characterize the nonlinear susceptibility of two-dimensional (2D) materials, we have developed a formalism to model SHG based on the Greens function with a 2D nonlinear sheet source. In addition, polarized SHG is demonstrated as a useful method to probe the structural symmetry and crystal orientation of 2D materials. To understand the large second order nonlinear susceptibility of monolayer WS2, density functional theory based calculation is performed. Our analysis suggests the origin of the large nonlinear susceptibility in resonance enhancement and a large joint density of states, and yields an estimate of the nonlinear susceptibility value deff = 0.77 nm/V for monolayer WS2, which shows good order-of-magnitude agreement with the experimental result.

Ultrashort optical pulse characterization using WS 2 monolayers

We demonstrate the application of two-dimensional materials for ultrashort optical pulse characterization. Monolayer transition metal dichalcogenides, such as tungsten disulfide (WS₂), possess extraordinarily large second-order nonlinear susceptibility, and due to their atomic thickness, have relaxed phase-matching requirements and, hence, an inherently wide bandwidth. Synthesized monolayer WS₂ triangular islands were used to characterize ultrashort optical pulses at the focal point of an objective lens through second-harmonic generation collinear frequency-resolved optical gating.

MoS2 monolayers on nanocavities: enhancement in light–matter interaction

Two-dimensional (2D) atomic crystals and van der Waals heterostructures constitute an emerging platform for developing new functional ultra-thin electronic and optoelectronic materials for novel energy-efficient devices. However, in most thin-film optical applications, there is a long-existing trade-off between the effectiveness of light–matter interactions and the thickness of semiconductor materials, especially when the materials are scaled down to atom thick dimensions. Consequently, enhancement strategies can introduce significant advances to these atomically thick materials and devices. Here we demonstrate enhanced absorption and photoluminescence generation from MoS2 monolayers coupled with a planar nanocavity. This nanocavity consists of an alumina nanolayer spacer sandwiched between monolayer MoS2 and an aluminum reflector, and can strongly enhance the light–matter interaction within the MoS2, increasing the exclusive absorption of monolayer MoS2 to nearly 70% at a wavelength of 450 nm. The nanocavity also modifies the spontaneous emission rate, providing an additional design freedom to control the interaction between light and 2D materials.

Label-Free Particle Sensing by Fiber Taper-Based Raman Spectroscopy

A fiber taper-based method for label-free Raman sensing is presented, which exploits the interaction between adsorbed specimen and the exposed evanescence tail of guided pump light. Chemical specificity and detection of microsphere specimens with diameters as small as 1 μm m are shown. To improve the sensitivity, we further propose resonator-enhanced Raman spectroscopy by taking advantage of the power build-up of a resonant mode in a cavity. Proof of concept is demonstrated by incorporating a fiber taper within a fiber ring resonator and locking a tunable laser to a circulating resonant mode. This demonstration shows a modest 40% enhancement of the Raman signal.

Tunable optical source based on divided pulse soliton self-frequency shift (Conference Presentation)

We presented a broadly tunable, power scalable, multi-line, ultrafast source. The source is based on combining principles of pulse division with the phenomenon of the soliton self-frequency shift. By using this system, interferometric pulse recombination is demonstrated showing that the source can decouple the generally limiting relationship between output power and center wavelength in soliton self-frequency shift based optical sources. Broadly tunable multi-color soliton self-frequency shifted pulses are experimentally demonstrated. Simultaneous dual-polarization second harmonic generation was performed with the source, demonstrating one novel imaging methodology that the source can enable.

Surface enhanced Raman scattering in whispering gallery mode microresonators

In this talk I will discuss surface enhanced Raman scattering in silica microsphere resonators based on whispering gallery mode resonance. Recently silica microspheres have attracted attention as a novel substrate for surface enhanced Raman scattering. Whispering gallery mode resonance has been identified as a major enhancement mechanism, along with other effects such as photonic nanojets. In most of the previous experiments, however, free space pumping of the microsphere has been used, which has low efficiency in coupling to the whispering gallery modes. In our approach, we use a tapered fiber coupler for a highly efficient coupling to the whispering gallery modes. Coupling to the microresonator is monitored using a tunable laser. We observe both pump enhancement and Purcell enhancement in the microresonator. Since the linewidth of the whispering gallery modes is much smaller than that of the Raman peaks, sharp peaks corresponding to the whispering gallery modes are overlaid on top of the Raman spectrum of the bulk material. To demonstrate the system’s potential for Raman analysis, I will present the whispering gallery mode surface enhanced Raman spectrum of rhodamine 6G thin film coated on a microsphere resonator.

Divided pulse soliton self-frequency shift: a multi-color, dual-polarization, power-scalable, broadly tunable optical source

A versatile, broadly tunable, power scalable, multi-line, ultrafast source is presented, the operation of which is based on combining principles of pulse division with the phenomenon of the soliton self-frequency shift (SSFS). Interferometric pulse recombination is demonstrated showing that the source can decouple the generally limiting relationship between the output power and the center wavelength in SSFS-based optical sources. Broadly tunable two- and four-color soliton self-frequency shifted pulses are experimentally demonstrated. Simultaneous dual-polarization second-harmonic generation was performed with the source, demonstrating one novel imaging methodology that the source can enable. It is expected that this source architecture will be useful for advancing current nonlinear optical imaging methodologies.

MoS 2 monolayers on nanocavities: Enhanced light-matter interaction within atomic monolayers

We report a fundamental strategy to enhance the light-matter interaction of atomically-thin films based on strong interference in planar nanocavities, which is validated experimentally by absorption and photoluminescence enhancement of MoS2 monolayers.

Raman sensing in optical microresonantors(Conference Presentation)

Ultrahigh-quality whispering gallery mode optical microresonators have been studied for their use as highly sensitive sensors. In this talk, we discuss the use of microsphere microresonators in Raman spectroscopy for interrogating particles adhered to the surface of the resonator. An external cavity diode laser is tuned to a resonant high-Q mode and the circulating optical field experiences a large buildup, resulting in enhanced Raman scattering. Here we present studies of Raman scattering spectroscopy of single particles. Raman sensing with different Qs is discussed.

Low wavenumber Raman spectroscopy using atomic filters(Conference Presentation)

Low-wavenumber Raman spectroscopy has long been demonstrated as a method of optical characterization in a variety of applications, such as thermal detection and semiconductor analysis. However, accessing low-wavenumber Raman shifts remains a challenge, usually requiring an expensive and complex multi-stage spectrographic system to measure several cm-1 Raman shifts. In this work, we demonstrate a method to measure low-wavenumber Raman shifts down to 1 cm-1 using atomic filters. By using a narrow-band Faraday anomalous dispersion optical filter to remove spontaneous emission noise from the laser cavity and a heated atomic cell as a notch filter to remove the excitation laser, the system is able to measure low Raman shifts (down to 1 cm-1). To demonstrate the capabilities, we measure the broadband Raman spectrum from a silica optical fiber with approximately 0.3 cm-1 resolution, detecting both Stokes and Anti-Stokes Raman shift as low as 0.7 cm-1.