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.
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.
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.
Layered two-dimensional (2D) materials possess unique optical properties. For example, the monolayer family of Transition Metal Dichalcogenide (TMD) materials (such as WS2 and MoS2) is well known for the existence of a direct band gap. In this talk, we discuss the characterization of the second order nonlinear susceptibility in mono- or few-layer TMDs, which have been recently shown to exhibit extraordinary second harmonic generation. We investigate the large nonlinear response due to resonance enhancement. Theoretical analysis will be discussed and compared with experimental results.
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 Q's is discussed.
High-quality whispering-gallery-mode optical resonators have garnered interest in particle sensing for a variety of applications. Here, we further explore the idea of using microresonators to enhance single-particle detection and identification by monitoring the Raman scattering from a particle adhered to a silica micro-sphere. A tunable diode laser is critically coupled into a resonant mode of the micro-sphere resonator, allowing circulating power to build up within the cavity for enhanced interaction with the attached particle. Experimental results of single particle Raman scattering in microsphere resonators are presented.
Two-Dimensional (2D) layered materials have garnered interest due to their novel optical and electronic properties. In
this work, we investigate Second Harmonic Generation (SHG) in Tungsten Disulfide (WS2) monolayers grown on
SiO2/Si substrates and suspended on a transmission electron microscopy grid; we find an unusually large second order
susceptibility, which is nearly three orders of magnitude larger than common nonlinear crystals. We have also developed
a Green’s function based formalism to model the harmonic generation from a 2D layer .
We investigate Raman spectroscopic sensing using whispering gallery microresonators as a label-free method toward single particle detection. Whispering gallery mode microresonators are used as platforms to perform sensitive particle detection by exploiting the strong, evanescent field of a resonant mode exposed on the surface
of the microresonator. Particles adhered to the microresonator surface interact with the field and scatter photons circulating within the resonator. In particular, Raman scattered photons are detected, providing
molecular-specific "fingerprint" information regarding the adhered particles. The exploitation of a resonant mode allows for enhancement of generated Raman signal over traditional methods of spontaneous Raman scattering. Preliminary proof-of-concept experimental results are shown.