Browsing by Subject "Tuning"

Active plasmonic nanostructures with tunable resonances promise to enable smart materials with multiple functionalities, on-chip spectral-based imaging and low-power optoelectronic devices. A variety of tunable materials have been integrated with plasmonic structures, however, the tuning range in the visible regime has been limited and small on/off ratios are typical for dynamically switchable devices. An all optical tuning mechanism is desirable for on-chip optical computing applications. Furthermore, plasmonic structures are traditionally fabricated on rigid substrates, restricting their application in novel environments such as in wearable technology.

In this dissertation, I explore the mechanisms behind dynamic tuning of plasmon resonances, as well as demonstrate all-optical tuning through multiple cycles by incorporating photochromic molecules into plasmonic nanopatch antennas. Exposure to ultraviolet (UV) light switches the molecules into a photoactive state enabling dynamic control with on/off ratios up to 9.2 dB and a tuning figure of merit up to 1.43, defined as the ratio between the spectral shift and the initial line width of the plasmonic resonance. Moreover, the physical mechanisms underlying the large spectral shifts are elucidated by studying over 40 individual nanoantennas with fundamental resonances from 550 to 720 nm revealing good agreement with finite-element simulations.

To fully explore the tuning capabilities, the molecules are incorporated into plasmonic metasurface absorbers based on the same geometry as the single nanoantennas. The increased interaction between film-coupled nanocubes and resonant dipoles in the photochromic molecules gives rise to strong coupling. The coupling strength can be quantified by the Rabi-splitting of the plasmon resonance at ~300 meV, well into the ultrastrong coupling regime.

Additionally, fluorescent emitters are incorporated into the tunable absorber platform to give dynamic control over their emission intensity. I use optical spectroscopy to investigate the capabilities of tunable plasmonic nanocavities coupled to dipolar photochromic molecules. By incorporating emission sources, active control over the peak photoluminescence (PL) wavelength and emission intensity is demonstrated with PL spectroscopy.

Beyond wavelength tuning of the plasmon resonance, design and characterization is performed towards the development of a pyroelectric photodetector that can be implemented on a flexible substrate, giving it the ability to be conformed to new shapes on demand. Photodetection in the NIR with responsivities up to 500 mV/W is demonstrated. A detailed plan is given for the next steps required to fully realize visible to short-wave infrared (SWIR) pyroelectric photodetection with a cost-effective, scalable fabrication process. This, in addition to real-time control over the plasmon resonance, opens new application spaces for photonic devices that integrate plasmonic nanoparticles and actively tunable materials.

Nonlinear generation of optical fields has enabled many exciting breakthroughs in light science such as nonlinear imaging, all-optical switching and supercontinuum generation. The intrinsic nonlinear response from bulk materials is extremely weak and phase matching conditions need to be satisfied for efficient generation. Plasmonic structures have proven to be a promising platform to investigate nonlinear optics due to the capability to enhance and localize electromagnetic fields within subwavelength volumes beyond the diffraction limit. However, the origin of the large nonlinear response observed in the plasmonic structures is not fully understood and the investigations of nonlinear processes involving multiple excitation wavelengths are limited. Furthermore, simultaneous enhancement and precise manipulation of multiple nonlinear optical processes have not been experimentally demonstrated.

In this dissertation, I describe a specific film-coupled plasmonic nanogap cavity structure consisting of arrays of nanoparticles separated from a metallic ground plane by an ultrathin dielectric layer. Polarization-dependent, dual-band and spatially-overlapped resonances are obtained with arrays of rectangle nanoparticles where the two resonances can be tuned independently. Highly efficient third harmonic generation (THG) is achieved by integrating dielectric materials in plasmonic nanogap cavities, resulting in more than six orders of magnitude enhancement in the THG response compared with a bare gold film. Utilizing comprehensive spectral analysis and finite-element simulation, it is concluded that the main contributing nonlinear source is dielectric material in the gap. Furthermore, I demonstrate simultaneous enhancement of three nonlinear responses from THG, sum frequency generation (SFG) and four wave mixing (FWM) by integrating 1-7 nm Al2O3 layer in the nanocavities formed by a gold ground plane and silver nanorectangles. Enhancement up to 106-fold for both THG and FWM and 104-fold for SFG is achieved when the excitation wavelengths overlap with the resonance wavelengths from transverse and longitudinal modes of the nanorectangles. Precise control of the relative strength of these nonlinear responses is demonstrated either actively by varying the ratio between excitation powers or passively by changing the Al2O3 gap thickness. Moreover, a metasurface-based efficient frequency mixer is realized utilizing diamond and a novel polymer transfer process is employed for creating nanoparticle arrays. This new insight into the nonlinear response in ultrathin gaps between metals is expected to be promising for both the fundamental understanding of nonlinear optics at the deep nanoscale and efficient on-chip nonlinear devices such as ultrafast optical switching and entangled photon sources. The capability to precisely manipulate nonlinear optical processes at the nanoscale could find important applications for nonlinear imaging and quantum communication.