Browsing by Author "Mikkelsen, Maiken H"

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.

Solid-state quantum emitters embedded in carefully engineered nanostructures could enable a new generation of quantum information and sensing technologies, including networked processors for quantum computing and precise monitors of temperature and strain at the nanoscale. The primary goal when designing these nanostructures is to utilize the Purcell effect to improve the emission rate, directionality and brightness of quantum emitters, as long decay times, nondirectional emission and weak fluorescence limit their applications. One particularly promising emitter is the silicon vacancy (SiV) in diamond, which offers excellent photostability and minimal spectral diffusion, in addition to coherent emission at its zero-phonon line (ZPL) comprising 80% of its total fluorescence. In this dissertation, up to 121-fold enhancement of the spontaneous emission rate of SiVs coupled to plasmonic nanogap cavities is demonstrated. The vacancy centers are implanted into a monolithic diamond thin film, which is then etched to nanometer-scale thickness, an approach with a clear path towards wafer-scale fabrication. A novel approach to creating film-coupled nanogap metasurfaces was developed to support this research and consists of transferring EBL-fabricated nanoparticles by using a PDMS stamp. Up to seven orders of magnitude of enhancement of nonlinear frequency conversion was also observed in diamond thin films coupled to these metasurfaces. Furthermore, a robust mechanism for actively tuning the nanocavity absorption resonance by integrating sub-10-nm films of the phase-change material vanadium dioxide. This platform opens up opportunities for on-chip quantum networks and nanoscale sensors based on nanocavity-coupled SiVs with the potential for in-situ frequency conversion to outcouple to photonic circuits and reconfigurable properties by incorporating VO2 thin films.

Fluorescence-based methodologies have been used extensively for biosensing and to analyze molecular dynamics and interactions. An emerging, promising diagnostic tool are fluorescence-based microarrays due to their high throughput, small sample volume and multiplexing capabilities. However, their low fluorescence output has limited their implementation for in vitro diagnostics applications in a point-of-care (POC) setting. Here, by integration of a sandwich immunoassay microarray within a plasmonic nanogap metasurface, we demonstrate strongly enhanced fluorescence enabling readout by a fluorescence microarray even at low sensitivities. We have named this plasmonic architecture the plasmonically enhanced D4 (PED4) assay. The immunoassay consists of inkjet-printed capture and fluorescently labeled detection antibodies on a polymer brush which is grown on a gold film. Colloidally synthesized silver nanocubes (SNCs) are placed on top of the brush through a polyelectrolyte layer and interacts with the underlying gold film creating a nanogap plasmonic structure supporting local electromagnetic field enhancements of ~100-fold. By varying the thickness of the brush between 5 and 20 nm, a 151-fold increase in fluorescence and a 14-fold improvement in the limit-of-detection (LOD) is observed for the cardiac biomarker B-type natriuretic peptide (BNP) compared to the unenhanced assay, paving the way for a new generation of point-of-care clinical diagnostics.

To move the PED4 towards a single step point of care test (POCT), SNCs are conjugated with a secondary antibody that attaches specifically to the detection antibody. This allows SNCs to deposit on the surface without the need of a charged polyelectrolyte layer. In addition, multiplexing capabilities are demonstrated in this plasmonic platform where NT-proBNP, Galectin-3, and NGAL are simultaneously detected and fluorescently enhanced. Microfluidics integration and use of a low-cost detector is also demonstrated.

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.

Thermal photodetectors are uniquely capable of sensing any incident wavelength and are only limited by the wavelength range of the integrated absorber, yet these detectors are commonly thought to be less sensitive and slower than their photoconductor (PC) or photovoltaic (PV) counterparts. As such, the use of thermal detectors for multispectral imaging has largely been ignored for spectral regions outside of the LWIR. However, recent theoretical and experimental developments have shown that thermal detectors with spectrally selective absorbers can outperform ideal PC and PV detectors in the MWIR and beyond. In this dissertation, thermal detectors integrated with spectrally selective, metasurface absorbers are theoretically and experimentally investigated. Large-area metasurface absorbers with resonances spanning from 340 to 2740 nm are developed that localize absorption and subsequent thermal generation in significantly subwavelength volumes. When integrated with a thermally sensitive pyroelectric film, the metasurface-pyroelectric detectors exhibit a sub-nanosecond response time with potential to match the response times of state-of-the-art, high-speed photodiodes. Overall, the spectrally selective thermal detectors developed here are a promising approach capable of disrupting the low cost, low form-factor spectral imaging market and providing an interesting platform to conduct research into photothermal generation and thermal diffusion at the nanoscale.

Quantum emitters and thermal photodetectors serve as key building blocks for future quantum information and sensing systems. Their properties can be significantly enhanced by integrating them with nanostructured elements, tailoring their interactions with incident optical fields. Quantum emitters integrated into nanophotonic cavities exhibit increased brightness and faster emission due to the Purcell effect, improving their maximum repetition rates as single photon sources, as well as decreasing saturation power leading to less pump induced noise. Further, the significant reduction in fluorescence lifetimes observed for quantum emitters coupled to nanophotonic cavities enhances their viability for use in quantum memory schemes. Many solid-state quantum memories suffer from short coherence times; however, if the fluorescence lifetime is much shorter than the decoherence time of the stored qubit, then it becomes far more likely that the output photon successfully carries the stored qubit. A popular solid-state defect for quantum memory type applications is the silicon vacancy center in diamond. Presented in this dissertation are results showing ensemble silicon vacancy containing diamond membranes integrated with ultrasmall mode volume plasmonic nanocavities. The cavity integrated silicon vacancies exhibit record instrument-limited 8 ps fluorescence lifetimes, corresponding to >135-fold lifetime enhancement factors, as well as 19-fold photoluminescence (PL) intensity enhancement. These results indicate promise of the approach for future fabrication of an ultrafast source of single photons. v Similar nanophotonic cavities can also be used as low heat capacity photothermal converters which, when codesigned with a thermally sensitive layer, act as a photodetector. Metallic metasurfaces have previously been considered less than ideal for polarization sensing, where they are usually operated in a reflection mode and require an external detector. Arrays of subwavelength plasmonic cavities, comprising a metasurface, are designed with spectral and polarization selectivity. Loss in the cavities converts light to heat, which then diffuses into an underlying pyroelectric sensing layer. The structures presented herein exhibit high extinction ratios of up to 19 for orthogonal linear polarization states, with extraction of stokes parameters coming within 12% of the theoretical value. These sensing structures are ultrathin, with active layer thicknesses of 290 nm, and fast with rise times of 2 ns. Preliminary results from smaller devices exhibit sub-nanosecond rise times with responsivity showing good agreement with absorbance. Smaller devices are also characterized for their bandwidth in the frequency domain and show -3dB bandwidths in excess of 1 GHz. These results are extremely fast for pyroelectric detectors and clear paths towards achieving higher responsivity and faster response are laid out. The metasurface absorber elements, or plasmonic cavities, used in these projects exhibit extremely small mode volumes showing promise for future highly integrated devices.

Two-dimensional transition metal dichalcogenides (TMDCs), with direct bandgaps in the visible to near-infrared wavelength, offer a tantalizing platform for making optoelectronic devices with enhanced and novel functionalities. In this dissertation, we explore the valley dynamics and various excitonic states in monolayer TMDCs, as well as demonstrate tunable and enhanced exciton emission using plasmonic nanocavities. First, we probe the origin of the excitonic and localized states in monolayer WSe2 using polarization-resolved PL spectroscopy at temperatures from 10 K to room temperature. Next, Kerr rotation experiments are used to investigate the temporal and spatial valley dynamics of monolayer TMDCs using femtosecond pump and probe pulses.

Despite these remarkable optical properties, atomically thin TMDC monolayer suffer from intrinsically weak light absorption (~3 %) and low photoluminescence (PL) quantum yield (~0.4 %). Furthermore, among the complex excitonic states of monolayer TMDCs, the B exciton emission is inherently weak compared to the dominant A exciton emission. Thus, we demonstrate a tunable plasmonic nanocavity where emitters are sandwiched in a sub-10-nm dielectric gap between a metallic film and colloidally synthesized silver nanocubes. When emitters with an intrinsic long lifetime are embedded in the gap region, the spontaneous emission rate enhancements can be exceeding 1,000 times while the structure maintains a high quantum efficiency (>50 %) and directional emission. Incorporating semiconductor quantum dots into the plasmonic cavity enable ultrafast spontaneous emission with emission rates exceeding 90 GHz. Finally, when MoS2 monolayers are integrated into this plasmonic nanocavity with tunable plasmon resonances, we observe a 1,200-fold enhancement for the A exciton emission and a 6,100-fold enhancement for the B exciton emission. Moreover, we show a strong modification of the PL emission peaks, which exhibits a strong correlation between the emission wavelengths and the nanocavity resonance. Manipulating the optical properties of these 2D materials using tunable plasmon resonances is promising for the design of novel optical devices with precisely tailored responses, which is critical for optimizing the performance of future optoelectronic and nanophotonic devices.