Active Infrared Nanophotonics in van der Waals Materials

Introduction

Nanophotonics

Plasmonics
Metasurfaces

The seminal work of Yu et al.11 uses v-shaped metal structures, which result in different phases of light based on the angle between the arms of the resonator. This can be generalized as shown in Figure 1.3 below: by changing the geometry of the resonant element (which can be plasmonic or dielectric) and its environment (which will be the focus of our work), different phases can be achieved20, 22- 24.

Figure 1.1: Dispersion relation of the coupled odd and even modes for an air/silver/air multilayer with a metal core of thickness 50 nm

Graphene
Black Phosphorus

A newer addition to the family of 2D materials, black phosphorus (BP), is the layered allotrope of phosphorus, which possesses a kinked lattice structure that is nominally stable under ambient conditions.33 Due to the relatively weak in-plane bonding nature of the material, it is highly susceptible to oxidation and degradation, requiring encapsulation for device fabrication. By varying the number of BP layers, the change in quantum confinement in the vertical direction largely tunes the band gap energy.

Figure 1.4: Honeycomb lattice of graphene with two atoms per unit, A and B, defined by lattice vectors a 1 and a 2 and with nearest neighbor vectors δ I , i = 1,2,3

The Scope of this Thesis

An atomic force microscope (AFM) image of the resulting graphene nanoresonators is shown in the inset of Fig. The carrier density of the graphene nanoresonators was determined from experimentally measured resonance peak frequencies.

Figure 1.7: A pictorial representation of this thesis. Chapter 2 presents tightly confined graphene plasmons

Graphene Plasmons for Tunable Light Matter Interactions

Highly Confined Tunable Mid-Infrared Plasmonics in Graphene

Introduction
Experimental Measurement of Tunable Infrared Graphene
Theoretical Description of Graphene Plasmons

A resistance versus gate voltage curve of the graphene sheet showing a peak in the resistance at the charge neutral point (CNP), when the Fermi level (EF) is aligned with the Dirac point. Another notable feature of the SPPP that can be observed in the distribution is that modes with high k vectors are not supported.

Figure 2.1: Schematic of experimental device. (a) SEM image of a 80 × 80 um 2 graphene nanoresonator array etched in a continuous sheet of CVD graphene

Hybrid Surface-Plasmon-Phonon Polariton Modes in

Introduction
Experimental Measurement of Coupled 2D Phonon-Plasmon
Modeling of Coupled Plasmon-Phonon Dispersion

The distribution of optical modes of the graphene/h-BN/SiO2 nanoresonator can be observed in this plot as the maximum at the transmission modulation, -∆T/TCNP. In this description, it is observed that when the graphene plasmon mode is brought into resonance with the h-BN phonon, the polarizations of the two modes cancel each other, creating a transparency window where absorption in the plasmonic modes does not occur.

Figure 2.4: (a) Schematic of device measured and modeled in this paper. Graphene nanoresonators are fabricated on a monolayer h-BN sheet on a SiO 2 (285 nm)/Si wafer

Tunable Enhanced Absorption in a Graphene Salisbury Screen

Introduction
Experimental Demonstration of Enhanced Absorption

This normalization removes the low frequency feature below 1200 cm- 1, which is due to the broad optical phonon absorption in the SiNx and is independent of graphene doping. Because the absorption increases with carrier density, we associate it with resonant absorption in the trapped plasmons of the nanoresonators.30,. The total absorption in the device for undoped (red dashed) and highly hole doped (blue solid) 40 nm nanoresonators.

Figure 2.7: (a) Schematic of experimental device. 70 × 70 µm 2 graphene nanoresonator array is patterned on 1 µm thick silicon nitride (SiN x ) membrane via electron beam lithography

Conclusions and Outlook

As the carrier density increases to cm-2, the absorption in graphene and nearby SiNx increases due to the excitation of the confined plasmon mode. Feature (I) corresponds to the band-filling effect of the E11 intersubband transition, and feature (II) corresponds to the blocking of the E22 intersubband transition, which is schematically shown in Fig. 2d. In the thesis, we discussed the control of the amplitude and polarization of thermal radiation in the far field.

Graphene-Based Active Control of Thermal Radiation

Electronic Modulation of Thermal Radiation in a Graphene

Introduction
Experimental Realization of Dynamically Tuned Thermal
Theoretical Interpretation of Results
Radiated Power and Device Considerations

As can be seen in the figure, increasing the carrier density of the graphene nanoresonators leads to increases in emissivity near 730 cm-1 and 1400 cm-1. When graphene is placed on top of the SiNx, the intraband and interband transitions in the graphene serve to change the surface impedance of the device. A voltage of 0 V corresponds to the charge neutral point of the graphene and therefore the measurement of an 'off' signal.

Figure 3.1: Device and experimental set-up (a) Schematic of experimental apparatus. 70 × 70 µm 2 graphene nanoresonator arrays are placed on a 1 µm thick SiN x membrane with 200 nm Au backreflector

Electronic Control of Polarized Emission

Introduction
Design of Dual-Resonant Structure for Polarization Control

By carefully choosing the carrier concentration and width of the graphene resonators, it is possible to match different resonators to mλ. Electron beam lithography is used to expose a window on the back of the handle and XeF2 is used to etch through to SiO2. Graphene is then transferred on top of h-BN and 100 keV electron beam lithography is used to pattern the banded or cross-linked structures.

Figure 3.6: Schematic illustration of graphene-dielectric dual resonant structure for controlling the polarization state of reflected or emitted infrared light

Conclusions and Outlook

We explain the first feature at 0.5 eV as arising from a shift of the BP band edge due to the quantum-confined Stark effect. As in the thin flake, significant modulation of the absorption is observed at each intersubband transition due to the QCSE redshift of the energy of the subbands. Calculations of the optical constants of BP are based on the formalism developed in ref[231].

Phase Modulation and Active Beam Steering with

Experimental Demonstration of >230° Phase Modulation in

Introduction
Design of Resonant Phase-Shifting Structure
Experimental Demonstration of Phase Modulation
Beam Steering Calculations

We therefore illustrate this smooth resonant tuning at a wavelength of 8.70 µm in Figures 4.1b and 4.1c, where we show the magnitude of the electric field at different Fermi energies of graphene. A comparison of the relative phase difference between interferograms recorded for different sample biases is performed to capture the phase shift as a function of EF. Considering only the array factor, we can analytically capture the beam steering characteristics of the metasurface as a function of the achievable phase tuning range of the element.

Figure 4.1: Tunable resonant gap-mode geometry. (a) Schematic of graphene-tuned antenna arrays with field concentration at gap highlighted

Multi-Element Graphene-Gold Meta-Device for Active Beam

We note that all calculations in Figure 4.5 are for an operating wavelength of 8.6 µm to simplify the comparison. a) Optical microscope image of completed device with 28 independently tunable elements. Two examples of designs currently in experimental use are shown in Figure 4.7. Bo

Figure 4.6: Fabricated tunable metadevice. (a) Optical microscope image of completed device with 28 independently gate-tunable elements

Conclusions and Outlook

In this chapter, we report measurements of the infrared optical response of thin black phosphor under field effect modulation. As in the previous sample, they grow in strength with increasing magnitude of the gate voltage, regardless of polarity. We propose that quantum-confined Franz-Keldysh effects lead to the emergence of the additional oscillatory spectral features we observe.

Field Effect Optoelectronic Modulation of Quantum-Confined

Introduction

A majority of studies on both the fundamental optical properties of black phosphorus and applications in optoelectronic devices have investigated only the visible frequency range. Therefore, little is known about the intrinsic optical response of black phosphorus in the infrared range. Theoretical investigations of black phosphorus have suggested new optical infrared phenomena such as anisotropic plasmons228, 229, field-effect tunable exciton strong shifts230, and strong Burstein-Moss231 and quantum-confined Franz-Keldysh effects232 that promise to open new directions for both fundamental nanophotonics research and applications.

Experimental Design

In an anisotropic Burstein-Moss (BM) shift, the optical band gap of the material changes due to band filling and the resulting Pauli blocking of intersubband transitions. When the carrier concentration of the sample changes, the Fermi level shifts into (out of) the conduction or valence band, resulting in a decrease (increase) in absorption due to the blocking (allowing) of optical transitions 271, 272. This can be well understood due to the dependence of the Stark effect on optical transition oscillator initial powers; because there are no inter-subband optical transitions.

Tuning of Infrared Absorption in Few-Layer Black Phosphorus

BP Thickness #1
BP Thickness #2
BP Thickness #3

Conclusions and Outlook

In this paper, we experimentally demonstrate that the application of a static electric field enables modulation of the linear dichroism of few-layer black phosphorus (BP). An isotropic resonant structure can be used, taking advantage of BP to introduce different degrees of anisotropy. The freezer at the Atwater Group laboratories (251a) is used to keep the MIBK cold and to improve the modeling resolution.

Electrical Control of Linear Dichroism in Black Phosphorus

Introduction

Ultrathin Van der Waals materials are especially promising for such applications, as they enable the control of light at the atomic scale and have properties that can be actively modulated using an external gate voltage56, 242. Of these, black phosphorus consists of single layers (BP ) is particularly notable for its high electronic mobility and a direct bandgap that can be tuned as a function of thickness from 0.3 eV to 2 eV34,243. This has enabled the realization of numerous high-performance optoelectronic devices, including photodetectors that can be easily integrated with other photonic elements such as waveguides.

Experimental Isolation of Electro-Optic Effects

Due to the strong field enhancement in the near field of the graphene resonators, it is also suggested that very small concentrations of molecules can be detected. By changing the degree of anisotropy of the BP, the propagation of a surface plasmon polariton could be redirected, as shown schematically in Figure 7.6 below. Due to the low losses of TiO2, almost all of the improvement in absorption is in WS2, shown in Figure 7.9 below.

Figure 6.1: Anisotropic electro-optical effects in few-layer BP. (a) Schematic figure of infrared modulation devices

Thickness-Dependent Electro-Optic Effects

Visible-Frequency Gate-Tunability

Finally, in Figure 6.5 we present the results of gate-tunable dichroism at visible frequencies in a 20 nm thick wafer, comparable to those considered for infrared modulation. To enable the transmission of visible light, a new device geometry is used, which is shown schematically in Fig. 6.5a and in the optical image in Fig. In this configuration, a SrTiO3 substrate is used to enable transmission mode measurements at visible wavelengths.

Conclusions and Outlook

For many years, a major goal of the graphene community has been the realization of tunable plasmon devices in the technologically important telecommunications band (1550 nm), or higher energies. A 2 nm Ti/200 nm Au back reflector/back gate was evaporated onto the back side of the membrane by electron beam deposition. Electrical access to the contacts is achieved by exposing the PMMA using electron beam lithography on top of the contacts and then by wire bonding.

Perspective and Future Works

Graphene Research

Control of Far-Field Thermal Radiation
Control of Near-Field Heat Transfer
Graphene-Based Sensors
Graphene Devices in the High-Carrier Concentration Limit

It would be very interesting to consider a combination of such projects, which would enable active control of the phase of the emitted thermal radiation. In addition, we will take advantage of ultra-high-resolution patterning of single-layer materials enabled by He+ focused ion beam lithography (Zeiss Orion FIB). The use of a Zeiss Orion FIB instrument to fabricate graphene nanoribbons with a feature size of 5 nm was demonstrated, as seen in Fig.

Figure 7.1: A conceptual representation of the steering of thermal radiation using a metasurface with a linear phase gradient on a heated polar substrate for steering of radiation

Graphene-Integrated Devices (Commercialization)

Visualization of the maturity phases of new technologies, useful (if not scientifically validated) for understanding the life cycle of graphene so far.

Black Phosphorus Research and Development

Black Phosphorus for In-Plane Beam Steering
Black Phosphorus for Far-Field Polarization Control

By exploiting the anisotropy of BP we can open an interesting new design space for integrated photonics. One of the simplest ways to take advantage of BP's anisotropic optical response is to integrate it into an external cavity for efficient switching of the absorbed polarization state of light. In this way, the polarization component of the absorbed light depends on the differential absorption between the two axes of the BP, enhanced by the resonant design.

Figure 7.6: A schematic proposal of using black phosphorus as an active dielectric material for steering of surface plasmons

Nanophotonics and other van der Waals Materials

In this case, we exploit the variation in peak absorption in the TiO2 resonators with resonator width combined with WSe2, which absorbs light strongly above the visible, in contrast to WS2 with a narrow exciton absorption line. By sweeping the width of the resonators from 135 to 295 nm, the WSe2 absorption can be enhanced above the visible. Aydin et al using silver trapezoidal absorbers.186 The simulated WSe2 absorption is shown in Figure 7.10 below.

Figure 7.8: Schematic of resonant geometry designed for enhancing absorption in monolayer TMDCs

Endless Opportunities

The foil is then placed on top of the ferric chloride etchant solution (Transene, CE-100) for approximately 20 minutes until the copper is completely removed, leaving the graphene/PMMA film floating on the surface. Finally, the graphene is transferred with a spoon to a smaller beaker with water for 20 minutes, after which it is scooped onto the chosen substrate (in this case 285nm SiO2/Si chips). The water is baked off the chip for at least 8 hours at 40 - 50°C) and then the PMMA is removed in acetone or Remover PG (Microchem) for 45 minutes. Control of the source, translation stage, pyroelectric effect detector, and the Keithley source used to bias the metasurface is performed through a Labview automation script.