Surface plasmons for enhanced mid-infrared graphene photodetection
Creators: name: Peng, Jingyang
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401809
subject: Nanophotonics
classificationCode: 510204
subject: Photonics, optoelectronics and optical communications

description: Information photonics, which deals with the manipulation, detection, generation and transmission of light across a broad wavelength range has undoubtedly shaped and transformed our daily life, from mobile and communication devices to computing and the internet. Although the research and application of photonic devices based on silicon and other semiconductor materials have shown potentials towards the next information revolution, several intrinsic limitations have shown up, resulting from the common drawbacks of semiconductors, such as large footprint, limited absorption bandwidth and complex and expensive manufacturing process. Therefore, the exploration of new material is highly demanded and the integration with other research fields, such as electronics, is crucial.<br><br>Integrated optoelectronics, which bridges optics and electronics through manipulating, detecting and generating optical signals with electronic signals, provides the ability to tackle some of the existing challenges, because it not only inherits the mature processing protocols and theories from well-developed integrated electronics but also assimilates advantages from optics. Among integrated optoelectronics devices, photodetectors, devices that transduce absorbed photons into measurable electrical signals, are ubiquitous. They are the key components for optical communications, imaging, security, night-vision, spectroscopy and motion detection. Current commercial photodetectors have thrived on the development of band theory and the maturity of semiconductor growth and manufacturing; however, current mid-infrared photodetectors are intrinsically limited by high dark current, low working temperature and responsivity and expensive cost resulting from the narrow band gap of semiconductor alloys and restrictive materials growth process. Nevertheless, the mid-infrared wavelength plays a crucial role in the applications of spectroscopy, security and industry, chemical and biological sensing. Therefore, exploration and investigation of novel materials for mid-infrared photodetection is critical.<br><br>Graphene, a one-atom-thick layer of sp2-bonded carbon atoms tightly packed in a hexagonal lattice, has shown potential as a substitute material for silicon for the next information revolution because of its extraordinary optical, electronic, mechanical and thermal properties. These exceptional properties make it a promising candidate for a comprehensive range of applications, such as energy and storage, electronics, biomedical, flexible and wearables, photonics and optoelectronics. For example, the intrinsic surface plasmons based on a graphene platform surpass conventional plasmons because of its exceptional characteristics including tuneability, adjustability and relatively low dissipation, which implies that novel manufactured devices based on graphene surface plasmons can operate with low power consumption and driving voltage, small footprint and unrivalled speed. Moreover, the broadband absorption from UV to THz, ultrafast carrier mobility and tuneable Fermi level are prominent for mid-infrared photodetection.<br><br>Common mid-infrared photodetection focuses on wavelength; if other states of light, such as polarisation and phase could be detected at same time, detection bandwidth could be broadened, and detection functionality increased. Compared with wavelength detection, the detection of circular polarisation states is much less explored, especially at the mid-infrared wavelength range. Chirality, a geometric property of a structure, can be utilised to detect polarisation states of light due to the reason that chiral structures behave differently for incident light with different circular polarisation states. <br><br>The main objective of this thesis is to explore both theoretically and experimentally new types of mid-infrared graphene photodetectors enhanced by surface plasmons to address the main drawbacks of mid-infrared photodetection, such as low working temperature, low responsivity, high costs and lacking detection of polarisation states. The primary achievements are summarised as follows: To simultaneously detect the spin angular momentum states of mid-infrared region from 3 to 5 µm, we employ a photodetection structure that contains a zigzag chiral metal structure and a graphene layer. The localised surface plasmons can be excited within this designed photodetection structure to enhance the light absorption of graphene layer and distinct the spin angular momentum states by producing photocurrent with different direction. First, to understand how the nano-engineered chiral structure affects spin angular momentum and the potential size of the circular dichroism, we performed a detailed theoretical simulation of the hybrid structure and optimised all geometric parameters. To unveil the physical mechanisms behind the high graphene absorption and circular dichroism, we performed an electric field analysis of the left-handed and right-handed structure with left circularly polarised (LCP) and right circularly polarised (RCP) light at resonant wavelength. It is shown that the hybrid structures with different chirality resonant differently for the incident light with left-circular polarised state and right-circular polarised state respectively. In addition, the circular dichroism originates from the different absorption for LCP and RCP because the zigzag chiral structures resonant differently.<br><br>Instructed by these theoretical investigations, we fabricated mid-infrared graphene photodetectors that contain a zigzag gold structure, graphene layer and SiO2/Si substrate, via steps including graphene transfer, photolithography, electron beam lithography and electron beam deposition. To qualitatively and quantitatively characterise our fabricated devices, we performed characterisations with SEM; the error of all measured structure sizes was less than 3.1% compared with designed values, which proves the high precision and uniformity of the fabricated devices. Moreover, to characterise the optical properties of our fabricated devices, FTIR measurements were taken. Considering that incident light is not pure plane wave and not all the light is vertically illuminated on the devices, the simulated and measured extinction spectra and circular dichroism spectra were highly consistent. <br><br>To electrically characterise these photodetectors, we conducted photocurrent measurements with our home-made photocurrent characterisation system. Results show that responsivity of 0.36 µA/Wwas achieved for left-handed (LHD) structure with LCP light illumination (0.33 µA/Wfor right-handed (RHD) structure with RCP light illumination) and the circular dichroism was approximately 75% both LHD and RHD structures. Most importantly, photocurrent with different circulation direction was produced when the incident light possessed different circular polarisation states, which is the novel contribution of our proposed simultaneous mid-infrared spin angular momentum photodetector. <br><br>Surface plasmons supported by a gold chiral structure possesses a parasitic absorption problem. Moreover, chirality originates from the chiral structure and the monolayer graphene only acts as a transport layer for the photodetector. Hence, to fully implement the potential of graphene and take advantage of its exceptional optical and electrical properties, we investigated a mid-infrared photodetector based on a grapheme nanomesh (GNM) structure. Tuneable intrinsic graphene plasmons is introduced to enhance light absorption (i.e., to enhance the responsivity of device). Further, band gap engineering was utilised to improve the electron-hole separation efficiency and restrict dark current. <br><br>To fully understand the impacts of geometric size and graphene properties on band gap engineering and intrinsic graphene plasmons and investigate whether they can work side by side, we performed a thorough theoretical analysis of this proposed GNMstructure. First, we calculated the band gap of four representative types of GNM structures and their dependence on the period length of the unit cell and diameter of the hole. Then we investigated the impact of these two parameters on absorption magnitude of the GNMlayer and resonant wavelength of photodetectors. The band gap energy of the GNM is larger than the energy of thermal fluctuation at room temperature implying the opened band gap can effectively restrict dark current. As it was smaller than graphene plasmons energy, this proves that incident mid-infrared light at resonant wavelength can be absorbed and excited electrons can jump from valence band to conduction band. Thus, this proves that band gap engineering and graphene plasmons can work side by side to improve performance. <br><br>Experimentally, GNM structures with period length of 100 nm and hole diameters from 40 nm to 80 nm were achieved. The geometric characterisation results show that the experimental size was within ± 5% of the design size. The resistance change under different back-gate voltages proved the tuneability of fabricated devices. Most importantly, the shifted resonant peaks shown on extinction spectra manifested the resonant graphene local surface plasmon supported by the GNM platform and the tunability of the Fermi level of the GNM layer. <br><br>In summary, this thesis demonstrates novel research on mid-infrared photodetection and offers two significant contributions. The first is the increase in detector functionality by simultaneously detecting the LCP and RCP light. The second is the improvement in performance of mid-infrared photodetectors by boosting responsivity and restricting dark current. A simultaneous mid-infrared spin angular momentum photodetector was proposed, theoretically investigated and experimentally achieved. Furthermore, a mid-infrared photodetector based on GNM was thoroughly explored. Band gap engineering and graphene plasmons were introduced and investigated and it was proved they can work side by side to improve responsivity and restrict dark current. Moreover, the device was fabricated and characterised according to geometric, electrical and optical aspects. All these contributions form the basis for future advances. <br>