Cristiane Nascimento Santos

Twisted Bi‐Layer Graphene: Microscopic Rainbows

Blue, pink, and yellow colorations appear from twisted bi-layer graphene (tBLG) when transferred to a SiO2 /Si substrate (SiO2 = 100 nm-thick). Raman and electron microscope studies reveal that these colorations appear for twist angles in the 9-15° range. Optical contrast simulations confirm that the observed colorations are related to the angle-dependent electronic properties of tBLG combined with the reflection that results from the layered structure tBLG/100 nm-thick SiO2 /Si.

Direct growth of graphene on Si(111)

Due to the need of integrated circuit in the current silicon technology, the formation of graphene on Si wafer is highly desirable, but is still a challenge for the scientific community. In this context, we report the direct growth of graphene on Si(111) wafer under appropriate conditions using an electron beam evaporator. The structural quality of the material is investigated in detail by reflection high energy electron diffraction, Auger electron spectroscopy, X-ray photoemission spectroscopy, Raman spectroscopy, high resolution scanning electron microscopy, atomic force microscopy, and scanning tunneling microscopy. Our experimental results confirm that the quality of graphene is strongly dependent on the growth time during carbon atoms deposition.

Terahertz and mid-infrared reflectance of epitaxial graphene

Graphene has emerged as a promising material for infrared (IR) photodetectors and plasmonics. In this context, wafer scale epitaxial graphene on SiC is of great interest in a variety of applications in optics and nanoelectronics. Here we present IR reflectance spectroscopy of graphene grown epitaxially on the C-face of 6H-SiC over a broad optical range, from terahertz (THz) to mid-infrared (MIR). Contrary to the transmittance, reflectance measurements are not hampered by the transmission window of the substrate, and in particular by the SiC Reststrahlen band in the MIR. This allows us to present IR reflectance data exhibiting a continuous evolution from the regime of intraband to interband charge carrier transitions. A consistent and simultaneous analysis of the contributions from both transitions to the optical response yields precise information on the carrier dynamics and the number of layers. The properties of the graphene layers derived from IR reflection spectroscopy are corroborated by other techniques (micro-Raman and X-ray photoelectron spectroscopies, transport measurements). Moreover, we also present MIR microscopy mapping, showing that spatially-resolved information can be gathered, giving indications on the sample homogeneity. Our work paves the way for a still scarcely explored field of epitaxial graphene-based THz and MIR optical devices.

Evaluation of laser dazzling induced task performance degradation

When studying the impact of laser dazzle on the human vision, three categories of parameters are of influence: those related to the human eye, the characteristics of the laser source itself, and environmental parameters such as target size and contrast, position of the target with respect to the laser dazzler, and ambient light level. The effects are most often translated into a decrease of the field-of-view of the eye but the question remains how this translates into human task performance degradation. More specifically, the research question for this study is the following: can we quantify the performance degradation of military specific tasks in a land environment when being dazzled by a portable laser system? Two main measurement campaigns have been organized to answer this question: a driving test to allow benchmarking with the literature, and a shooting test. The registered time and scoring of the specific tests have been statistically analyzed to determine the significant effects. The results of the driving test confirm those described in the literature; the results of the shooting test give insight in the main parameters of interest. Moreover, the outcomes of the two trials show that there is a need for a specific test protocol to find the correct compromise between environmental validity of a trial and the number of independent variables that can be controlled.

In-band low-power laser dazzle and pixel damage of an uncooled LWIR thermal imager

Infrared imaging sensors are a vital component of modern military weapons and surveillance systems, which for land operations is dominated by uncooled thermal imagers using microbolometer array detectors. These sensors, just as for all electro-optical and infrared devices, are vulnerable to the ever-increasing threat of in-band laser weapons, which can perturb or destroy their operational effectiveness. Importantly, this can happen even for relatively low laser output power. In this article, we analyze the experimentally measured results of laser dazzle and subsequent damage on an uncooled long-wave thermal imager. The imager has a vanadium oxide microbolometer array of size 640x480 pixels. A tunable quantum cascade laser is used with power output less than 100mW and fixed at a wavelength of 10.6 micron. The laser power was increased in incremental steps with the imager positioned only a few meters away. We discovered that the pixels covered by the laser spot saturate, leading to damage from accumulated exposures of only a few seconds. Additionally, we recorded circular diffraction effects and blooming of the array. In summary, we observed that damage was inflicted to pixels on the microbolometer array well before any significant dazzling was achieved.