Simeon Bikorimana

Nonlinear optical responses in two-dimensional transition metal dichalcogenide multilayer: WS 2 , WSe 2 , MoS 2 and Mo 0.5 W 0.5 S 2

Synthetic two-dimensional transition metal dichalcogenides such as, tungsten disulphide (WS₂), tungsten diselenide (WSe₂), molybdenum disulphide (MoS₂) as well as mixed molybdenum tungsten disulphide (Mo_0.5W_0.5S₂) single crystals were grown by the chemical vapor transport method using halogens (bromine or chlorine) as transport agents. Multi- layer samples were cleaved from the single crystals, and their nonlinear optical (NLO) properties were obtained from both open aperture and closed aperture Z-scan measurements using a picosecond mode-locked Nd: YAG laser operating at a wavelength of 1064 nm, with pulse duration of 25 ps, and 20 Hz repetition rate. Both WS₂ and MoS₂ exhibited nonlinear saturable absorption (SA), whereas WSe₂ and Mo_0.5W_0.5S₂ showed nonlinear two-photon absorption (2PA). A large 2PA coefficient β as high as + 1.91x10^-8 cm/W was obtained for the Mo_0.5W_0.5S₂, and an index of refraction coefficient γ = -2.47x10^-9 cm²/W was obtained for the WSe₂ sample.

A bow-tie photoconductive antenna using a low-temperature-grown GaAs thin-film on a silicon substrate for terahertz wave generation and detection

This paper presents heterogeneously integrated bow-tie emitter–detector photoconductive antennas (PCAs) based on low-temperature grown-gallium arsenide (LTG-GaAs) thin-film devices on silicon-dioxide/silicon (SiO2/Si) host substrates for integrated terahertz (THz) systems. The LTG-GaAs thin-film devices are fabricated with standard photolithography and thermal evaporation of metal-contact layers of chromium (Cr), nickel (Ni) and gold (Au). They are etched selectively and separated from their growth GaAs substrate. The LTG-GaAs thin-film devices are then heterogeneously integrated on bow-tie antenna electrodes patterned on the surface of a SiO2/Si host substrate for THz emitters and THz detectors. Cost-effective and selective integration of LTG-GaAs thin-film devices on a Si platform is demonstrated. THz radiation from the fabricated THz PCAs is successfully measured using a pump–probe THz time-domain configuration. The THz temporal duration was measured at full width half maximum of 0.36 ps. Its frequency spectrum exhibits a broadband response with a peak resonant frequency of about 0.31 THz. The demonstration illustrates the feasibility of creating heterogeneously integrated THz systems using separately optimized LTG-GaAs devices and Si based electronics.

Synthesis of selenium nanoparticles and their nonlinear optical properties

Selenium nanoparticles were fabricated using a physical deposition method based on heterogeneous condensation. The Selenium nanoparticles were deposited on a quartz substrate and the ultrafast nonl...

Beam Combining of SOA-Based Bidirectional Tunable Fiber Nested Ring Lasers With Continuous Tunability Over the C-band at Room Temperature

Nested ring cavities are used to develop a simple compact, low-cost semiconductor optical amplifier (SOA)-based bidirectional tunable fiber ring laser source. We propose a bidirectional tunable fiber ring laser structure based on (N > 1) number of SOAs that has a great potential for achieving a high power laser source that uses low-power optical components by coherently coupling the nested ring cavities. A commercial tunable filter is used to deliver a wavelength tuning range of 30 nm. The lasing tunable range can be further changed by using SOAs with operating wavelengths in different regions of the optical spectrum. The output power of the bidirectional fiber ring resonator with three SOAs was around 3.5 times larger than the output of a single SOA fiber ring laser (i.e., 17.7 versus 5.14 mW) at 1550 nm wavelength. The optical signal-to-noise ratio was measured to be up to +49 dB, wavelength stability was ±0.015 nm, and optical power stability was better than ±0.2 dB.

Non-mechanical beam steering with a dynamic lithography of tunable metasurface

Beam steering has been traditionally achieved by mechanical means. Even though these mechanical techniques have evolved over the past few decades, non-mechanical approaches, due to benefits such as increased speed, lower costs and reduced complexity, have gained considerable interest. In this work, we propose a non-mechanical beam steering of terahertz radiation method using a transient, or dynamic lithography process. The experimental setup is based on a pump-probe technique, with a Ti:Sa pulse laser at 800 nm wavelength as laser source. The high power pump beam is used to “write” a metasurface pattern composed of v-shaped antennas on a high purity float zone silicon substrate. The wavefront of the pump beam is modulated by a liquid crystal spatial light modulator (LCSLM), such that, by the time the beam reaches the silicon substrate, the illumination pattern has the appearance of an array of antennas. Upon incidence on the silicon substrate, the beam generates electron-hole pairs in the illuminated areas, therefore creating structures with metallic-like properties. The presence of carriers and implicitly the metallic quality of the structures are ensured as long the radiation is incident on the substrate. This process was named transient or dynamic lithography, due to its non-destructive property relative to the silicon wafer. The probe beam, less powerful, is used to generate the terahertz signal. This is achieved by a photo-conductive antenna. Subsequently, the terahertz beam probes the pattern projected on the silicon substrate by the pump beam. Due to the electron-hole pairs previously generated in the substrate, the antenna structures will respond to the terahertz radiation in a way similar to metallic antennas. The terahertz beam is therefore steered by the pseudo-metallic antenna array. The detection is achieved with an un-cooled bolometer terahertz video camera. This beam steering technique is very promising due to its flexibility in quickly changing the direction of the steered beam, by “rewriting” the antenna arrays on the silicon substrate, without any mechanical movement of optical elements.

Continuous Tunable Terahertz Wave Generation via a Novel CW Optical Beat Laser Source

A novel technique of generating two colors or dual-wavelength in a fiber hybrid compound-ring resonator is discussed. Generation of continuous-wave terahertz radiation is demonstrated by using a dual-wavelength widely tunable C-band SOA-based fiber compound-ring laser as a light source, which excites a continuouswave terahertz photomixer operating at 1.55 μm telecom optical wavelengths. The proposed dualwavelength fiber laser has a hybrid compound-ring resonator structure and external reflectors that allow output power upscaling and single or dual-output port operation, respectively. Wavelength selection and continuous tunability are achieved by a widely tunable optical filter sandwiched between two fiber-Bragg grating filters of similar Bragg center wavelength. The difference wavelength tuning range of 20.42 nm (i.e., 2.51 THz) is demonstrated in the C-band. Continuous-wave terahertz radiation with continuous tunability between 0.8 and 2.51 THz at room temperature using only a fiber laser source is achieved via photomixing.

Beam Combining of SOA-based Bidirectional Tunable Fiber Compound-ring Lasers with External Reflectors

A simple, stable and inexpensive dual- output port widely tunable semiconductor optical amplifier-based fiber compound-ring laser structure is demonstrated. This unique nested ring cavity enables high optical power to split into different branches where amplification and wavelength selection are achieved by using low-power SOAs and a tunable filter. Furthermore, two Sagnac loop mirrors which are spliced at the two ends of the ring cavity not only serve as variable reflectors but also channel the optical energy back to the same port without using any high power combiner. More than 98% coherent beam combining efficiency of two parallel nested fiber ring resonators is achieved over the C-band tuning range of 30 nm. Optical signal to noise ratio (OSNR) of + 45 dB, and optical power fluctuation of less than ± 0.02 dB are measured over three hours at room temperature.

Terahertz-Wave Characterization of LTG-GaAs Thin-Film Photoconductive Antenna

Low temperature grown (LTG) Gallium Arsenide (GaAs) thin-film based photoconductive antennas (PCAs) are fabricated and their terahertz (THz) performance is compared with conventional bulk LTG-GaAs PCAs. To fabricate the LTG-GaAs thin-film based PCAs, LTG-GaAs thin-films are separated from their semi-insulating (SI)-GaAs substrate and integrated onto bow–tie PCA electrode tips patterned on a silicon dioxide (SiO2) layer thermally grown on a silicon (Si) substrate. Conventional bulk LTG-GaAs PCAs are fabricated by direct photolithography and metallization on the surface of a conventional bulk LTG-GaAs material. THz waves radiated from each of the excited PCA emitters are characterized by a variation of applied bias voltages maintaining the incident optical pump power constant at 10 mW. It was observed that the peak THz radiation signal from the LTG-GaAs thin-film based PCA is higher than the one obtained from the conventional bulk LTG-GaAs PCA structure with the same bias voltage and optical pump laser beam conditions. The performance difference was analyzed using numerical simulations. Based on the unique integration structure, the LTG-GaAs thin-film based PCAs have a potential to improve the THz radiation and enable heterogeneously integrated THz systems.

Nonlinear optical responses in two-dimensional transition metal dichalcogenide multilayer: WS 2 , WSe 2 , MoS 2 and Mo 0.5 W 0.5 S 2 : publisher’s note

This publishers note amends the Funding section of a recent publication [Opt. Express24, 20685 (2016].

Dynamic lithography of v-shaped antennas for beam steering applications

A method to generate an optical metasurface is developed. In our experimental setup, we use a pump-probe technique, where the pump beam is used to project patterns of v-shaped antennas on the surface of a silicon substrate. In the areas illuminated with the images of v-shaped antennas electron-hole pairs are created. Therefore, the antenna structures on silicon will have metallic-like properties, we classify this structure as a metasurface. The THz beam probes refraction and reflection on the metasurface generated on the silicon substrate. The dynamic change of these patterns of metasurface causes the beam steering effects of THz radiation.