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We investigated THz phonon-polaritons in a lead halide perovskite film with metamaterials. Rabi splitting occurred when we tune the metamaterial resonance with the phonon resonance of the perovskite film, due to the strong coupling between the resonances. The polaritonic branches exhibited the dispersion curve with a clear anti-crossing behavior as a function of the metamaterial resonance. Rabi splitting versus crystal volume fraction were obtained with in-situ THz spectroscopy in which we found a unique power-law scaling, depending on the crystal growth dimensions.

We investigate the effect of the effective substrate refractive index on the sensitivity of terahertz (THz) metamaterials for dielectric sensing by substrate etching. We found the effective refractive index near the metamaterials can be actively controlled by etching, allowing optimization of the sensitivity. Our experimental findings are in good agreement with finite element method (FEM) simulation results.

We investigated the THz dielectric constant of a wide range of microbial species in conjunction with their cell wall compositions. In general, the dielectric constant of the molds was lower than that of bacteria, whereas the yeasts exhibited particularly high dielectric constants, which were higher than that of water. The measurement on the dielectric constants of peptidoglycan and polysaccharides such as chitin, $\alpha$-glucan, and $\beta$-glucans revealed that cell wall composition is the main cause of the observed differences in dielectric constants for different types of microorganisms.

We demonstrate THz metamaterials as efficient label-free detectors for microorganisms such as fungi, bacteria, and viruses. A shift in the resonant frequency is observed following the deposition of various living and viable microorganisms, which arises due to the change in the effective dielectric constant in a capacitive gap of the metamaterials. The metamaterial sensing is a universal method, while a selective detection is also possible by functionalizing the substrates with antibodies specific to the target substances. Dielectric constants of the various microorganisms have been recorded using the metamaterial sensors, which will help to achieve selective and sensitive identification of the pathogens without the help of receptors.

Most semiconductors have surface dynamics radically different from its bulk counterpart due to the surface state. However, direct observation of the surface carrier dynamics has been limited to nano structures where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nanoscopy to investigate surface dynamics of bulk semiconductors, using metallic nano gap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically as the gap size decreases, and that they return to their bulk values once the nano patterns are removed.

We fabricated THz metamaterial sensors for sensitive, on-site detection of microorganisms such as fungi and bacteria. Strong field localization and enhancement in a gap area enables us to detect the microorganisms with high sensitivity, in aqueous and ambient conditions. The resonant frequency shift is investigated as a function of the dielectric constant and shape of the microorganisms. To optimize microbial sensors, the sensitivity has been studied with respect to substrate dielectric constant and geometrical parameters of the gap structures.

Passive mode-locking of a Ti:sapphire laser employing monolayer graphene as saturable absorber was demonstrated for the first time. The large-area, high-quality monolayer graphene was fabricated by chemical vapor deposition method. Without any additional treatment, monolayer graphene shows ultrafast recovery time and nonlinear absorption properties appropriate for the bulk laser mode-locking near 800 nm. The graphene mode-locked Ti:sapphire laser delivers 63-fs pulses with a maximum output power of 480 mW.

We demonstrate enhanced terahertz wave transmission through carbon nanotube films with subwavelength apertures. The peak frequency of the transmitted waves matches well with the shape resonance, mainly determined by the length of the apertures and the refractive index of surrounding media.

We performed time-domain terahertz (THz) spectroscopy on single-walled carbon nanotube films. We found that a filtration method provides an efficient way of fabricating thick nanotube films which is capable of effective shielding of THz waves. Shielding effectiveness of more than 40 has been achieved from our method. In addition, all of the films we fabricated demonstrate a good metallic behavior with a dispersion of optical constant relevant to simple Drude model.

Novel transmission- and reflection-type ultrafast saturable absorbers were developed based on single-walled carbon nanotubes (SWCNTs). Their nonlinear optical characteristics, crucial for mode-locking, such as modulation depth, saturation fluence and recovery time were studied. Additionally, we demonstrated passive mode-locking of different bulk solid-state lasers operating above 1 μm using these SWCNT-SAs and achieved stable femtosecond pulses without q-switching instabilities.

We report huge field accumulations in rectangular aperture arrays on thin metal film by using shape resonance in THz frequency region. A large far-field transmittance enhancement is observed in the samples of various widths ranging from 10 μm to 1.8 μm which corresponds to only an order of λ/100. Theoretical calculations based on vector diffraction theory indicates 230 times field enhancement in the near-field for the 1.8 μm rectangular aperture.

We report efficient terahertz (THz), pulse generation in a newly-developed single organic crystal OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile). The THz output characteristics were compared with those generated in inorganic nonlinear crystals such as ZnTe and CdZnTe. We achieved quasi single-cycle terahertz pulses at 1 kHz with an amplitude of ~8 kV/cm (peak-to-peak) at the central frequency of 0.8 THz. We believe that such high-power THz pulses can be effectively used for investigation of various nonlinear phenomena in the THz regime.

We performed time-domain terahertz spectroscopy on thin layers of single-walled carbon nanotubes (SWCNTs) coated on flexible films. The SWCNT layers demonstrate good shielding of electromagnetic waves in the terahertz range while maintaining good transparency for visible light. The shielding efficiency can be engineered through the thickness control and/or chemical treatment of SWCNT layers. The frequency-dependent dielectric constants of SWCNT layers are in good agreement with the Drude free-electron model.

Ultrafast single-walled carbon nanotube saturable absorbers were fabricated and optimized for mode-locking solid-state lasers near 1 mum. Their typical modulation depth and recovery time were measured to be Gt 0.3% and ~ 750 fs, respectively.

We report optical scanning measurements on photocurrent in individual Germanium nanowire transistors. Photoconductivity measurements reveal that Germanium nanowire can be used as an ultrasensitive, nanoscale photodetectors, especially in the visible range. The photoconductivity shows a strong saturation behavior unlike Schottky photocurrents observed at the metal contacts.

We present low-power, femtosecond laser machining on vertically aligned array of carbon nanotube (CNT) films synthesized by two different growth techniques. Arrays of holes can be efficiently fabricated for a possible application as CNT based metamaterials, showing completely different features for the two samples.

Solid single-walled carbon nanotube (SWCNT)/silica composites were prepared by a sol-gel method. Their third-order susceptibilities were estimated by using z-scan technique at 532 nm and 1064 nm, respectively. The results reveal negative nonlinear refraction and saturable absorption, which magnitudes depend on kinds of used SWCNTs prepared by different techniques.

We demonstrate cathodoluminescence measurements of individual ZnO nanorods synthesized by wet chemical method. Strong UV emission and relatively weak orange-red emission are observed. Energy-selective imaging technique reveals that the UV emissions are spatially modulated due to the stacking fault defects, while the orange-red luminescence is uniform over the entire nanorods.

Scanning photocurrent measurements are demonstrated in individual carbon nanotube field effect transistors. Photocurrent images in conjunction with the electrical conductance measurement elucidate the properties of metal-CNT interfaces, especially the electron band alignment at the contact.

Even in this day of proliferation of widely tunable femtosecond laser sources, severe limitations exist when one tries to perform two color femtosecond experiments with two independently tunable femtosecond lasers, One can generate, by clever cavity manipulation, two perfectly synchronized femtosecond pulses, but with very limited tunability. On the other hand, commercially available systems deliver completely independent tunability over the entire tuning range, but with timing jitters in the range of 5-10 ps. Here, we have developed a novel scheme which allows true two color femtosecond experiments with independent tunability of the two lasers.

We demonstrate a transition from exciton resonance to frequency mixing for the first time, in the nondegenerate femtosecond four wave mixing (FWM) of a GaAs-AlGaAs multiple quantum well. The frequency mixing which is dominant at high intensity and high detuning survives only within the pulse-width.