Joong Wook Lee

Shape resonance omni-directional terahertz filters with near-unity transmittance

Terahertz transmission filters have been manufactured by perforating metal films with various geometric shapes using femtosecond laser machining. Two dimensional arrays of square, circular, rectangular, c-shaped, and epsilon-shaped holes all support over 99% transmission at specific frequencies determined by geometric shape, symmetry, polarization, and lattice constant. Our results show that plasmonic structures with different geometric shaped holes are extremely versatile, dependable, easy to control and easy to make terahertz filters.

Near field imaging of terahertz focusing onto rectangular apertures

We performed terahertz near-field experiments on single rectangular holes with various lengths grown on an electro-optic crystal substrate with lambda/100 resolution. We find that the near-field amplitude becomes proportionally larger as the rectangle becomes narrower, strongly suggesting that a constant energy passes through even for a very narrow slit. The occurrence of a large field enhancement at the fundamental localized resonance is discussed confirming the funneling of energy at the near-field.

Fabry–Perot effects in THz time-domain spectroscopy of plasmonic band-gap structures

Using terahertz time-domain spectroscopy, we study transmission in one-dimensional arrays of slits fabricated on metal plates by laser machining. The enhanced peaks of zero-order transmission spectra are attributed to the combined effects of Fabry–Perot and surface plasmon resonances. Angle dependence of transmission spectra shows that the strongly surface plasmon-enhanced peaks appear when the Fabry–Perot-type resonance is located nearby in energy. This means that surface waves traveling in the horizontal direction couple with nearest Fabry–Perot resonance to generate enhanced peaks. These results are in excellent agreement with theoretical calculations.

Terahertz transparency at Fabry-Perot resonances of periodic slit arrays in a metal plate: experiment and theory

We report on a terahertz transparency in periodic arrays of metallic slits using terahertz time- domain spectroscopy. Experimental results and theoretical calculations reveal that Fabry-Perot resonance appearing in spectral region below first Rayleigh minimum strongly enhances terahertz transmission and a symmetric eigenmode inside the slits is excited under the condition of terahertz transparency.

Organic conjugated material-based broadband terahertz wave modulators

A simple and efficient broadband terahertz (THz) wave modulator based on an organic conjugated material thin film, 200-nm thick organic copper phthalocyanine (CuPc) film, deposited on a Si wafer was proposed. External laser beams significantly decrease the transmittance of THz pulses through the CuPc film over all frequency of the pulse. Modulation efficiency reaches as high as 55% under a cw-laser beam of 80 mW.

Polarization dependent transmission through asymmetric C-shaped holes

We report that asymmetric c-shaped holes show mode switching between single- and double-resonance frequencies in the terahertz region, when the polarization direction of the incident light is controlled. The number of resonant frequencies is attributed to fundamental shape modes such as circle-, coaxial-, and rectangular-shaped holes activated under each polarization condition. All of these resonant peaks also show a transmission of over 90% with the assistance of the periodically arranged holes. These experimental results are in close agreement with theoretical simulations.

Highly efficient terahertz wave modulators by photo-excitation of organics/silicon bilayers

Using hybrid bilayer systems comprising a molecular organic semiconductor and silicon, we achieve optically controllable active terahertz (THz) modulators that exhibit extremely high modulation efficiencies. A modulation efficiency of 98% is achieved from thermally annealed C60/silicon bilayers, due to the rapid photo-induced electron transfer from the excited states of the silicon onto the C60 layer. Furthermore, we demonstrate the broadband modulation of THz waves. The cut-off condition of the system that is determined by the formation of efficient charge separation by the photo-excitation is highly variable, changing the system from insulating to metallic. The phenomenon enables an extremely high modulation bandwidth and rates of electromagnetic waves of interest. The realization of near-perfect modulation efficiency in THz frequencies opens up the possibilities of utilizing active modulators for THz spectroscopy and communications.

Terahertz modulation on angle-dependent photoexcitation in organic-inorganic hybrid structures

The characteristics of terahertz (THz) modulation in organic copper phthalocyanine thin films deposited on a Si wafer were investigated by angle-dependent photoexcitation. We reveal that the efficiency of THz modulation reflects not only the angle-dependent reflectivity of the organic thin films that undergo a change of complex refractive index due to photoexcitation but also the laser-induced birefringence induced by a charge density grating in the direction normal to the polarization of the excitation beams.

Terahertz transmission properties of an individual slit in a thin metallic plate

We report on the terahertz transmission properties through a single slit in a thin metallic film. The properties are studied by comparing the transmissions of TE- and TM-polarized electromagnetic waves over a broad spectral range from the geometrical regime to the subwavelength limit. In the geometrical regime, the remarkable terahertz transmission due to guided modes is observed even without the contribution of surface waves. Whereas in the subwavelength limit, the surface charge oscillations associated with the TM-polarized guided mode give rise to strong transmission enhancement. The nature of the mechanisms for the terahertz transmission is elucidated using theoretical simulations of the near-field distributions and electromagnetic energy flow.

Optimum areal coverage for perfect transmission in a periodic metal hole array

We investigate the conditions for perfect transmission of terahertz radiation in periodic two-dimensional plasmonic system of square holes. Changing the period in the direction perpendicular to the incident light polarization reveals the optimum areal coverage for perfect transmission at a specific period. The simulated near-zone energy flow distributions show that all incident light is accumulated on the apertures, under the condition that the period is shorter than the resonant wavelength, acting as static local capacitors restricted to a wavelength-confined region. Our finding can be exploited for designing cost-effective terahertz filters and may be applicable to terahertz spectroscopy requiring strong local field enhancement.