Kanghee Lee

Graphene–ferroelectric metadevices for nonvolatile memory and reconfigurable logic-gate operations

Memory metamaterials are artificial media that sustain transformed electromagnetic properties without persistent external stimuli. Previous memory metamaterials were realized with phase-change materials, such as vanadium dioxide or chalcogenide glasses, which exhibit memory behaviour with respect to electrically/optically induced thermal stimuli. However, they require a thermally isolated environment for longer retention or strong optical pump for phase-change. Here we demonstrate electrically programmable nonvolatile memory metadevices realised by the hybridization of graphene, a ferroelectric and meta-atoms/meta-molecules, and extend the concept further to establish reconfigurable logic-gate metadevices. For a memory metadevice having a single electrical input, amplitude, phase and even the polarization multi-states were clearly distinguishable with a retention time of over 10 years at room temperature. Furthermore, logic-gate functionalities were demonstrated with reconfigurable logic-gate metadevices having two electrical inputs, with each connected to separate ferroelectric layers that act as the multi-level controller for the doping level of the sandwiched graphene layer.

Terahertz waves emitted from an optical fiber.

We report a simple method of creating terahertz waves by applying the photo-Dember effect in a (100)-oriented InAs film coated onto the 45-degree wedged-end facet of an optical fiber. The terahertz waves are generated by infrared pulses guided through the optical fiber which is nearly in contact with a sample and then measured by a conventional photo-conductive antenna detector. Using this alignment-free terahertz source, we performed proof-of-principle experiments of terahertz time-domain spectroscopy and near-field terahertz microscopy. We obtained a bandwidth of 2 THz and 180-microm spatial resolution. Using this method, the THz imaging resolution is expected to be reduced to the size of the optical fiber core. Applications of this device can be extended to sub-wavelength terahertz spectroscopic imaging, miniaturized terahertz system design, and remote sensing.

Subwavelength silicon through-hole arrays as an all-dielectric broadband terahertz gradient index metamaterial

Structuring at subwavelength scales brings out artificial media with anomalous optical features called metamaterials. All-dielectric metamaterials have high potential for practical applications over the whole electromagnetic spectrum owing to low loss and optical isotropy. Here, we report subwavelength silicon through-hole arrays as an all-dielectric gradient index metamaterial with broadband THz operation. The unit cell consists of a single subwavelength through-hole on highly resistive monocrystalline silicon. Depending on the fill-factor and period, the effective index was linearly modulated at 0.3–1.6 THz. The experimental results also demonstrate silicon gradient refractive index (Si-GRIN) lenses with parabolic index profiles through the spatial modification of a single unit cell along the radial direction. Si-GRIN lenses either focus 0.4–1.6 THz beam to the diffraction-limit or serve as a flat and thin solid immersion lens on the backside of THz photoconductive antenna for highly efficient pulse extraction. This all-dielectric gradient index metamaterial opens up opportunities for integrated THz GRIN optics.

Single-pixel coherent diffraction imaging

We demonstrate single-pixel coherent diffraction imaging, whereby broadband terahertz waveforms passed through a slanted phase retarder (SPR), diffracted from an object, were measured by a terahertz detector located in the far field. For one dimensional imaging, the fixed-location single-pixel broadband detector simultaneously measured all the spatial frequency components of the object because the frequency components of the source maintain a one-to-one correspondence with the object’s spatial frequency. For two dimensional imaging, the angular position of the SPR enabled the diffracted terahertz wave to carry an angular projection image of the object.

Coherent optical computing for T-ray imaging

Single-point imagery of 2D objects is proposed by exploiting the extreme broadband nature of an ultrafast terahertz wave. In the proposed imagery, a collimated terahertz beam is illuminated on an object, and the scattered fields are measured through a hole at the Fourier plane in a conventional terahertz time-domain spectroscope. This arrangement allows conversion of radial spatial frequencies of the object to the temporal spectrum of the pulse. Hence, a 2D image can be readily obtained by rotating a hole around the optical axis. Experimental results confirm that a complicated object can be reliably imaged using only 30 waveform measurements.

Terahertz phase microscopy in the sub-wavelength regime

Gouy phase shift is a well-known behavior that occurs when a propagating light is focused, but its behavior in the sub-wavelength confinement is not yet known. Here, we report the theoretical and experimental study of the aperture-size dependency of the Gouy phase shift in the sub-wavelength diffraction regime. In experiments carried out with laser-induced terahertz (THz) wave emission from various semiconductor apertures, we demonstrate the use of Guoy phase shit for sub-wavelength THz microscopy.

THz near-field spectral encoding imaging using a rainbow metasurface

We develop a spectral encoding image technique in the terahertz range using a space-frequency converting metasurface. From our developed technique, 2-dimensional images are successfully reconstructed using only 1-dimensional data acquisition processes.

Polarization shaping of few-cycle terahertz waves

We present a polarization shaping technique for few-cycle terahertz (THz) waves. For this, N femtosecond laser pulses are generated from a devised diffractive optical system made of as-many glass wedges, which then simultaneously illuminate on various angular positions of a sub-wavelength circular pattern of an indium arsenide thin film, to produce a THz wave of tailor-made polarization state given as a superposition of N linearly-polarized THz pulses. By properly arranging the orientation and thickness of the glass wedges, which determine the polarization and its timing of the constituent THz pulses, we successfully generate THz waves of various unconventional polarization states, such as polarization rotation and alternation between circular polarization states.

Tailoring the Spectra of Terahertz Emission from CdTe and ZnTe Electro-Optic Crystals

We compare the spectral amplitude of terahertz pulses generated using difference-frequency mixing, via the second-order nonlinear process in CdTe and ZnTe, with numerical model calculations using the nonlinear Maxwell equation. We have found that the spectral details of the generated terahertz pulses from CdTe are well explained by considering the effects of optical and terahertz absorptions, diffraction, and the frequency dependence of nonlinear coefficients. This method allows us to design tailored compound materials with improved versatility for generating specific terahertz pulse shapes.

Phase-shift anomaly caused by subwavelength-scale metal slit or aperture diffraction

Terahertz time-domain spectroscopy probes anomalous phase-shift caused by wave diffraction from a subwavelength-scale metal slit or aperture. Carrier frequency phase measurements in the far-field region reveals that nearly 30° phase advance is induced from a subwavelength slit diffraction and that 180° phase-advance from a subwavelength aperture. These results indicate that the conventional 90° phase advance of diffracted waves in the far-field region, known as the Gouy phase shift, is not valid for subwavelength diffraction phenomena. The physical origin of these phase-shift anomalies is attributed to induced electric currents or magnetic dipole radiation, and theoretical analyses based on these factors are in good agreement with the experimental results.