Terahertz radiation from magnetoresistive Pr0.7_{\text{0.7}}Ca0.3_{\text{0.3}}MnO3_{\text3} thin films

Terahertz (THz) radiation with its spectrum extending up to 1 THz has been observed by an illumination of femtosecond optical pulses to optical switching devices fabricated on magnetoresistive manganite thin films; Pr$_{0.7}$Ca$_{0.3}$MnO$_3$. The THz radiation strongly depends on temperature $T$ and its $T$ trend reverses sign across charge-orbital and spin ordering $T$'s.Comment: Revtex4, 4 pages including 3 figure

Partial and macroscopic phase coherences in underdoped Bi2{}_{2}Sr2{}_{2}CaCu2{}_{2}O8+δ{}_{8+{\delta}} thin film

A combined study with use of time-domain pump-probe spectroscopy and time-domain terahertz transmission spectroscopy have been carried out on an underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+{\delta}}$ thin film. It was observed that the low energy multi-excitation states were decomposed into superconducting gap and pseudogap. The pseudogap locally opens below $T^*{\simeq}210$ K simultaneously with the appearance of the high-frequency partial pairs around 1.3 THz. With decreasing temperature, the number of the local domains with the partial phase coherence increased and saturated near 100 K, and the macroscopic superconductivity appeared below 76 K through the superconductivity fluctuation state below 100 K. These experimental results indicate that the pseudogap makes an important role for realization of the superconductivity as a precursor to switch from the partial to the macroscopic phase coherence.Comment: Revtex4, 4 pages, 4 figure

Sub-diffraction thin-film sensing with planar terahertz metamaterials

Planar metamaterials have been recently proposed for thin dielectric film sensing in the terahertz frequency range. Although the thickness of the dielectric film can be very small compared with the wavelength, the required area of sensed material is still determined by the diffraction-limited spot size of the terahertz beam excitation. In this article, terahertz near-field sensing is utilized to reduce the spot size. By positioning the metamaterial sensing platform close to the sub-diffraction terahertz source, the number of excited resonators, and hence minimal film area, are significantly reduced. As an additional advantage, a reduction in the number of excited resonators decreases the inter-cell coupling strength, and consequently the resonance Q factor is remarkably increased. The experimental results show that the resonance Q factor is improved by 113%. Moreover, for a film with a thickness of \lambda/375 the minimal area can be as small as 0.2\lambda by 0.2\lambda. The success of this work provides a platform for future metamaterial-based sensors for biomolecular detection.Comment: 8 pages, 6 figure

Collective Antenna Effects in the Terahertz and Infrared Response of Highly Aligned Carbon Nanotube Arrays

We study macroscopically-aligned single-wall carbon nanotube arrays with uniform lengths via polarization-dependent terahertz and infrared transmission spectroscopy. Polarization anisotropy is extreme at frequencies less than $\sim$3 THz with no sign of attenuation when the polarization is perpendicular to the alignment direction. The attenuation for both parallel and perpendicular polarizations increases with increasing frequency, exhibiting a pronounced and broad peak around 10 THz in the parallel case. We model the electromagnetic response of the sample by taking into account both radiative scattering and absorption losses. We show that our sample acts as an effective antenna due to the high degree of alignment, exhibiting much larger radiative scattering than absorption in the mid/far-infrared range. Our calculated attenuation spectrum clearly shows a non-Drude peak at $\sim$10 THz in agreement with the experiment.Comment: 5 pages, 5 figure

Terahertz physics and applications

[No abstract available

Sub-wavelength terahertz beam profiling of a THz source via an all-optical knife-edge technique

Terahertz technologies recently emerged as outstanding candidates for a variety of applications in such sectors as security, biomedical, pharmaceutical, aero spatial, etc. Imaging the terahertz field, however, still remains a challenge, particularly when sub-wavelength resolutions are involved. Here we demonstrate an all-optical technique for the terahertz near-field imaging directly at the source plane. A thin layer (<100 nm-thickness) of photo carriers is induced on the surface of the terahertz generation crystal, which acts as an all-optical, virtual blade for terahertz near-field imaging via a knife-edge technique. Remarkably, and in spite of the fact that the proposed approach does not require any mechanical probe, such as tips or apertures, we are able to demonstrate the imaging of a terahertz source with deeply sub-wavelength features (<30 μm) directly in its emission plane

Laser Terahertz Emission Microscope

Abstract: Laser terahertz (THz) emission microscope (LTEM) is reviewed. Femtosecond lasers can excite the THz waves in various electronic materials due to ultrafast current modulation. The current modulation is realized by acceleration or deceleration of photo-excited carriers, and thus LTEM visualizes dynamic photo-response of substances. We construct free-space type and scanning probe one with transmission or reflection modes. The developed systems have a minimum spatial resolution better than 2 µm, which is defined by the laser beam diameter. We also present some examples of LTEM applications

Probing low-density carriers in a single atomic layer using terahertz parallel-plate waveguides

As novel classes of two-dimensional (2D) materials and heterostructures continue to emerge at an increasing pace, methods are being sought for elucidating their electronic properties rapidly, non-destructively, and sensitively. Terahertz (THz) time-domain spectroscopy is a well-established method for characterizing charge carriers in a contactless fashion, but its sensitivity is limited, making it a challenge to study atomically thin materials, which often have low conductivities. Here, we employ THz parallel-plate waveguides to study monolayer graphene with low carrier densities. We demonstrate that a carrier density of ~2 × 1011 cm−2, which induces less than 1% absorption in conventional THz transmission spectroscopy, exhibits ~30% absorption in our waveguide geometry. The amount of absorption exponentially increases with both the sheet conductivity and the waveguide length. Therefore, the minimum detectable conductivity of this method sensitively increases by simply increasing the length of the waveguide along which the THz wave propagates. In turn, enabling the detection of low-conductivity carriers in a straightforward, macroscopic configuration that is compatible with any standard time-domain THz spectroscopy setup. These results are promising for further studies of charge carriers in a diverse range of emerging 2D materials

Graphene field-effect-transistors with high on/off current ratio and large transport band gap at room temperature

Graphene is considered to be a promising candidate for future nano-electronics due to its exceptional electronic properties. Unfortunately, the graphene field-effect-transistors (FETs) cannot be turned off effectively due to the absence of a bandgap, leading to an on/off current ratio typically around 5 in top-gated graphene FETs. On the other hand, theoretical investigations and optical measurements suggest that a bandgap up to a few hundred meV can be created by the perpendicular E-field in bi-layer graphenes. Although previous carrier transport measurements in bi-layer graphene transistors did indicate a gate-induced insulating state at temperature below 1 Kelvin, the electrical (or transport) bandgap was estimated to be a few meV, and the room temperature on/off current ratio in bi-layer graphene FETs remains similar to those in single-layer graphene FETs. Here, for the first time, we report an on/off current ratio of around 100 and 2000 at room temperature and 20 K, respectively in our dual-gate bi-layer graphene FETs. We also measured an electrical bandgap of >130 and 80 meV at average electric displacements of 2.2 and 1.3 V/nm, respectively. This demonstration reveals the great potential of bi-layer graphene in applications such as digital electronics, pseudospintronics, terahertz technology, and infrared nanophotonics.Comment: 3 Figure