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

Invited Article: Terahertz microfluidic chips sensitivity-enhanced with a few arrays of meta-atoms

We present a nonlinear optical crystal (NLOC)-based terahertz (THz) microfluidic chip with a few arrays of split ring resonators (SRRs) for ultra-trace and quantitative measurements of liquid solutions. The proposed chip operates on the basis of near-field coupling between the SRRs and a local emission of point like THz source that is generated in the process of optical rectification in NLOCs on a sub-wavelength scale. The liquid solutions flowing inside the microchannel modify the resonance frequency and peak attenuation in the THz transmission spectra. In contrast to conventional bio-sensing with far/near-field THz waves, our technique can be expected to compactify the chip design as well as realize high sensitive near-field measurement of liquid solutions without any high-power optical/THz source, near-field probes, and prisms. Using this chip, we have succeeded in observing the 31.8 fmol of ion concentration in actual amount of 318 pl water solutions from the shift of the resonance frequency. The technique opens the door to microanalysis of biological samples with THz waves and accelerates development of THz lab-on-chip devices

Distributed source model for the full-wave electromagnetic simulation of nonlinear terahertz generation

The process of terahertz generation through optical rectification in a nonlinear crystal is modeled using discretized equivalent current sources. The equivalent terahertz sources are distributed in the active volume and computed based on a separately modeled near-infrared pump beam. This approach can be used to define an appropriate excitation for full-wave electromagnetic numerical simulations of the generated terahertz radiation. This enables predictive modeling of the near-field interactions of the terahertz beam with micro-structured samples, e.g. in a near-field time-resolved microscopy system. The distributed source model is described in detail, and an implementation in a particular full-wave simulation tool is presented. The numerical results are then validated through a series of measurements on square apertures. The general principle can be applied to other nonlinear processes with possible implementation in any full-wave numerical electromagnetic solver

Scanning point terahertz source microscopy of unstained comedo ductal carcinoma in situ

Terahertz imaging is an emerging candidate to diagnose breast cancers in a labelfree manner. However, detailed terahertz analysis of early stage breast cancers is difficult to achieve owing to its low spatial resolution. In this study, utilizing a probe-less terahertz near-field microscope named scanning point terahertz source microscope, we visualize an unstained comedo ductal-carcinoma-in-situ including an architectural structure (comedo necrosis) measuring ∼ϕ500 μm, which is known as highly-malignant early-stage breast cancer, in terahertz images for the first time. The outcome is a critical step toward the label-free diagnosis of single early stage cancer lesions with terahertz waves

Label-Free Observation of Micrometric Inhomogeneity of Human Breast Cancer Cell Density Using Terahertz Near-Field Microscopy

Terahertz-light imaging is attracting great attention as a new approach in non-invasive/non-staining biopsy of cancerous tissues. Positively, terahertz light has been shown to be sensitive to the cell density, the hydration content, and the chemical composition of biological samples. However, the spatial resolution of terahertz imaging is typically limited to several millimeters, making it difficult to apply the technology to image biological tissues which have sub-terahertz-wavelength-scale inhomogeneity. For overcoming the resolution, we have recently developed a terahertz near-field microscope with a spatial resolution of 10 µm, named scanning point terahertz source (SPoTS) microscope. In contrast to conventional far-field terahertz techniques, this microscope features the near-field interactions between samples and point terahertz sources on a sub-terahertz-wavelength scale. Herein, to evaluate the usefulness of terahertz imaging in cancer tissue biopsy in greater detail, we performed terahertz near-field imaging of a paraffin-embedded human-breast-cancer section having sub-terahertz-wavelength-scale inhomogeneity of the cancer cell density using the SPoTS microscope. The observed terahertz images successfully visualized local (~250 µm) inhomogeneities of the cell density in breast invasive ductal carcinoma. These results may bypass the terahertz limitation in terms of spatial resolution and may further motivate the application of terahertz light to cancer tissue biopsy

Probing photocarrier dynamics in a Bi2Te3–Te eutectic p–n junction with a laser terahertz emission microscope

Bismuth telluride (Bi2Te3)-based heterostructures have attracted considerable attention owing to their interesting anisotropic properties and expected higher thermoelectric performance. Therefore, exploring the nature of the carrier dynamics in these heterostructures has been an important subject in the design and optimization of advanced materials. In the present study, hot carrier injection and its subsequent spatiotemporal behavior in a multilayered crystalline Bi2Te3–Tellurium (Te) eutectic composite were studied using a laser terahertz (THz) emission microscopy (LTEM). The THz emission electric fields at the Bi2Te3–Te interface were polarized perpendicular to the interface. The polarities of these waveforms reveal the direction of the electric field between the Bi2Te3 and Te regions, indicating the carrier types of these components and the p–n junction formed at the interface. In addition, in the Te region, a strong THz emission with an electric field polarized parallel to the interface was observed. This unique THz emission can be qualitatively explained through hot photocarrier anisotropic transport by considering the effective mass of electrons and holes. LTEM clarified the local carrier dynamics in the microstructures and revealed the potential distribution and anisotropic transport properties. These findings contribute to the exploration of eutectic heterostructures as new functional materials and provide new avenues for cutting-edge thermoelectric and photovoltaic devices

Media 1: Distributed source model for the full-wave electromagnetic simulation of nonlinear terahertz generation

Originally published in Optics Express on 30 July 2012 (oe-20-16-18397