Hironaru Murakami

Terahertz nonlinear superconducting metamaterials

We investigate the nonlinear effect of a planar superconducting metamaterial made from niobium nitride (NbN) at terahertz frequencies. As the variation of the incident intense terahertz field alters the intrinsic conductivity in the NbN, a consequent giant amplitude modulation is observed due to the strong nonlinearities. The high sensitivity of the chosen metamaterial even allows observing the nonlinear behaviors at various temperatures, but the resonance modulation induced by the nonlinear effect was distinct from that induced by the heating effect. The presented results illustrate a clever implementation of strongly enhanced nonlinearities and thus may bring nonlinear metamaterials into novel applications.

Imaging of a Polycrystalline Silicon Solar Cell Using a Laser Terahertz Emission Microscope

We employed a laser terahertz (THz) emission microscope (LTEM) as a novel tool for evaluating solar cells. LTEM images are obtained by exciting a polycrystalline silicon solar cell with femtosecond laser illumination and visualizing the local distribution of the optical response. THz emission signals also provide various types of information, such as the screening effect of the built-in electrical field near pn junctions. These results indicate that this technique can be used to evaluate the local photoelectric conversion efficiency distribution and dynamic behavior of optically excited carriers in solar cells.

Observation of Multi-Stage Superconducting Gap States in Bi2Sr2CaCu2Ox Crystal Surface by LT-STM/STS

Low temperature STM/STS observation for BSCCO single crystal was carried out to investigate an intrinsic superconducting gap state of CuO 2 layer. The STM observation revealed out a mid-level-layer between the top and bottom BiO surfaces of the half unit cell in the crystal c-axis direction. The STS study on this mid-level surface brought a reproducible definite superconducting gap structure which shows a finite gap region with a low conductance level of about 5%, and which also shows a nature of wide range gap parameter of Δ max /Δ min ∼4 with Δ min ∼10 meV. This definite gap structure is considered to be a more direct reflection of the superconducting electronic states of the CuO 2 layer than the superconducting gap structure observed at the top BiO layer.

Nonlinear response of superconducting NbN thin film and NbN metamaterial induced by intense terahertz pulses

We present the nonlinear response of superconducting niobium nitride (NbN) thin film and NbN metamaterial with different thicknesses under intense terahertz pulses. For NbN thin film, nonlinearity emerges and superconductivity is suppressed with increasing incident terahertz electric field, and the suppression extent weakens as the film thickness increases from 15 to 50?nm. As the variation in intense terahertz fields alters the intrinsic conductivity in NbN, a consequent remarkable amplitude modulation in NbN metamaterial is observed due to the strong nonlinearity. Absorbed photo density in either NbN film or NbN metamaterial is estimated and used to understand the mechanism of nonlinear response. With a thicker NbN film element of 200?nm, the resonance of the metamaterial shows similar nonlinear modulation accompanied by a lower loss and a higher quality factor compared with a thinner NbN film element of 50?nm, which demonstrates the innovative implementation of strongly enhanced nonlinearity with thick superconducting film elements and the potential for novel applications using nonlinear metamaterial.

Resonant terahertz radiation from Tl2Ba2CaCu2O8+δ thin films by ultrafast optical pulse excitation

We have observed the free-space radiation of the resonant terahertz wave from c-axis oriented Tl2Ba2CaCu2O8+δ thin films by femtosecond optical pulse excitation under a radial magnetic field of about 100 Oe nearly parallel to the c axis of the thin film. The observed wave form showed clear oscillations up to 80 K. The frequency was shifted from ∼620 GHz at 24 K to ∼300 GHz at 80 K, corresponding to an increase in c-axis penetration depth, λc, from ∼26 μm to ∼53 μm, and disappeared above TC as expected for the Josephson plasma resonance.

Transmission-Type Laser THz Emission Microscope Using a Solid Immersion Lens

A recently developed laser terahertz (THz) emission microscope (LTEM) is a new type of inspection tool for semiconductor integrated circuits by 2-D mapping of THz emission excited by femtosecond (fs) laser pulse. For high-resolution imaging, we demonstrate the combination of a hemispherical solid immersion lens (SIL) and an LTEM with transmission-type detection mode. Unlike reflective LTEM geometries, in which both the probing fs laser pulses and the generated THz emissions utilize the same optical components, the transmission system enables flexibility in the construction of both the fs laser and the THz detection optics: The optics on the incident side of the sample are used only to manipulate the fs pulses, while those on the transmission side are meant only for manipulating the THz emission. We could improve the spatial resolution by less than 1.5 mum by combining the objective lens with the hemispherical SIL with a refractive index n = 1.98 at 780 nm.

Backside observation of large-scale integrated circuits with multilayered interconnections using laser terahertz emission microscope

We have developed a laser terahertz emission microscope utilizing excitation laser pulses at 1.06 μm wavelength for the inspection and localization of electrical failures in large-scale integrated circuits with multilayered interconnection structures. The system enables to measure terahertz emission images from the backside of a large-scale integrated circuits chip with a multilayered interconnection structure that prevents the observation from the front side. By comparing the terahertz emission images, we successfully distinguish a normal circuit from damaged ones with different positions of the interconnection defects without any electrical probing.

Bandwidth tunable THz wave generation in large-area periodically poled lithium niobate

A new scheme of optical rectification (OR) of femtosecond laser pulses in a periodically poled lithium niobate (PPLN) crystal, which generates high energy and bandwidth tunable multicycle THz pulses, is proposed and demonstrated. We show that the number of the oscillation cycles of the THz electric field and therefore bandwidth of generated THz spectrum can easily and smoothly be tuned from a few tens of GHz to a few THz by changing the pump optical spot size on PPLN crystal. The minimal bandwidth is 17 GHz that is smallest ever of reported in scheme of THz generation by OR at room temperature. Similar to the case of Cherenkov-type OR in single-domain LiNbO₃, the spectrum of THz generation extends from 0.1 THz to 3 THz when laser beam is focused to a size close to half-period of PPLN structure. The energy spectral density of narrowband THz generation is almost independent of the bandwidth and is typically 220 nJ/THz for ~1 W pump power at 1 kHz repetition rate.

Tetrahertz Pulse Radiation Properties of a Bi2Sr2CaCu2O8+δ Bowtie Antenna by Optical Pulse Illumination

We demonstrate, for the first time, terahertz (THz) pulse radiation properties from a current biased Bi2Sr2CaCu2O8+δ (BSCCO) bowtie antenna by femtosecond laser pulse illumination. The bias current, excitation laser power and temperature dependence of the radiated amplitude of THz pulse are basically in good agreement with those observed on YBa2Cu3O7-δ (YBCO). However, the detailed properties of the THz pulse from BSCCO are considerably different from those for YBCO. The observed amplitude of the terahertz (THz) radiation pulse was very strong due to the small index value of BSCCO thin film even for the small bias current, and the Fourier components of the pulse contains the frequencies lower than 0.4 THz with a central frequency around 0.1–0.16 THz reflecting its long relaxation time of photo-excited quasi-particle in BSCCO.

Terahertz generation by optical rectification in lithium niobate crystal using a shadow mask

A simple approach to generate high energy, frequency and bandwidth tunable multicycle THz pulses by optical rectification (OR) of spatially shaped femtosecond laser pulses in the lithium niobate (LN) crystal is proposed and demonstrated. A one dimensional binary shadow mask is used as a laser beam shaper. By building the masks image in the bulk LN crystal with various demagnifications, the frequency of THz generation was tuned in the range of 0.3 - 1.2 THz. There exist also an opportunity to tune the bandwidth of THz generation from 20 GHz to approximately 1 THz by changing the optical beam size on the crystal. The energy spectral density of narrowband THz generation is almost independent of the bandwidth and is typically 0.18 μJ/THz for ~1 W pump power at 1 kHz repetition rate.