Research

Ⅰ. Current Research Interests

1. Open new research field “Terahertz Nanoscience”. We have established the international symposium on Terahertz Nanoscience since Nov. 2011. THz frequencies cover from 100 GHz to 30 THz in general, which correspond to the wavelengths between 3micrometer and 3mm. Nanoscience dose not work for us at nano-point but functions in devices which sizes are micrometer scales. So the relation between nanoscience and THz science is very important. Alternatively, THz field can coherently controls the large body of nano-functions.

HFD: Hierarchic Functional Development, SCF: Stimulated Cooperative Function, SC: Superconductor, QW: Quantum Well, NC: Nanocarbon.

2. THz science in nanomaterials such as nanotubes, graphene, other atomic layers, and advanced materials such as strongly correlated electron systems, multiferroics, high-Tc superconductors, and metamaterials.

Temperature-programed THz emission spectroscopic imaging THz conductivity of graphene, optical response of multiferroic, BiFeO3, and THz electromagnetic response of metamaterials.

3. THz-Bio science. THz waves can sense the dynamic inter interaction of large molecules, hydrogen-bonding, the dynamic hydration of alive biological matters such as protein. We develop extreme high sensitive THz bio-chips for the aforementioned.

THz Bio-Chip

4．New tools for THz nanoscience, THz-Bio sensing, and industrial applications.
Followings are our original tools, only one system in the world

Scanning laser near field THz emission time domain spectroscopy and imaging system.
System, LTEM image of GaN with PL image, pump&probe LTEM(Dynamic THz emission microscope(DTEM)

Scanning laser near field THz emission time domain spectroscopy and imaging system, human single hair LTEM image, and human blood glucose vs optical constant.

Temperature-programed THz emission spectroscopic imaging system.

THz parallel plate waveguide spectroscopy system for ultrathin conductive materials. Transmittance of low carrier density graphene and 4nm gold film transmittance with fits by Drude-Smith model.

Scanning laser magneto-optical microscope, single vortex trapped in BSCCO and flux quanta in Superconductor device.

Ⅱ. Introduction

Tonouchi laboratory studies material science, novel devices and systems for ultrafast photonics and terahertz applications. One of the important missions is to open new research field of "Superconductive Photonics”, in which, for instance, high-speed optical interface to single flux quantum circuits, terahertz emission devices, optical-terahertz signal generation and detection systems, and so on are included. For the material science, we prepare high quality thin films made of, mainly, strongly correlated electron systems by means of laser ablation technique. We characterize them in terms of both optical and terahertz functionality, carrier dynamics, and new phenomenon. As for the devices and systems, we develop the optical interface, photomixing local oscillator, basic systems for wireless terahertz wave communications, and quantum computing elements based on the magnetic flux quantum control. The experimental tools for those researches have been proposed and developed, such laser-terahertz emission microscope. Shown in "Project/Fund" section is our main research.

Ⅲ. THz Science in Advanced Materials

A. Materials of our current interest
Semiconductors:
GaAs, InP, InAs, InSb, LT-GaAs, InGaAs, Quantum Structures, Carbone Nanotube
Superconductors:
YBa2Cu3O7-δ, Y1-xPrxBa2Cu3O7-δ, MgB2, Bi2Sr2CaCu2O8+δ, Tl2Ba2CaCu2O8+δ
Manganites:
La0.7Ca0.3MnO3, La0.7Sr0.3MnO3, Pr1-xCaxMnO3, La2-2xSr1+2xMn2O7 (x=0.3)
Dielectrics and Ferroelectrics:
SrTiO3, BaTiO3, Ba1-xSrxTiO3, SrBi2Ta2O9
Multiferroics:
BiFeO3, BiFe1-xMnxO3
Conductors:
SrRuO3, Carbone Nanotube

We grow most of the perovskite thin films by laser ablation, and characterize them by means of X-ray diffraction, magnetization, AFM, X-ray fluorescence spectrometer, femtosecond optical pump and probe transient reflection measurement,terahertz emission spectroscopy, pump and probe terahertz emission spectroscopy, dielectric parameter measurements using inter-digital electrodes, and so on.
Example
　D. S. Rana, K. Takahashi, K. R. Mavani, I. Kawayama, H. Murakami, M. Tonouchi, T. Yanagida, H. Tanaka, and T. Kawai, "Thickness dependence of the structure and magnetization of BiFeO3 thin films on (LaAlO3)(0.3)(Sr2AlTaO6)(0.7) (001) substrate" Phys. Rev. B, 75(6), 060405(FEB 2007)

B. Terahertz Time-Domain Spectroscopy
The prepared films are characterized by means of Terahertz time-domain spectroscopy (THz-TDS) at various temperatures. The followings are notable findings we obtained so far; Pseudo-gap opening above 200 K in BSCCO thin films was observed in the temperature dependence of the imaginary part of conductivity; Pseudogap was observed in La2-2xSr1+2xMn2O7 (x=0.3); Charge density waves were excited in PCMO; Charge density wave are excited by THz waves, and so on.
Example
　N. Kida and M. Tonouchi “Spectroscopic evidence for a charge-density-wave condensate in a charge-ordered manganite:Observation of a collective excitation mode in Pr0.7Ca0.3MnO3-d by using THz time-domain spectroscopy”, Phys. Rev. B, 66(2), 24401(2002)

C. Terahertz Emission Spectroscopy
Terahertz emission properties are studied to understand carrier dynamics in various electronic materials. We found that moderate decrease in carrier density of YBCO enhances THz emission efficiency.The THz emission from high Tc superconductors obeys the rules that the maximum frequency of the emitted waves never exceed Josephson plasma frequencies of the materials. THz emission properties reflect spin behavior of PCMO. Photo-induced phase transition from metal state to insulating could provide unique function of the THz switches made of PCMO thin films. Recently we dicovered THz emission effect multiferroic materials with memorry effect.
Examples
　D.S. Rana, I. Kawayama, K.R. Mavani, K. Takahashi, H. Murakami, M. Tonouchi "Understanding the nature of ultrafast polarization dynamics of ferroelectric memory in the multiferroic BiFeO3 thin films&" Adv. Mater., 21, 2009, online
　M. Suzuki, M.Tonouchi, K. Fujio, H. Ohtake, T. Hirosumi,"Excitation wavelength dependence of terahertz emission from semiconductor surface" Appl. Phys. Lett., 89(9), 091111(AUG 28 2006)
　K. Takahashi, N. Kida, and M. Tonouchi, "Terahertz radiation by an ultrafast spontaneous polarization modulation of multiferroic BiFeO3 thin films", Phys. Rev. Lett., Vol.96(2006)art.no.117402.
　Y. Tominari, T. Kiwa, H. Murakami, M. Tonouchi, H. Wald, P. Seidel, H. Schneiderwind “Resonant terahertz radiation from thin films Tl2Ba2CaCu2O8+d by ultrafast optical pulse excitation” Applied Physics Letters Vol. 80, No. 17, April 29, 2002, 3147-3149
　N. Kida, M.Tonouichi, "Terahertz radiation from magnetoresistive Pr0.7Ca0.3MnO3 thin films" Appl. Phys. Lett., 78(26), 4115-4117(Jun 25 2001)

D. Pump and Probe Terahertz Emission Spectroscopy
We develop the system of pump and probe terahertz emission spectroscopy (PPTES) at low temperatures. Semiconductors, GaAs, InP, and LT-GaAs, are well characterized by PPTES, which has been proven as a powerful tools to study ultrafast carrier dynamics while exciting terahertz waves. PPTES has been employed to explore the physics in YBCO and PCMO as well as semiconductors.
Example
　M. Tonouchi, N. Kawasaki, T. Yoshimura, H. Wald, P. Seidel, "Pump and probe terahertz generation study of ultrafast carrier dynamics in low-temperature grown-GaAs" Jpn. J. Appl. Phys., 41(6B), L706-L709(JUN 15 2002)

Ⅳ. Novel Photonic Devices

A. Untrafast Optical Respnse of High-Tc Superconductor Photonic Devices
General physics is organized on the basis of the minimum free energy of the ensemble system. Recent progress in ultrafast laser is now breaking the rules of the ensemble system. As one of the interesting approach, we try to manipulate magnetic flux quantum in superconductors by illumination with femtosecond laser. As a new effect, we realize vortex generation and its control inside high-Tc superconductor devices, breaking the rule of superconductivity.We also develop flux-flow-transitor for optical interface.
Examples
　M. Tonouchi, "Magnetic flux quanta in YBa2Cu3O7-delta thin-film loops controlled by femtosecond optical pulses", Jpn. J. Appl. Phys., 40(6A), L542-L544(JUN 1 2001)
　M. Tonouchi, N. Wada, M. Hangyo, M. Tani, K. Sakai, "Control of magnetic flux in a YBa2Cu3O7-delta thin film loop using femtosecond laser pulses" Appl. Physc. Lett., 71(16), 2364-2366(OCT 20 1997)

B.Semiconductor Terahertz Switches
We develop a-Ge and InGaAs-photoconductive-switches for femtosecond fiber laser excitation at a wavelength of 1.5μm. Fe implanted InGaAs works efficiently as a THz emitter but inefficiently as a detector. Optimization of both annealing and implantation is now underway.
Examples
M. Suzuki, M. Tonouchi “Fe-implanted InGaAs photoconductive terahertz detectors triggered by 1.56 mu m femtosecond optical pulses“ Appl. Phys. Lett., 86(16), 163504 (APR 18 2005)　
M. Suzuki, M, Tonouchi,"Fe-implanted InGaAs terahertz emitters for 1.56 mu m wavelength excitation" Appl. Phys. Lett., 86(5), 051104(JAN 31 2005)

C. Terahertz Devices for ICT
A photomixer coupled with a high-Tc Josephson junction has been developed for photonic local oscillator and optical input interface to SFQ circuits.We generate over 100GHz electromagnetic waves by photomixing and the waves which travel in free space are detected by high-Tc Josephson junction.This provides important techniques and information for the research of Terahertz wireless communication.

Ⅴ. Application Systems

A. Compact THz-TDS System
We develop compact THz-TDS systems; mobile THz- imaging head coupled with optical fiber and all 1.5-μm-wavelenght fiber-laser-based THz-TDS. Both system enable us to apply the system for many applications. For instance, the system fits in cryostat for the study of physics in strongly correlated electron systems at low temperatures.

B. Laser Terahertz Emission Microscope
One can observe terahertz emission from various kinds of materials excited with a femtosecond laser. Images can be obtained by scanning laser spot on them, which we refer as to “Laser Terahertz Emission Microscope (LTEM)”. Since terahertz emission properties reflect carrier dynamics and intrinsic nature in the materials, we will be able to visualize the new type of physical information.
Example
S. Kim, H. Murakami, M. Tonouchi ” Transmission-type Laser THz Emission Microscope using a Solid Immersion Lens” IEEE J Select. Topic Quant. Electron., 14(2), 498 (2008).

C. Magneto-Optical Microscope
We are developing a new type of magneto-optical (MO) microscope to study vortex dynamics in the optically excited high-Tc superconductors. The system will be applied to develop the optical output interface of SFQ.

Ⅵ. Summary

Tonouchi laboratory explores ultrafast science and looks for novel terahertz/photonic functions in new materials. The final targets of the research are for ultrafast phtonics and terahertz applications, especially in the field of solid state photonics/electronics such as Terahertz Wireless Communications.