Do you know Terahertz wave? It’s electromagnetic wave with frequency of terahertz. “Tera” means 12th power of 10 (i.e. 1012), so 1THz wave is electromagnetic wave which oscillates 1trillion times in a second. Please see figure1.
Fig.1, Electromagnetic waves and their applications.
This figure shows electromagnetic waves from radio waves to X-ray. These waves are all called electromagnetic waves only with different frequencies. Terahertz waves with frequency of 0.1-10THz are located at between radio waves and light waves. Although terahertz waves have been unexplored frequency region, some applications are expected because of recent progress of technical development at this frequency region.
Applications of THz waves include wireless communications, medical applications, security, atmospheric/astronomical research, and so on. For example, high-speed wireless communications are expected using very high frequency as carrier. We show an example for atmospheric research. We have developed SMILES (Superconducting Submillimeter-Wave Limb-Emission Souder) as shown in figure 2.
These sounders, which carry 600GHz low noise superconducting receiver, were developed to observe ozone and ozone destruction-related molecules in the stratosphere of the Earth. BSMILES (Balloon-borne SMILES) was developed in NICT and launched by using large balloon of JAXA/ISAS up to 35km altitude, and made observations in 2003, 2004, 2006. JEM/SMILES (Japanese Experiment Module/SMILES) was launched in 2009 by H-IIB rocket mounted on HTV(H-II Transfer Vehicle called “KOUNOTORI”). JEM/SMILES caries a 4K mechanical cooler, and made observations in half a year.
Fig.2, ISS-borne SMILES (JEM/SMILES)
(JEM/SMILES is seen at the second from the front)
A new type of semiconductor laser at THz frequency called THz-QCL (THz Quantum Cascade Laser) was proposed and developed as high power THz source. THz-QCL is different from conventional type of laser in terms of using inter sub-bands transition of carrier, whereas conventional laser uses inter bands transition. The carrier which contributes light emission is reused many times by inducing to the next emission units. Therefore, THz-QCL can emit high power terahertz waves.
The device of this THz laser is fabricated in our laboratory. The device is composed of multilayer structure of GaAs/AlGaAs with different thickness of the order of nanometer. Figure 3 show SEM (Scanning Electron Microscope) image of THz-QCL device and chip carrier of the device. We measured 3.1THz emission with output power of about 100μW by cooling this device to 15-45K and applying bias voltage. We also confirmed the output frequency can be tuned by changing the voltage.
Fig.3, (a) SEM image of fabricated THz-QCL device. (b) Photograph of chip carrier.
Heterodyne receiver system
We are developing superconducting low noise receiver called HEBM (Hot Electron Bolometer Mixer). This device is consists of NbN thin film on Si substrate. When terahertz wave is fed to this device, temperature of electron in the superconducting film increases, then “Hot Electron” is generated. The “Hot Electron” is cooled very rapidly in the order of nanosecond because this device is very small (0.4μmx4μm) and very thin (3nm). When two terahertz waves with slightly different frequency of GHz is fed to this device, the electron temperature varies with frequency of beat frequency of these two terahertz waves. We can detect this beat signal due to high speed response of the device. Therefore, we can measure not only total power but also (b) frequency spectrum of the terahertz waves.
The HEBM device is fabricated in our clean room of Kobe NICT. Figure 4 show photographs of (a) SEM image of center part of the device, (b) whole image of the device, and (c) HEBM mixer mount. A log-spiral antenna with wide band characteristics is used to detect terahertz waves.
Fig. 4, Photographs of (a) SEM image of center part of HEBM device, (b) whole image of the device, and (c) HEBM mixer mount.
We are developing a heterodyne receiver system to measure spectrum of terahertz waves. Figure 5 shows block diagram of the heterodyne receiver system. When we measure terahertz waves, RF (radio frequency) and LO (local oscillator) signals are both fed to the mixer. The mixer converts RF signal down to IF (intermediate frequency) signal with frequency of 1-3GHz. It is difficult to amplify the terahertz signal directly, however, we can easily amplify and measure spectrum of IF signal. Figure 6 shows photograph of measurement setup of a heterodyne receiver system. THz-QCL is used as a local oscillator. 3THz source which consists of microwave synthesizer and multipliers is used as simulated THz signal coming to the antenna shown in figure 5. Figure 7 shows measured spectrum of 3THz signal. This result indicates this system can work as a terahertz spectrum analyzer.
Fig.5, Block diagram of heterodyne receiver system
Fig.6, Photograph of a measurement setup of a heterodyne receiver system
Fig.7, Measured spectrum of 3THz signal by using a heterodyne receiver.
The resolution of this terahertz spectrum analyzer is limited to a few MHz - a few tens of MHz because THz-QCL frequency is unstable due to bias noise and temperature instability. To solve this problem, phase-locking of THz-QCL is necessary. We need frequency-stabilized terahertz source as a reference. Beat signal between the THz reference and THz-QCL is detected by using a hot electron bolometer mixer. The beat signal is compared with a microwave reference, and the error signal is applied to PLL with feedback to the bias voltage of the THz-QCL. We have several THz references such as microwave source followed by multipliers, CW THz reference generated by frequency comb, and THz comb. Figure 8 show result of phase-locking. Figure 8 (a) shows unlocked signal, (b) locked signal, and (c) locked signal with resolution band width (RBW) of 1Hz. The line width of the phase-locked signal of better than 1Hz (0.1Hz) which is limited by RBW of a spectrum analyzer was achieved.
Fig.8, (a) unlocked signal / (b) locked signal
(c) locked signal with RBW of 1Hz
Senior Researcher Irimajiri study using an electron microscope.
We need further development to apply this receiver system to atmospheric/astronomical observations from the sky. We plan to measure emission line spectra of molecules by using gas cell system. In order to detect weak emission from molecules, the receiver performance should be more improved. Furthermore, the receiver system is required to be small size, to have low power consumption and long term stability, and should be robust system, i.e., it should work in the low temperature and vacuum environment. We continue the development dreaming our receiver system would get to the sky someday.
To be continued…
Senior Researcher Irimajiri carrying out research and development.
Yoshihisa IrimajiriRemote Sensing Laboratory
Shoichiro KojimaRemote Sensing Laboratory
Aoi NakamizoSpace Environment Laboratory
Maya MizunoElectromagnetic Compatibility Laboratory
Koki WakunamiElectromagnetic Applications Laboratory
Makoto AokiRemote Sensing Laboratory
Seiji KawamuraRemote Sensing Laboratory
Miho FujiedaSpace-Time Standards Laboratory
Kensuke SasakiElectromagnetic Compatibility Laboratory