April 11, 2016
Nippon Telegraph and Telephone (NTT)
Tokyo University of Science
Nippon Telegraph and Telephone (NTT) Corporation (Head office, Chiyoda-ku, Tokyo, Japan; Hiroo Unoura, President and CEO) and Tokyo University of Science (Kagurazaka Campus, Shinjuku-ku, Tokyo, Japan; Akira Fujishima, University President) successfully observed electronic oscillation (dipole oscillation) with attosecond (as: 10-18 of a second) periodicity using gallium nitride (GaN) wide-bandgap semiconductor. A few-cycle near-infrared pulse induces the ultrafast electric interband polarization. The dipole oscillation with 860-as periodicity in the GaN electron and hole system is revealed by an extremely short isolated attosecond pulse. The resultant dipole frequency reaches 1.16 PHz (1015 Hz), making this the first time the petahertz frequency barrier has been exceeded with semiconductor. This study shows the potential of future petahertz signal processing technology based on ordinary wide-bandgap semiconductor devices.
This achievement is reported in Nature Physics. In addition, the study is introduced in “News and Views” of the journal on April 11, 2016.
This work was supported by JSPS KAKENHI Grant No. 25706027.
The growing demand for unprecedented speeds and capacities in the advanced information society has been creating challenges in signal processing technology based on modern semiconductor photonics and electronics. The high-speed operation depends on ultrafast control of physical properties with radio-frequency (RF) electric fields. This principle is common to semiconductor applications ranging from the field-effect transistor, which is one of the building blocks for high-clock-rate logic operation in digital electronics, to the electro-absorption modulator for high-bit-rate transport in optical communications.
In such high-speed semiconductor devices, the current maximum operational frequency is in the terahertz (1012 Hz) regime, which is limited by the response time of band energy modulation with RF electric fields. To extend the operational frequency into the petahertz (1015 Hz) regime, lightwave field control has potential. However, to observe the ultrafast electron motion with petahertz frequency, extremely high temporal resolution is required. For example, as a camera needs a high-speed shutter to take stop-motion snap shots, an instantaneous strobe light is necessary in order to observe an electron with ultrafast motion.
We demonstrate optical drive at 1.16-PHz using gallium nitride (GaN) wide-bandgap semiconductor1. A few-cycle near-infrared (NIR) pulse induces the ultrafast electric interband polarization with the multi-photon process. The dipole oscillation2 with 860-as periodicity in the GaN electron and hole system is revealed by an extremely short isolated attosecond pulse (IAP)3. The resultant dipole frequency reaches 1.16 PHz, making this the first time the petahertz frequency barrier has been exceeded with semiconductor.
The larger the electron transition energy is, the shorter the periodicity becomes. Since the GaN semiconductor has a wide bandgap (energy gap between the VB and CB) of 3.4 eV, the induced dipole oscillation can achieve attosecond periodicity. Transient absorption spectroscopy with the IAP is a powerful method for monitoring ultrafast electron dynamics because the absorption strongly depends on the dipole oscillation.
In this experiment, the duration of the IAP needs to be shorter than the dipole oscillation periodicity of 860 as. In addition, its center photon energy should be reduced, because the lower photon energy is close to the VB and CB states in GaN semiconductor. However, the higher center photon energy gives shorter pulse duration. Here to satisfy both conditions, the IAP with 20-eV center photon energy in VUV region and 660-as duration is generated using the DOG technique.
The demonstrated petahertz optical drive using wide-bandgap GaN semiconductor has the potential for use in constructing high-clock-rate logic operation systems in digital photonics and electronics. In addition, this ultrafast property revealed by the direct time-domain observation will provide an ultrafast manipulation technology for dipole oscillation. The benefit is directly linked to controlling absorption, reflection, refractive indices, photocurrent, photoemission, and diffraction, which are important for implementing photonic and electronic devices with unprecedented speed in the future.
Hiroki Mashiko, Katsuya Oguri, Tomohiko Yamaguchi, Akira Suda and Hideki Gotoh
“Petahertz optical drive with wide-bandgap semiconductor”
Nature Physics (2016)
1. Nippon Telegraph and Telephone (NTT) Corporation
Science and Core Technology Laboratory Group,
NTT Has Instituted a Logo to Represent R&D Activities.
2. Tokyo University of Science
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