Terahertz surprises

Noriaki Horiuchi, 


https://www.nature.com/articles/s41566-018-0115-6

The observation of terahertz nonlinearities in graphene and quantum wells and the emission of terahertz waves from water show that terahertz science is still a fertile area of research that is full of surprises.

There were many eye-opening findings and applications reported at the International Symposium on Microwave/Terahertz Science and Applications (MTSA 2017) held in Okayama, Japan from 19 to 23 November 2017. Interestingly, the symposium was combined and co-located with two other conferences. The first was the International Symposium on Terahertz Nanoscience (Tera Nano 8). The second was Current Trends in Optical and X-Ray Metrologies of Key Enabling Nanomaterials/Devices for the Ubiquitous Society, Renewable Energy and Health (Opto X Nano) symposium. The joint event provided a rich forum for discussing new developments in terahertz (THz) science and technology.

While graphene has been theoretically predicted to exhibit a strong nonlinearity in the THz frequency range due to its unusual linear electronic band structure, experimental investigations are lacking. Manfred Helm of Helmholtz-Zentrum Dresden-Rossendorf told the conference how his group has been exploring the properties of graphene using a THz free-electron laser (FEL) and reported his recent experimental results.

“Our idea was that the resonant enhancement in a magnetic field may increase the chance to observe the nonlinearity. In addition, this nonlinearity is frequency tunable with the magnetic field,” explained Helm. In a magnetic field, graphene’s band structure splits into non-equidistant Landau levels, which give rise to resonant optical properties. The tunable and narrow radiation from the THz FEL at Dresden-Rossendorf is thus convenient for probing the spectrally narrow Landau-level resonance.

His group implemented resonantly enhanced four-wave mixing at a photon energy of 78 meV and a strong magnetic field of B = 4.5 T. The German researchers measured the intensity and pulse duration of the two incident pulses from the FEL and from that they calculated the electric-field strength in a 40-layer graphene sample. The sample was found to have a third-order nonlinear optical susceptibility of 9 × 10–20m3 V−2.

In the second part of his talk, Helm described the observation of the Autler–Townes splitting of intersubband transitions in a single GaAs/AlGaAs quantum well. Autler–Townes splitting has never been observed in the THz frequency range before (it is usually seen at near or mid-infrared wavelengths in quantum wells) and is thought to be related to the origin of third-order nonlinear optical susceptibility.

The team employed the THz FEL in combination with THz time-domain spectroscopy to realize a true narrow-band pump–broadband probe experiment. While pumping one of the intersubband transitions in the quantum well at 3.5 THz, the researchers probed the entire THz absorption up to 4 THz. The experiment makes it possible to extract the change in transmission as a function of the pump–probe time delay as well as monitor the spectral shape of the transmission change. “By performing a detailed data analysis, we have indeed observed transient changes in the transmitted spectrum which can be interpreted as Autler–Townes splitting,” commented Helm. “However, the observed signatures are not as clear as we had hoped for.”

To address the growing demand for greater data transmission capacity in future mobile networks, THz wireless communication in a frequency range above 275 GHz is being developed. While this frequency band is not yet allocated for a public communication channel, it is an atmospheric window that is less affected by moisture in the air than millimetre-wave wireless communication and is expected to transmit information more than ten times faster. That said, THz wireless communication currently has two main problems. First, the output power of THz transmitters is not very strong and second, free-space path loss and ohmic loss in metals are inevitable. Both of the issues impact on the opportunity for long-distance wireless communication.

To direct the majority of the radiated power from a source to a THz receiver, Withawat Withayachumnankul from the University of Adelaide has developed two types of all-dielectric THz antenna — a dielectric resonator antenna and a tapered rod array — fed by photonic crystal waveguides. The antennas and photonic crystal waveguides were fabricated on a single float-zone high-resistivity silicon wafer without metal, because intrinsic high-purity silicon offers negligible loss and moderate permittivity.

The optical characteristics of each THz antenna were measured with a continuous-wave terahertz electronic platform. For the dielectric resonator antenna, the gain was ~10 dBi and the 3 dB gain bandwidth was 315–334 GHz. The tapered rod array offered a gain of nearly 20 dBi at 325 GHz and the 3 dB gain bandwidth was 315–342 GHz.

“Ultimately, the array can be used for communications in a short range, for example, between a kiosk and mobile devices,” commented Withayachumnankul. “However, the rod antenna array can be developed further so that the antenna gain is high enough for point-to-point wireless transfer between fixed base stations separated by a distance of up to a kilometre.”

To date, most of the experiments on free-space THz communications are limited to distances of less than 1 m. Kenta Iwamoto of Osaka University reported a real-time 100 m wireless transmission at 70 Gbit s−1 in the 300 GHz band with quadrature phase-shift keying (QPSK) modulation. “The record-breaking transmission distance is achieved thanks to a high-gain reflector antenna. The gain is 53 dBi, while that of a horn antenna is 25 dBi maximum,” Iwamoto said.

In the transmitter, a single-frequency laser was modulated at 26.67 GHz with two electro-optic phase modulators to generate the optical frequency comb. Two optical signals with a frequency difference of 320 GHz were selected by using an optical filter. One of the signals was QPSK modulated with the electro-optic phase modulator driven by a pulse pattern generator, and combined with the other optical signal by using an optical coupler. Then, these two optical signals were injected into a uni-travelling carrier photodiode, and a THz wave at 320 GHz was generated. By employing QPSK modulation instead of simple on–off keying, the transmission data rate was nearly doubled.

In the receiver, local oscillator signals with a phase difference of π/2 are applied to two subharmonic mixers to separate in-phase and quadrature signals. Iwamoto used a single subharmonic mixer and one of the separated in-phase and quadrature signals. The signal was amplified with a preamplifier, reshaped with a limiting amplifier and the bit error rate was measured with an error detector.

A data rate of up to 70 Gbit s−1 was confirmed for a transmitter power of −15.5 dBm and a forward error correction rate (bit error rate <2 × 10–3). The main factor limiting the data rate is thought to be the residual phase noise of the carrier frequency. To enhance the data rate towards 100 Gbit s−1, Iwamoto believes that introduction of multiple carriers or polarization multiplexing would be effective.

Such high-speed THz communication systems would potentially be very useful for applications such as wireless transmission of uncompressed 8K resolution video data or use as an alternative communication system in a situation where a fibre-optic link is not feasible. “To realize such applications, a transmission distance of 1 km and transmission data rate of 100 Git s−1 would be required,” Iwamoto told Nature Photonics.

One of the longstanding issues for many THz applications is the strong THz absorption by moisture. However, Xi-Cheng Zhang of the University of Rochester, USA, surprisingly told conference attendees that he has now found that a thin film of water can be used to generate broadband THz waves by illuminating it with femtosecond laser pulses. “Since the absorption and emission are related through Einstein relations, we thought why don’t we look at the emission THz waves from water, even though it has very high absorption?” he told Nature Photonics.

A film of water, a few hundred micrometres thick, was produced by a jet nozzle. A femtosecond laser (central wavelength of 800 nm, pulse duration of 600 fs, pulse energy on the order of mJ, repetition rate on the order of kHz) was focused onto the water film. Zhang’s team observed a threshold of THz wave intensity versus the pulse energy. The spectrum of the generated THz waves spanned from 0.1 to 4.0 THz. In contrast to THz wave generation from air plasma, the bandwidth of the THz wave generated from the water film was narrower, but the generation efficiency was more than one order of magnitude larger. In addition, a longer pulse duration laser generated a stronger THz wave from the liquid film, with the optimal pulse duration being around 500 fs. These optical features imply a new THz wave-generation mechanism in liquid materials.

“We are very glad to see the stronger THz signal generated from water than from the air, under the comparable laser excitation condition,” said Zhang. “Liquid, especially water, could be the last piece of the puzzle of THz generation in all four states of matter; solid, liquid, gas, and plasma.” The next MTSA conference will be held in Korea in 2019.

More than 200 scientists from all over the world attended the joint MTSA2017/Opto X Nano/Tera Nano 8 conferences in Okayama, Japan.

Credit: Toshihiko Kiwa, Okayama University