Sussex scientists develop ultra-thin terahertz source, paving the way to next generation of communication tech

 

Alice Ingall

http://www.sussex.ac.uk/broadcast/read/55078

Physicists from the University of Sussex have developed an extremely thin, large-area semiconductor surface source of terahertz, composed of just a few atomic layers and compatible with existing electronic platforms.

Terahertz sources emit brief light pulses oscillating at ‘trillion of times per second’. At this scale, they are too fast to be handled by standard electronics, and, until recently, too slow to be handled by optical technologies. This has great significance for the evolution of ultra-fast communication devices above the 300GHz limit – such as that required for 6G mobile phone technology – something that is still fundamentally beyond the limit of current electronics.

Researchers in the Emergent Photonics (EPic) Lab at Sussex, led by the Director of the Emergent Photonics (EPic) Lab Professor Marco Peccianti, are leaders in surface terahertz emission technology having achieved the brightest and thinnest surface semiconductor sources demonstrated so far. The emission region of their new development, a semiconductor source of terahertz, is 10 times thinner than previously achieved, with comparable or even better performances.

The thin layers can be placed on top of existing objects and devices, meaning they are able to place a terahertz source in places that would have been inconceivable otherwise, including everyday object such as a teapot or even a work of art – opening up huge potential for anti-counterfeiting and ‘the internet of things’ - as well as previously incompatible electronics, such as a next generation mobile phone.

Dr Juan S. Totero Gongora, Leverhulme Early Career Fellow at the University of Sussex, said: “From a physics perspective, our results provide a long-sought answer that dates back to the first demonstration of terahertz sources based on two-colour lasers. Semiconductors are widely used in electronic technologies but have remained mostly out of reach for this type of terahertz generation mechanism. Our findings therefore open up a wide range of exciting opportunities for terahertz technologies.”

Dr Luke Peters, Research Fellow of the European Research Council project TIMING at the University of Sussex, said: “The idea of placing terahertz sources in inaccessible places has great scientific appeal but in practice is very challenging. Terahertz radiation can have a superlative role in material science, life science and security. Nevertheless, it is still alien to most of the existing technology, including devices that talk to everyday objects as part of the rapidly expanding ‘internet of things’. This result is a milestone in our route to bring terahertz functions closer to our everyday lives.”

Lying between microwaves and infrared in the electromagnetic spectrum, terahertz waves are a form of radiation highly sought in research and industry. They have a natural ability to reveal the material composition of an object by easily penetrating common materials like paper, clothes and plastic in the same way X-rays do, but without being harmful. Terahertz imaging makes it possible to ‘see’ the molecular composition of objects and distinguish between different materials. Previous developments from Prof Peccianti’s team showcased the potential applications of terahertz cameras, which could be transformative in airport security, and medical scanners – such as those used to detect skin cancers.

One of the biggest challenges faced by scientists working in terahertz technology is that what is commonly accepted as an ‘intense terahertz source’ is faint and bulky when compared with, for example, a light bulb. In many cases, the need for very exotic materials, such as nonlinear crystals, makes them unwieldy and expensive. This requirement poses logistical challenges for integration with other technologies, such as sensors and ultrafast communications.

The Sussex team have overcome these limitations by developing terahertz sources from extremely thin materials (about 25 atomic layers). By illuminating an electronic-grade semiconductor with two different types of lasers light, each oscillating at different frequency or colour, they were able to elicit the emission of short bursts of Terahertz radiation.

This scientific breakthrough has been long-sought by scientists working in the field since the first demonstration of terahertz sources based on two-colour lasers in the early 2000s. Two-colour terahertz sources based on special mixtures of gas, such as nitrogen, argon or krypton, are among the best performing sources available today. Semiconductors, widely used in electronic technologies, have remained mostly out of reach for this type of terahertz generation mechanism.

The research was developed within the framework of the European Research Council project “TIMING”.

The full research paper, titled, ‘All-Optical Two-Color Terahertz Emission from Quasi-2D Nonlinear Surfaces’ is published in the four star journal, Physical Review Letters, and can be read in full here: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.263901   

Abstract-Terahertz control of air lasing

M. Clerici, A. Bruhács, D. Faccio, M. Peccianti, M. Spanner, A. Markov, B. E. Schmidt, T. Ozaki, F. Légaré, F. Vidal,  R. Morandotti,

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.99.053802

The coherent emission from ionized nitrogen molecules is of interest for remote sensing and astronomical applications. To initiate the lasing process, we used an intense ultrashort near-infrared (NIR) pulse overlapped with a terahertz (THz) single-cycle pulse. We observed that coherent emission could be seeded and modulated by the amplitude of the THz field, which is the result of a combined effective second-order nonlinear polarization and the nonlinear effects induced by the NIR pump. Our results shed light on the role of intense transient fields in the coherent emission from photoexcited gas molecules.

• 
• 
• 
• 
• 

Abstract-Terahertz control of air lasing

M. Clerici, A. Bruhács, D. Faccio, M. Peccianti, M. Spanner, A. Markov, B. E. Schmidt, T. Ozaki, F. Légaré, F. Vidal, and R. Morandotti

https://journals.aps.org/pra/accepted/8007bY2dMa71016661fa2ff421161a92659e17711

The coherent emission from ionized nitrogen molecules is of interest for remote sensing and astronomical applications. To initiate the lasing process, we used an intense ultrashort near-infrared (NIR) pulse overlapped with a terahertz (THz) single-cycle pulse. We observed that coherent emission can be seeded and modulated by the amplitude of the THz field, which is the result of a combined effective second-order nonlinear polarization and the nonlinear effects induced by the NIR pump. Our results shed new light on the role of intense transient fields in the coherent emission from photoexcited gas molecules. One of the key phenomena accompanying the focusing of an intense laser pulse in air is the fluorescence from gas molecules.()()()..~[1–3]. Under appropriate excitation conditions, those molecules produce coherent radiation, which is appealing for standoff spectroscopy applications, especially when emitted in the opposite direction of the ionizing laser pulse..().()()..~[4,5]. Owing to nitrogen’s abundance in our atmosphere, one of the most investigated effects has been the ultraviolet (UV) forward emission from photoexcited molecular nitrogen ions, first described as lasing by Luo and co-workers in 2003..()…….()..~[6]. A number of experiments report narrowband coherent emission at 391 nm and 428 nm, corresponding to the transitions ${N_2^{+}(B}^2{\Sigma }_u^{+}(\nu =0))\to {N_2^{+}(X}^2{\Sigma }_g^{+}({\nu }^{\prime }=0,1\left)\right)$ (see also Fig. 1(c) for potential energy diagrams). Such observations show narrowband amplification at the frequencies corresponding to molecular transitions. Those are seeded, for instance, by harmonics of the near-infrared (NIR) pump pulse…()….(..)(()())(..)…….()..~[7–9], white light…..()…()..~[10], or self-seeded by the pump’s supercontinuum itself…..()()()..~[10–12]. Such reports have reinforced the idea that a lasing process is at the origin of the coherent emission. However, the mechanism responsible for the gain is still not fully understood, although it is likely that laser-driven couplings between electronic states in the ion..().().()……..(..)…..()..~[13,14] and rotational effects()..~[15 –21] play a role Here, we report our experimental observation and analysis of the effect of a strong terahertz (THz) electric field on the coherent emission from photoexcited nitrogen ion molecules, as outlined in Figs. 1(a) and 1(b). We show a correlation between the THz field amplitude and the coherent emission at both 391 nm and 428 nm wavelengths. We interpret our results as consequence of the THz electric field induced symmetry-breaking of the gas molecules. This is turn leads to a THz-controlled seeding of the coherent emission by means of the THz induced second-harmonic of the near-infrared pump pulse. In our investigations we used a single-cycle pulse at THz frequencies as a strong electric field. The THz transient was generated by the transverse photocurrents induced through gas ionization via a laser pulse at 1.8 $$m carrier combined with its second harmonic. 

Abstract-Terahertz absorption by cellulose: Application to ancient paper artifacts

M. Peccianti, R. Fastampa, A. Mosca Conte, O. Pulci, C. Violante, J. Łojewska, M. Clerici, R. Morandotti, and M. Missori

https://journals.aps.org/prapplied/accepted/ce07fYc6Le011c53637216614e26e35315d5a921c

Artifacts made of cellulose, such as ancient documents, pose a significant experimental challenge in the THz transmission spectra interpretation due to their small optical thickness. In this Letter we describe a method to recover the complex refractive index of cellulose fibers from the THz transmission data obtained on single freely standing paper sheets in the 0.2\textendash 3.5~THz range. By using our technique, we were able to eliminate Fabry-Perot effects and recover the absorption coefficient of the cellulose fibers. The obtained THz absorption spectra are explained in terms of absorption peaks of the cellulose crystalline phase superimposed to a background contribution due to a disordered hydrogen bonds network. The comparison between the experimental spectra with THz vibrational properties simulated by density functional theory calculations confirms this interpretation. In addition, evident changes in the THz absorption spectra are produced by natural and artificial aging on paper samples, whose final stage is characterized by a spectral profile with only two peaks at about 2.1~THz and 3.1~THz. These results could be used to provide a quantitative assessment of the state of preservation of cellulose artifacts.