We review the development of terahertz (THz) technology and describe a typical system used in biomedical applications. By considering where the THz regime lies in the electromagnetic spectrum, we see that THz radiation predominantly excites vibrational modes that are present in water. Thus, water absorption dominates spectroscopy and imaging of soft tissues. However, there are advantages of THz methods that make it attractive for pharmaceutical and clinical applications. In this review, we consider applications ranging from THz spectroscopy of crystalline drugs to THz imaging of skin cancer.

https://iopscience.iop.org/article/10.1088/0022-3727/39/17/R01/pdf

Biomedical applications of terahertz technology

Exposing a fused silica sample to a strong, waveform-controlled, few-cycle optical field increases the dielectric’s optical conductivity by more than 18 orders of magnitude in less than 1 femtosecond, allowing electric currents to be driven, directed and switched by the instantaneous light field. Two studies published in this issue highlight the potential for ultrafast signal manipulation in dielectrics using optical fields. When it comes to electrical signal processing, semiconductors have become the materials of choice. However, insulators such as dielectrics could be attractive alternatives: they have a fast response in principle, but usually have extremely low conductivity at low electric fields and break down in large fields. The electronic properties of dielectrics can be controlled with few-cycle laser pulses that permit damage-free exposure of dielectrics to high electric fields. Agustin Schiffrin et al. demonstrate that strong optical laser fields with controlled few-cycle waveforms can reversibly transform a dielectric insulator into a conductor within the optical period (within one femtosecond). Martin Schultze et al. address the crucial issue of ultrafast reversibility, demonstrating that the dielectric can be repeatedly switched 'on' and 'off' with light fields, without degradation. The time it takes to switch on and off electric current determines the rate at which signals can be processed and sampled in modern information technology1,2,3,4. Field-effect transistors1,2,3,5,6 are able to control currents at frequencies of the order of or higher than 100 gigahertz, but electric interconnects may hamper progress towards reaching the terahertz (1012 hertz) range. All-optical injection of currents through interfering photoexcitation pathways7,8,9,10 or photoconductive switching of terahertz transients11,12,13,14,15,16 has made it possible to control electric current on a subpicosecond timescale in semiconductors. Insulators have been deemed unsuitable for both methods, because of the need for either ultraviolet light or strong fields, which induce slow damage or ultrafast breakdown17,18,19,20, respectively. Here we report the feasibility of electric signal manipulation in a dielectric. A few-cycle optical waveform reversibly increases—free from breakdown—the a.c. conductivity of amorphous silicon dioxide (fused silica) by more than 18 orders of magnitude within 1 femtosecond, allowing electric currents to be driven, directed and switched by the instantaneous light field. Our work opens the way to extending electronic signal processing and high-speed metrology into the petahertz (1015 hertz) domain.

Optical-field-induced current in dielectrics

Many non-equilibrium phenomena have been discovered or predicted in optically driven quantum solids1. Examples include light-induced superconductivity2,3 and Floquet-engineered topological phases4–8. These are short-lived effects that should lead to measurable changes in electrical transport, which can be characterized using an ultrafast device architecture based on photoconductive switches9. Here, we report the observation of a light-induced anomalous Hall effect in monolayer graphene driven by a femtosecond pulse of circularly polarized light. The dependence of the effect on a gate potential used to tune the Fermi level reveals multiple features that reflect a Floquet-engineered topological band structure4,5, similar to the band structure originally proposed by Haldane10. This includes an approximately 60 meV wide conductance plateau centred at the Dirac point, where a gap of equal magnitude is predicted to open. We find that when the Fermi level lies within this plateau the estimated anomalous Hall conductance saturates around 1.8 ± 0.4 e2/h. A transient topological response in graphene is driven by a short pulse of light. When the Fermi energy is in the predicted band gap the Hall conductance is around two conductance quanta. An ultrafast detection technique enables the measurement.

/pdf/light-induced-anomalous-hall-effect-in-graphene-4m1acwe9gt.pdf

Light-induced anomalous Hall effect in graphene.

In recent years, the field of terahertz (THz) science and technology has entered a completely new phase of unprecedented expansion that is generating ever growing levels of broad-based international attention. In particular,there have been important advances in state-of-the-art THz technology and very enthusiastic growth in research activities associated with related scientific and industrial applications. One can legitimately argue that the potential payoffs of THz sensing and imaging (THz S&I) to application areas such as defense, security, biology and medicine are the major drivers of this new phenomenon. However, there remain major science and technology "gaps" in the THz regime that must be reconciled before many of the perceived payoffs ever become realizable. Therefore, it is natural to ask the question "Is now the time for THz?" or rather, are these recent events just a repeat of previous cycles in THz overenthusiasm that have been witnessed during the last century? The main goal of this paper is to consider some of the most promising THz S&I applications within the specific context of their particular science and technology challenges in an attempt to credibly judge (or speculate on) their future potential.

Terahertz Frequency Sensing and Imaging: A Time of Reckoning Future Applications?

We review pioneering and recent studies of the conductivity of solid state systems at terahertz frequencies. A variety of theoretical formalisms that describe the terahertz conductivity of bulk, mesoscopic and nanoscale materials are outlined, and their validity and limitations are given. Experimental highlights are discussed from studies of inorganic semiconductors, organic materials (such as graphene, carbon nanotubes and polymers), metallic films and strongly correlated electron systems including superconductors.

A Review of the Terahertz Conductivity of Bulk and Nano-Materials

Quasimetallic photoconductivity produced by the absorption of picosecond optical pulses in silicon transmission line structures has been used to devise electronic switches and gates which can be turned on and off in a few picoseconds. Electrical signals as large as 100 V can be switched by a few microjoules of optical energy. The switching speed was measured by correlating the response of two transmission gates in tandem, each having an aperture time of 15 psec.

D. H. Auston

Papers

Picosecond optoelectronic switching and gating in silicon