Introduction: Terahertz (THz) waves with frequencies in the range of 0.1 to 10 THz are electromagnetic radiation with growing applications in various fields of science and technology. Attractive features of this radiation have brought out many novel possibilities for medical diagnostics and treatments with considerable advantages compared to other conventional methods. Methods: In this paper, we present a review of more recent reports on practical applications of THz radiation for diagnostic, biosensing and clinical treatments. The review includes the diagnosis of breast, skin, mouth, cervical, lungs, small intestine, prostate, colon, and stomach cancers, the evaluation of biomolecules, the detection of genetic mutations, the determination of burn depth, the diagnosis of tooth decay, diabetes, and emotional-psychological states, the evaluation of corneal water to diagnose visual diseases, and wound healing monitoring. Further, it embraces the use of THz therapy in reducing the size of the tumor, treating skin cancer, and healing burn wounds, cardiovascular disease, corneal epithelium, angina, and THz heating. Results: This review has emphasized the capabilities of THz waves as a novel tool for future clinical diagnostics and treatments. Conclusion: The paper provides a comprehensive understanding of the feasible potential application of THz waves for clinical purposes and its advantages in comparison with other conventional tools.

Terahertz (THz) radiation generation by the interaction of two co-propagating high intensity laser beams with a warm collisional inhomogeneous plasma is analytically investigated. By presenting the dielectric permittivity of plasma and taking into account the ponderomotive force, the nonlinear current at THz frequency is obtained. A secondary resonant enhancement of THz radiation is observed, in addition to that occurs at the plasma frequency, which can be tuned by plasma density and temperature. Moreover, we show that for each beat frequency, there exists an optimum temperature at which THz radiation is maximized. It is also shown that the power and efficiency of THz radiation decrease by increasing the collision frequency.

The nonlinear interaction of an ultra-short intense frequency-chirped laser pulse with an underdense plasma is studied. The effects of plasma inhomogeneity and laser parameters such as chirp, pulse duration, and intensity on plasma density and wakefield evolutions, and electron acceleration are examined. It is found that a properly chirped laser pulse could induce a stronger laser wakefield in an inhomogeneous plasma and result in higher electron acceleration energy. It is also shown that the wakefield amplitude is enhanced by increasing the slope of density in the inhomogeneous plasma.

Light absorption and scattering in biological tissue are significant variables in optical imaging technologies and regulating them enhances optical imaging quality. Optical clearing methods can decrease light scattering and improve optical imaging quality to some extent but owing to their limited efficacy and the potential influence of optical clearing agents on tissue functioning, complementing approaches must be investigated. In this paper, a new strategy of optical clearing proposed as time-dependent or temporal tissue optical clearing (TTOC) is described. The absorption and scattering in light interaction with tissue are regulated in the TTOC technique by altering the pulse width. Here, the dependence of optical properties of matter on the pulse width in a gelatin-based phantom was investigated experimentally. Then, a semi-classical model was introduced to computationally study of Ultra-short laser/matter interaction. After studying phantom, the absorption and scattering probabilities in the interaction of the pulse with modeled human skin tissue were investigated using the proposed model for pulse widths ranging from 1µs to 10fs. The propagation of the pulse through the skin tissue was simulated using the Monte Carlo technique by computing the pulse width-dependent optical properties (absorption coefficient µ_a, scattering coefficient µ_s, and anisotropy factor g). Finally, the penetration depth of light into the tissue and reflectance for different pulse widths was found.

Electron dynamics in the field of a chirped linearly polarized laser pulse is investigated. Variations of electron energy gain versus chirp parameter, time duration, and initial phase of laser pulse are studied. Based on maximizing laser pulse asymmetry, a numerical optimization procedure is presented, which leads to the elimination of rapid fluctuations of gain versus the chirp parameter. Instead, a smooth variation is observed that considerably reduces the accuracy required for experimentally adjusting the chirp parameter.

A widely tunable regenerative terahertz parametric amplifier is proposed based on degenerate four-wave mixing in an asynchronously pumped highly nonlinear medium. To enhance the Kerr nonlinearity, a hybrid structure of cyclic olefin co/polymer (COC/P) TOPAS integrated by a graphene oxide film is utilized as the gain medium and embedded into a cavity. A low-power terahertz seed wave is injected inside the cavity and sequentially pumped by four asynchronous coupled beams of identical powers. Our results reveal that the output power, the tuning frequency range and the power conversion efficiency of terahertz waves could be considerably improved in the proposed design. Terahertz waves with maximum peak power of 641 W, the accumulated conversion efficiency of 3.8% and amplification gain of 320.5 are obtained at a frequency of 9.61 THz, which could be tuned over a range of 1.17 − 19.736 THz.