## 1 Introduction
### Penetration Depth of THz: Bio-Medical Applications  
Terahertz (THz) radiation is highly absorbed by liquid water, as such, this presents the possibility to image skin using the difference in water content between normal skin and pathological skin lesions. However, the potential depth is presently limited to 0.2-0.3 mm. A unique feature of water is that in the 0.1-2.0 THz radiation window, ice has an absorption coefficient around 1.0-5.0 cm\({}^{-1}\) while 150-200 cm\({}^{-1}\) in the liquid phase [11; 12; 13; 14; 15; 16]. Consequently, 90% of the incident 0.45 THz signal reaches a 1 mm depth in ice, whereas only \(<0.0001\%\) does in liquid. Except for the uppermost skin layer, the stratum corneum, human skin contains 70-72% free water [17], whilst many cancers, including melanomas, contain on the order of 82-85% free water [18]. The \"dry\" skin elements do not show the same absorption coefficient change on freezing as water does. Thus, when skin is frozen, the water content changes from being the principal absorber, to becoming a minor contributor of THz radiation absorption. The resulting greater contrast is potentially useful for local assessment of skin pathology to a depth of 5.0 mm in frozen skin. The _in vivo_ therapeutic freezing of skin is an established and common medical procedure for treatment of skin diseases such as solar keratoses, warts, and some basal cell cancers. THz imaging techniques using frozen skin would be built upon established clinical cryotherapy methods. In this regard, the high brilliance synchrotron radiation is of especial interest for the proof of the concept experiments for large penetration depth; the brilliance \(B\) is defined as the number of photons \(N_{\lambda}\) of energy \(h\
u\) (J) and direction within the solid angle \(\Omega\) (mrad\({}^{2}\)) per unit of time \(t\) (s), area \(A\) (mm\({}^{2}\)), and 0.1% of the bandwidth, \(BW\) (J), as \(B=N_{\lambda}h\
u/(tA\Omega 0.1\%BW)\). High brilliance of synchrotron radiation is essential for a more than one order of magnitude better signal-to-noise ratio of absorbance quantification as compared with conventional thermal sources used in standard desktop Fourier transform IR (FTIR) spectrometers at comparable data acquisition conditions. It is foreseeable that THz-IR diagnostics facilities can be established at the synchrotrons which are operational 24/7. High intensity THz beams can be generated on-demand using tightly focused ultra-short laser pulses irradiating micro-jets of water or other liquids [19; 20]. Sub-wavelength \(<1\) mm localisation of THz emission from the waist of the focal region can be a useful feature for engineering of practical THz emitters. Interestingly, hard X-rays are simultaneously emitted at the similar focusing conditions and might be used for a sub-surface characterisation [21]. It is imperative that accurate dielectric properties of frozen and non-frozen skin (and its appendages such as hair and sweat ducts) be better understood over the 0.1 to 2.0 THz band. While normal melanocytes export the melanin to surrounding cells, the melanin tends to accumulate in the cancerous cell with melanin levels reaching up to 80 mg/cm\({}^{3}\)[22]. The dielectric properties of frozen melanin also need to be measured as well as the dielectric properties of frozen skin at various hydration levels. The likely method for the deployment of the \"THz-skin freeze\" technique for measuring melanoma depth would be performed just before the actual surgery. This would require robust, predictable, and fast THz imaging technology. The current capacity of THz systems to image surfaces in sufficient detail and with adequate speed is restricted by both expense and technical limitations. The impetus for the deployment of faster and more sensitive THz systems will only come if there is a practical application that demands such development.