THz Ap­p­li­ca­ti­on Overview

The se­arch for THz ap­p­li­ca­ti­ons is a field of on­go­ing re­se­arch es­pe­ci­al­ly since the in­ven­ti­on of ul­tras­hort pulse laser sys­tems in the end of the 1980. The big po­ten­ti­al for new sys­tems and ap­p­li­ca­ti­ons is most­ly at­tri­bu­ted to the fol­lowing cha­rac­te­ris­tics of THz ra­dia­ti­on:

• Low en­er­gy: In con­trast to x-rays the ra­dia­ti­on bet­ween 100G­Hz and 10THz is non-io­ni­zing and the­re­fo­re not harm­ful to hu­mans and / or ma­te­ri­al. But this comes at a price as the back­ground ra­dia­ti­on at room tem­pe­ra­tu­re is in the same en­er­gy range and gives a high noise level.
• Many op­ti­cal­ly opaque ma­te­ri­als are trans­pa­rent for THz ra­dia­ti­on: Paper, card­work, cloth and plas­tics are most­ly ma­te­ri­als which are trans­pa­rent in the THz re­gime. This gives huge op­por­tu­nities to im­ple­ment sys­tems for qua­li­ty con­trol and se­cu­ri­ty ap­p­li­ca­ti­ons. Howe­ver there is no mass mar­ket for those sys­tems up to now.
• High ab­sorp­ti­on in water: Boon and bane of the THz ra­dia­ti­on. The high ab­sorp­ti­on of water makes very sen­si­ti­ve hu­mi­di­ty me­a­su­re­ments pos­si­ble (e.g. test for skin can­cer) but is a big chal­len­ge for data trans­mis­si­on at high fre­quen­cies. The hu­mi­di­ty in the air re­stricts data trans­mis­si­on to nar­row com­mu­ni­ca­ti­on bands and / or to short dis­tan­ces.
• Spec­tral fin­ger­prints: Many mole­cu­les ex­hi­bit cha­rac­te­ris­tic ab­sorp­ti­on lines in the fre­quen­cy range above 1THz. This enables the iden­ti­fi­ca­ti­on of many un­known sub­stan­ces like ex­plo­si­ves or phar­maceu­ti­cals.

Fi­gu­re 1: Pos­si­ble ap­p­li­ca­ti­on in qua­li­ty con­trol: At 300G­Hz the struc­tu­res in­si­de of the pen-ta­blet are cle­ar­ly vi­si­ble.

Up to now the kil­ler ap­p­li­ca­ti­on for THz ra­dia­ti­on has not been found. One re­a­son for this is the high in­vest­ment cost for a THz sys­tem. No­nethe­l­ess there is a big di­s­cus­sion about pos­si­ble areas of ope­ra­ti­on:

• Me­di­ci­ne: Es­pe­ci­al­ly du­ring the ni­n­e­ties of the last cen­tu­ry the ap­p­li­ca­ti­on of THz for me­di­cal ex­ami­na­ti­on was di­s­cus­sed. The big­gest pro­blem here is the high water ab­sorp­ti­on con­fi­ning the ap­p­li­ca­ti­on to the upper skin lay­ers. Alt­hough the iden­ti­fi­ca­ti­on of skin can­cer is pos­si­ble with such a sys­tem the costs are too high com­pa­red to the tre­at­ment by a ex­pe­ri­en­ced phy­si­ci­an
• Se­cu­ri­ty: There are two main areas for se­cu­ri­ty ap­p­li­ca­ti­ons: The spec­trosco­pic in­ves­ti­ga­ti­on of pos­tal en­ve­lo­pes and packa­ges to find il­le­gal / ex­plo­si­ve sub­stan­ces, and the iden­ti­fi­ca­ti­on of hi­d­den ob­jects under clo­thes. In the for­mer case the ac­qui­si­ti­on rate is the big­gest chal­len­ge while se­ver­al lay­ers of clo­thes are a big chal­len­ge in the lat­ter one.
• Qua­li­ty con­trol: The in-li­ne qua­li­ty con­trol of plas­tic tubes is an es­ta­blis­hed ap­p­li­ca­ti­on no­wa­days. More thick­ness me­a­su­ring ap­p­li­ca­ti­ons for op­ti­cal­ly opaque ma­te­ri­als are under de­ve­lop­ment. Mul­ti­lay­er coa­tings can be me­a­su­red even du­ring the dry­ing pro­cess.
• Phar­maceu­ti­cals: The on­line ob­ser­va­ti­on of the coa­ting pro­cess of phar­maceu­ti­cals is one ap­p­li­ca­ti­on which is done with THz sys­tems even no­wa­days. Fur­ther­mo­re Ra­man-spec­trosco­py is a wi­de­ly used tech­ni­que wor­king with fre­quen­cies in the THz range. But here the me­a­su­re­ment takes place in­di­rect­ly in the op­ti­cal re­gi­on en­ab­ling the use of very cost-ef­fec­tive ele­ments.
• Com­mu­ni­ca­ti­on: The in­crea­se of data rates brings the need for hig­her mo­du­la­ti­on fre­quen­cies into the range of se­ver­al 100G­Hz. Un­for­t­u­n­a­te­ly the water ab­sorp­ti­on in the at­mo­s­phe­re at­te­nua­tes the si­gnal quite dras­ti­cal­ly. Fur­ther­mo­re the trans­mis­si­on needs a line of sight which makes ad­van­ced tech­ni­ques ne­cessa­ry for in­door com­mu­ni­ca­ti­ons.