Terahertz Spectroscopy NIMISHA ARORA Background • • • • • • • • • THz range ~ 0.3 to 20 THz, 10 to 600 𝑐𝑚−1 1THz ~ 33.33 𝑐𝑚−1 , 0.004 𝑒𝑉, 300 𝜇𝑚 Continuous source of far IR source: Arc lamps or globars Pulsed sources: Free electron laser or synchrotron (~ 3ps duration or greater) Advantage of THz spectroscopy: Measure transient electric field which is make up of phase and amplitude. The phase and amplitude is directly related to the absorption coefficient and refraction coefficient thus allow us to obtain complex permittivity of the sample without carry out a kramer-kronig analysis It is a table top experiment with brightness equal to or exceeding that of synchrotron sources Coherent detection allows pulses below the blackbody radiation level to be measured without the use of specialized detectors. THz-TDS does not measure dynamics From Maxwell eq. , a time varying electric current will radiate an electromagnetic pulse. Background • Application in: medical science, imaging of concealed items, time domain spectroscopy, defense application earth and space science, basic science, space instrumentations, agriculture, semiconductor wafer inspection, and air pollution checking. • Characteristics: – Penetration(can pass through different materials with different attenuation) – Resolution(increase with the decrease in wavelength, better than microwave) – Spectroscopy(Various gases and solids exhibit terahertz signature in 0.5-3 THz, used for detection) – Non-ionization(low power levels, exhibit low ionization in biological tissues) – Scattering(inversely proportional to the wavelength thus low in comparison to light wave) – Intensity(Collimation is easier in the terahertz regime in comparison with microwave) Advantage of THz in communication system • • • • • • • • Microwave band is almost preoccupied by different services and its bandwidth limited. Terahertz offer a wider bandwidth. The diffraction of the THz wave is low in comparison with that of microwave , advantageous in point to point link and line of sight Presently, the licenses have been allocated up to 250 GHz, and thereafter, it is license free. This band offers high degree of information security, especially in the spread spectrum technology In comparison with infrared, THz has low attenuation of the signal in certain atmospheric conditions like fog. The time-varying refractive index of the atmospheric path increases the scintillation effect in the infrared link, and it can be reduced in the THz communication link. It is a viable solution to the last and first mile problem. The significant development to enhance the data rate in the infrared wireless communication is slow due to the requirement of the advanced modulation formats like orthogonal frequency division multiplexing (OFDM), coherent transmission and requirement of multiple-input-multiple-output (MIMO) technique. Obstacle in THz communication • • • • The atmospheric path loss is the main obstacle to the realization of the commercial THz wireless communication system. with the increase in the frequency, the attenuation of the signal increases. there are certain low atmospheric attenuation windows in the terahertz region where the successful wireless communication can be established. Below 1 THz, these windows exist around 300, 350, 410, 650, and 850 GHz Apart from this, above 16 km height where the effect of moisture is negligible, the attenuation is also insignificant, and at this height, hence the inter-satellite communication can be established. https://www.google.com/url?sa=i&source=images&cd=&cad=rja&uact=8&ved=2ahUKEwjGg5yHlNHgAhU MqI8KHQLyBF0QjRx6BAgBEAU&url=%2Furl%3Fsa%3Di%26source%3Dimages%26cd%3D%26ved%3D%26u rl%3Dhttps%253A%252F%252Fwww.researchgate.net%252Ffigure%252FAtmospheric-attenuation-inTHz-and-infrared-band12_fig2_255979456%26psig%3DAOvVaw2ngGlMI5q8qNukM_u4wpQ7%26ust%3D1550987048067418&ps ig=AOvVaw2ngGlMI5q8qNukM_u4wpQ7&ust=1550987048067418 THz Sources and Detection Photoconductive Antennas • Material of the PCA is according to the excitation wavelength • Generated carriers formed an electromagnetic pulse under DC biased. • Can provide a bandwidth as high as 5THz • Resonant excitation method • Limited by the excitation laser pulse width and response time of the material. • The transmitter consist of a transmission line deposited onto an undoped GaAs wafer. • Emitted THz pulse is collimated by a crystalline bquartz or silicon hyperhemisphrical lens and an offaxis paraboidal mirror. THz Sources and Detection • As grown GaAs is highly nonstiochiometric, contain excess As, and its properties are governed by point defects such as As interstitials as As antisites. • In LT-GaAs, metallic As clusters embedded in a GaAs semiconductor. • LT-GaAs proved better for receiver. • The material used for the receiver is radiation damaged silicon on sapphire or LT-GaAs. • Electrons can not travel back across the gap because the carriers have been trapped or have recombined due to short lifetime. • The full waveform is mapped out by stepping the variable delay line incrementally. THz Sources and Detection • Planar Antennas • THz pulse generation by depositing thing metal film or growing p-i-n diodes. • A p-i-n diode is comprised of an n-type semiconductor substrate with a ~ 500nm semi-insulating semiconductor material, followed by about 20 nm of p-type material. • Extremely high bias fields can be generated because the insulating layer is thin. • The applied bias voltage is along the surface normal, the wafer must be tilted to generate a usable signal.
