1THz Science, Technology, and Systems• The best motivation for students and researchers entering a new field is often the scientific history and systems applications.• This first set of notes provides some highlights from three scientific and application areas: (1) spectroscopy, (2) radio astronomy, and (3) concealed-object imaging, and (4) biomedical imaging.• We will also contrast the investment perspectives from the Government and from Private Industry. Both sectors are activelyinvolved in the THz field, more so than any other time in history.• Will also summarize some of the “grand challenges” that THz researchers and engineers are presently facing – challenges that certainly qualify as good Ph.D. Thesis topics !2THz Spectroscopy• Historically was the first application in THz region and based on the rotationaltransitions of many vapor-phase molecules that occur in this region.• 1950s: THz spectroscopy of gas-phase molecules started using frequencymultiplication of microwave and mm-wave vacuum-tubes (W. Gordy et al).Continual growth of frequency multiplier technology since that time.• 1960s: THz spectroscopy of solids and liquids started in the 1960s with the advent of a simple instrument – the Fourier transform spectrometer. Much pioneering work done on the dielectric properties of plastics, semiconductors, and ceramics (H. Gebbie, P. Richards, K. Button, et al.)• 1970s: High-resolution (i.e., “line”) astronomy with advent of ultrasensitivecryogenic heterodyne receivers above 100 GHz the discovery of CO rotational ladder and other small molecules in nebular regions (T.G. Phillips et al.) •1970s: Low-resolution astronomy with the advent of ultrasensitive cryogenic(composite) bolometers and the discovery of the cosmic background blackbodypeak (P. Richards et al).• 1990s: Spectrometric imaging becomes possible with the advent of ultrafastphotoconductive techniques: time-domain and photomixer spectrometry3(from Website of Prof. F. DeLucia; http://www.physics.ohio-state.edu/~uwave/energyspec.html)An Early High-Resolution THz Spectrometer4Spectroscopic Figures-of-Merit• Resolution, δν (instantaneous linewidth)- Low resolution δν > 0.1 cm-1(3 GHz or higher); Fourier Transform and Time-Domain- Moderate resolution 0.0001 cm-1 (3 MHz) < δν < 0.1 cm-1 (3 GHz)photomixer spectroscopy, FASSST (vacuum-tube) - High resolution δν < 3 MHzfrequency multiplier-based spectroscopy• Frequency tuning ∆ν = νmax- νmin- Broadband incoherent (frequency multiplexed), ∆ν >> 1 THzFourier transform and time-domain- Broadly tunable coherent ∆ν > 1 THz (photomixing)- Moderatlly tunable coherent - ∆ν > 100 GHz (BWOs)- Slightly tunable coherent ∆ν < 100 GHz (Frequency multiplier chains)• Average power Pave (at ~ 1 THz)- “High”: Pave> 1 mW (BWOs)- “Moderate” 0.1 < Pave< 1 mW (Frequency multiplier chains)-“Low”: Pave < 0.1 mW (time domain switches, photomixers)5Interference fringesSpectrumInSb detector 1InSbdetector 2Ring cavity: L~15 mMylar beam splitter 1Mylar beam splitter 2High voltagepower supplySlow wave structuresweeperAluminum cell: length 6 m; diameter 15 cmTrigger channel /Triangular waveform channel Signal channelBWOMagnetLensFilament voltagepower supplyLength ~60 cmSteppermotorReference channelLensStainless steel railsPath of microwaveradiationPreamplifierFrequency roll-offpreamplifierReferencegas cellGlass rings used to suppress reflectionsData acquisition systemComputerFASSST SpectrometerCourtesy Prof. F. DeLucia6BWO-Based FASSST SpectrometerThe FAst Scan Submillimeter Spectroscopic Technique(FASSST) spectrometer takes advantage of the high spectral purity of the backward wave oscillator (BWO) and uses a fast sweep (~ 105linewidths/sec) to "freeze" instabilitesassociated with power supply ripples, thermal drift, etc. Because ~ 106points are recorded (in 1 - 10 seconds), it is not possible to display a spectrum in its entirety here. However, the sequence of successive blow-ups illustrates the results. Because of the fast sweep, ~ 106Hz of detection bandwidth was used to record the spectrum. The use of a bandwidth typical of high resolution microwave spectra (1 Hz) would increase the signal to noise ratio by ~ 10007FASSST Spectrum of the Classical Weed< 0.01 second of dataMethyl FormateCourtesy Prof. F. DeLucia8Hydrogen Peroxide Spectrum vs PressureCourtesy Prof. F. DeLucia9Fixed DFBLaser λ ≈ 780 nmTunableLaser λ >780 nmOptical SpectrumAnalyzer or WavemeterPhotomixer ChipLock-inAmplifierFrequency TuningLap-TopComputerSampleHolderChopperFree-space-toFiber coupler+LowNoiseAmpIsolatorIsolatorU-Bench-10 dBSplitterSignal+ControlMicroscopeObjectiveBeamCombinerHyperhemisphericalLensRoom Tempor CryobolometerPhotomixing Spectrometer: Direct Detection10400 600 800 1000 1200 1400Frequency [GHz]80 GHzZoom-In30 GHzZoom-InWater Vapor Spectroscopy11The modified Tera View TPI-1000THz spectrometer with hybrid scanning mode. Room Temperature.Transmissive THz Time-Domain Spectrometer120123450.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Frequency [THz]Attenuation Coefficient [1/cm]Photomixer vs Time Domain Spectrometers(Lactose Monohydrate)0.02.04.06.08.00.3 0.5 0.7 0.9 1.1 1.3 1.5Frequency [THz]Attenuation Coefficient [1/cm]PhotomixingTime Domain13Normalized Transmission: Dilute Bacillus subtilisOptical Attenuation Signatures of Bacillus Subtilis in the THz Region,” E.R. Brown, J.E. Bjarnason, T.L.J. Chan, A.W.M. Lee, and M.A. Celis, Appl. Phys. Lett., vol. 84 (no 18), p 3438-3440.9.0 µm0.5 micron14Fewer standing waves,but much lower resolutionthan photomixer spectroscopy“THz Absorption Spectrumof Bacillus subtilus spores,”B. Yu, A. Alimova, A. Katz,And R. R. Alfano, Proc. SPIEpaper 5727-3, 2005. Time-Domain Results for Bacillus subtilis15FixedLaser λ ≈ 780 nmTunableLaser λ >780 nmOptical SpectrumAnalyzer or WavemeterPhotomixingTransmitter ChipFiber PhaseShifterLock-inAmplifierFrequency TuningLap-TopComputer(w MATLABfor signalprocessing)SampleChopperFree-space-toFiber coupler+CurrentAmpIsolatorIsolatorU-Bench-10 dBSplitterCollimating LensSignal+Control3-dBSplitter3-dBSplitterFiberCombinerHyperhemisphericalLensPhotomixingReceiver ChipφPhotomixing Coherent Transceiver16Amplitude Response Phase ResponsePhotomixing Coherent Transceiver ResultsWater Vapor at Standard Temperature and PressureVoigt Model(from HITRAN96 database)17Commercial Photomixing Spectrometer18Photomixer Source ModuleDilute H2O2Vapor SpectrumCommercial Photomixing