Time-Domain Terahertz - Toptica

Pick one: cw or Pulsed Terahertz?Highest bandwidth with time-domain systemsTime-domain and frequency-domain techniques each have merit. Time-domain terahertz spectroscopy offers the advantage of avery broad bandwidth: spectra acquired with photoconductive switches extend to 4 Thz, and crystal emitters even achieve 10 Thz.measurement speed provides the second advantage. The acquisition of a spectrum typically takes 0.1 .. 10 seconds, depending onthe number of traces averaged. Time-domain systems also lend themselves to the investigation of multi-layered samples, where depthinformation is retrieved via time-of-flight measurements.Best resolution with frequency-domain systemsFrequency-domain spectroscopy is the preferred choice for applications requiring highest spectral resolution. While a pulsedspectrometer usually offers a resolution on the 10 Ghz level, cw systems allow frequency control with single-megahertz precision.cw systems also provide the highest dynamic ranges, with peak values greater than 80 dB. Last but not least, frequency-domainspectroscopy enables spectrally-selective measurements. It is thus possible to zoom in on one spectral signature (e.g., to measurethe strength of a single absorption line).Dynamic range of a frequency-domainsystem (left) and a time-domainsystem (right). The dips are absorptionlines of water vapor. The circleshighlight the same set of lines.Specifications cw terahertz systems Pulsed terahertz systemsBandwidth 0.05 – 2 Thz, limited by lasers 0.1 – 4 .. 10 Thz, depending on emitterPeak dynamic range ~ 85 dB ~ 75 dBFrequency resolution 10 mhz Typ. 10 GhzSpectral sensitivity yes noAcquisition time(complete spectrum)minutes to hours, depending on resolutionand lock-in timemilliseconds to seconds, depending onnumber of averagesFemtoFErb 1560• Wavelength 1560 nm• Power > 100 mW• Pulse width < 100 fsInGaAs antennas• Bandwidth > 4 THz• 1 µW average power• SM/PM fiber pigtailTeraFlash• Bandwidth > 4 THz• Peak dynamic range > 70 dB• Resolution 4 .. 40 GHz, selectable