Time-Domain Terahertz - Toptica

TeraFlash TMTime-Domain Terahertz Spectroscopy PlatformThe new TeraFlash system combines TOPTICA’s established FemtoFErb laser andstate-of-the-art InGaAs photoconductive switches into a table-top terahertz platform,based on mature 1.5 µm technology. Owing to a highly precise mechanical delay stage,the TeraFlash achieves a peak dynamic range of more than 70 dB. In “precise scan”mode, the system attains a bandwidth of 4 THz and a resolution better than 10 GHz.Alternatively, in “fast scan” mode, the system acquires a pulse trace in only 50 ms.Equipped with SM/PM fiber pigtails, the antenna modules can be flexibly arrangedaccording to the requirements of the experiment in question.Key features· Versatile time-domain terahertzplatform· Fiber-coupled InGaAsphotoconductive switches· High bandwidth > 4 THz,peak dynamic range > 70 dBVoice coil +position sensorfree space collimators fibersPath lengthcompensationto emitterbias voltageFemtoFErb 1560+ fiber deliveryto detectorSM/PMFC/APCSMBSM/PMFC/APCApplications· High-bandwidth spectroscopy(e.g., explosives or liquid crystals)· Industrial process controlReal-time data processing boardSMAdetector signalPCSchematic diagram of the TeraFlash platform. Blue lines depict electricsignals, red lines the optical signals.Amplitude (a.u)Electric field amplitude (a.u.)20Reference10α-Lactose0-10-20-30-400 20 40 60 80 100Time (ps)1E-101E-111E-120.0 0.5 1.0 1.5 2.0 2.5Frequency (THz)Pulse trace and absorption spectrumof α-lactose monohydrate (blue) andan air reference (black)Precise timekeepingA fast Fourier transform converts the time trace of the terahertz field amplitude into thecorresponding spectrum. The dynamic range of the spectrum (i.e., the ratio of the peaksignal to the noise level) depends on the timing accuracy with which the terahertz pulseis sampled, which in turn is determined by the length jitter of the delay stage used.To achieve a dynamic range above 70 dB, the delay length, given by the momentaryposition of the moveable retro-reflector, must be known with submicron accuracy. TheTeraFlash uses a voice-coil delay with a timing resolution of 1.2 fs, thus measuring thedelay length to better than 400 nm – the secret behind the superb dynamic range ofthe instrument.Flexible scan speedFourier theory determines not only the dynamic range, but also the frequency resolutionof a time-domain terahertz spectrum: the longer the delay stage scan (i.e., the samplingtime of the pulse), the better the resolution. The maximum scan range of the TeraFlashis 200 ps, which translates into a resolution better than 5 GHz. In addition, the user canchoose to average a number of traces in order to exploit the maximum dynamic range.If, however, the measurement speed matters most, then the scan time can be reducedto 25 ps, and 20 pulse traces per second can be acquired – still at a peak dynamicrange of 55 dB.The control software can flexibly adjust the scan time and the number of averages.Dipole antenna with mesa structure(Fraunhofer Heinrich-Hertz Institute,Berlin / Germany)14 | www.toptica.com