The QWP is based on the one single photon design and the core region consists of 30-period AlGaAs/GaAs (80/18 nm) quantum well structure. The central 10 nm of the GaAs well is doped with Si to 5 × 1016 cm−3. The core region is sandwiched between the top and bottom GaAs contact layers. The whole QWP structure is grown using a molecular beam epitaxy (MBE) system on a semi-insulating GaAs (100) substrate. The aluminum concentration of the AlGaAs barrier is of great importance for determining the peak response wavelength. Here the aluminum fraction is set to 1.9%. The MBE-grown wafer is processed into mesa structures with top and bottom electrodes. The detailed fabrication process of the 45° edge facet QWP can be found in ref. 41.
The QCL used in this work is based on a Al0.25Ga0.75As/GaAs material system grown by a MBE system on a semi-insulating GaAs (100) substrate. The growth starts with a 400-nm thick n-type GaAs bottom contact layer followed by the active region which consists of 76 cascade periods. Finally, a 50-nm thick GaAs top contact layer with 5 × 1018 cm−3 doping is grown on top of the active region. The MBE-grown QCL wafer is processed into single plasmon waveguide lasers using the standard semiconductor laser processing technology, for instance, the optical lithography, wet and dry etching, electron beam evaporation, lift-off, and thermal annealing.
The as-cleaved QWP and QCL chips are indium-soldered on the copper base. Wire bonds are then used for the electrical injection. Finally the packaged devices are screwed onto the cold fingers of continuous-flow liquid Helium cryostats for low temperature measurements.
The photoresponse spectra of terahertz QWPs with a spectral resolution of 4 cm−1 are
measured using a Fourier Transform Infrared Spectrometer (FTIR)
equipped with a Globar far-infrared broadband source. The peak
responsivity of the QWP is calibrated using a 1000-K blackbody source.
The noise spectral density is measured using a spectrum analyser with a
resolution bandwidth of 1 Hz. To accurately measure the noise, a low
noise current preamplifier (Stanford Research, SR570) with a sensitivity
of 200 nA/V and a bandwidth of 2 kHz is used. The readout quantity from
the spectrum analyser is in the unit of V/
The microwave rectification of the terahertz QWP is measured by employing the experimental setup shown in Fig. 2c. To read the rectified voltage using a lock-in amplifier, we amplitude modulate the RF source at a frequency of 20 kHz by a wave function generator. In order to get a strong modulation signal, we increase the peak-to-peak voltage of the wave function generator until a 95% modulation depth is obtained.
The emission spectrum of the 6-mm long terahertz QCL is measured using a FTIR. The terahertz light emitted from the QCL is coupled into the FTIR via a side input port using an off-axis parabolic mirror. The spectral resolution is set as 0.1 cm−1. To reduce the water absorption, the FTIR is under vacuum and the beam path out side the FTIR is purged with dry air.
The optical inter-mode beat note of the terahertz QCL is measured using the fast terahertz QWP as shown in Fig. 1. The beat note spectra are recorded using a spectrum analyser with a bandwidth of 26 GHz. To measure the weak beat note signal, the optical alignment is critical. We do the alignment as follows: Firstly, we use a red diode laser with a piece of filter paper and a small aperture placed in sequence at the output port to imitate the QCL point source. It would be much easier to align the two parabolic mirrors using the visible analogous point source. After the optics are well aligned, we replace the diode laser with our QCL. Here we have to put the QCL facet exactly at the same position of the diode laser. Then, we run the QCL to do the fine alignment using a terahertz camera (NEC, IR/V-T0831C). Finally we replace the terahertz camera with the fast QWP to perform the optical beat note measurement.
The authors thank Stefano Barbieri for discussions on fast terahertz detectors. This work was supported by the “Hundred-Talent” Program of Chinese Academy of Sciences, the 973 Program of China (2014CB339803), the Major National Development Project of Scientific Instrument and Equipment (Grant No. 2011YQ150021), the National Natural Science Foundation of China (61575214 and 61405233), and the Shanghai Municipal Commission of Science and Technology (14530711300, 15560722000, 15ZR1447500, 15DZ0500103 and 17YF1430000).
H.L. conceived the experiment, W.J.W. conducted the molecular beam epitaxy growth and fabricated the terahertz QCLs, Z.L.F. and H.X.W. fabricated the terahertz QWPs, Z.Y.T., Z.L.F., C.W. and H.L. performed the electrical, optical and microwave rectification characterizations for the terahertz QWP, H.L., W.J.W., and Z.P.L. conducted the optical inter-mode beat note measurements for the terahertz QCL using the fast terahertz QWP, H.L. modelled the frequency-dependent rectified voltage for terahertz QWPs, T.Z. debugged the LabView programs for the automated measurements and provided assistance during the measurements, X.G.G. designed the QWP structure, H.L. wrote the manuscript, H.L. and J.C.C. supervised the experiment, and all authors reviewed the manuscript.
The authors declare that they have no competing interests.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Hua Li, Email: nc.ca.mis.liam@il.auh.
Jun-Cheng Cao, Email: nc.ca.mis.liam@oaccj.
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