Before demonstrating the fast terahertz detection, we first characterize the electrical and optical performances of the terahertz QWP. The measured dark current versus voltage characteristics of the 400 × 400 μm2 terahertz QWP at 5 K is shown in Fig. 2a, where the electrical hysteresis can be clearly observed. The QWP device is working at low current level of 10−7 A below 200 mV to achieve moderate background noise. Figure 2b plots the photoresponse spectra of the QWP at different bias voltages measured using a fast-scan Fourier Transform Infrared Spectroscopy. It can be seen that the spectra are peaked around 4.2 THz. The inset of Fig. 2b gives the peak responsivity as a function of voltage calibrated with a blackbody source. In the working voltage range between 100 and 150 mV, the QWP device shows a peak responsivity of 0.7A/W. By measuring the noise spectral density expressed in A/Hz−−−√, we finally obtain the noise equivalent power (NEP) of 0.4 pW/Hz−−−√. Both the responsivity and NEP parameters show that the QWP is a sensitive detector for terahertz emission.
Device characterizations of the terahertz QWP. (a) The dark current versus voltage characteristics of the 400 × 400 μm2 terahertz QWP measured at 5 K. (b) Photoresponse spectra of the QWP device at different ...
Due to the complexity or the difficulty of performing optical heterodyne measurements in the teraherz regime, the microwave rectification technique18 is an useful and simple way to investigate the high frequency characteristics of the QWP device. Instead of using two terahertz sources with a frequency separation in microwave range for heterodyne measurements, as shown in Fig. 2c we apply a microwave signal to the QWP via a Bias-T and measure the change in the DC voltage. The rectification relies on the nonlinearity of the voltage-current (V-I) curve and the microwave modulation amplitude. The rectified DC voltage at current I 0, V rect, can be written as19, 20
Vrect=12|V''|I0I2RF,QWP’
1
where |V″|I0 is the second derivative of the V − I curve at I 0 and I RF,QWP is the effective current amplitude applied by the radio frequency (RF) source to the QWP device. The inset gives the parallel resistance-capacitance (R-C) equivalent circuit of the QWP device for modeling that will be discussed in detail later. The inductance L is introduced by the wire bonds used for connecting the QWP mesa and the microwave transmission line. R and C are resistance and capacitance of the QWP mesa, respectively.
Figure 2d shows the measured rectified voltage as a function of frequency for the QWP device at various injecting microwave power. The bias voltage is set to an optimal value of 120 mV which corresponds to an electric field of 0.41 kV/cm. The measured 3 dB response bandwidth f 3dB increases from 4.3 to 5.3 GHz with increasing the microwave power from 0 to 15 dBm. The data in Fig. 2d indicate that the QWP device is capable of responding to a high frequency modulated terahertz light with a carrier frequency around 4.2 THz. If we assume the rectified voltage has a frequency dependent roll off behavior of 1/[1 + (ωτ)2], where ω is the microwave frequency and τ is a characteristic time including contributions from the intrinsic carrier relaxation time and the R-C circuit, the characteristics time τ at 0 dBm microwave power can be derived as τ = 1/(2πf 3dB) = 37 ps. It has been known that for the intersubband devices the intrinsic carrier relaxation time is normally in few picoseconds level. Therefore, the current QWP device is working in the R-C dominant mode.
In order to prove that the QWP device can work at high GHz frequency speed, we should find a suitable terahertz source for the fast detection. Actually the electrically-pumped terahertz QCL is an ideal source for this application due to the following reasons: (1) The emission frequency of a terahertz QCL can be intentionally designed to match the response frequency of the QWP, which therefore can result in strong photocurrent to improve the detection signal-to-noise ratio. (2) The terahertz light emitted from a terahertz QCL is naturally modulated at the cavity round trip frequency. Therefore by changing the cavity length, we are able to modify the laser modulation frequency and then evaluate the detection speed of the QWP device. As shown in Fig. 2d the response modulation bandwidth of the 400 × 400 μm2 QWP is measured to be 4.2 GHz at 0 dBm RF power. So we need a long cavity terahertz QCL to lase with a modulation frequency close to the response modulation bandwidth ~4.2 GHz. Using a combined active region design of bound-to-continuum transition and resonant phonon scattering21, 22 for terahertz QCLs, we achieve an ultralow laser threshold current density of 50 A/cm2 and thus the continuous wave (cw) operation of long cavity terahertz QCL is made possible23. Finally in this work we choose a 6-mm long terahertz QCL as a terahertz source for the fast detection. A typical emission spectrum of the QCL is shown in Fig. 3a (bottom X and left Y) measured using a Fourier transform infrared (FTIR) spectrometer with a spectral resolution of 3 GHz (0.1 cm−1). As a reference, the response spectrum of the QWP is also plotted in Fig. 3a (top X and right Y). The reddish area (from 4.1 to 4.35 THz) represents the peak response range of the QWP. We can see that the QWP photoresponse spectrum exactly covers the QCL emission spectrum and the devices are perfectly spectral matched. The measured mode spacing of the terahertz QCL emission is 6 GHz which is roughly following c/2ln.
Fast detection of terahertz light. (a) The emission spectrum of a 6-mm long terahertz QCL recorded in continuous wave mode at 15 K. The photoresponse spectrum of the terahertz QWP is shown for comparison. The reddish region shows the frequency ...
Using the experimental setup depicted in Fig. 1, we successfully detect the fast modulated terahertz light. In Fig. 3b we show a typical RF spectrum measured with a spectrum analyser. The resolution bandwidth (RBW) used for this measurement is 100 kHz and the spectrum is obtained after 20 times average. A single line at 6.19896 GHz with a signal to noise ratio of 17 dB is clearly observed, which indicates that the QWP device can work as fast as 6.2 GHz although the rectification shows a response modulation bandwidth of 4.3 GHz. In frequency domain, we can say that the RF line shown in Fig. 3 is originated from the inter-mode beating of the QCL longitudinal modes. Like the electrical beat note measurements14, 24, the optical beat note signal presented in this work therefore can indirectly characterize the coherence properties of terahertz modes. In Fig. 3c we show the RF spectrum in “Max hold” mode. With a time duration of 3 minutes, we see the spectrum spans over 150 kHz. The “Max hold” linewidth can be regarded as an indicator of the laser frequency stability. It is worth noting that the optical beat note measurement is an alternative to the electrical beat note for accurately characterizing the repetition rate and mode stability of QCLs. Since the source and the detector are spatially separated, this space can be used for other applications; for instance, the optical beat note imaging25. Currently, the optical beat note signal obtained in this work is weak, i.e., −75 dBm power and 17 dB signal to noise ratio as shown in Fig. 3b. The weak signal would strongly prevent the technique from high resolution imaging applications. However, if the phase locking26, 27 is implemented for the system, we can significantly improve the imaging resolution and contrast by employing the coherent imaging technique28.
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