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Abstract
A high-efficiency active bidirectional electrically-controlled terahertz device based on DMSO-doped PEDOT:PSS with low-power photoexcitation is investigated. Under low-power optical excitation of 30 mW (0.5 W/cm2) and under bias voltages ranging from −0.6 V to 0.5 V, spectrally broadband modulation of THz transmission over a range from −54% to 60% is obtained over the frequency range from 0.2 to 2.6 THz in a MEH-PPV/PEDOT:PSS:DMSO/Si/PEDOT:PSS:DMSO hybrid structure. By considering the combined carrier density characteristics of the proposed device, it is found that the large-scale amplitude modulation can be ascribed to the electrically-controlled carrier density in the silicon layer with the assistance of the p-n junction that consists of the DMSO-doped PEDOT:PSS and silicon. Bidirectional modulation has a larger modulation range and is easier to use in communications applications when compared with unidirectional modulation. These results show great potential for application to the design of active broadband terahertz devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Functional terahertz devices such as modulators, filters, and switches are essential for acceleration of the use of THz technology in spectroscopy, imaging, and communications applications [1–5]. In recent years, various methods have been proposed to produce efficient THz modulators, including use of electrically-induced perturbed intraband transitions in graphene [6–9], thermally-induced phase changes in vanadium dioxide (VO2) [10–14], and optically-excited composite metamaterial structures formed using split-ring resonators and silicon [15–20]. In electrically-controlled THz devices in particular, Mao et al. presented a broadband THz wave modulator with improved modulation depth produced by careful selection of the gate dielectric materials in a large-area graphene-based field-effect transistor, which achieved a modulation depth of 22% over a frequency range from 0.4 THz to 1.5 THz [21]. Li et al. demonstrated transmission modulation of 83% in a graphene-silicon hybrid structure under various voltage biases and photoexcitation using a continuous wave green laser [8,22]. Liu et al. showed that the enhanced THz transmission caused by application of a forward bias can be interpreted well using a proposed model that considered the substantial difference between the carrier mobilities of the Si and BiFeO3 films in the heterojunction [23]. Du et al. presented a modulator that exhibited voltage-controlled bidirectional modulation and reached maximum transmission modulation depths of −23% to 62% in the 0.25 THz–0.65 THz range under high-power photoexcitation [24]. However, the efficiency of the existing THz modulators is limited by process complexity, high-power photoexcitation requirements, low bidirectional modulation depths and narrow bandwidths.
Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) offers high transparency, excellent thermal stability, high carrier mobility and high conductivity [25]. Specifically, the conductive properties of PEDOT:PSS can be tuned via chemical doping using ethylene glycol (EG) and dimethylsulfoxide (DMSO). Kim et al. reported on the fabrication and characterization of an organic light-emitting device that was made using a highly conductive form of PEDOT:PSS as a hole-conducting layer [26]. Consequently, PEDOT:PSS has been widely used as a transparent conductive layer in a manner similar to graphene in organic electronics and in THz functional devices, such as capacitors and transparent electrodes. Yan et al. proposed the potential application of 6% DMSO-doped PEDOT:PSS to low-cost and broadband THz antireflection coatings for use on quartz and silicon substrates [27]. Du et al. reported the use of DMSO-doped-PEDOT:PSS films as transparent electrodes for an electrically tunable THz liquid crystal phase shifter [28].Mixing PEDOT:PSS with various solvents, such as DMSO, allows its conductivity to be improved by up to two or even three orders of magnitude [29]. Therefore, DMSO-doped-PEDOT:PSS film with ease of soluble processing, high transparent and conductivity can be replace graphene film in terahertz functional devices.
In this work, a high-efficiency active bidirectional electrically-controlled THz device based on DMSO-doped PEDOT:PSS with low-power photoexcitation was investigated. Under a low optical excitation power of 30 mW (0.5 W/cm2) and under bias voltages ranging from −0.6 V to 0.5 V, spectrally broadband modulation of THz transmission over a range from −54% to 60% is obtained within the 0.2 to 2.6 THz frequency range in a poly [2-methoxy-5-(2′-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV)/PEDOT:PSS:DMSO/Si/PEDOT:PSS:DMSO (MPSP) hybrid structure. By considering the combined carrier density characteristics of the device materials, it is found that the large-scale amplitude modulation can be ascribed to the electrically-controlled carrier density in the silicon layer with the assistance of the p-n junction composed of the DMSO-doped PEDOT:PSS and silicon.
2. Experimental details
A schematic of the measurement setup and the detailed structure of the electrically-controlled THz device are illustrated in Fig. 1(a). 30-nm-thick 10 vol.% DMSO-doped PEDOT:PSS layers were spin-coated on the front and back sides of a clean 2-mm-thick silicon wafer with high resistance of 10,000 Ω∕sq. The DMSO-doped PEDOT:PSS layers were dried at 100°C for 15 min. Two 200-nm-thick square silver rings were then carefully fabricated by vapor deposition on both sides of the PEDOT:PSS:DMSO-Si-PEDOT:PSS:DMSO hybrid structure to act as electrodes and allow application of a bias voltage to the electrically-controlled THz device. In the next step, a 100-nm-thick MEH-PPV film was spin-coated on the prefabricated wafer. This spin-coated MEH-PPV film was then dried in a vacuum at 80°C for 1 h.
Fig. 1 (a) Proposed MPSP hybrid structure. (b) THz transmission time-domain signal and (c) corresponding frequency-domain signal of bidirectional electrically-controlled terahertz device at various bias voltages (−1, −0.2, −0.1, 0, 0.05, 0.1, and 0.5 V) under photoexcitation power of 0.5 W/cm2.
A terahertz time-domain spectroscopy (THz-TDS) system was used to measure the transmission spectra of the resulting polymer–silicon hybrid device under application of various bias voltages (Vg) and 450 nm continuous wave (CW) optical pump illumination with power P at normal incidence. For reference, PEDOT:PSS:DMSO/Si/PEDOT:PSS:DMSO and MEH-PPV/Si/PEDOT:PSS:DMSO hybrid structures were also photoexcited using the same optical power and voltage bias conditions.
3. Results and discussion
The proposed structure can be actively electrically controlled using the bias voltage and optical excitation. Figure 1(b) and 1(c) show the measured normalized time-domain and frequency-domain signals from the MPSP sample under different bias voltages (−1, −0.2, −0.1, 0, 0.05, 0.1, and 0.5 V) and CW optical illumination power of 0.5 W/cm2. It was observed that changes in the bias voltage also caused significant changes in the transmission intensity of THz radiation over the range from 0.2 THz to 2.6 THz. The THz transmission gradually and clearly decreased with a change in the negative voltage bias from 0 V to −1 V. The THz transmission gradually increased with increasing positive voltage bias from 0 V to 0.5 V, rising to almost complete recovery at the bias of 0.5 V.
Figure 2(a) shows the measured THz transmission of the MPSP structure at different bias voltages under optical pump powers of 0, 0.4, 0.5 and 2.3 W/cm2 at 450 nm. Without photo-excitation, the applied bias did not cause any significant change in the THz transmission through the MPSP sample. At the low photo-excitation power (0.4 W/cm2), an increasing positive bias led to a small change in transmission, whereas a dramatic decline in THz transmission was observed under increasingly negative bias. In contrast, under high-power photo-excitation (2.3 W/cm2), increasing negative bias led to negligible changes in transmission, whereas a dramatic increase in THz transmission was observed with increasing positive bias. Under photo-excitation at 0.5 W/cm2, the THz transmission drops to almost 50% of its original value. THz transmission decreased gradually as the negative voltage bias changed from 0 V to −0.6 V. In contrast, the THz transmission gradually increased as the positive voltage bias increased from 0 V to 0.5 V. Figure 2(b) shows that bidirectional electric modulation based on a PEDOT:PSS:DMSO/Si/PEDOT:PSS:DMSO structure was also achieved under photo-excitation at 2.9 W/cm2, which is at a much higher power than the photo-excitation of the MPSP sample. Figure 2(c) shows that the MEH-PPV/Si/PEDOT:PSS:DMSO structure loses the ability to perform bidirectional modulation. The different properties are attributed to carrier concentration and hetero-junction structure of different devices which will be discussed in detail below. The results thus show that a bidirectional electrically-controlled THz device based on an MPSP structure with low photo-excitation power and low bias voltage was achieved.
Fig. 2 (a) Transmittance of MEH-PPV/PEDOT:PSS:DMSO/Si/PEDOT:PSS:DMSO structure at bias voltages ranging from −0.6 V to 0.5 V under various levels of photoexcitation (0, 0.4, 0.5 and 2.3 W/cm2). (b) PEDOT:PSS:DMSO/Si/PEDOT:PSS:DMSO structure under bias voltages ranging from −1 V to 0.3 V under 2.9 W/cm2 photo-excitation and (c) MEH-PPV/Si/PEDOT:PSS:DMSO structure under bias voltages ranging from −2V to 2V under photo-excitation at 0.45 W/cm2. The insets show schematics of the measured devices.
Further measurements of the bidirectional modulation characteristics of the MPSP sample were performed. The setup used for measurement of the signals of the modulated THz beam based on THz time domain spectroscopy is shown in Fig. 3(a). In the experimental system, a square-wave voltage that alternated between 0 and 0.5 V or 0 and −0.5 V was used to drive the MPSP sample. Then, an oscilloscope (TDS 2012C) was used to record the modulated THz response waveform to the application of this square modulation voltage. Figure 3(b) shows the modulated THz beam signals from the MPSP sample when the bias voltage is positive (red line) and negative (blue line). When the bias voltage is positive, the modulated THz beam signal is the same as the bias voltage that is used for reference. In contrast, when the bias voltage is negative, the modulated THz beam signal is the opposite to the bias voltage that is used for reference. Therefore, it is more intuitive show the bidirectional modulation characteristics of the MPSP sample.
Fig. 3 (a) Setup for measurement of the signals of the modulated THz beam based on THz-TDS. (b) Measured signals of the modulated THz beam when the bias is positive (red line) and negative (blue line) for the MPSP sample.
The modulation speed of the MPSP sample was measured using a system that consists of a CW terahertz source (TeraSence Inc.) with a central output at approximately 180 GHz, and a 170–260 GHz zero-bias Schottky diode-type intensity detector. Figure 4(a)–4(d) show the modulated terahertz transmission signals when the driving bias signals were square waves at frequencies of 10 Hz, 6 kHz, 12 kHz, and 15 kHz, respectively. The transmission of the THz modulated signal is consistent with that of a square wave at low frequencies. The modulated signal gradually becomes a triangular wave as the bias frequency increases. The modulation frequency limit is close to 15 kHz. Furthermore, the dynamic electrical behavior of the MPSP sample was also characterized. Using the conductivity decay method, we see that the conductivity of the MPSP structure to light illumination of 0.5 W/cm2 changes by periodically square wave using a signal generator, as shown in Fig. 4(e). The typical multi-cycle is inserted in Fig. 4(e). It is clearly seen that a sharp response peak, is induced at the moment the 0.5 V positive bias is appeared. Then the conductivity gradually reaches a steady plateau due to the stable voltage. A reverse sharp conductivity-response peak is induced once the positive bias is replaced by negative −0.5 V. Last, the electronic flow stays stable, and the conductivity returns to constants. The response times at the edge are calculated to be 80 μs [30]. Therefore, the conductivity response times of 80 μs means that the modulation rate limit is larger than 12.5 kHz. This result is consistent with the above conclusion. In addition, an ASCII code waveform that represents the acronym ‘CNU’ was sent and received in this experimental system based on the MPSP sample, as shown in Fig. 4(f).
Fig. 4 (a)–4(d) Modulated terahertz beam signals for a carrier frequency of 180 GHz at different modulation frequencies. (e) Half typical cycle of the conductivity-time curve. The inset shows multi-cycle of curve (f) Received ASCII code waveforms for the word ‘CNU’.
The modulation factor (MF) can describe the performance of the MPSP sample accurately. We introduce the MF, which is defined as the relative change in the integrated transmitted THz power that is caused by the photo-excitation intensity, and can be expressed as [19]
where and are the transmitted powers of the THz radiation field when the bias voltage is off and on, respectively. Figure 5(c) shows the MF characteristics of the MPSP sample as a function of the bias voltage. Under low-power optical excitation at 0.5 W/cm2 and at low bias voltages ranging from −0.6 V to 0.5 V, a spectrally broadband MF for THz transmission over the range from −54% to 60% is obtained within the frequency range from 0.2 to 2.6 THz.Fig. 5 (a) Real and 5(b) imaginary parts of the conductivities of the MPSP structure at various bias voltages under photo-excitation at 0.5 W/cm2. (c) Calculated carrier density (blue line) and modulation factor (black line) as a function of bias voltage.
To confirm our explanation of the modulation mechanism, we calculated the carrier density for the MPSP structure under photo-excitation and an applied bias voltage. In calculating the carrier density, we extracted the complex dielectric constants of the samples from their transmission spectra measured using THz-TDS at various applied bias voltage. The transmission ratio T(ω) of the Fourier transforms of these two waveforms is related to the complex conductivity as follows [19]:
where Z0 = 377 Ω is the impedance of free space, n = 3.4 is the refractive index of the non-excited MPSP sample in the THz range, and d is the thickness of the photo-excited layer. Figure 5(a) and 5(b) show the real part and the imaginary part of the calculated photoconductivity, respectively, of the MPSP sample at various biases under external laser irradiation of 0.5 W/cm2. Based on the Drude model, the carrier density N can be calculated using the following equation [19]:where is the effective electron mass, is the permittivity of a vacuum, is the electronic charge, and is the plasma frequency, which can be expressed approximately as follows:where and are the real and imaginary parts of the dielectric constant, respectively. is the THz frequency; the value of was chosen to be 1 THz, at which the THz power is at a maximum in the transmission spectroscopy scheme shown in Fig. 1(a). As noted above, and are the real and imaginary parts of the dielectric constant, respectively, and are related to the complex photoconductivity as follows: We can now describe the trends for variation of the photo-excited carrier density and the photoconductivity with respect to the applied bias voltage qualitatively. Without the applied bias voltage, the average carrier density in the MPSP structure was estimated to be N0V = 1.08 × 1018 /m3 under photoexcitation at 0.5 W/cm2. At the same pump power, the carrier density increased to N−1V = 1.79 × 1018 /m3 at a bias voltage of −1 V and decreased to N0.5V = 5.38 × 1017 /m3 at bias voltage of 0.5 V, as shown in Fig. 5(c).For further clarification of the modulation mechanism of this MPSP structure, we could also interpret the sample characteristics as being those of an electronic ‘p-n’ junction, as shown in Fig. 6. The mismatch of the alignment between the conduction band of Si (~−3.1 eV from vacuum) and the LUMO level of PEDOT:PSS (~−3.6 eV from vacuum) and the bended energy-band relationship at the interface causes the free charge carriers to move toward the Si/PEDOT:PSS interface [20]. This diffusion of the electrons and holes then formed a depletion layer and a built-in electric field. Under the positive bias, the depletion layer became thinner. Majority carrier diffusion dominates the drift processes [22]. Electrons were injected from the n-type (silicon) to the p-type semiconductor (PEDOT:PSS) and the holes moved from P to N in general, as shown in Fig. 6(a). The reason for the increased terahertz transmission is the reduction in the number of carriers in the silicon. Beyond a specific threshold bias, the electrons were able to form a current flow in the electronic circuit loop and it thus became difficult for them to accumulate in the silicon. Therefore, obvious transmission enhancement was observed when the voltage was increased from 0 to 0.5 V. In contrast, when a negative bias voltage was applied, the carrier diffusion was weakened while the depletion layer was broadened, as shown in Fig. 6(b). More electrons would then be injected into the Si as the negative bias increased. Simultaneously, the conductivity increased further, which caused increased attenuation of the THz waves as a result. Further increases in the negative bias led to the saturation of the carrier density under a negative bias of up to 1 V. MEH-PPV could broaden the depletion layer under the same pump excitation conditions shown in Fig. 2(b). Accumulation of the photo-generated charge carrier density occurs at the Si/MEH-PPV interface under photo-excitation because the carrier mobility of MEH-PPV is lower than that of Si. Therefore, more charge carriers gather in the silicon, which leads to a wider p-n junction. In contrast, the bidirectional modulation process did not work in the MEH-PPV/Si/PEDOT:PSS:DMSO structure because no p-n junction was formed within the MEH-PPV/Si structure, as shown in Fig. 2(c).
Fig. 6 Movement of electrons and holes in a p-n junction when (a) negative bias and (b) positive bias are applied.
4. Conclusions
A high-efficiency active bidirectional electrically-controlled THz device based on a DMSO-doped PEDOT:PSS structure with low-power photo-excitation was investigated. Under low-power optical excitation and applied bias voltages ranging from −0.6 V to 0.5 V, spectrally broadband modulation of the THz transmission over a range from −54% to 60% is obtained within the frequency range from 0.2 to 2.6 THz in the hybrid MEH-PPV/PEDOT:PSS:DMSO/Si/PEDOT:PSS:DMSO structure. From the combined carrier density characteristics of the device, it was found that the large amplitude modulation could be ascribed to the electrically-controlled carrier density in the silicon layer with the assistance of the PEDOT:PSS:DMSO layer and the p-n junction. The results reported here show considerable potential for application to the design of active broadband THz devices.
Funding
National Natural Science Foundation of China (Grant No. 61505125) and the National Instrumentation Program of China (Grant No. 2012YQ140005), Youth Innovative Research Team of Capital Normal University and High-level Teachers in Beijing Municipal Universities in the Period of 13th Five–year Plan.
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