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Abstract
We present a telecommunication-compatible photoconductive terahertz detector realized without using any short-carrier-lifetime photoconductor. By utilizing plasmonic contact electrodes on a thin layer of high-mobility photoconductor, the presented detector offers a short transit time for the majority of the photocarriers in the absence of a short-carrier-lifetime photoconductor. Consequently, high-sensitivity terahertz detection is achieved with a record-high signal-to-noise ratio of 122 dB over a 3.6 THz bandwidth under an optical probe power of 10 mW. To achieve such a high sensitivity, the device geometry is chosen to maintain a high resistance and low Johnson Nyquist noise. This design approach can be widely applied for terahertz detection using various semiconductors and optical wavelengths, without being limited by the availability of short-carrier-lifetime photoconductors.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photoconductive detectors are extensively used in terahertz time-domain spectroscopy (THz-TDS) systems for a wide range of applications including chemical identification, material characterization, non-destructive industrial evaluation, and medical imaging [1–15]. A photoconductive terahertz detector typically consists of a terahertz antenna fabricated on a short-carrier-lifetime photoconductive substrate. When a pulsed terahertz radiation and a femtosecond optical probe beam are simultaneously incident on the active area of the detector, the induced terahertz field across the antenna terminals drifts the photo-generated carriers. Consequently, an ultrashort photocurrent proportional to the terahertz field intensity is generated, which is routed to the output terminal of the detector.
Short-carrier-lifetime photoconductors are used to realize photoconductive terahertz detectors because they offer device response times less than a single terahertz oscillation cycle by introducing a large density of deep-level defects, which provides a photocarrier recombination time of a few hundreds of femtoseconds. They are usually implemented by molecular beam epitaxy (MBE) at a low temperature [16–18], rare-earth element or transition metal doping [19–23], heavy-ion irradiation [24–26], and multilayer heterostructures (MLHS) [27], etc. However, introducing defects also reduces optical responsivity, carrier mobility, and thermal conductivity of the photoconductors, which will degrade the signal-to-noise ratio (SNR) and reliability of the detector. In addition, many of these techniques are not widely accessible due to their low repeatability and limited availability in only a few fabrication facilities. Some of these restrictions are more limiting when developing telecommunication-compatible photoconductive terahertz detectors operating at optical wavelengths of ∼1550 nm, at which cost-effective, compact, and high-performance femtosecond fiber lasers are readily available. In order to absorb 1550 nm photons, the photoconductor bandgap energy should be less than ∼0.8 eV. Photoconductors with such narrow bandgap energy levels have high conductivity levels, which result in high Johnson-Nyquist noise current and low detector sensitivity levels, consequently.
To address some of the limitations of photoconductive terahertz detectors based on short-carrier-lifetime photoconductors, we recently proposed an alternative approach to provide an ultrafast detector response by reducing carrier transit time instead of carrier lifetime [28,29]. By utilizing plasmonic nanostructures and exciting surface plasmon waves, the intensity of the optical probe beam is significantly enhanced near the contact electrodes of the photoconductive detectors [30–43]. As a result, most of the photo-generated carriers are tightly confined within a few tens of nanometers distance from the contact electrodes, where the induced terahertz field drifts them to the contact electrodes in a sub-picosecond time-scale. In other words, the transport path length and transit time of the photo-generated carriers to the contact electrodes are significantly reduced to obtain an ultrafast terahertz detection operation. Consequently, this approach eliminates the need for a short-carrier-lifetime photoconductor, and photoconductors with a high carrier mobility can be used to realize high-responsivity terahertz detectors. This approach was first demonstrated with a large-area terahertz detector based on plasmonic nanoantenna arrays designed for operation at an 800 nm optical wavelength [28]. A combination of the plasmonic nanoantennas and an AlAs/AlGaAs distributed Bragg reflector (DBR) was used to form a nanocavity to confine photocarrier generation inside a thin GaAs layer under the plasmonic nanoantennas. Following this work, another study extended the operation wavelength of these large-area plasmonic photoconductive terahertz detectors to ∼1 µm by using a plasmonic nanoantenna array to confine photocarrier generation inside a thin In0.24Ga0.76As layer under the plasmonic nanoantennas [29]. Despite the high performance terahertz detection achieved in these studies, the utilized terahertz detector architecture based on a large-area plasmonic nanoantenna array does not provide an acceptable SNR when adopted for operation at optical wavelengths around 1550 nm. This is because of the considerably lower resistivity of photoconductive substrates at ∼1550 nm wavelengths, which results in a substantially lower detector resistance and, thus, higher Johnson-Nyquist noise current when adopting the detector architecture based on a large-area plasmonic nanoantenna array [44].
As an example, a terahertz detector prototype based on a 250 µm × 250 µm plasmonic nanoantenna array is designed for operation at 1550 nm wavelength (Fig. 1(a)). The detector is fabricated on an epitaxial semiconductor structure consisting of a 200-nm-thick undoped In0.53Ga0.47As layer and a 200-nm-thick AlAs layer grown on a semi-insulating GaAs (SI-GaAs) substrate by MBE. The In0.53Ga0.47As layer is the photo-absorbing active region of the terahertz detector. The indium concentration of 53% is chosen to obtain a high absorption coefficient at 1550 nm and a low substrate conductivity simultaneously. In addition, limiting the thickness of this layer prevents the generation of photocarriers deeper inside the semiconductor substrate, enabling a short transit time to the nanoantenna arrays for the majority of the photo-generated carriers. The AlAs layer serves as a high resistivity buffer to lower the substrate conductivity. The resistance of the detector, however, is not determined only by the substrate conductivity, but also by the geometry of the nanoantenna array. In contrary to the previous designs optimized for operation at shorter wavelengths [28,29], for which photoconductors with higher bandgap energies and much lower conductivity levels were used, the fabricated terahertz detector based on the nanoantenna-array optimized for operation at 1550 nm wavelength has a much lower device resistance (tens of Ω as opposed to tens of kΩ for detectors optimized for operation at shorter wavelengths at tens of mW optical power). As shown in Fig. 1(b), the SNR and bandwidth of a fabricated detector prototype with this nanoantenna array architecture is severely limited by the high Johnson-Nyquist noise current caused by the low resistance of the detector. After capturing and averaging 1000 time-domain traces, the detector offers only 60 dB SNR over a ∼2 THz bandwidth. To address this performance limitation, we present a new architecture for telecommunication-compatible photoconductive terahertz detectors without short-carrier-lifetime photoconductors that uses a broadband logarithmic spiral antenna with a small active area and, hence, a large resistance to offer more than a 122 dB SNR over a 3.6 THz detection bandwidth. In principle, this generic approach can be applied to design terahertz detectors operating at any given optical wavelength without being limited by the availability of short-carrier-lifetime photoconductors.
Fig. 1. (a) Schematic of the terahertz detector prototype based on a nanoantenna array optimized for operation at 1550 nm. The nanoantennas have a 4 μm arm length and a 2 μm tip-to-tip gap size. Each arm of the nanoantennas is connected to a 2-μm-wide metal stripe on one side to route the generated photocurrent to the output pad. 4-μm-wide metal stripes are placed on the Si3N4 anti-reflection coating to shadow the gaps between the nanoantenna arrays and prevent photocarrier generation. (b) The measured power spectra corresponding different number of captured and averaged time-domain traces. Inset shows the time-domain terahertz waveform.
2. Device design and fabrication
As illustrated in Fig. 2, the terahertz detector consists of a photoconductor with plasmonic contact electrodes integrated with a broadband logarithmic spiral antenna. The detector is fabricated on the same epitaxial semiconductor structure consisting of a 200-nm-thick undoped In0.53Ga0.47As layer and a 200-nm-thick AlAs layer grown on an SI-GaAs substrate. The plasmonic contact electrodes, which are in the form of one-dimensional metallic gratings separated by a small tip-to-tip gap, have a 5 µm × 10 µm area each. They are designed to enhance optical intensity inside the In0.53Ga0.47As layer near the tip of the gratings by excitation of surface plasmon waves. The In0.53Ga0.47As layer is etched everywhere except under the plasmonic electrodes to form a mesa structure, which prevents photocarrier generation outside the device active area and increases the resistance of the detector. Covered with a Si3N4 anti-reflection coating (ARC), the plasmonic electrodes are optimized to offer the largest absorption of a transverse-magnetic (TM) incident optical beam at 1550 nm in the In0.53Ga0.47As layer.
A numerical software based on finite difference time-domain method (Lumerical) is used to analyze the interaction of the optical beam with the plasmonic grating to optimize its geometry. The optimum geometry has a 460 nm periodicity, an 80 nm grating gap, a 3/77 nm Ti/Au height, and a 240 nm Si3N4 ARC thickness. Using this grating geometry, an ∼50% absorption is estimated for a TM-polarize optical beam at 1550 nm (Fig. 3(a)). Figure 3(b) shows the optical absorption profile in the x-z and y-z cross-sections of the substrate near the tips of the gratings for a tip-to-tip gap size of 1 µm. Since the optical coupling is enhanced by the excitation of surface plasmon waves, most of the absorption is confined to the areas near the metal/semiconductor interface, providing a sub-picosecond transit time for the majority of the photo-generated carriers in the In0.53Ga0.47As layer.
Fig. 3. (a) Estimated optical absorption spectrum inside the 200 nm In0.53Ga0.47As layer using Lumerical. (b) Estimated optical absorption profile in the x-z and y-z cross-sections of the substrate.
To investigate the effect of the tip-to-tip gap between the plasmonic contact electrode gratings on the detector performance, the impact of the gap size on the induced terahertz voltage at the input port of the logarithmic spiral antenna, the induced terahertz electric field and optical absorption inside the In0.53Ga0.47As layer is analyzed. Figures 4(a) and 4(b) show the estimated induced voltage and electric field distribution across the antenna terminals, by using two numerical software packages based on the finite element method (HFSS and COMSOL, respectively). Figure 4(c) shows the estimated optical absorption across the antenna terminals for a TM-polarized optical excitation at a 1550 nm wavelength with a 1/e2 diameter of 2 µm along the y-axis, by using Lumerical. The induced electric field and optical absorption are both enhanced at the tip of the antenna terminals, especially when the tip-to-tip gap between the plasmonic contact electrode gratings is reduced. Therefore, higher responsivity values are expected for detectors utilizing smaller tip-to-tip gap sizes with the drawback of a steeper responsivity roll-off at higher terahertz frequencies. In addition, detectors with smaller gap sizes are more susceptible to carrier screening and bleaching and hence, output saturation at lower optical powers due to the generation and separation of a larger number of electrons and holes within a smaller volume. Considering these performance tradeoffs, we choose detectors with 0.5 and 1 µm tip-to-tip gap sizes for further experimental characterization.
Fig. 4. (a) Estimated induced voltage across the antenna terminals for tip-to-tip gap sizes of 0.5, 1, and 2 µm as a function of frequency. (b) Induced electric field intensity at 0.6 THz and (c) optical absorption across the antenna gap at a 5 nm depth below the In0.53Ga0.47As surface.
In order to have a strong overlap between the optical generation profile and the induced terahertz field across the antenna terminals, the optimum optical beam shape should be a narrow ellipse that covers the entire gap between the two plasmonic contact electrodes and is tightly confined around the grating tips. Such a tight optical focus would result in a high optical intensity, leading to possible optical absorption bleaching as the photocarriers fill up the limited available states inside the In0.53Ga0.47As layer. To further investigate how this bleaching effect could impact our detector performance, we use Lumerical to estimate the optical intensity profile inside the In0.53Ga0.47As layer, for an elliptical optical beam spot with 2 µm (the minimum achievable spot size with our objective lens) and 10 µm 1/e2 diameters along the y-axis and x-axis, respectively, and calculate the bleaching threshold. The bleaching threshold is reached when the number of absorbed photons per unit volume is equal to the total density of states of In0.53Ga0.47As that can be filled by absorbed 1550 nm photons, which is ∼1017 cm-3. Figure 5 shows the estimated number of absorbed photons per unit volume at a 190 nm depth inside the In0.53Ga0.47As layer at optical power levels ranging from 0.01 mW to 10 mW. The areas where bleaching occurs are clearly shown in light yellow. Since the optical intensity drops as the depth increases, the bleached areas close to the bottom of the In0.53Ga0.47As layer provide a very good assessment of the extent of bleaching. No bleaching is observed at a 0.01 mW optical power for both 0.5-µm-gap and 1-µm-gap detectors. When the optical power is increased to ∼0.1 mW, small bleached areas are spotted at the tip-to-tip gap between the plasmonic contact electrodes, which experiences the highest optical intensity. As the optical power is increased from 0.1 mW to 1 mW, a substantial part of the active area is bleached for both detectors. Therefore, we expect the detector output to be significantly saturated at optical powers beyond 1 mW, which is supported by our experimental results.
Fig. 5. The estimated number of absorbed photons per unit volume at a 190 nm depth inside the In0.53Ga0.47As layer at optical powers levels ranging from 0.01 mW to 10 mW for the 0.5-µm-gap and 1-µm-gap detectors are shown in (a) and (b), respectively. The areas where bleaching occurs are shown in light yellow.
The optical and scanning electron microscopy images of a fabricated detector prototype are shown in Fig. 6. The fabrication process starts with electron-beam lithography patterning, electron-beam evaporation of 3/77 nm Ti/Au, and liftoff to realize the plasmonic contact electrodes. Next, the In0.53Ga0.47As layer is dry etched by using Cl2/Ar chemistry with the plasmonic electrodes masked by a maN-2405 electron-beam resist. The 240 nm-thick Si3N4 ARC is then deposited globally using plasma-enhanced chemical vapor deposition. Then, contact vias at the edge of both plasmonic electrodes are patterned by optical lithography and opened by CHF3/O2 reactive ion etching chemistry. Finally, the logarithmic spiral antenna and bonding pads are patterned by optical lithography followed by 50/550 nm Ti/Au deposition and liftoff. The fabricated terahertz detector prototypes are mounted on hyper-hemispherical silicon lenses to better focus the incident terahertz radiation onto the device active area.
Fig. 6. The optical microscopy image of a fabricated detector prototype and the scanning electron microscopy image of the plasmonic contact electrodes at the center of the detector.
3. Experimental results
Characterization of the terahertz detector prototypes is performed using a THz-TDS system with an optical parametric oscillator, which converts femtosecond pulses from a Ti-Sapphire laser (Coherent Mira HP) to femtosecond pulses at ∼1550 nm center wavelength with a 190 fs pulse-width and a 76 MHz repetition rate. The laser is split into two branches to pump a terahertz source and probe the terahertz detector prototypes, respectively. A motorized delay stage is used to control the time-delay between the pump and probe beams. An InAs-based bias-free nanoantenna array is used as the terahertz source [45]. The combination of a cylindrical lens and a 100x objective lens is used to focus the optical probe beam down to a narrow elliptical spot with 2 µm and 10 µm 1/e2 diameters to cover the entire gap between the two plasmonic contact electrodes. The output photocurrent of the characterized detectors is amplified using a transimpedance amplifier (FEMTO DLPCA). The amplified signal is acquired while varying the time-delays between the optical pump and probe beams to resolve the time-domain terahertz field. The detected spectrum is obtained by calculating the Fourier transform of the time-domain data.
Figure 7 shows the obtained time-domain terahertz waveforms and the corresponding power spectra of the 1-µm-gap detector under 1 mW, 3 mW and 5 mW optical probe power levels at an average terahertz power of 206 µW. 10 time-domain traces are captured and averaged to obtain these results. As predicted by our numerical analysis, a strong saturation in the detector output is observed when the optical probe power exceeds 3 mW, at which most of the available states in the 200-nm-thick In0.53Ga0.47As layer are occupied by photocarriers, limiting further increase in the induced photocurrent.
Fig. 7. (a) Time-domain terahertz waveforms and (b) the corresponding power spectra obtained by the 1-µm-gap detector under 1 mW, 3 mW and 5 mW optical probe power levels at a terahertz power of 206 µW. 10 time-domain traces are captured and averaged to obtain these results.
Figure 8 shows the obtained time-domain terahertz waveforms and the corresponding power spectra of the 1-µm-gap and 0.5-µm-gap detectors under a 5 mW optical probe power and 206 µW average terahertz power. 10 time-domain traces are captured and averaged to obtain these results. A comparison between the power spectra clearly shows a stronger roll-off in the frequency response of the 0.5-µm-gap detector, which is explained by the larger capacitive loading to the antenna when a 0.5 µm gap between the plasmonic contact electrodes is used. Since the detector resistance under illumination is much larger than the antenna radiation resistance, the RC time constant of these detectors is determined by the radiation resistance and parasitic capacitance. While the antennas of both detectors have the same radiation resistance, the estimated capacitance between the plasmonic contact electrodes for gap sizes of 0.5 µm and 1 µm is 1.53 fF and 1.27 fF, respectively. The slightly lower responsivity of the 0.5-µm-gap detector is attributed to a more severe carrier screening due to the stronger optical intensity enhancement and the separation of a larger number of electrons and holes in the active area of this detector.
Fig. 8. (a) Time-domain terahertz waveforms and (b) the corresponding power spectra obtained by the 0.5-µm-gap and 1-µm-gap detectors under a 5 mW optical probe power and 206 µW terahertz power. 10 time-domain traces are captured and averaged to obtain these results.
The peak terahertz field, noise power, and SNR obtained by the 1-µm-gap and 0.5-µm-gap detectors under different probe power levels are shown in Fig. 9. As expected by our theoretical analysis, the output signal from both detectors saturates when increasing the optical probe power due to bleaching and carrier screening. However, the output of the 0.5-µm-gap detector is saturated at a lower optical probe power due to a stronger carrier screening and bleaching in the device active area, leading to a slightly lower responsivity offered by this detector (Fig. 9(a)). Since the detector noise is dominated by the Johnson-Nyquist noise, the measured noise power of both detectors shows a linear dependence on the optical probe power (Fig. 9(b)). The 0.5-µm-gap detector has two times higher noise power levels compared to the 1-µm-gap detector since its resistance is approximately half of the 1-µm-gap detector. As predicted by the measured peak terahertz field and noise power values, the measured SNR saturates when increasing the optical probe power for both detectors and higher SNR levels are offered by the 1-µm-gap detector (Fig. 9(c)). The maximum SNR values of 102 dB and 97 dB are achieved at 5 mW and 10 mW optical probe powers for the 0.5-µm-gap and 1-µm-gap detectors, respectively. For these SNR measurements 10 time-domain traces are captured and averaged.
Fig. 9. The measured (a) peak terahertz field, (b) noise power, and (c) spectrum SNR as a function of the optical probe power for the 0.5-µm-gap and 1-µm-gap detectors. 10 time-domain traces are captured and averaged for the SNR measurements.
To further enhance the SNR, we increase the number of the time-domain traces that are captured and averaged to obtain the SNR. Figure 10 shows the resolved power spectra by the 1-µm-gap detector when changing the number of the time-domain traces from 10 to 1000, corresponding to 4 s to 400 s data acquisition time. As expected, increasing the number of traces reduces the noise power, leading to higher SNR and bandwidth. As clearly shown in Fig. 10, the noise level is reduced by 10 dB for every 10x increase in the number of traces, resulting in an SNR increase from 102 dB to 122 dB when increasing the data acquisition time from 4 s to 400 s. More than a 3.6 THz of detection bandwidth is achieved, as confirmed by the concurrence of the experimentally detected spectral dips and water vapor absorption lines taken from the HITRAN database [46]. It should be mentioned that the achieved bandwidth is limited by the pulse-width of the optical beam used to pump/probe the terahertz source/detector in the THz-TDS system and hence, broader detection bandwidths would be achieved when using shorter optical pulse-widths.
Fig. 10. The resolved power spectra by the 1-µm-gap detector when changing the number of the time-domain traces that are captured and averaged to obtain the spectra.
Table 1 shows a comparison between the performance of the demonstrated terahertz detector and state-of-the-art photoconductive detectors operating at ∼1550 nm wavelength based on short-carrier-lifetime substrates, including InGaAs/InAlAs multilayer heterostructures, transition-metal-doped InGaAs, and ErAs:InGaAs. The demonstrated terahertz detector offers much higher SNR levels compared with the state-of-the-art, while requiring lower optical probe powers. This enhancement in SNR is partly due to the use of plasmonic contact electrodes, which enhance carrier concentration near the antenna terminals, and partly due to the absence of a short-carrier-lifetime photoconductor, which boosts the responsivity by reducing carrier recombination and increasing carrier mobility. On the other hand, the demonstrated terahertz detector provides a narrower detection bandwidth compared with the state-of-the-art. This reduction in bandwidth is partly due to the larger laser pulse-width used for characterizing the detector. However, the long transit time of the photocarriers generated away from the plasmonic contact electrodes is another factor that results in this reduced bandwidth. Therefore, the use of an In0.53Ga0.47As layer with a smaller thickness could enhance the detection bandwidth further.
Table 1. Terahertz detection performance comparison
4. Conclusion
In conclusion, we introduce a telecommunication-compatible plasmonic photoconductive terahertz detector that utilizes short carrier transit time, instead of short carrier lifetime, to offer high responsivity levels. A photoconductor with plasmonic contact electrodes integrated with a broadband logarithmic spiral antenna is used to offer highly concentrated optical generation around the tips of the contact electrodes, where a strong overlap with the induced terahertz field is achieved, to provide an ultrashort transit time for most of the photo-generated carriers. Therefore, this terahertz detector architecture eliminates the need for a short-carrier-lifetime photoconductor. Hence, a long-carrier-lifetime and high mobility semiconductor can be used as the photoconductive substrate to offer high responsivity levels by reducing carrier recombination and increasing carrier mobility. We demonstrate terahertz detection with a record-high SNR of 122 dB and a 3.6 THz bandwidth when using an optical wavelength of 1550 nm, probe power of 10 mW, and pulse-width of 190 fs. The terahertz detection bandwidth could be increased by the use of shorter optical pulse-widths. In addition, reducing the In0.53Ga0.47As layer thickness could also enhance the terahertz detection bandwidth, while having a negligible impact on the SNR since the optical absorption is tightly confined near the plasmonic contact electrodes. Furthermore, reducing the In0.53Ga0.47As layer would reduce the Johnson-Nyquist noise as a result of an increase in the device resistance. Importantly, this work introduces a generic and reliable approach for designing photoconductive terahertz detectors that utilize various semiconductors and optical wavelengths, without being limited by the availability of short-carrier-lifetime photoconductors.
Funding
U.S. Department of Energy (DE-SC0016925); Office of Naval Research (N000141912052).
Disclosures
The authors declare no conflict of interest.
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