We present a photoconductive terahertz detector operating at the 1 µm wavelength range at which high-power and compact Ytterbium-doped femtosecond fiber lasers are available. The detector utilizes an array of plasmonic nanoantennas to provide sub-picosecond transit time for the majority of photo-generated carriers to enable high-sensitivity terahertz detection without using a short-carrier-lifetime substrate. By using a high-mobility semiconductor substrate and preventing photocarrier recombination, the presented detector offers significantly higher sensitivity levels compared with previously demonstrated broadband photoconductive terahertz detectors operating at the 1 µm wavelength range. We demonstrate pulsed terahertz detection over a 4 THz bandwidth with a record-high signal-to-noise ratio of 95 dB at an average terahertz radiation power of 6.8 µW, when using an optical pump power of 30 mW.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Terahertz time-domain spectroscopy (THz-TDS) systems are extensively used in different scientific and industrial applications for non-invasive material characterization, chemical sensing, quality control, and medical imaging [1–9]. A common way to detect pulsed terahertz radiation in THz-TDS systems is to use a photoconductive detector [10–13]. A photoconductive terahertz detector consists of an antenna fabricated on a photo-absorbing semiconductor substrate. When a femtosecond optical pump beam is incident on the semiconductor, photo-generated electron-hole pairs drift to the antenna terminals due to the electric field induced by the incident terahertz radiation. As a result, an ultrafast photocurrent proportional to the terahertz electric field is induced at the photoconductive detector output terminals [14,15].
A photoconductive detector should have a sub-picosecond response time in order to detect terahertz radiation with a high signal-to-noise-ratio (SNR) [15,16]. To provide the required sub-picosecond response time, conventional photoconductive terahertz detectors utilize semiconductors with a high concentration of defects. The defects in the semiconductor lattice are introduced through semiconductor growth at low temperatures [11,12,17–20], incorporating rare-earth elements during the semiconductor growth [10,21,22], or high-energy ion implantation [23,24]. The introduced defects in the semiconductor lattice enable ultrafast operation by recombining the photocarriers that cannot drift to the antenna terminals in a sub-picosecond time-scale. However, the responsivity of photoconductive detectors based on short-carrier-lifetime semiconductors is limited since most of the photo-generated carriers get recombined in the substrate before reaching the antenna terminals. Moreover, the introduced defects reduce the carrier mobility and drift velocity significantly, which further reduces the number of the photocarriers that reach the antenna terminals in a sub-picosecond time-scale. In addition, short-carrier-lifetime substrate growth techniques are often not scalable to many photoconductive semiconductors, limiting the types of semiconductors and pump lasers that can be used for developing photoconductive terahertz detectors.
Alternatively, the ultrafast operation of a photoconductive detector can be achieved by reducing the transit time of the photocarriers to the output terminals. This approach eliminates the need for short-carrier-lifetime semiconductors and, consequently, increases the terahertz detection sensitivity by preventing the recombination of the photocarriers and enabling the use of high-mobility semiconductors. Such a short-carrier-lifetime-semiconductor-free photoconductive detector operating at an 800 nm pump wavelength was recently implemented by forming a plasmonic nanocavity around an undoped 170-nm-thick GaAs layer . Here, we present a high-performance photoconductive terahertz detector that operates at the 1 µm optical pump wavelength, at which high-power, low-cost, and compact Ytterbium (Yb)-doped femtosecond fiber lasers are commercially available. Despite their great promise, the sensitivity of previously-demonstrated photoconductive terahertz detectors operating at this wavelength range has been limited by the shortcomings of the short-carrier-lifetime semiconductors used for implementing these detectors, including low-temperature (LT) grown GaAs , LT-GaBiAs [27,28], LT-InGaAs [19,29,30], LT-In0.53Al0.28Ga0.20As , or LT-In0.53Ga0.47As/ In0.52Al0.48As superlattices [31,32]. Instead of relying on a short-carrier-lifetime substrate, we utilize In0.24Ga0.76As/AlAs epilayers grown on a semi-insulating (SI) GaAs substrate. An array of plasmonic nanoantennas are used to induce terahertz electric field inside the substrate when illuminated by terahertz radiation. The plasmonic nanoantennas are designed to concentrate the optical pump beam very close to the nanoantenna tips, where the induced terahertz electric field is maximized, to ensure that most of the photo-generated carriers drift to the nanoantenna terminals in a sub-picosecond time scale. By providing a high spatial overlap between the photo-generated carrier and terahertz electric field profiles, a broad terahertz detection bandwidth exceeding 4 THz and more than a 95 dB SNR level is achieved without using a short-carrier-lifetime substrate, demonstrating more than a 10 dB sensitivity enhancement compared with previously-reported broadband photoconductive detectors operating at the 1 µm wavelength range.
2. Design and fabrication
Figure 1 shows the schematic of the photoconductive terahertz detector, which consists of a two-dimensional array of plasmonic nanoantennas fabricated on undoped In0.24Ga0.76As/AlAs epilayers grown on an SI-GaAs substrate. The In0.24Ga0.76As layer serves as the photo-absorbing active region. The mole fraction of indium is chosen as 0.24 to provide a cut-off wavelength larger than 1.1 µm . Even though an InGaAs layer with a larger In fraction would provide higher optical absorption, the cut-off wavelength is chosen close to the operating wavelength to have high substrate resistivity and, thus, low Johnson-Nyquist noise current levels. Similarly, the AlAs layer serves as a high-resistivity buffer to suppress any Johnson-Nyquist noise current induced outside the photo-absorbing active region. The In0.24Ga0.76As layer is grown undoped to offer high carrier mobility and dark resistance for high-sensitivity terahertz detection. While a thicker In0.24Ga0.76As layer is expected to provide a larger number of photocarriers, the photocarriers generated deeper in the In0.24Ga0.76As layer have a longer average transit time to the nanoantennas. The subsequent tradeoff between the terahertz detection bandwidth and sensitivity is analyzed by investigating two different thicknesses for the In0.24Ga0.76As layer, 50 nm and 200 nm. Length of the nanoantennas is selected much smaller than terahertz wavelengths (4 µm) to provide a broad terahertz detection bandwidth . The tip-to-tip gap between the nanoantenna terminals is chosen to induce a strong terahertz electric field, while providing a high spatial overlap between the terahertz electric field and photo-generated carrier profiles [34,35]. Geometry of the nanoantennas, which are designed in the form of dipole gratings covered with a Si3N4 anti-reflection coating is chosen to enhance optical absorption inside the In0.24Ga0.76As layer at the 1 µm pump wavelength range . This enhancement is achieved through the excitation of surface plasmon waves, which increases the optical pump intensity and, thus, the photocarrier concentration at the interface between the In0.24Ga0.76As layer and nanoantennas [36–44]. An array of shadow metal stripes is deposited on the Si3N4 coating to prevent photocarrier generation at the gaps between the adjacent nanoantenna arrays. The shadow metals prevent photocurrent generation in the opposite direction to that of the nanoantennas.
A finite-difference time-domain (FDTD) method-based electromagnetic solver, Lumerical, is used to analyze the interaction between the optical pump beam and the designed plasmonic nanoantennas. Two nanoantenna geometries are optimized for the detectors fabricated on the substrates with 50 nm and 200 nm In0.24Ga0.76As layer thicknesses (Fig. 2 inset). The nanoantenna geometries are optimized by maximizing the optical absorption inside the In0.24Ga0.76As layer when a TM-polarized (x-polarized) optical beam is incident on the nanoantennas at a 1.04 µm wavelength. As illustrated in Fig. 2, the designed nanoantennas enable absorption of 15% and 25% of the incident optical pump photons at a 1.04 µm wavelength inside the 50-nm and 200-nm-thick In0.24Ga0.76As layers under the nanoantennas, respectively, showing the tradeoff between the number of the photo-generated carriers and the average photocarrier transport time to the nanoantennas. Since the optical absorption is obtained through the excitation of surface plasmon waves by an x-polarized optical pump beam, the optical absorption at the edge of the nanoantennas is not affected by the tip-to-tip gap size between the nanoantenna terminals (Fig. 3).
However, the tip-to-tip gap size between the nanoantenna terminals has a significant impact on the intensity of the induced terahertz electric field at the nanoantenna edges, where the photocarrier concentration is maximized. Figure 4(a) shows the estimated electric field enhancement factor for a y-polarized terahertz radiation incident on the nanoantennas as a function of the tip-to-tip gap size between the nanoantenna terminals. The results show a substantial increase in the induced terahertz electric field as the tip-to-tip gap size is reduced from 2 µm to 1 µm, and 0.5 µm. Tip-to-tip gap sizes smaller than 0.5 µm are not considered in our analysis because they dramatically reduce optical absorption between the nanoantenna terminals due to the diffraction limit. The electric field enhancement factor is significantly enhanced in close proximity to the nanoantenna edges, where the photocarrier concentration is also enhanced . The high spatial overlap between the induced terahertz electric field and the photocarrier concentration near the nanoantenna edges provides sub-picosecond transit times for the majority of the photo-generated carriers. Since the terahertz electric field is in parallel with the nanoantennas, the incident terahertz radiation can efficiently couple to the nanoantenna terminals if the effective radiation wavelength is much larger than the periodicity of the nanoantennas along the y-axis [34,35]. Therefore, the induced electric field is reduced as the terahertz frequency is increased, as shown in Fig. 4(b).
Detector prototypes with the designed nanoantennas are fabricated on 200/200 nm and 50/200 nm In0.24Ga0.76As/AlAs epilayers grown on two SI-GaAs substrates in a molecular beam epitaxy (MBE) chamber at 510 °C. Photoluminescence spectroscopy is performed after the growth to confirm that the cut-off wavelength of the grown substrates is above 1.1 µm [ Fig. 5(a)]. The roughness of the grown substrates is then analyzed using an atomic force microscope, to confirm that the substrate surface is smooth enough for the subsequent device fabrication steps [Fig. 5(b)]. Then, the nanoantennas are patterned using electron-beam lithography followed by a 3/77 nm Ti/Au deposition and lift-off. Next, the nanoantenna connection lines and output pads are patterned via optical lithography, followed by a 20/180 nm Ti/Au evaporation and lift-off. The Si3N4 antireflection coating is deposited via plasma-enhanced chemical vapor deposition. Then, shadow metal stripes are patterned by optical lithography, followed by a 10/90 nm Ti/Au deposition and lift-off. Finally, the Si3N4 coating that covers the contact pads is cleared using reactive ion etching to open vias for electric connection. In order to investigate the impact of the geometrical parameters, plasmonic nanoantenna arrays with tip-to-tip gap sizes of 0.5 µm, 1 µm, 2 µm and array sizes of 0.5×0.5 mm2 and 0.25×0.25 mm2 are fabricated. Figure 5(c) shows the microscopy image of a fabricated detector prototype and the scanning electron microscopy image of the fabricated nanoantennas with a tip-to-tip gap size of 0.5 µm.
The fabricated photoconductive detectors are characterized in a THz-TDS setup with an Yb-doped fiber laser that generates 88-fs-wide pulses with an 80 MHz repetition rate and a 1.04 µm center wavelength. Performance of the detectors is characterized in response to terahertz pulses generated by a bias-free plasmonic photoconductive source pumped by the Yb-doped fiber laser. Polarization of the optical pump beam is set to be perpendicular to the nanoantennas (along x-axis) and polarization of the terahertz radiation is adjusted to be in parallel with the nanoantennas (along y-axis). An optical pump 1/e2 diameter of 300 µm is used for all of the characterization measurements. To investigate the impact of the tip-to-tip gap size between the nanoantenna terminals, the time-domain electric field profiles of the detected radiation by the detector prototypes with tip-to-tip gap sizes of 0.5 µm, 1 µm, and 2 µm are measured [ Fig. 6(a)]. All of the detectors used for these measurements are fabricated on a 200-nm-thick In0.24Ga0.76As layer and have a 0.5×0.5 mm2 active area. All of these measurements are performed at a 10 mW average optical pump power and a 6.8 µW average terahertz power. For each measurement, 10 time-domain traces are captured and averaged to reduce the background noise. As predicted by our theoretical analysis, the highest responsivity is achieved when using a 0.5 µm gap size, which offers the highest induced terahertz electric field to drift the photo-generated carriers. Figure 6(d) shows the detected radiation spectra by the detectors with tip-to-tip gap sizes of 0.5 µm, 1 µm, and 2 µm. Johnson-Nyquist noise is the dominant noise mechanism in the presented photoconductive detectors, which is determined by the average device resistance under optical pump illumination [12,15,16,45]. Since the majority of the photo-generated carriers are tightly confined to the nanoantenna edges, the average device resistance is dominated by the concentration of these photocarriers. Since the photocarrier concentration near the nanoantenna edges is not considerably affected by the tip-to-tip gap size (Fig. 3), similar noise current levels are obtained for all three detectors with tip-to-tip gap sizes of 0.5 µm, 1 µm, and 2 µm. Overall, the detector with the tip-to-tip gap size of 0.5 µm offers the highest SNR level, as shown in Fig. 6(d).
The impact of the nanoantenna array size on the performance of the presented detector is also analyzed by comparing the performance of two detectors with 0.25×0.25 mm2 and 0.5×0.5 mm2 active areas. Both detectors are fabricated on a 200-nm-thick In0.24Ga0.76As layer and have a tip-to-tip gap size of 0.5 µm. All of these measurements are performed at a 10 mW average optical pump power and a 6.8 µW average terahertz power. The optical pump spot size on the nanoantenna arrays is adjusted to maximize responsivity of each detector. For each measurement, 10 time-domain traces are captured and averaged to reduce the background noise. As illustrated in Fig. 6(b), the detector with a larger array size offers higher responsivity because it can capture a larger fraction of the incident terahertz radiation. It should be noted that the spot size of the terahertz radiation incident of the nanoantenna arrays is diffraction limited. Therefore, the lower frequency components of the incident terahertz radiation have a larger spot size. As a result, the responsivity enhancement of the detector with a 0.5×0.5 mm2 active area is more pronounced at lower frequencies, as shown in Fig. 6(e). Under the same optical pump power, the same total number of photocarriers is generated in close proximity to the nanoantennas of both detectors, resulting in similar photocurrent and, thus, noise current levels in both devices. Overall, the detector with a 0.5×0.5 mm2 active area offers higher SNR level, as shown in Fig. 6(e). The impact of the In0.24Ga0.76As layer thickness on the performance of the presented detector is also analyzed by comparing the performance of two detectors with 50 nm and 200 nm In0.24Ga0.76As layer thicknesses. Both detectors have a 0.5×0.5 mm2 active area and a tip-to-tip gap size of 0.5 µm. All of these measurements are performed at a 10 mW average optical pump power and a 6.8 µW average terahertz power. For each measurement, 10 time-domain traces are captured and averaged to reduce the background noise. Figure 6(c) shows that the detector fabricated on the 200-nm-thick In0.24Ga0.76As layer provides higher responsivity because it absorbs a larger fraction of the optical pump photons, as illustrated in Fig. 2. Therefore, a larger number of photocarriers drift to the nanoantennas to contribute to terahertz detection. Since the photocarriers generated deeper in the In0.24Ga0.76As layer experience longer transit times to the nanoantennas, the observed responsivity enhancement of the detector fabricated on the 200-nm-thick In0.24Ga0.76As layer is more pronounced at lower terahertz frequencies [Fig. 6(f)]. Both detectors show similar noise current levels, since they have the same number of photocarriers generated in close proximity to the nanoantennas. Overall, the detector with a 200-nm-thick In0.24Ga0.76As layer offers higher SNR level, as shown in Fig. 6(f).
In addition to the device geometry, the optical power level used for pumping the photoconductive terahertz detectors has a significant impact on their responsivity and noise performance. Figure 7(a) shows the time-domain electric field profiles of the detected terahertz radiation for the device fabricated on a 200-nm-thick In0.24Ga0.76As layer with a 0.5 µm tip-to-tip gap size and a 0.5×0.5 mm2 active area, at different optical pump power ranging from 10 mW to 50 mW. This device geometry is chosen because of the high SNR values it provides, as shown in Fig. 6. All of these measurements are performed at a 6.8 µW average terahertz power. For each measurement, 10 time-domain traces are captured and averaged to reduce the background noise. As expected, the detected electric field increases as the optical pump power is increased above 10 mW, since more photocarriers drift to the nanoantennas to contribute to terahertz detection. However, the detected electric field drops at optical powers above 30 mW because the increased number of photocarriers accumulating at the edge of the nanoantennas screen the induced terahertz electric field.
As mentioned before, Johnson-Nyquist noise is the dominant noise mechanism in the presented photoconductive detectors. Therefore, the average noise current is expected to be inversely proportional to the square root of the average device resistance under optical pump illumination. Since the average device resistance, dominated by the tightly confined photocarriers near the nanoantenna edges, is inversely proportional to the optical pump power, the average noise current is expected to be proportional to the square root of the optical pump power. This dependence is confirmed experimentally, as shown in Fig. 7(c), where the dotted line shows the square root of the optical pump power. Overall, the highest SNR level is achieved at a 30 mW optical pump power.
The SNR level is increased further by increasing the number of the time-domain traces that are captured and averaged. The radiation spectra obtained with different number of traces are shown in Fig. 8. As expected, the noise level is reduced when the number of traces is increased. A terahertz detection bandwidth of more than 4 THz and a peak SNR level of 95 dB is provided by the designed detector when 1000 time-domain traces are captured and averaged, demonstrating a 10 dB SNR enhancement compared with previously reported broadband photoconductive detectors operating at the 1 µm optical pump wavelength range [19,26–32]. The observed absorption dips in the spectra are due to the water vapor present in the ambient, which match the water vapor absorption lines reported in HITRAN database .
In conclusion, we present a photoconductive terahertz detector that operates at the 1 µm optical wavelength range at which high-power, low-cost and compact Yb-doped femtosecond fiber lasers are commercially available. The detector is equipped with an array of plasmonic nanoantennas that enables ultrafast device operation by reducing the transit time of the photo-generated carriers. As a result, short-carrier-lifetime substrates that reduce the responsivity of conventional photoconductive terahertz detectors are eliminated and higher SNR levels are achieved. The geometric parameters of the nanoantennas are selected to provide a high spatial overlap between the profiles of the photo-generated carriers and the induced terahertz electric field in close proximity to the nanoantenna terminals. By optimizing the nanoantenna geometry, peak SNR levels exceeding 95 dB and detection bandwidths exceeding 4 THz are achieved when using a 30 mW optical pump power. The demonstrated SNR is 10 dB higher than previously-reported broadband photoconductive detectors operating at the 1 µm optical pump wavelength range. It should be noted that the 95 dB SNR is achieved at a 6.8 µW terahertz radiation power and much higher SNR levels can be achieved under higher terahertz radiation power levels. In addition, the terahertz detection sensitivity can be further increased by embedding the photo-absorbing active region in a plasmonic nanocavity to increase the quantum efficiency [25,47]. Therefore, the presented photoconductive terahertz detector architecture driven by Yb-doped femtosecond fiber lasers could be very effective for future THz-TDS system used in practical applications.
Office of Naval Research (N000141912052); Moore Foundation; U.S. Department of Energy (DE-SC0016925).
We thank Dr. Baolai Liang for useful discussions and technical assistance for the MBE growth and characterization of the semiconductor substrate.
The authors declare no conflict of interest.
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