- Email: [email protected]
Plasma wave field effect transistor as a resonant detector for 1 terahertz imaging applications
Optics Communications 282 (2009) 3055–3058
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Plasma wave field effect transistor as a resonant detector for 1 terahertz imaging applications A. El Fatimy a,*, J.C. Delagnes a, A. Younus a, E. Nguema a, F. Teppe b, W. Knap b, E. Abraham a, P. Mounaix a,* a b
Université Bordeaux 1 and CNRS, CPMOH, UMR 5798, 351 Cours de la Libération, 33405 Talence cedex, France Université Montpellier 2 and CNRS, GES, UMR 5650, Place E. Bataillon, 34095 Montpellier, France
a r t i c l e
i n f o
Article history: Received 27 February 2009 Received in revised form 23 April 2009 Accepted 24 April 2009
Keywords: Plasma wave HEMT detectors Imaging systems Far infrared or terahertz Ultrafast lasers
a b s t r a c t The possibility of using plasma wave field effect transistor in a time domain terahertz (THz) spectroscopy setup is presented. We demonstrate that High Electron Mobility Transistors (HEMTs) is an efficient device for detection of pulsed terahertz electric fields generated with a femtosecond laser oscillator. The response was observed in the frequency range of about 1 THz, far above the cutoff frequency of the transistors at room temperature. We show that the physical mechanism of the detection is related to the plasma waves excited in the transistor channel and that significant improvement of the active device can be achieved by increasing the drain current. The two-dimensional terahertz imaging applications clearly demonstrate that plasma wave nanometer HEMT should be employed as efficient future detectors in a matrix configuration. Ó 2009 Elsevier B.V. All rights reserved.
xN ¼ x0 ð1 þ 2NÞ
1. Introduction Since the first demonstrations of all-optoelectronic THz imaging with a pulsed time-domain setup in 1995 [1], various all-optoelectronic systems have been developed, based on either pulsed [2], continuous-wave (cw) [3], or quasi-cw [4,5] near-infrared laser sources. Recently, one have demonstrated real-time imaging capabilities and a number of researchers have begun to move away from THz optoelectronics and to focus on the development of faster cw imaging systems based on purely passive detection [3,4]. In this work we present an alternative approach which combines a broadband terahertz source and a resonant detection by HEMTs. Nonlinearities related to plasma wave excitations in twodimensional electron gas in a nanometer-size field effect transistor (FET) have a simple linear dispersion law,
x¼sk
ð1Þ
where k is the wave vector and x is the angular frequency. Plasma wave velocity s is defined as:
s¼
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi eðV gs V th Þ m
ð2Þ
where Vgs is gate-to-source voltage, Vth is threshold voltage, e is electron charge and m* is electron effective mass. For these waves a HEMT channel of a given length Lg acts as a resonant ‘‘cavity” with eigenfrequencies xN given by: * Corresponding authors. E-mail addresses: [email protected], [email protected] (A. El Fatimy), [email protected] (P. Mounaix). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.04.054
ð3Þ
where N = 1,2,3,. . .. The fundamental plasma frequency x0 can be tuned by changing the gate voltage Vgs.
x0 ¼
ps 2Lg
ð4Þ
For a submicron gate length, x0 can reach the terahertz range. Dyakonov and Shur [6] showed that nonlinear properties of such a cavity can be exploited for selective tuneable terahertz detection. They demonstrated that a nanometer size HEMT subjected to a terahertz radiation with a frequency x develops a constant source-todrain voltage:
DU /
x20 ðx x0 Þ2 þ 1=s2
ð5Þ
where s is the momentum relaxation time. In the absence of external current the width of the resonance curve is determined mainly by the inverse of the momentum relaxation time 1/s. When x0s « 1 the plasma oscillations are overdamped and the HEMT response is a smooth function of frequency as well as of the gate voltage non-resonant broadband detection. In the regime such that x0s » 1, the field effect transistor operates as a resonant and tuneable detector. The fundamental frequency of plasma wave oscillations can be tuned by changing the gate voltage. Tuneable resonant detection of THz radiation by two-dimensional plasma waves was recently demonstrated using HEMTs at cryogenic [7,8] and room temperatures [9]. Room temperature imaging with HEMTs was demonstrated only for sub-THz frequencies with an incoherent GaAs HEMTs
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detector [10]. In most experiments the detected signal decreases dramatically with increasing of radiation frequency either because of a strong reduction in coupling efficiency and/or because of water vapour absorption. In this letter, we present images taken in a transmission mode up to 1 THz using a coherent GaAs HEMTs detector at room temperature with a broadband THz source. The regime of operation of the HEMTs with a constant drain bias applied, allowed us to increase device sensitivity by more than three orders of magnitude. More details about the transistor design and performance can be found in Ref. [11]. We demonstrate a fast THz pulse imaging system without optical components usually adopted in pulsed imaging systems. In this work, the system is relatively compact since it does not require any sampling laser pulse for time resolved detection of the THz field. Moreover, HEMT detectors can be integrated in a matrix form for future approach in real-time imaging. 2. Technology A source of pulsed THz radiation based on the p-InAs surface radiation emitter excited by a Ti:Sapphire laser oscillator with 800 nm central wavelength, 200 mW mean power, 60 fs pulse duration and repetition rate of 76 MHz was used. The physical mechanism of THz generation relies on the photo-Dember effect [12]. Fig. 1 shows the frequency bandwidth obtained by a Fourier Transform of a measured time-domain THz pulse. A photoconductive dipole antenna fabricated on a low-temperature-grown GaAs was used for detection. The range of available bandwidth is between 0.2 and 4 THz [13]. The imaging experiments use two high-density polyethylene lenses with 50 mm focal lengths. The emitted THz radiation was collimated and focused onto a GaAs HEMT detector with 250 nm gate length. The experimental setup used for the HEMT imaging is shown in the insert of the Fig. 1. The sample was placed on an adjustable three-axes sample holder. The HEMT was covered by polyethylene filters to prevent any parasitic visible and infrared background illumination. No special guiding/collecting antennas were used and the THz radiation was coupled to the device through contact pads. Radiation intensity was modulated with a mechanical chopper at 1.5 kHz. The device was tuned/driven either by a gate-to-source voltage or a drain-to-source current. The source terminal was always grounded and the induced dc drain voltage DU (see Eq. (5)), which appeared as a response to the THz radiation, was measured by a standard lock-in technique.
Fig. 2. Drain response as a function of gate voltage for different values of current from 5 to 40 mA. Vertical arrows: position of resonance maxima as a function of gate voltage. The inset represents the drain source current Ids as a function of gateto-source voltage Vgs of a HEMT with 250 nm gate length.
Results of DU response of a GaAs/AlGaAs transistor exposed to broadband THz source as a function of gate voltage measured at various drain sources current Ids (from 5 to 40 mA) are shown in Fig. 2. Threshold voltage extracted from the transfer current voltage characteristics at low drain bias was Vth = 0.41 V (inset of the Fig. 2). One can see that detection efficiency was strongly enhanced from 40 lV (Ids = 0, data not shown here) to 8.3 mV (Ids = 40 mA) by increasing the drain current, especially when driving the transistor into the saturation region. Below 20 mA, only non-resonant detection was observed as a broadband peak around 0.41 V. When drain current is increased above 20 mA, an additional peak appears as a shoulder on the non-resonant detection. We attribute this behavior to the resonant detection of THz radiation by plasma waves. This result showed that the HEMT works as a ‘‘tunable” detector since the device sensitivity strongly increases at higher drain current (between 20 and 40 mA). Even if the problem of broadening of plasma resonances considered in the above part is still unclear [14], plasma oscillations in nanotransistors are very promising for THz detection applications. The resonant plasma wave effect can be active in THz range providing an efficient mechanism to improve detection efficiency especially at room temperature. We use this approach of resonant detection by associating a surface field emission source gated with a Ti:Sapphire femtosecond laser and a tunable HEMT detector. We have confirmed that the response of the detector is sufficient to provide THz images with high enough signal-to-noise ratio. We measured typically more than three orders of magnitude dynamic range in the detected voltage with and without the THz beam. Moreover by changing the drain current, it is possible to strongly improve the responsitivity of the detector. For example at Vgs = 0.2 V and Ids < 5 mA, we detected less than 100 lV (noise less than 13 nV/Hz½) whereas for Ids = 40 mA, the response is about 8 mV (noise less than 31 nV/ Hz½, Fig. 2). The operation regime of the HEMT with a constant dc drain bias makes it possible to increase the device response by more than three orders of magnitude and keeps it the noise level in nano-Volt range. 3. Two-dimensional terahertz imaging
Fig. 1. Spectrum of THz generation by a p-InAs surface radiation emitter activated by a femtosecond laser. Insert: experimental setup for plasma wave imaging with a pulsed THz source.
Using the HEMT, we performed raster-scan imaging in transmission mode as shown in insert of Fig. 1. The object under test is a metallic paper clip (850 lm width) inserted in an envelope and mounted onto a mechanical stage. The object is raster-scanned
A. El Fatimy et al. / Optics Communications 282 (2009) 3055–3058
in two dimensions through a 0.5-mm iris diaphragm located in a focus of a polyethylene lens and is 2 mm in front of the test object. Intensity of each pixel in the final image is current intensity measured by lock-in detection with an integration time of 30 ms without further averaging. The integration time can be less than microsecond [15] with a stable and fast XYZ mechanical stage. An image consisting of 41 41 pixels, 0.3 mm pixel size was obtained in about 10 min. Fig. 3 shows different THz images of the metallic paper clip inside the envelope. The image sizes are (15 11) mm2. Fig. 3-right was obtained with the HEMT detector. The presence of the clip is clearly revealed. We selected Vgs = 0.2 V and Ids = 40 mA, which corresponds to a plasma frequency of 1 THz. In this polarization state, the HEMT response is mainly due to plasma wave oscillation effect rather than non-resonant broadband detector. In that case, the obtained image contrast could be supposed connected to 1 THz frequency response of the object under investigation. To support our suggestion of the possibility to use HEMT as a tunable detector, we compare the present detector scheme with a conventional raster-scanning THz-TDS imaging system in regards to imaging quality and pixel rate. Fig. 3-left is a THz-TDS image obtained at 1 THz by a raster-scanning system. This frequency value corresponds to the calculated plasma resonance from Eq. (2) and (4) for Vgs = 0.2 V and Ids = 40 mA and also to the maximum power generated by the surface field emitter system (see Fig. 1). This means that the image in Fig. 3-right (HEMT for a plasma frequency at 1 THz) can be compared to the image of Fig. 3-left (THz-TDS at 1 THz). No significant differences are revealed with good image quality and high signal-to-noise ratio. We can even notice that the HEMT image exhibits a better signal-to-noise ratio in the region outside of the metallic clip. In this region, the image intensity is more uniform and less noisy compared to the THzTDS image. We also qualitatively studied images constructed at lower frequency (for TDS imaging and under lower plasma wave resonance conditions for HEMT). We observed the same behavior down to 600 GHz. Below this value; resolution was insufficient to clearly conclude on the imaging capabilities. On the acquisition time consuming with the THz-TDS system, if we measure one temporal waveform in 1 s per pixel, it requires 600 s to acquire an image with 20 by 30 pixels. The resulting rate is only 1 pixel/s. For the HEMT imaging system with the same sample, the resulting pixel rate is as high as 100 pixel/s, which is 100 times higher than that obtained in the THz-TDS system. In this example, the speed is only limited by the scanning speed of the XY translation stage. We conclude that the HEMT system is a very fast THz imaging device with good imaging quality, similar to that of a THz-TDS system. Another experiment was made with using a hologram stripe of a 10 Euros banknote as a sample (Fig. 5a) Using a THz-TDS spectrometer, we performed spectroscopic analysis of the different dielectric responses of the complex note structure. We found that, both in time and frequency domains, it is possible to discriminate between the region with the ‘‘10 surrounded by stars” and the region where the € symbol changes into 10 depending on the orientation of the
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Fig. 4. Temporal THz data through the hologram stripe of a 10 euros banknote obtained with a standard THz-TDS spectrometer.
banknote. The analysis of Fig. 4 makes it possible to observe small differences in the temporal and the FFT spectrum through the hologram stripe. Particularly, in the region of the 10 surrounded by stars, we observed that around 0.8 THz there is a decrease in the transmission attributed to the small holes in this part of the hologram stripe. Since the measured transmitted THz waves have not the same maxima according to their spatial position, we can use the HEMT detector to perform a fast efficient THz imaging of the banknote. The imaging portion of the banknote is similar to precedent one: the euro symbol presents in the hologram stripe (see photograph in the Fig. 5a and the imaging region indicated by the rectangular black box). Fig. 5b presents a THz images obtained with the THzTDS setup using a photoswitch as detector (left) and with HEMT as detector (right). The image size is (11.25 6.5 mm2) consisting in 45 26 pixels with a spatial step of 250 lm. With the THz-TDS setup, we choose Imax Imin parameters to elaborate the contrast (Imax Imin of the transmitted field). We can observe that the euro symbol € is correctly resolved with THz-TDS obtained from Imax Imin (Fig. 5b, left). Using the HEMT detector (Fig. 5b, right), THz image is presented with Vgs = 0.2 V and Ids = 40 mA (plasma frequency 1 THz). The images with HEMT detector present an excellent signal-tonoise ratio with a good contrast than with THz-TDs system. By using HEMT detector, the presences of the small holes in the 10 Euros banknote are clearly revealed. To show a better representation of the euro symbol obtained with the HEMT, we finally represented the image obtained with Vgs =0.2 V and Ids = 40 mA using a false 3D color map (Fig. 5c). The 3D representation makes it possible to clearly distinguish the euro symbol surrounded by the metallic hologram stripe.
Fig. 3. THz imaging of a metallic paper clip inside an envelope. (Right) HEMT with Vgs = 0.2 V, Ids = 40 mA (plasma frequency 1 THz); (left) THz-TDS imaging at 1 THz.
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of miniature THz detector based on nanotransistors. HEMT-based detectors have been shown to be efficient for THz imaging applications. Raster-scanned two dimensional images exhibit spatial resolution and signal-to-noise ratio comparable to TDS-THz imaging. The main advantage of these new detectors is that they can be easily integrated into arrays and enable development of future realtime THz camera. Acknowledgments This work is supported by the ‘‘Conseil Régional d’Aquitaine”, in collaboration with the Labri UMR 5800, and Innovative Imaging Solutions corporation: http://www.i2s-corp.com I2S. This work is also partially supported by JSPS International Fellowship Program for Research in Japan. The authors from Montpellier University acknowledge the support from CNRS GDR/GDR-E project Semiconductor sources and detectors of THz frequencies, Région of Languedoc-Roussillon through the ‘‘Terahertz Platform” project as well as the European Union MTKD-CT-2005-029671 grant. References Fig. 5. THz Imaging of a small part of 10 Euros banknote. (a) Top-visible imaging of a 10 Euros banknote (rectangular black box in the photograph), (b) left – by TDS imaging using Imax Imin; right – THz image with HEMT detector (Vgs = 0.3 V, Ids = 30 mA; plasma frequency 1 THz), (c) 3D false color representation of the Euro symbol using the HEMT detector with Vgs = 0.2 V, Ids = 40 mA (plasma frequency calculated is 1 THz).
The next step further is realization of real-time imaging THz cameras. In this context, field effect transistors can be treated as the most promising option. In particular, silicon- based devices displaying high sensitivity of 200 V/W, NEP values close to 0.1 nW/ p Hz at room temperature [16] and the bandwidth higher than 500 GHz. 4. Conclusion In summary, experimental results of THz imaging based on voltage-tunable THz detection using GaAs nanometer transistors are presented. Inserted in a time-domain THz spectrometer, we demonstrate that HEMT is an efficient device for detection of pulsed terahertz electric field. Experimental results prove the possibility
[1] B.B. Hu, M.C. Nuss, Opt. Lett. 20 (1995) 1716. [2] Q. Wu, T.D. Hewitt, X.-C. Zhang, Appl. Phys. Lett. 69 (1996) 1026. [3] T. Kleine-Ostmann, P. Knobloch, M. Koch, S. Hoffmann, M. Breede, M. Hofmann, G. Hein, K. Pierz, M. Sperling, K. Donhuijsen, Electron. Lett. 37 (2001) 1461. [4] K. Kawase, Y. Ogawa, H. Minamide, H. Ito, Semicond. Sci. Technol. 20 (2005) S258. [5] T. Yasui, K.I. Sawanaka, A. Ihara, E. Abraham, M. Hashimoto, T. Araki, Opt. Express 16 (2008) 1208. [6] M. Dyakonov, M. Shur, Phys. Rev. Lett. 71 (1993) 2465. [7] A. El Fatimy, F. Teppe, N. Dyakonova, W. Knap, D. Seliuta, G. Valusis, A. Shchepetov, Y. Roelens, S. Bollaert, A. Cappy, S. Rumyantsev, Appl. Phys. Lett. 89 (2006) 131926. [8] A. El Fatimy, S. Boubanga Tombet, F. Teppe, W. Knap, D.B. Veksler, S. Rumyantsev, M.S. Shur, N. Pala, R. Gaska, Q. Fareed, X. Hu, D. Seliuta, G. Valusis, C. Gaquiere, D. Theron, A. Cappy, Electron. Lett. 42 (23) (2006) 1342. [9] F. Teppe, M. Orlov, A. El Fatimy, A. Tiberj, W. Knap, J. Torres, V. Gavrilenko, A. Shchepetov, Y. Roelens, S. Bollaert, Appl. Phys. Lett. 89 (2006) 222109. [10] A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, H.G. Roskos, Electron. Lett. 44 (2008) 408. [11] A. El Fatimy, PhD. Thesis, Terahertz emission and detection by plasma waves, Hall, 00182726, June 2007. [12] Ping Gu et al., J. Appl. Phys. 91 (2002) 9. [13] E. Nguema, V. Vigneras, J.L. Miane, P. Mounaix, Eur. Polym. J. 44 (2008) 124. [14] V.V. Popov, O.V. Polischuk, W. Knap, A. El Fatimy, Appl. Phys. Lett. 93 (2008) 263503. [15] M. Ortolani et al., J. Eur. Opt. Soc. – Rapid Publications 4 (2009) 09006. [16] R. Tauk el al, Appl. Phys. Lett. 89 (2006) 253511.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Plasma wave field effect transistor as a resonant detector for 1 terahertz imaging applications A. El Fatimy a,*, J.C. Delagnes a, A. Younus a, E. Nguema a, F. Teppe b, W. Knap b, E. Abraham a, P. Mounaix a,* a b
Université Bordeaux 1 and CNRS, CPMOH, UMR 5798, 351 Cours de la Libération, 33405 Talence cedex, France Université Montpellier 2 and CNRS, GES, UMR 5650, Place E. Bataillon, 34095 Montpellier, France
a r t i c l e
i n f o
Article history: Received 27 February 2009 Received in revised form 23 April 2009 Accepted 24 April 2009
Keywords: Plasma wave HEMT detectors Imaging systems Far infrared or terahertz Ultrafast lasers
a b s t r a c t The possibility of using plasma wave field effect transistor in a time domain terahertz (THz) spectroscopy setup is presented. We demonstrate that High Electron Mobility Transistors (HEMTs) is an efficient device for detection of pulsed terahertz electric fields generated with a femtosecond laser oscillator. The response was observed in the frequency range of about 1 THz, far above the cutoff frequency of the transistors at room temperature. We show that the physical mechanism of the detection is related to the plasma waves excited in the transistor channel and that significant improvement of the active device can be achieved by increasing the drain current. The two-dimensional terahertz imaging applications clearly demonstrate that plasma wave nanometer HEMT should be employed as efficient future detectors in a matrix configuration. Ó 2009 Elsevier B.V. All rights reserved.
xN ¼ x0 ð1 þ 2NÞ
1. Introduction Since the first demonstrations of all-optoelectronic THz imaging with a pulsed time-domain setup in 1995 [1], various all-optoelectronic systems have been developed, based on either pulsed [2], continuous-wave (cw) [3], or quasi-cw [4,5] near-infrared laser sources. Recently, one have demonstrated real-time imaging capabilities and a number of researchers have begun to move away from THz optoelectronics and to focus on the development of faster cw imaging systems based on purely passive detection [3,4]. In this work we present an alternative approach which combines a broadband terahertz source and a resonant detection by HEMTs. Nonlinearities related to plasma wave excitations in twodimensional electron gas in a nanometer-size field effect transistor (FET) have a simple linear dispersion law,
x¼sk
ð1Þ
where k is the wave vector and x is the angular frequency. Plasma wave velocity s is defined as:
s¼
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi eðV gs V th Þ m
ð2Þ
where Vgs is gate-to-source voltage, Vth is threshold voltage, e is electron charge and m* is electron effective mass. For these waves a HEMT channel of a given length Lg acts as a resonant ‘‘cavity” with eigenfrequencies xN given by: * Corresponding authors. E-mail addresses: [email protected], [email protected] (A. El Fatimy), [email protected] (P. Mounaix). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.04.054
ð3Þ
where N = 1,2,3,. . .. The fundamental plasma frequency x0 can be tuned by changing the gate voltage Vgs.
x0 ¼
ps 2Lg
ð4Þ
For a submicron gate length, x0 can reach the terahertz range. Dyakonov and Shur [6] showed that nonlinear properties of such a cavity can be exploited for selective tuneable terahertz detection. They demonstrated that a nanometer size HEMT subjected to a terahertz radiation with a frequency x develops a constant source-todrain voltage:
DU /
x20 ðx x0 Þ2 þ 1=s2
ð5Þ
where s is the momentum relaxation time. In the absence of external current the width of the resonance curve is determined mainly by the inverse of the momentum relaxation time 1/s. When x0s « 1 the plasma oscillations are overdamped and the HEMT response is a smooth function of frequency as well as of the gate voltage non-resonant broadband detection. In the regime such that x0s » 1, the field effect transistor operates as a resonant and tuneable detector. The fundamental frequency of plasma wave oscillations can be tuned by changing the gate voltage. Tuneable resonant detection of THz radiation by two-dimensional plasma waves was recently demonstrated using HEMTs at cryogenic [7,8] and room temperatures [9]. Room temperature imaging with HEMTs was demonstrated only for sub-THz frequencies with an incoherent GaAs HEMTs
3056
A. El Fatimy et al. / Optics Communications 282 (2009) 3055–3058
detector [10]. In most experiments the detected signal decreases dramatically with increasing of radiation frequency either because of a strong reduction in coupling efficiency and/or because of water vapour absorption. In this letter, we present images taken in a transmission mode up to 1 THz using a coherent GaAs HEMTs detector at room temperature with a broadband THz source. The regime of operation of the HEMTs with a constant drain bias applied, allowed us to increase device sensitivity by more than three orders of magnitude. More details about the transistor design and performance can be found in Ref. [11]. We demonstrate a fast THz pulse imaging system without optical components usually adopted in pulsed imaging systems. In this work, the system is relatively compact since it does not require any sampling laser pulse for time resolved detection of the THz field. Moreover, HEMT detectors can be integrated in a matrix form for future approach in real-time imaging. 2. Technology A source of pulsed THz radiation based on the p-InAs surface radiation emitter excited by a Ti:Sapphire laser oscillator with 800 nm central wavelength, 200 mW mean power, 60 fs pulse duration and repetition rate of 76 MHz was used. The physical mechanism of THz generation relies on the photo-Dember effect [12]. Fig. 1 shows the frequency bandwidth obtained by a Fourier Transform of a measured time-domain THz pulse. A photoconductive dipole antenna fabricated on a low-temperature-grown GaAs was used for detection. The range of available bandwidth is between 0.2 and 4 THz [13]. The imaging experiments use two high-density polyethylene lenses with 50 mm focal lengths. The emitted THz radiation was collimated and focused onto a GaAs HEMT detector with 250 nm gate length. The experimental setup used for the HEMT imaging is shown in the insert of the Fig. 1. The sample was placed on an adjustable three-axes sample holder. The HEMT was covered by polyethylene filters to prevent any parasitic visible and infrared background illumination. No special guiding/collecting antennas were used and the THz radiation was coupled to the device through contact pads. Radiation intensity was modulated with a mechanical chopper at 1.5 kHz. The device was tuned/driven either by a gate-to-source voltage or a drain-to-source current. The source terminal was always grounded and the induced dc drain voltage DU (see Eq. (5)), which appeared as a response to the THz radiation, was measured by a standard lock-in technique.
Fig. 2. Drain response as a function of gate voltage for different values of current from 5 to 40 mA. Vertical arrows: position of resonance maxima as a function of gate voltage. The inset represents the drain source current Ids as a function of gateto-source voltage Vgs of a HEMT with 250 nm gate length.
Results of DU response of a GaAs/AlGaAs transistor exposed to broadband THz source as a function of gate voltage measured at various drain sources current Ids (from 5 to 40 mA) are shown in Fig. 2. Threshold voltage extracted from the transfer current voltage characteristics at low drain bias was Vth = 0.41 V (inset of the Fig. 2). One can see that detection efficiency was strongly enhanced from 40 lV (Ids = 0, data not shown here) to 8.3 mV (Ids = 40 mA) by increasing the drain current, especially when driving the transistor into the saturation region. Below 20 mA, only non-resonant detection was observed as a broadband peak around 0.41 V. When drain current is increased above 20 mA, an additional peak appears as a shoulder on the non-resonant detection. We attribute this behavior to the resonant detection of THz radiation by plasma waves. This result showed that the HEMT works as a ‘‘tunable” detector since the device sensitivity strongly increases at higher drain current (between 20 and 40 mA). Even if the problem of broadening of plasma resonances considered in the above part is still unclear [14], plasma oscillations in nanotransistors are very promising for THz detection applications. The resonant plasma wave effect can be active in THz range providing an efficient mechanism to improve detection efficiency especially at room temperature. We use this approach of resonant detection by associating a surface field emission source gated with a Ti:Sapphire femtosecond laser and a tunable HEMT detector. We have confirmed that the response of the detector is sufficient to provide THz images with high enough signal-to-noise ratio. We measured typically more than three orders of magnitude dynamic range in the detected voltage with and without the THz beam. Moreover by changing the drain current, it is possible to strongly improve the responsitivity of the detector. For example at Vgs = 0.2 V and Ids < 5 mA, we detected less than 100 lV (noise less than 13 nV/Hz½) whereas for Ids = 40 mA, the response is about 8 mV (noise less than 31 nV/ Hz½, Fig. 2). The operation regime of the HEMT with a constant dc drain bias makes it possible to increase the device response by more than three orders of magnitude and keeps it the noise level in nano-Volt range. 3. Two-dimensional terahertz imaging
Fig. 1. Spectrum of THz generation by a p-InAs surface radiation emitter activated by a femtosecond laser. Insert: experimental setup for plasma wave imaging with a pulsed THz source.
Using the HEMT, we performed raster-scan imaging in transmission mode as shown in insert of Fig. 1. The object under test is a metallic paper clip (850 lm width) inserted in an envelope and mounted onto a mechanical stage. The object is raster-scanned
A. El Fatimy et al. / Optics Communications 282 (2009) 3055–3058
in two dimensions through a 0.5-mm iris diaphragm located in a focus of a polyethylene lens and is 2 mm in front of the test object. Intensity of each pixel in the final image is current intensity measured by lock-in detection with an integration time of 30 ms without further averaging. The integration time can be less than microsecond [15] with a stable and fast XYZ mechanical stage. An image consisting of 41 41 pixels, 0.3 mm pixel size was obtained in about 10 min. Fig. 3 shows different THz images of the metallic paper clip inside the envelope. The image sizes are (15 11) mm2. Fig. 3-right was obtained with the HEMT detector. The presence of the clip is clearly revealed. We selected Vgs = 0.2 V and Ids = 40 mA, which corresponds to a plasma frequency of 1 THz. In this polarization state, the HEMT response is mainly due to plasma wave oscillation effect rather than non-resonant broadband detector. In that case, the obtained image contrast could be supposed connected to 1 THz frequency response of the object under investigation. To support our suggestion of the possibility to use HEMT as a tunable detector, we compare the present detector scheme with a conventional raster-scanning THz-TDS imaging system in regards to imaging quality and pixel rate. Fig. 3-left is a THz-TDS image obtained at 1 THz by a raster-scanning system. This frequency value corresponds to the calculated plasma resonance from Eq. (2) and (4) for Vgs = 0.2 V and Ids = 40 mA and also to the maximum power generated by the surface field emitter system (see Fig. 1). This means that the image in Fig. 3-right (HEMT for a plasma frequency at 1 THz) can be compared to the image of Fig. 3-left (THz-TDS at 1 THz). No significant differences are revealed with good image quality and high signal-to-noise ratio. We can even notice that the HEMT image exhibits a better signal-to-noise ratio in the region outside of the metallic clip. In this region, the image intensity is more uniform and less noisy compared to the THzTDS image. We also qualitatively studied images constructed at lower frequency (for TDS imaging and under lower plasma wave resonance conditions for HEMT). We observed the same behavior down to 600 GHz. Below this value; resolution was insufficient to clearly conclude on the imaging capabilities. On the acquisition time consuming with the THz-TDS system, if we measure one temporal waveform in 1 s per pixel, it requires 600 s to acquire an image with 20 by 30 pixels. The resulting rate is only 1 pixel/s. For the HEMT imaging system with the same sample, the resulting pixel rate is as high as 100 pixel/s, which is 100 times higher than that obtained in the THz-TDS system. In this example, the speed is only limited by the scanning speed of the XY translation stage. We conclude that the HEMT system is a very fast THz imaging device with good imaging quality, similar to that of a THz-TDS system. Another experiment was made with using a hologram stripe of a 10 Euros banknote as a sample (Fig. 5a) Using a THz-TDS spectrometer, we performed spectroscopic analysis of the different dielectric responses of the complex note structure. We found that, both in time and frequency domains, it is possible to discriminate between the region with the ‘‘10 surrounded by stars” and the region where the € symbol changes into 10 depending on the orientation of the
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Fig. 4. Temporal THz data through the hologram stripe of a 10 euros banknote obtained with a standard THz-TDS spectrometer.
banknote. The analysis of Fig. 4 makes it possible to observe small differences in the temporal and the FFT spectrum through the hologram stripe. Particularly, in the region of the 10 surrounded by stars, we observed that around 0.8 THz there is a decrease in the transmission attributed to the small holes in this part of the hologram stripe. Since the measured transmitted THz waves have not the same maxima according to their spatial position, we can use the HEMT detector to perform a fast efficient THz imaging of the banknote. The imaging portion of the banknote is similar to precedent one: the euro symbol presents in the hologram stripe (see photograph in the Fig. 5a and the imaging region indicated by the rectangular black box). Fig. 5b presents a THz images obtained with the THzTDS setup using a photoswitch as detector (left) and with HEMT as detector (right). The image size is (11.25 6.5 mm2) consisting in 45 26 pixels with a spatial step of 250 lm. With the THz-TDS setup, we choose Imax Imin parameters to elaborate the contrast (Imax Imin of the transmitted field). We can observe that the euro symbol € is correctly resolved with THz-TDS obtained from Imax Imin (Fig. 5b, left). Using the HEMT detector (Fig. 5b, right), THz image is presented with Vgs = 0.2 V and Ids = 40 mA (plasma frequency 1 THz). The images with HEMT detector present an excellent signal-tonoise ratio with a good contrast than with THz-TDs system. By using HEMT detector, the presences of the small holes in the 10 Euros banknote are clearly revealed. To show a better representation of the euro symbol obtained with the HEMT, we finally represented the image obtained with Vgs =0.2 V and Ids = 40 mA using a false 3D color map (Fig. 5c). The 3D representation makes it possible to clearly distinguish the euro symbol surrounded by the metallic hologram stripe.
Fig. 3. THz imaging of a metallic paper clip inside an envelope. (Right) HEMT with Vgs = 0.2 V, Ids = 40 mA (plasma frequency 1 THz); (left) THz-TDS imaging at 1 THz.
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of miniature THz detector based on nanotransistors. HEMT-based detectors have been shown to be efficient for THz imaging applications. Raster-scanned two dimensional images exhibit spatial resolution and signal-to-noise ratio comparable to TDS-THz imaging. The main advantage of these new detectors is that they can be easily integrated into arrays and enable development of future realtime THz camera. Acknowledgments This work is supported by the ‘‘Conseil Régional d’Aquitaine”, in collaboration with the Labri UMR 5800, and Innovative Imaging Solutions corporation: http://www.i2s-corp.com I2S. This work is also partially supported by JSPS International Fellowship Program for Research in Japan. The authors from Montpellier University acknowledge the support from CNRS GDR/GDR-E project Semiconductor sources and detectors of THz frequencies, Région of Languedoc-Roussillon through the ‘‘Terahertz Platform” project as well as the European Union MTKD-CT-2005-029671 grant. References Fig. 5. THz Imaging of a small part of 10 Euros banknote. (a) Top-visible imaging of a 10 Euros banknote (rectangular black box in the photograph), (b) left – by TDS imaging using Imax Imin; right – THz image with HEMT detector (Vgs = 0.3 V, Ids = 30 mA; plasma frequency 1 THz), (c) 3D false color representation of the Euro symbol using the HEMT detector with Vgs = 0.2 V, Ids = 40 mA (plasma frequency calculated is 1 THz).
The next step further is realization of real-time imaging THz cameras. In this context, field effect transistors can be treated as the most promising option. In particular, silicon- based devices displaying high sensitivity of 200 V/W, NEP values close to 0.1 nW/ p Hz at room temperature [16] and the bandwidth higher than 500 GHz. 4. Conclusion In summary, experimental results of THz imaging based on voltage-tunable THz detection using GaAs nanometer transistors are presented. Inserted in a time-domain THz spectrometer, we demonstrate that HEMT is an efficient device for detection of pulsed terahertz electric field. Experimental results prove the possibility
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