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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 14 — Jul. 9, 2007
  • pp: 8943–8950
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Emission characteristics of ion-irradiated In0.53Ga0.47As based photoconductive antennas excited at 1.55 μm

J. Mangeney, N. Chimot, L. Meignien, N. Zerounian, P. Crozat, K. Blary, J.F. Lampin, and P. Mounaix  »View Author Affiliations


Optics Express, Vol. 15, Issue 14, pp. 8943-8950 (2007)
http://dx.doi.org/10.1364/OE.15.008943


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Abstract

We present a detailed study of the effect of the carrier lifetime on the terahertz signal characteristics emitted by Br+-irradiated In0.53Ga0.47As photoconductive antennas excited by 1550 nm wavelength femtosecond optical pulses. The temporal waveforms and the average radiated powers for various carrier lifetimes are experimentally analyzed and compared to predictions of analytical models of charge transport. Improvements in bandwidth and in average power of the emitted terahertz radiation are observed with the decrease of the carrier lifetime on the emitter. The power radiated by ion-irradiated In0.53Ga0.47As photoconductive antennas excited by 1550 nm wavelength optical pulses is measured to be 0.8 μW. This value is comparable with or greater than that emitted by similar low temperature grown GaAs photoconductive antennas excited by 780 nm wavelength optical pulses.

© 2007 Optical Society of America

1. Introduction

The generation of coherent terahertz radiation from photoconductive antenna (PA) has attracted considerable interest since it is a way to reach the intermediate terahertz frequency range. The best terahertz performance is achieved by photoconductive antenna excited by ∼0.8μm optical pulses and made from low-temperature-grown (LTG) GaAs material, because this material associates both subpicosecond carrier lifetime and high resistivity [1

1. A.C. Warren, N. Katzenellenbogen, D. Grisckowsky, J. M. Woodall, M. R. Melloch, and N. Otsuka, “Subpicosecond, freely propagating electromagnetic pulse generation and detection using GaAs:As epilayers,” Appl. Phys. Lett 58, 1512–1514 (1991). [CrossRef]

]. The use of a lower-bandgap semiconductor, such as In0.53Ga0.47As, allows cheap, compact and turnkey terahertz spectroscopy setups based on erbium fiber (Er:fiber) lasers, which can produce sub-picosecond pulses at a central wavelength λ=1.55 μm. Moreover, as the bandgap of the In0.53Ga0.47As is smaller than the GaAs one, In0.53Ga0.47As semiconductor shows a lower Γ-valley-effective mass which result in a higher electron mobility and greater terahertz power is expected [2

2. J. Lloyd-Hughes, E. Castro-Camus, and M. B. Johnston, “Simulation and optimisation of terahertz emission from InGaAs and InP photoconductive switches,” Solid State Commun. 136, 595–599 (2005). [CrossRef]

]. Photoconductive antennas based on Fe-implanted or heavy ion-irradiated In0.53Ga0.47As excited by 1.55μm laser pulses have been investigated and their efficiencies either as terahertz emitter [3

3. M. Suzuki and M. Tonouchi, “Fe-implanted InGaAs terahertz emitters for 1.56 μm wavelength excitation,” Appl. Phys. Lett. 86, 1104–1106 (2005). [CrossRef]

,4

4. N. Chimot, J. Mangeney, L. Joulaud, P. Crozat, H. Bernas, K. Blary, and J. F. Lampin, “Terahertz radiation from heavy-ion-irradiated In0.53Ga0.47As photoconductive antenna excited at 1.55 μm,” Appl. Phys. Lett. 87, 193510–193512 (2005). [CrossRef]

] or as terahertz detector [5

5. M. Suzuki and M. Tonouchi, “Fe-implanted InGaAs photoconductive terahertz detectors triggered by 1.56 μm femtosecond optical pulses,” Appl. Phys. Lett. 86, 163504–163506 (2005). [CrossRef]

] have been demonstrated. However, whereas ionic irradiation or implantation is an efficient method to reduce in a controlled way the carrier lifetime in semiconductor layers, only few works [6

6. Tze-An Liu, M. Tani, and Ci-Ling Pan, “THz radiation emission properties of multienergy arsenic-ion-implanted GaAs and semi-insulating GaAs based photoconductive antennas,” J. Appl. Phys. 93, 2996–3001 (2003). [CrossRef]

] focus on the study of how the carrier lifetime impacts on the emitted terahertz signal characteristics in In0.53Ga0.47As PA. Such investigation turns out to be crucial for applications as carrier lifetime strongly influences the spectral distribution and the power of radiation emitted by PA [7–10

7. B. Salem, D. Morris, V. Aimez, J. Beauvais, and D. Houde, “Improved characteristics of a terahertz set-up built with an emitter and a detector made on proton-bombarded GaAs photoconductive materials,” Semicond. Sci. Technol. 21, 283–286 (2006). [CrossRef]

]. In this letter, we present a detailed study of the effect of the carrier lifetime on the terahertz signal characteristics emitted by PA made on Br+-irradiated In0.53Ga0.47As material; we investigated four In0.53Ga0.47As emitters with different carrier lifetimes, namely > 1ns, 4.2 ps, 0.7 ps and 0.3 ps. The variations of the temporal waveforms and the average radiated powers for various carrier lifetimes are experimentally analyzed and compared to predictions of analytical models of charge transport. The spectral bandwidth is found to increase with the decrease of the carrier lifetime on the emitter. Improvement in the average power of the emitted terahertz radiation is also observed for emitters with short carrier lifetimes as a consequence of the increase of the maximum possible bias voltage with the increase of the ion irradiation dose. The power radiated by ion-irradiated In0.53Ga0.47As PA excited by 1550 nm wavelength optical pulses is compared to that emitted by similar LTG-GaAs PA excited by 780 nm wavelength optical pulses.

2. Devices

Undoped 1-μm-thick n-type In0.53Ga0.47As layers were epitaxially grown by gas-source MBE on semi-insulating InP:Fe substrates. A mesa etching process was used to define In0.53Ga0.47As absorbing area of 82×14 μm2 and 7×14 μm2 on the InP substrate for the emitter and the detector, respectively. The layers were then irradiated by 11 MeV heavy ions (Br+) at irradiation doses from 4×1010cm-2 to 1×1012cm-2. Furthermore, a thin cap layer of optical transparent silicon nitride is grown to protect the device from oxidation and to provide an antireflection coating. The electrode patterns were fabricated by metal evaporation and a conventional lift-off photolithographic technique. Then, the antenna structure is located at the center of a 20-mm-long coplanar transmission line consisting of simple coplanar striplines patterned onto the InP substrate. The latter are made of two 5-μm-wide, 0.5-μm-thick Ti/Au strips separated by 80 μm and 30 μm for the emitters and the detector, respectively. For the detector, a gap of 5 μm between the contacts is added.

3. Time domain measurements

In the time domain experiment, 250 fs optical pulses with a repetition rate of 14.3 MHz, delivered by a passively mode-locked fiber laser (Calmar Optcom) operating at 1550 nm were used to excite the emitter and the detector. The optical pump beam, with an average power of 3 mW, was focused on the photoconductive antennas on a spot size of about 10 μm, near the anode of the antennas. High-resistivity Si hyperhemispherical substrate lenses, with a 10mm diameter, were attached back to the emitter and the receiver antennas. The intensity of the terahertz radiation was modulated using a mechanical chopper. The detector was placed 5 cm away from the emitter. The optical probing beam, with an average power of 3 mW, was focused on the PC antennas on a spot size of about 5 μm. The current induced by the probe beam and the terahertz radiation in the detector is amplified and processed with a lock-in digital amplifier. Note that the InP substrate do not absorb the 0.8 eV photons (since the laser fluences are relatively low[11

11. D. Vignaud, J. F. Lampin, and F. Mollot, “Two-photon absorption in InP substrates in the 1.55 m range,” Appl. Phys. Lett. 85, 239–241 (2004). [CrossRef]

]) and its contribution on the measured waveforms is negligible.

Due to their high initial energy, the irradiating ions are implanted in the InP substrate beyond 3 μm, and uniform damage profiles through the In0.53Ga0.47As layer are created, as suggested by calculations using the “Stopping Range of Ions in the Matter” software [12

12. J. P. Biersack and L. G. Haggmark, “A Monte Carlo program for the transport of energetic ions in amorphous targets,” Nucl. Instrum. Methods 174, 257 (1980). [CrossRef]

]. The damages in the In0.53Ga0.47As layers are only host atom displacements distributed essentially in defect condensates. These defect clusters have deep energy levels acting as efficient capture and recombination centers for free carriers. The lifetimes of electrons in the conduction band have been determined by pump-probe differential transmission experiments. Four In0.53Ga0.47As emitters with carrier lifetimes of >1 ns, 4.2 ps, 0.7 ps and 0.3 ps were made by varying the Br+ irradiation dose. The carrier lifetime in the detector was 0.3 ps. The signal waveforms emitted from the four In0.53Ga0.47As PA are shown in Fig. 1. These results are obtained using an applied bias voltage of 7 V for the long carrier lifetime sample and bias voltages higher than 15V for samples with picosecond carrier lifetimes respectively.

Fig. 1. Terahertz radiation waveforms from Br+-irradiated In0.53Ga0.47As emitters. The solid lines represent the measured waveforms and the dashed lines the calculated waveforms. The carrier lifetime reported on each graph is the carrier lifetime extracted from optical pump-probe differential transmission measurements.

To correlate these experimental evidences of the effect of the carrier lifetime on the emitted waveforms to theoretical predictions, the detected THz waveforms were fitted by the analytical expression of the detected photocurrent given by Duvillaret et al [15

15. L. Duvillaret, F. Garet, J.-F. Roux, and J.-L. Coutaz, “Analytical modeling and optimization of terahertz time-domain spectroscopy experiments using photoswitches as antennas,” IEEE J. Sel. Top. Quantum Electron. 7,615–623 (2001). [CrossRef]

]:

jrec(t)(τem+τrec)exp(τ˜las22τ˜em2tτ˜em)erfc(τ˜las2tτ˜em2τ˜emτ˜las)+(τemτ˜em)exp(τ˜las22τrec2tτrec)erfc(τ˜las2+tτrec2τrecτ˜las)(τrec+τ˜em)exp(τ˜las22τem2tτem)erfc(τ˜las2+tτem2τemτ˜las)

with 1/τ˜em = 1/τem + 1/δτem and τem, τrec, τlas, δτem are the carrier lifetime in the emitter, the carrier lifetime in the detector, the laser pulse duration and the carrier collision time respectively. This model is based on the assumption that the free-carrier relaxation in both emitter and detector is governed by a single exponential decay law. Since Fe:InP material shows dispersion in the terahertz domain, a broadening of the pulses due to their travel in the Fe:InP substrates of the two antennas (the terahertz beam propagates through a total thickness of 0.7mm of Fe:InP) must be accounted. The detected photocurrent is then calculated using the convolution integral of j(t)=dtjrec(t)exp((tt)22Δt) with Δt = Δnl/c ≈ 200fs since we measured, by time domain terahertz spectroscopy, the maximum variation of refraction index Δn in Fe:InP in the considered frequency range to be approximatively 0.09. The saturation effects are not taken into account as the optical excitation fluences involved in these measurements are relatively low. Note that this model does not integrate high pass and low pass filter to account for the effect of the detector size.

Figure 2 displays the normalized Fourier transform amplitude spectra of the temporal waveforms. Just as the temporal behaviour depends on the carrier lifetime in the emitter, the spectral peaks depend on these carrier lifetimes, being shifted to higher frequency when decreasing the carrier lifetime. The maximum of the spectrum is shifted from a frequency inferior to 0.05 THz for the un-irradiated In0.53Ga0.47As PA to a frequency of 0.38 THz for the most irradiated In0.53Ga0.47As PA (1×1012cm-2).

Fig. 2. Normalized spectra of the terahertz waveforms for emitters with different carrier lifetime. Inset: Frequency of peak emitted terahertz power as a function of carrier lifetime. The experimental data are represented by triangles and the values extracted from the model by solid line.

The inset of Fig. 2 shows how the frequency at which the terahertz emission peaks when varying the carrier lifetime, and as one can see, theory and experiment agree fairly well. Therefore we have shown that higher bandwidth terahertz emission is obtained with the decrease of the carrier lifetime, consistently with theoretical expectations based on Duvillard et al’s model.

5. Time integrated measurements

Fig. 3. Bolometer output measured as a function of the average laser power driving the emitter for the photoconductive antennas with different carrier lifetime. The solid curve is the theoretical curve fitted to the data.

For all samples, the radiated power increases quadratically at low incident optical powers and saturates at higher incident optical powers. This saturation is attributed to the screening of the applied bias field by the radiation field and the space-charge field, which contribute to the collapse of the total electric field acting on the carriers at high carrier density. For small optical size excitation (small-aperture emitters), models based on Monte Carlo method have shown that the space charge induced by separating carriers is the dominant contribution to the screening of the applied electric field [18

18. E. Castro-Camus, J. Lloyd-Hughes, and M. B. Johnston, “Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches,” Phys. Rev. B 71, 195301–195307 (2005). [CrossRef]

,19

19. D. S. Kim and D. S. Citrin, “Coulomb and radiation screening in photoconductive terahertz sources,” Appl. Phys. Lett. 88, 161117–161119 (2006). [CrossRef]

]. Therefore, to interpret the observed saturation of the average radiated powers with incident optical powers, we considered the following simple model. As the pulse duration is smaller than the carrier lifetime (except for the most irradiated sample), the concentration of photoexcited electron-hole pairs is given by n 0 = α(1-e-αl)Flas/hν where Flas is the fluence of the incident optical pulse beam. Whatever the incident optical power, the maximum of the transient current is achieved at the end of the pulse duration τlaser and is thus given by j max = -en 0 v(τlas), neglecting for the sake of simplicity the minor current contribution from the holes. The photoexcited carrier density is high and then scattering works very efficiently to restore thermal equilibrium among carriers in characteristic time less than the optical pulse duration. The average velocity is then expressed by v(τ max) = μEloc(τlas). The local field can be estimated from the Poisson’s equation when considering that during the time τlas, the carriers have propagated a distance l=las. The resulting local electric field is given by Eloc(τlas) = Ebias - qn 0 v(τlas)τlas/ε. As hole velocity is much lower than electron velocity, the hole contribution to the space charge field can be neglected. The average velocity is then written by v(τlas) = μEbias/(1 + qμn 0 τlas/ε) and the resulting peak current is given by j max = -en 0 μEbias/(1 + qμn 0 τlas/ε). As the shape of the waveform is not affected by the saturation effects, the peak amplitude of the terahertz waveform can then be expressed as EmaxFlaserF0+Flaser with F0 defined by εh ν/((1-e-αl)μτlas and the average terahertz power as PTHz(FlaserF0+Flaser)2. Note that this theoretical law is very close to the one obtains in large-aperture photoconductive antenna for which screening by the radiated field only is considered. The excellent fit of the experimental data by this theoretical law contributes to validate our simple analysis. Assuming that the only irradiation fluence-dependent parameter of F 0 is the photoexcited carrier mobility, from the fit, a photoexcited carrier mobility ratio of 3 between the un-irradiated sample and the sample with 0.3 ps carrier lifetime was estimated. This ratio is clearly lower than the Hall carrier mobility ratio of 23 measured between these two samples. The difference is explained because Hall mobility is the low electrical field mobility obtained with a carriers in low density (1015cm-3), whereas the mobility involved in these measurements results from higher electrical field and from a high concentration of photocreated carriers (few 1017cm-3). The irradiation process affects much more the electron Hall mobility due to the defect scattering than the effective mobility of photoexcited carriers in the early stage after their creation. Moreover, as the ion irradiation process is found to increase the maximum possible bias voltage of In0.53Ga0.47As material, the ion irradiation of In0.53Ga0.47As layers results in an increase of both the bandwidth and the maximum power delivered by PA emitters.

Fig. 4. Emitted terahertz power as a function of the carrier lifetime at 0.1 THz (circle), 0.38 THz(triangle), 0.84 THz (diamond) and 1.2 THz (square) computed from time domain measurements with Bolometer detector normalization.

This tendency is illustrated by Fig. 4 showing the radiated power as a function of the carrier lifetime in In0.53Ga0.47As PA, computed for different frequencies from time domain measurements with Bolometer detector normalization. The increase of the radiated terahertz power with the decrease of the carrier lifetime at high frequency has important practical implications for terahertz emitter design: the carrier lifetime of the emitter can be adjusted to reach a specific terahertz spectral range.

Conclusion

The influence of the carrier lifetime in Br+-irradiated In0.53Ga0.47As photoconductive antennas excited by 1550 nm wavelength femtosecond optical pulses on the characteristics of the emitted terahertz signal has been investigated. We demonstrate that the spectral bandwidth increase when decreasing the carrier lifetime and that the average power is improved in short carrier lifetime devices as the ion irradiation process is found to increase the maximum possible bias voltage of In0.53Ga0.47As material. The maximum power radiated by ion-irradiated In0.53Ga0.47As photoconductive antennas excited by 1550 nm wavelength optical pulses is measured to be 0.8 μW. This value is comparable with or greater than that emitted by similar low temperature grown GaAs photoconductive antennas excited by 780 nm wavelength optical pulses. This study has important practical implications for the design of ion–irradiated In0.53Ga0.47As photoconductive antennas used as terahertz emitter.

Acknowledgments

The authors gratefully thank S. Henry (CSNSM) for ionic irradiation and G. Fishman for fruitful discussions. This work has been carried out in the frame of the french RTB network.

References and links

1.

A.C. Warren, N. Katzenellenbogen, D. Grisckowsky, J. M. Woodall, M. R. Melloch, and N. Otsuka, “Subpicosecond, freely propagating electromagnetic pulse generation and detection using GaAs:As epilayers,” Appl. Phys. Lett 58, 1512–1514 (1991). [CrossRef]

2.

J. Lloyd-Hughes, E. Castro-Camus, and M. B. Johnston, “Simulation and optimisation of terahertz emission from InGaAs and InP photoconductive switches,” Solid State Commun. 136, 595–599 (2005). [CrossRef]

3.

M. Suzuki and M. Tonouchi, “Fe-implanted InGaAs terahertz emitters for 1.56 μm wavelength excitation,” Appl. Phys. Lett. 86, 1104–1106 (2005). [CrossRef]

4.

N. Chimot, J. Mangeney, L. Joulaud, P. Crozat, H. Bernas, K. Blary, and J. F. Lampin, “Terahertz radiation from heavy-ion-irradiated In0.53Ga0.47As photoconductive antenna excited at 1.55 μm,” Appl. Phys. Lett. 87, 193510–193512 (2005). [CrossRef]

5.

M. Suzuki and M. Tonouchi, “Fe-implanted InGaAs photoconductive terahertz detectors triggered by 1.56 μm femtosecond optical pulses,” Appl. Phys. Lett. 86, 163504–163506 (2005). [CrossRef]

6.

Tze-An Liu, M. Tani, and Ci-Ling Pan, “THz radiation emission properties of multienergy arsenic-ion-implanted GaAs and semi-insulating GaAs based photoconductive antennas,” J. Appl. Phys. 93, 2996–3001 (2003). [CrossRef]

7.

B. Salem, D. Morris, V. Aimez, J. Beauvais, and D. Houde, “Improved characteristics of a terahertz set-up built with an emitter and a detector made on proton-bombarded GaAs photoconductive materials,” Semicond. Sci. Technol. 21, 283–286 (2006). [CrossRef]

8.

S. G. Park, A. M. Weiner, M. R. Melloch, C. W. Siders, J. L. W. Siders, and A. J. Taylor, “High-power narrow-band terahertz generation using large-aperture photoconductors,” IEEE J. Quantum Electron. 35, 1257 (1999). [CrossRef]

9.

M. Tani, S. Matsura, K. Sakai, and S. Nakashima, “Emission characteristics of photoconductive antennas based on low-temperature-grown GaAs and semi-insulating GaAs,” Appl. Opt. 36, 7853–7859 (1997). [CrossRef]

10.

T.-A. Liu, G.-R. Lin, Y.-C. Lee, S. C. Wang, M. Tani, H.-H. Wu, and C.-L. Pan, “Dark current and trailing-edge suppression in ultrafast photoconductive switches and terahertz spiral antennas fabricated on multienergy arsenic-ion-implanted GaAs,” J. Appl. Phys. 98, 013711–013714 (2005). [CrossRef]

11.

D. Vignaud, J. F. Lampin, and F. Mollot, “Two-photon absorption in InP substrates in the 1.55 m range,” Appl. Phys. Lett. 85, 239–241 (2004). [CrossRef]

12.

J. P. Biersack and L. G. Haggmark, “A Monte Carlo program for the transport of energetic ions in amorphous targets,” Nucl. Instrum. Methods 174, 257 (1980). [CrossRef]

13.

P. U. Jepsen, R. H. Jacobsen, and S. R. Keiding, “Generation and detection of terahertz pulses from biased semiconductor antennas,” J. Opt. Soc. Am. B 13, 2424–2436 (1996). [CrossRef]

14.

S.-G. Park, M. R. Melloch, and A. M. Weiner, “Analysis of terahertz waveforms measured by photoconductive and electrooptic sampling,” IEEE J. Quantum Electron. 35, 810–819 (1999). [CrossRef]

15.

L. Duvillaret, F. Garet, J.-F. Roux, and J.-L. Coutaz, “Analytical modeling and optimization of terahertz time-domain spectroscopy experiments using photoswitches as antennas,” IEEE J. Sel. Top. Quantum Electron. 7,615–623 (2001). [CrossRef]

16.

This value is consistent with the values given by P.Y. Yu and M. Cardona, “Fundamentals of Semiconductors,” 2nd Edition, (Springer, 1999) p. 290.

17.

L. Joulaud, J. Mangeney, J.-M. Lourtioz, P. Crozat, and G. Patriarche “Thermal stability of ion-irradiated InGaAs with (sub-) picosecond carrier lifetime,” Appl. Phys. Lett. 82, 856–8582003. [CrossRef]

18.

E. Castro-Camus, J. Lloyd-Hughes, and M. B. Johnston, “Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches,” Phys. Rev. B 71, 195301–195307 (2005). [CrossRef]

19.

D. S. Kim and D. S. Citrin, “Coulomb and radiation screening in photoconductive terahertz sources,” Appl. Phys. Lett. 88, 161117–161119 (2006). [CrossRef]

OCIS Codes
(160.5140) Materials : Photoconductive materials
(320.7130) Ultrafast optics : Ultrafast processes in condensed matter, including semiconductors

ToC Category:
Materials

History
Original Manuscript: March 23, 2007
Revised Manuscript: April 23, 2007
Manuscript Accepted: May 10, 2007
Published: July 3, 2007

Citation
J. Mangeney, N. Chimot, L. Meignien, N. Zerounian, P. Crozat, K. Blary, J. F. Lampin, and P. Mounaix, "Emission characteristics of ion-irradiated In0.53Ga0.47As based photoconductive antennas excited at 1.55 µm," Opt. Express 15, 8943-8950 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-14-8943


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References

  1. A. C. Warren, N. Katzenellenbogen, D. Grisckowsky, J. M. Woodall, M. R. Melloch, and N. Otsuka, "Subpicosecond, freely propagating electromagnetic pulse generation and detection using GaAs:As epilayers," Appl. Phys. Lett 58, 1512-1514 (1991). [CrossRef]
  2. J. Lloyd-Hughes, E. Castro-Camus, and M. B. Johnston, "Simulation and optimisation of terahertz emission from InGaAs and InP photoconductive switches," Solid State Commun. 136, 595-599 (2005). [CrossRef]
  3. M. Suzuki and M. Tonouchi, "Fe-implanted InGaAs terahertz emitters for 1.56 µm wavelength excitation," Appl. Phys. Lett. 86, 1104-1106 (2005). [CrossRef]
  4. N. Chimot, J. Mangeney, L. Joulaud, P. Crozat, H. Bernas, K. Blary, J. F. Lampin, "Terahertz radiation from heavy-ion-irradiated In0.53Ga0.47As photoconductive antenna excited at 1.55 µm," Appl. Phys. Lett. 87, 193510-193512 (2005). [CrossRef]
  5. M. Suzuki and M. Tonouchi, "Fe-implanted InGaAs photoconductive terahertz detectors triggered by 1.56 µm femtosecond optical pulses," Appl. Phys. Lett. 86, 163504-163506 (2005). [CrossRef]
  6. Tze-An Liu, M. Tani, and Ci-Ling Pan, "THz radiation emission properties of multienergy arsenic-ion-implanted GaAs and semi-insulating GaAs based photoconductive antennas," J. Appl. Phys. 93, 2996-3001 (2003). [CrossRef]
  7. B. Salem, D. Morris, V. Aimez, J. Beauvais and D. Houde, "Improved characteristics of a terahertz set-up built with an emitter and a detector made on proton-bombarded GaAs photoconductive materials," Semicond. Sci. Technol. 21, 283-286 (2006). [CrossRef]
  8. S. G. Park, A. M. Weiner, M. R. Melloch, C. W. Siders, J. L. W. Siders, and A. J. Taylor, "High-power narrow-band terahertz generation using large-aperture photoconductors," IEEE J. Quantum Electron. 35, 1257 (1999). [CrossRef]
  9. M. Tani, S. Matsura, K. Sakai, and S. Nakashima, "Emission characteristics of photoconductive antennas based on low-temperature-grown GaAs and semi-insulating GaAs," Appl. Opt. 36, 7853-7859 (1997). [CrossRef]
  10. T.-A. Liu, G.-R. Lin, Y.-C. Lee, S. C. Wang, M. Tani, H.-H. Wu, and C.-L. Pan, "Dark current and trailing-edge suppression in ultrafast photoconductive switches and terahertz spiral antennas fabricated on multienergy arsenic-ion-implanted GaAs," J. Appl. Phys. 98, 013711-013714 (2005). [CrossRef]
  11. D. Vignaud, J. F. Lampin and F. Mollot, "Two-photon absorption in InP substrates in the 1.55 m range," Appl. Phys. Lett. 85, 239-241 (2004). [CrossRef]
  12. J. P. Biersack and L. G. Haggmark, "A Monte Carlo program for the transport of energetic ions in amorphous targets," Nucl. Instrum. Methods 174, 257 (1980). [CrossRef]
  13. P. U. Jepsen, R. H. Jacobsen, and S. R. Keiding, "Generation and detection of terahertz pulses from biased semiconductor antennas," J. Opt. Soc. Am. B 13, 2424-2436 (1996). [CrossRef]
  14. S.-G. Park, M. R. Melloch, and A. M. Weiner, "Analysis of terahertz waveforms measured by photoconductive and electrooptic sampling," IEEE J. Quantum Electron. 35, 810-819 (1999). [CrossRef]
  15. L. Duvillaret, F. Garet, J.-F. Roux, and J.-L. Coutaz, "Analytical modeling and optimization of terahertz time-domain spectroscopy experiments using photoswitches as antennas," IEEE J. Sel. Top. Quantum Electron. 7, 615-623 (2001). [CrossRef]
  16. This value is consistent with the values given by P.Y. Yu, M. Cardona, "Fundamentals of Semiconductors," 2nd ed., (Springer, 1999) p. 290.
  17. L. Joulaud, J. Mangeney, J.-M. Lourtioz, and P. Crozat, and G. Patriarche "Thermal stability of ion-irradiated InGaAs with (sub-) picosecond carrier lifetime," Appl. Phys. Lett. 82, 856-858 (2003). [CrossRef]
  18. E. Castro-Camus, J. Lloyd-Hughes and M. B. Johnston, "Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches," Phys. Rev. B 71, 195301-195307 (2005). [CrossRef]
  19. D. S. Kim and D. S. Citrin, "Coulomb and radiation screening in photoconductive terahertz sources," Appl. Phys. Lett. 88, 161117-161119 (2006). [CrossRef]

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