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Terahertz pulse emission from semiconductor surfaces illuminated by femtosecond Yb:KGW laser pulses
ARTICLE IN PRESS Physica B 404 (2009) 3386–3390
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Terahertz pulse emission from semiconductor surfaces illuminated by femtosecond Yb:KGW laser pulses , V. Pacˇebutas, A. Krotkus % A. Bicˇiunas Semiconductor Physics Institute, 01108, A. Gostauto 11, Vilnius, Lithuania
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 December 2008 Accepted 14 May 2009
The amplitudes of terahertz pulses emitted from the surfaces of InAs, InSb, InGaAs, GaAs and Ge after their excitation by femtosecond 1 mm laser pulses was compared. It has been found that this effect is most efficient in p-type InAs. The mechanisms leading to the terahertz emission are investigated and discussed. It has been concluded that in the majority of the investigated semiconductors the main contribution to THz pulse emission comes from the electrical-field-induced optical rectification effect. & 2009 Elsevier B.V. All rights reserved.
PACS: 07.57.Hm 42.65.Re 87.50.U 61.82.Fk Keywords: Terahertz InAs InSb InGaAs Ge Electrical-field-induced optical rectification
1. Introduction Terahertz (THz) pulse generation and detection using optoelectronic semiconductor components excited by femtosecond laser pulses has become lately a powerful experimental technique finding numerous applications in THz time-domain spectroscopy and THz imaging [1]. Traditionally, mode-locked Ti:sapphire lasers emitting at the wavelengths around 800 nm are used for this purpose because they are more advanced technically and commercially more mature than other similar products and because their photon energy perfectly matches the absorption band of GaAs. Nonstoichiometric GaAs obtained either by molecular-beam-epitaxy growth at reduced temperatures (LTG GaAs) [2] or by high-energy, heavy ion implantation [3] is characterized by unique set of material parameters [4] that facilitates its applications in pulsed THz components. However, because Ti:sapphire lasers require a rather complicated, manystage optical pumping arrangement, these systems are quite bulky and complicated, which prevents their wider applications.
Corresponding author. Tel.: +370 5 262 74 69; fax: +370 5 262 71 23.
% E-mail address: biciunas@pfi.lt (A. Bicˇiunas). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.05.017
One solution of this problem would be using the lasers emitting in the spectral ranges close to 1 or 1.5 mm, which can be directly pumped by diode laser bars. Recently, several compact, efficient and cost-effective solid-state and fiber laser systems that generate femtosecond pulses at these near-infrared wavelengths have been developed [5,6] and have been employed for activating THz time-domain spectroscopy systems [7–9]. The main obstacle in further development of these systems is the absence of material with appropriate bandgap and parameters high dark resistivities and subpicosecond carrier lifetimes—similar to those of LTG GaAs, which could be used for manufacturing photoconductive THz emitters and detectors. In the case of THz detectors activated by 1 mm wavelength femtosecond lasers, epitaxial layers of LTG InGaAs [7] or LTG GaBiAs [9] were employed, whereas no material appropriate for photoconductive THz emitters has been identified as yet. It is known [10] that THz radiation can be emitted also from unbiased semiconductor surfaces photoexcited by femtosecond laser pulses. At Ti:sapphire laser wavelength this effect is most efficient when using p-type InAs crystals [11]. Here we present a systematic study of THz pulse emission from the surfaces of various semiconductors illuminated by femtosecond Yb:KGW (potassium gadolinium vanadate) laser. It is shown that p-InAs and other narrow-gap semiconductors are most efficient THz
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emitters also in this case. Investigation of the physical mechanism leading to this effect has shown the domination of the electricalfield-induced optical rectification (EFIOR) for the majority of the materials studied.
2. Experimental details As the optical pulse source for THz pulse generation and sampling the oscillator from a high repetition rate femtosecond laser system Pharos (Light Conversion Ltd) was used. The oscillator was based on directly diode-pumped Yb:KGW (KGa(WO3)) crystal. Kerr-lens mode-locking was used to generate optical pulses with the central wavelength of 1030 nm. The average output power of the oscillator was 2 W, the pulse duration was 63 fs (Gaussian fit), the spectral width (FWHM) was 28 nm, and the pulse repetition rate was 75 MHz. Different semiconductor surfaces have been illuminated by the laser beam at the angle of 451 and THz radiation emitted from these surfaces was monitored in quasireflection direction. Azimuthal angle dependences of the emitted THz pulse amplitude were measured for all investigated semiconductors by rotating the crystal around the normal to its surface. All measurements were performed at room temperature. In our previous work, we have proposed to manufacture optoelectronic THz range components sensitive to near-infrared radiation from LTG GaBiAs alloy [9]. Epitaxial GaBi0.06As0.94 layer was grown on semi-insulating (1 0 0)-oriented GaAs substrate in a solid-state molecular-beam-epitaxy (MBE) system at 280 1C substrate temperature at the growth rate of 2 mm/h. The thickness of the layer was 0.4 mm; it has been used for the manufacturing coplanar Hertzian dipole antennae with 15 mm wide photoconducting gap between Ti–Au electrodes and equipped with a hemispherical lens made from high resistivity Si. S and P-polarized THz signals were separated by rotating the detector that was polarization sensitive and inserting into the THz beam path at a Brewster angle a semi-insulating InP wafer. The performance of the experimental system is illustrated in Fig. 1, where THz pulse and its corresponding Fourier spectrum radiated from the surface of p-type InAs crystal—most efficient THz emitter from all materials investigated in the present work—are shown. Besides p- and n-type InAs crystals, THz emission from the surfaces of InSb (intrinsic conduction at the room temperature) and p- and n-type Ge were investigated. All these crystals were cut parallel to the (111) crystallographic plane. On the other hand, epitaxial layers of CdxHg1xTe (with x ¼ 0.2) grown on CdTe substrates, and In0.5Ga0.5As on InP substrate had the symmetry of (1 0 0). The same symmetry had the wafers of semi-insulating GaAs that were also investigated in the experiments.
Fig. 1. THz pulse and its Fourier spectrum radiated from the surface of p-type InAs crystal and measured using GaBiAs detector.
3. Results and discussion Diagram presented in Fig. 2 compares THz pulse amplitudes as emitted from the surfaces of various semiconductor materials under the same experimental conditions. As it was already mentioned before, most efficiently THz pulses are generated at the surface of p-InAs, followed by several other narrow-gap semiconductors. There are two main groups of the physical mechanisms leading to THz emission from semiconductor surfaces excited by femtosecond laser pulses: surface photocurrent surge and optical rectification of femtosecond laser pulses due to their nonlinear optical interaction with the material [12]. Standard way to separate these mechanisms is measuring azimuthal angle dependences of the radiated THz pulse amplitudes: for semiconductor crystals that, typically, have a cubic symmetry the photocurrent is isotropic, whereas THz pulses
Fig. 2. Relative comparison of THz pulse amplitudes, emitted from the surface of various semiconductors illuminated by femtosecond 1 mm wavelength laser pulses. Light intensity on surface emitters is 0.63 W/cm2.
radiated due to the optical rectification effect exhibit complex dependences on the azimuthal angle determined by the symmetry of the photoexcited crystallographic plane and on the
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order of the optical nonlinearity (second or third order) that causes this effect [13]. In the following we will discuss the azimuthal angle dependences of THz radiation from different semiconductors separately.
3.1. InAs and InSb When excited with Ti:sapphire laser pulses, these narrow-gap semiconductors are the best THz pulse emitters because of large difference in the velocities of photoexcited electrons and holes that causes efficient spatial separation of these current carriers and because of a significant third order optical nonlinearity leading to the EFIOR effect at the surface of these materials [12]. Similar features of this effect were observed also when femtosecond 1 mm wavelength optical pulses were used for the photoexcitation. Fig. 3 shows azimuthal angle f dependences of P- and S-polarized THz pulse amplitude measured on two differently doped InAs crystals. All curves show the variation proportional to cos 3f that is typical for the optical rectification effect on (111) surfaces; the presence of S-polarized THz radiation, which cannot be generated by any mechanisms related to the photocurrent surge, is also evidencing the significance of the nonlinear optical effects. As in the case of 850 nm excitation [13], the most probable cause of THz emission is the optical rectification induced by the electric field that is appearing at the crystal surface due to the
photoexcited electron and hole separation. THz pulses radiated from p-type InAs surface have more than twice larger amplitudes than those radiated from n-InAs. This difference can be explained by the presence of a built-in electric field of the inversion layer at p-InAs surface [11]. Surface electric field in p-InAs is of the same polarity as the field arising due to the photoexcited electron and hole separation; however, it interacts with the whole optical pulse and not with its part as in n-InAs, where optical rectification effect is due to the electric field induced by the movement of the electrons excited during the initial phase of the laser pulse. This conclusion about the mechanisms of THz emission from n- and p-type InAs is supported by the measurements of THz pulse (S-polarized) amplitude dependences on the laser beam intensity presented in Fig. 4. As can be seen from this figure, the dependence measured on p-InAs shows a strong tendency to saturation, whereas in the case of n-InAs it remains linear to the largest intensities employed. This difference can arise due to the built-in surface field screening in the former crystal by carriers generated during the slowly varying initial part of the laser pulse. Azimuthal angle dependences of THz signal radiated from the surfaces of InSb (Fig. 5) are similar to those measured on InAs but the amplitudes of THz pulses are smaller. Although the excess
Fig. 4. THz pulse amplitude as a function of the average optical power for two InAs crystals.
Fig. 3. Azimuthal angle of P (full lines) and S-polarized (dashed lines) THz pulse amplitudes emitted from the (111) surfaces of p-InAs (a) and n-InAs (b) crystals.
Fig. 5. Azimuthal angle of P (full line) and S-polarized (dashed line) THz pulse amplitudes emitted from the (111) surface of InSb.
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energy of photoexcited electrons in InSb is larger than in InAs, which has a larger energy bandgap, electric field induced by the electron and hole separation is weaker because of the intense electron scattering to subsidiary conduction band valleys laying at L and X points of the Brillouin zone [14,15]. 3.2. GaAs and InGaAs The symmetry of the surfaces of semi-insulating GaAs crystal and epitaxial In0.5Ga0.5As layers corresponded to (1 0 0) crystallographic planes for which the EFIOR effect has only an azimuthal angle independent component. Therefore, azimuthal angle dependences of THz pulse amplitude proportional to cos 2f that were experimentally observed on the samples of these materials (Fig. 6) evidence the presence of the second-order nonlinear optical rectification effect [16]. For the case of GaAs transparent to 1030 nm wavelength radiation, this effect is the only physical mechanism leading to THz emission. For In0.5Ga0.5As that is absorbing laser radiation, besides the optical rectification contribution there is a significant azimuthal angle independent component, which can be caused by the EFIOR or/and photoDember effects.
Fig. 7. Azimuthal angle dependences of P-polarization (full line) and S-polarization (dashed line) THz pulse emitted from (111) plane of p-Ge with the resistivity r ¼ 0.05 O cm.
3.3. Germanium In contrast to the semiconducting compounds discussed above, elemental semiconductor crystal germanium do not possess a symmetry center, therefore clear dependences of THz signal radiated from its surface on the azimuthal angle shown in Fig. 7 evidence that the main cause of this effect is third-order optical nonlinearity or the EFIOR effect. This has been discovered before also for the case of the Ti:sapphire laser excitation of Ge [17]. The amplitudes of THz pulses are larger for the crystals with a higher doping level, for which surface electric fields are stronger. Additional proof of the significance of the EFIOR effect in Ge was obtained after measuring the pump-and-probe traces presented in Fig. 8. In this experiment the semiconductor surface was illuminated by two parts of the laser beam delayed with respect to each other. The weaker (average power of 45 mW) part of the laser beam was used for the generation of THz pulses, whereas the stronger part (average power of 600 mW) that was
Fig. 6. Azimuthal angle dependences of P-polarization THz pulse emitted from (1 0 0) planes of GaAs (full line) and In0.5Ga0.5As (dashed line).
Fig. 8. Amplitude of THz pulse generated by the probe pulse as a function of optical delay between the pump and the probe pulses. Full line n-Ge (r ¼ 0.2 O cm), dashed line p-Ge (r ¼ 0.05 O cm).
incident on the surface in the direction coinciding with its normal has created non-equilibrium current carriers that were screening the built-in surface field. Only the probe beam was mechanically chopped and S-polarized THz pulses were measured by the lock-in amplifier. As it can be seen from Fig. 8, THz pulse amplitude generated by the probe pulse is significantly reduced during 2 ps after the illumination with the pump pulse. In n-Ge THz pulses do even change their sign and the traces measured on n- and p-type crystals coincide with each other for some time period. These observations can be exploited for evaluating relative contributions of different physical mechanisms to the THz pulse emission from Ge surfaces. Because S-polarized radiation is registered, only the nonlinear optical effects, namely EFIOR effect, should be taken into account. The reduction of the THz pulse amplitude is, evidently, caused by the surface electric field screening by the carriers created by the pump pulse. When the surface field is screened and the energy bands are flat the only physical mechanism that could cause THz radiation is the EFIOR effect in the field induced by the carrier separation (photoDember field). It follows from the results presented in Fig. 8 that
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the polarity of this field corresponds to the case when holes, not electrons, are displaced farther towards the bulk, i.e., the mobility of the holes photoexcited by 1 mm wavelength laser pulses is larger than that of the electrons. In equilibrium conditions, when the carriers occupy the energy levels close to the band edge, the electron mobility in germanium is approximately 2 times larger than the hole mobility. However, the excitation with photons with energy much larger than the bandgap will create electrons in upper conduction band D-valleys where their mobility is 10 times smaller. Because the return of these electrons to the lowest L-valleys takes much longer time (4 ps [18]) than the duration of the THz pulse emission, the effective hole mobility right after the photoexcitation can be larger than the effective electron mobility.
4. Conclusions In conclusion, we have compared the amplitudes of THz pulses emitted from the surfaces of various semiconductors after their excitation by femtosecond 1 mm laser pulses. It has been found that this effect is most efficient in p-type InAs. THz time-domain spectroscopy system utilizing p-InAs emitter and detector made from low-temperature grown GaBiAs, when activated by 70 fs duration laser pulses, has demonstrated the signal-to-noise ratio of 60 dB and the bandwidth of 5 THz. In the majority of the investigated semiconductors the main contribution to THz pulse emission comes from the optical rectification effect induced by the simultaneous action of the third-order optical nonlinearity and the electric fields arising due to photoexcited electron and
hole separation and/or the band bending at the semiconductor surface. References [1] B. Ferguson, X.-C. Zhang, Nat. Mater. 1 (2002) 26. [2] S. Gupta, M.Y. Frankel, J.A. Valdmanis, J.F. Whitaker, G.A. Mourou, F.W. Smith, A.R. Calawa, Appl. Phys. Lett. 59 (1991) 3276. [3] C. Jagadish, H.H. Tan, A. Krotkus, S. Marcinkevicius, K.P. Korona, M. Kaminska, Appl. Phys. Lett. 68 (1996) 2225. [4] A. Krotkus, J.L. Coutaz, Semicond. Sci. Technol. 20 (2005) S142. [5] P. Polynkin, A. Polynkin, M. Mansuripur, J. Moloney, N. Peyghambarian, Opt. Lett. 30 (2005) 2745. % [6] L. Giniunas, J. Pocius, R. Danielius, in: Proceedings of CLEO/Europe-IQEC 2007. European Conference on Lasers and Electro-Optics and the International Quantum Electronics Conference, Munich, Germany, 17–22 June, 2007, p. 79. [7] G. Matthaus, T. Schreiber, J. Limpert, S. Nolte, G. Torosyan, R. Beigang, S. Riehemann, G. Notni, A. Tunnermann, Opt. Commun. 261 (2006) 114. [8] N. Chimot, J. Mangeney, L. Joulaud, P. Crozat, H. Bernas, K. Blary, J.F. Lampin, Appl. Phys. Lett. 87 (2005) 193510. % [9] G. Molis, R. Adomavicˇius, A. Krotkus, K. Bertulis, L. Giniunas, J. Pocius, R. Danielius, Electron. Lett. 43 (2007) 190. [10] X.-C. Zhang, D.H. Auston, J. Appl. Phys. 71 (1992) 326. [11] R. Adomavicˇius, A. Urbanowicz, G. Molis, A. Krotkus, E. Sˇatkovskis, Appl. Phys. Lett. 85 (2004) 2463. [12] V.L. Malevich, R. Adomavicˇius, A. Krotkus, C. R. Phys. 9 (2008) 130. [13] M. Reid, I.V. Cravetchi, R. Fedosejevs, Phys. Rev. B 72 (2005) 035201. [14] P. Gu, M. Tani, S. Kono, K. Sakai, X.-C. Zhang, J. Appl. Phys. 91 (2002) 5533. [15] R. Adomavicius, G. Molis, A. Krotkus, V. Sirutkaitis, Appl. Phys. Lett. 87 (2005) 261101. [16] X.-C. Zhang, Y. Jin, K. Yang, L.J. Schowalter, Phys. Rev. Lett. 69 (1992) 2303. [17] A. Urbanowicz, A. Krotkus, R. Adomavicˇius, V.L. Malevich, Phys. B Condensed Matter 398 (2007) 98. [18] A. Urbanowicz, R. Adomavicˇius, A. Krotkus, V.L. Malevich, Semicond. Sci. Technol. 20 (2005) 1010.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Terahertz pulse emission from semiconductor surfaces illuminated by femtosecond Yb:KGW laser pulses , V. Pacˇebutas, A. Krotkus % A. Bicˇiunas Semiconductor Physics Institute, 01108, A. Gostauto 11, Vilnius, Lithuania
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 December 2008 Accepted 14 May 2009
The amplitudes of terahertz pulses emitted from the surfaces of InAs, InSb, InGaAs, GaAs and Ge after their excitation by femtosecond 1 mm laser pulses was compared. It has been found that this effect is most efficient in p-type InAs. The mechanisms leading to the terahertz emission are investigated and discussed. It has been concluded that in the majority of the investigated semiconductors the main contribution to THz pulse emission comes from the electrical-field-induced optical rectification effect. & 2009 Elsevier B.V. All rights reserved.
PACS: 07.57.Hm 42.65.Re 87.50.U 61.82.Fk Keywords: Terahertz InAs InSb InGaAs Ge Electrical-field-induced optical rectification
1. Introduction Terahertz (THz) pulse generation and detection using optoelectronic semiconductor components excited by femtosecond laser pulses has become lately a powerful experimental technique finding numerous applications in THz time-domain spectroscopy and THz imaging [1]. Traditionally, mode-locked Ti:sapphire lasers emitting at the wavelengths around 800 nm are used for this purpose because they are more advanced technically and commercially more mature than other similar products and because their photon energy perfectly matches the absorption band of GaAs. Nonstoichiometric GaAs obtained either by molecular-beam-epitaxy growth at reduced temperatures (LTG GaAs) [2] or by high-energy, heavy ion implantation [3] is characterized by unique set of material parameters [4] that facilitates its applications in pulsed THz components. However, because Ti:sapphire lasers require a rather complicated, manystage optical pumping arrangement, these systems are quite bulky and complicated, which prevents their wider applications.
Corresponding author. Tel.: +370 5 262 74 69; fax: +370 5 262 71 23.
% E-mail address: biciunas@pfi.lt (A. Bicˇiunas). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.05.017
One solution of this problem would be using the lasers emitting in the spectral ranges close to 1 or 1.5 mm, which can be directly pumped by diode laser bars. Recently, several compact, efficient and cost-effective solid-state and fiber laser systems that generate femtosecond pulses at these near-infrared wavelengths have been developed [5,6] and have been employed for activating THz time-domain spectroscopy systems [7–9]. The main obstacle in further development of these systems is the absence of material with appropriate bandgap and parameters high dark resistivities and subpicosecond carrier lifetimes—similar to those of LTG GaAs, which could be used for manufacturing photoconductive THz emitters and detectors. In the case of THz detectors activated by 1 mm wavelength femtosecond lasers, epitaxial layers of LTG InGaAs [7] or LTG GaBiAs [9] were employed, whereas no material appropriate for photoconductive THz emitters has been identified as yet. It is known [10] that THz radiation can be emitted also from unbiased semiconductor surfaces photoexcited by femtosecond laser pulses. At Ti:sapphire laser wavelength this effect is most efficient when using p-type InAs crystals [11]. Here we present a systematic study of THz pulse emission from the surfaces of various semiconductors illuminated by femtosecond Yb:KGW (potassium gadolinium vanadate) laser. It is shown that p-InAs and other narrow-gap semiconductors are most efficient THz
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emitters also in this case. Investigation of the physical mechanism leading to this effect has shown the domination of the electricalfield-induced optical rectification (EFIOR) for the majority of the materials studied.
2. Experimental details As the optical pulse source for THz pulse generation and sampling the oscillator from a high repetition rate femtosecond laser system Pharos (Light Conversion Ltd) was used. The oscillator was based on directly diode-pumped Yb:KGW (KGa(WO3)) crystal. Kerr-lens mode-locking was used to generate optical pulses with the central wavelength of 1030 nm. The average output power of the oscillator was 2 W, the pulse duration was 63 fs (Gaussian fit), the spectral width (FWHM) was 28 nm, and the pulse repetition rate was 75 MHz. Different semiconductor surfaces have been illuminated by the laser beam at the angle of 451 and THz radiation emitted from these surfaces was monitored in quasireflection direction. Azimuthal angle dependences of the emitted THz pulse amplitude were measured for all investigated semiconductors by rotating the crystal around the normal to its surface. All measurements were performed at room temperature. In our previous work, we have proposed to manufacture optoelectronic THz range components sensitive to near-infrared radiation from LTG GaBiAs alloy [9]. Epitaxial GaBi0.06As0.94 layer was grown on semi-insulating (1 0 0)-oriented GaAs substrate in a solid-state molecular-beam-epitaxy (MBE) system at 280 1C substrate temperature at the growth rate of 2 mm/h. The thickness of the layer was 0.4 mm; it has been used for the manufacturing coplanar Hertzian dipole antennae with 15 mm wide photoconducting gap between Ti–Au electrodes and equipped with a hemispherical lens made from high resistivity Si. S and P-polarized THz signals were separated by rotating the detector that was polarization sensitive and inserting into the THz beam path at a Brewster angle a semi-insulating InP wafer. The performance of the experimental system is illustrated in Fig. 1, where THz pulse and its corresponding Fourier spectrum radiated from the surface of p-type InAs crystal—most efficient THz emitter from all materials investigated in the present work—are shown. Besides p- and n-type InAs crystals, THz emission from the surfaces of InSb (intrinsic conduction at the room temperature) and p- and n-type Ge were investigated. All these crystals were cut parallel to the (111) crystallographic plane. On the other hand, epitaxial layers of CdxHg1xTe (with x ¼ 0.2) grown on CdTe substrates, and In0.5Ga0.5As on InP substrate had the symmetry of (1 0 0). The same symmetry had the wafers of semi-insulating GaAs that were also investigated in the experiments.
Fig. 1. THz pulse and its Fourier spectrum radiated from the surface of p-type InAs crystal and measured using GaBiAs detector.
3. Results and discussion Diagram presented in Fig. 2 compares THz pulse amplitudes as emitted from the surfaces of various semiconductor materials under the same experimental conditions. As it was already mentioned before, most efficiently THz pulses are generated at the surface of p-InAs, followed by several other narrow-gap semiconductors. There are two main groups of the physical mechanisms leading to THz emission from semiconductor surfaces excited by femtosecond laser pulses: surface photocurrent surge and optical rectification of femtosecond laser pulses due to their nonlinear optical interaction with the material [12]. Standard way to separate these mechanisms is measuring azimuthal angle dependences of the radiated THz pulse amplitudes: for semiconductor crystals that, typically, have a cubic symmetry the photocurrent is isotropic, whereas THz pulses
Fig. 2. Relative comparison of THz pulse amplitudes, emitted from the surface of various semiconductors illuminated by femtosecond 1 mm wavelength laser pulses. Light intensity on surface emitters is 0.63 W/cm2.
radiated due to the optical rectification effect exhibit complex dependences on the azimuthal angle determined by the symmetry of the photoexcited crystallographic plane and on the
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order of the optical nonlinearity (second or third order) that causes this effect [13]. In the following we will discuss the azimuthal angle dependences of THz radiation from different semiconductors separately.
3.1. InAs and InSb When excited with Ti:sapphire laser pulses, these narrow-gap semiconductors are the best THz pulse emitters because of large difference in the velocities of photoexcited electrons and holes that causes efficient spatial separation of these current carriers and because of a significant third order optical nonlinearity leading to the EFIOR effect at the surface of these materials [12]. Similar features of this effect were observed also when femtosecond 1 mm wavelength optical pulses were used for the photoexcitation. Fig. 3 shows azimuthal angle f dependences of P- and S-polarized THz pulse amplitude measured on two differently doped InAs crystals. All curves show the variation proportional to cos 3f that is typical for the optical rectification effect on (111) surfaces; the presence of S-polarized THz radiation, which cannot be generated by any mechanisms related to the photocurrent surge, is also evidencing the significance of the nonlinear optical effects. As in the case of 850 nm excitation [13], the most probable cause of THz emission is the optical rectification induced by the electric field that is appearing at the crystal surface due to the
photoexcited electron and hole separation. THz pulses radiated from p-type InAs surface have more than twice larger amplitudes than those radiated from n-InAs. This difference can be explained by the presence of a built-in electric field of the inversion layer at p-InAs surface [11]. Surface electric field in p-InAs is of the same polarity as the field arising due to the photoexcited electron and hole separation; however, it interacts with the whole optical pulse and not with its part as in n-InAs, where optical rectification effect is due to the electric field induced by the movement of the electrons excited during the initial phase of the laser pulse. This conclusion about the mechanisms of THz emission from n- and p-type InAs is supported by the measurements of THz pulse (S-polarized) amplitude dependences on the laser beam intensity presented in Fig. 4. As can be seen from this figure, the dependence measured on p-InAs shows a strong tendency to saturation, whereas in the case of n-InAs it remains linear to the largest intensities employed. This difference can arise due to the built-in surface field screening in the former crystal by carriers generated during the slowly varying initial part of the laser pulse. Azimuthal angle dependences of THz signal radiated from the surfaces of InSb (Fig. 5) are similar to those measured on InAs but the amplitudes of THz pulses are smaller. Although the excess
Fig. 4. THz pulse amplitude as a function of the average optical power for two InAs crystals.
Fig. 3. Azimuthal angle of P (full lines) and S-polarized (dashed lines) THz pulse amplitudes emitted from the (111) surfaces of p-InAs (a) and n-InAs (b) crystals.
Fig. 5. Azimuthal angle of P (full line) and S-polarized (dashed line) THz pulse amplitudes emitted from the (111) surface of InSb.
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energy of photoexcited electrons in InSb is larger than in InAs, which has a larger energy bandgap, electric field induced by the electron and hole separation is weaker because of the intense electron scattering to subsidiary conduction band valleys laying at L and X points of the Brillouin zone [14,15]. 3.2. GaAs and InGaAs The symmetry of the surfaces of semi-insulating GaAs crystal and epitaxial In0.5Ga0.5As layers corresponded to (1 0 0) crystallographic planes for which the EFIOR effect has only an azimuthal angle independent component. Therefore, azimuthal angle dependences of THz pulse amplitude proportional to cos 2f that were experimentally observed on the samples of these materials (Fig. 6) evidence the presence of the second-order nonlinear optical rectification effect [16]. For the case of GaAs transparent to 1030 nm wavelength radiation, this effect is the only physical mechanism leading to THz emission. For In0.5Ga0.5As that is absorbing laser radiation, besides the optical rectification contribution there is a significant azimuthal angle independent component, which can be caused by the EFIOR or/and photoDember effects.
Fig. 7. Azimuthal angle dependences of P-polarization (full line) and S-polarization (dashed line) THz pulse emitted from (111) plane of p-Ge with the resistivity r ¼ 0.05 O cm.
3.3. Germanium In contrast to the semiconducting compounds discussed above, elemental semiconductor crystal germanium do not possess a symmetry center, therefore clear dependences of THz signal radiated from its surface on the azimuthal angle shown in Fig. 7 evidence that the main cause of this effect is third-order optical nonlinearity or the EFIOR effect. This has been discovered before also for the case of the Ti:sapphire laser excitation of Ge [17]. The amplitudes of THz pulses are larger for the crystals with a higher doping level, for which surface electric fields are stronger. Additional proof of the significance of the EFIOR effect in Ge was obtained after measuring the pump-and-probe traces presented in Fig. 8. In this experiment the semiconductor surface was illuminated by two parts of the laser beam delayed with respect to each other. The weaker (average power of 45 mW) part of the laser beam was used for the generation of THz pulses, whereas the stronger part (average power of 600 mW) that was
Fig. 6. Azimuthal angle dependences of P-polarization THz pulse emitted from (1 0 0) planes of GaAs (full line) and In0.5Ga0.5As (dashed line).
Fig. 8. Amplitude of THz pulse generated by the probe pulse as a function of optical delay between the pump and the probe pulses. Full line n-Ge (r ¼ 0.2 O cm), dashed line p-Ge (r ¼ 0.05 O cm).
incident on the surface in the direction coinciding with its normal has created non-equilibrium current carriers that were screening the built-in surface field. Only the probe beam was mechanically chopped and S-polarized THz pulses were measured by the lock-in amplifier. As it can be seen from Fig. 8, THz pulse amplitude generated by the probe pulse is significantly reduced during 2 ps after the illumination with the pump pulse. In n-Ge THz pulses do even change their sign and the traces measured on n- and p-type crystals coincide with each other for some time period. These observations can be exploited for evaluating relative contributions of different physical mechanisms to the THz pulse emission from Ge surfaces. Because S-polarized radiation is registered, only the nonlinear optical effects, namely EFIOR effect, should be taken into account. The reduction of the THz pulse amplitude is, evidently, caused by the surface electric field screening by the carriers created by the pump pulse. When the surface field is screened and the energy bands are flat the only physical mechanism that could cause THz radiation is the EFIOR effect in the field induced by the carrier separation (photoDember field). It follows from the results presented in Fig. 8 that
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the polarity of this field corresponds to the case when holes, not electrons, are displaced farther towards the bulk, i.e., the mobility of the holes photoexcited by 1 mm wavelength laser pulses is larger than that of the electrons. In equilibrium conditions, when the carriers occupy the energy levels close to the band edge, the electron mobility in germanium is approximately 2 times larger than the hole mobility. However, the excitation with photons with energy much larger than the bandgap will create electrons in upper conduction band D-valleys where their mobility is 10 times smaller. Because the return of these electrons to the lowest L-valleys takes much longer time (4 ps [18]) than the duration of the THz pulse emission, the effective hole mobility right after the photoexcitation can be larger than the effective electron mobility.
4. Conclusions In conclusion, we have compared the amplitudes of THz pulses emitted from the surfaces of various semiconductors after their excitation by femtosecond 1 mm laser pulses. It has been found that this effect is most efficient in p-type InAs. THz time-domain spectroscopy system utilizing p-InAs emitter and detector made from low-temperature grown GaBiAs, when activated by 70 fs duration laser pulses, has demonstrated the signal-to-noise ratio of 60 dB and the bandwidth of 5 THz. In the majority of the investigated semiconductors the main contribution to THz pulse emission comes from the optical rectification effect induced by the simultaneous action of the third-order optical nonlinearity and the electric fields arising due to photoexcited electron and
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