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Terahertz wave generation from GaSe crystals and effects of crystallinity
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 605–607 www.elsevier.com/locate/jpcs
Terahertz wave generation from GaSe crystals and effects of crystallinity Atsushi Kenmochia,, Tadao Tanabea, Yutaka Oyamaa, Ken Sutob, Jun-ichi Nishizawab a
Department of Materials Science and Engineering, Graduate School of Engineering, Tohoku University, Aoba-yama 6-6-21-1021, Sendai 980-8579, Japan b Semiconductor Research Institute, Aramaki Aza Aoba 519-1176, Sendai 980-0845, Japan
Abstract Terahertz (THz)-wave output power generated by GaSe crystals was investigated with respect to crystallinity. GaSe crystals with an epsilon crystal structure have carrier densities of 1010 or 1014 cm3. According to X-ray diffraction measurements, the two crystals have an almost identical lattice strain, while the lower carrier density crystal has a greater dislocation density than that of the crystal with higher carrier density. At frequencies of less than 1.3 THz, the THz wave output power of the GaSe crystal with the lower carrier density was higher than that of the crystal with the higher density. At frequencies greater than 1.3 THz, the crystal with the lower dislocation had a higher output than the other crystal. Therefore, a GaSe crystal with a low carrier density and few lattice defects should generate more powerful THz waves. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Semiconductor; A. Optical materials; C. Infrared spectroscopy; D. Optical properties
1. Introduction The terahertz (THz) region is located between the light and millimeter waves, and is relatively underutilized, compared to other regions, because no effective source of light or detector has been reported in the region. THz waves have straightly propagating light-wave properties, and a diffracted electric wave. In addition, this region contains many vibration modes in macromolecules, such as polymers and biomolecules. Therefore, THz spectroscopy [1] and THz imaging technology [2] could potentially be useful in the fields of medicine and pharmacy. In 1963, Nishizawa proposed that THz waves could be generated by exciting phonons in compounds such as semiconductors and dielectric substances [3,4]. Based on this proposal, Nishizawa and Suto [5] in 1979, succeeded in producing oscillation of the Raman laser with a GaP semiconductor in 1983, they used this GaP Raman oscillator, containing a GaAs mixing crystal, to generate Corresponding author. Tel.: +81 22 795 7330; fax: +81 22 795 7329.
E-mail addresses: [email protected] (A. Kenmochi), [email protected] (T. Tanabe). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.07.047
a THz wave (12 THz) with a peak power as high as 3 W [6]. In 2000, Nishizawa [7] suggested that THz waves could be applied to the diagnosis and treatment of cancer. In 2002, Tanabe, Suto, and Nishizawa generated THz waves by exiting a transverse optical (TO) phonon in a GaP crystal [8,9]. A THz spectral measurement system has been constructed with an automatic control (GaP Raman THz: GRT spectrometer) [10]. In a previous study, we measured the absorption spectra of biomolecules at frequencies of 0.8–5.8 THz [1]. We also generated THz waves based on difference frequency generation (DFG) in GaSe [11], ZnGeP2, and GaP. GaSe has a high second-order nonlinear optical (NLO) coefficient (d22 ¼ 54 pm V1) [12], and therefore this crystal is the most efficient semiconductor for THz wave generation. Furthermore, GaSe yields a simpler collinear phase-matched THz wave generation system than the non-collinear phase-matched DFG from a GaP crystal. Generated THz wave output may be greatly dependent on crystallinity [13,14]. The GaSe crystal has a layered structure, and is thus prone to defects and dislocations. However, researchers have not investigated how crystallinity affects THz wave output power.
ARTICLE IN PRESS 606
A. Kenmochi et al. / Journal of Physics and Chemistry of Solids 69 (2008) 605–607
In this study, we examined the crystallinity of GaSe crystals using X-ray diffraction, and investigated how it affected THz wave output. Intensity (a. u.)
We used z-cut undoped GaSe crystals with a length of 2 mm along the c-axis, and carrier densities of either 1010 or 1014 cm3. These crystals were grown using the Bridgman method. Lattice parameters were measured using (00018) X-ray symmetric diffraction. Lattice strain and dislocation were measured using the (00018) reciprocal space map. The pump source of the DFG in the GaSe crystals was a b-BaB2O4 (BBO)-based OPO (l ¼ 1042–1064 nm) and the signal source was a Nd:YAG laser (l ¼ 1064 nm) (Fig. 1). The OPO was pumped with 355 nm line from the third harmonic generation of the Nd:YAG laser. The Nd:YAG and OPO had pulse widths and line widths of 11 ns and 0.003 cm1 and 6 ns and 0.2 cm1, respectively. The repetition rate was 10 Hz. Pulse energies of the YAG and OPO were attenuated to 3 mJ (3 mm beam diameter). Two beams were mixed using a cube beam splitter. The pump and signal beams had vertical and horizontal polarization directions, respectively. An optical pass was adjusted by a delay line. In the case of GaSe with birefingent, the collinear phase-matching condition was satisfied by rotating the GaSe crystal. The generated output signals were detected using a liquid helium-cooled Si bolometer. A Ge filter was used to filter out near-infrared incidence beams.
100
10
120.72 120.76 2 theta - omega (deg)
120.80
Fig. 2. (00018) X-ray symmetric diffraction curves. Solid line: lower carrier density crystal; broken line: higher carrier density crystal.
0.5 0.4 0.3 0.2 0.1 -0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.10
3. Results and discussion
-0.05
0.00
0.05
0.10
0.05
0.10
Omega/2 Theta 0.5 0.4
Omega
Fig. 2 shows the (00018) X-ray symmetric diffraction rocking curves. The c-axes of GaSe crystals with carrier densities of 1010 and 1014 cm3 had lattice constants of 15.9633 and 15.9636 A˚, respectively. We confirmed that both GaSe crystals had epsilon structures; the full-width at half-maximum (FWHM) was almost equal, suggesting that the two crystals shared an almost identical lattice strain. Fig. 3 presents the (00018) reciprocal space maps. The lattice strain was almost identical in the two maps, but the crystal with the lower carrier density had a greater dislocation than the other crystal; it also contained twins,
1010 cm-3 1014 cm-3
Omega
2. Experiment
1000
0.3 0.2 0.1 -0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.10
-0.05
0.00
Omega/2 Theta
Fig. 3. (00018) reciprocal space maps of GaSe crystals for: (a) lower carrier density crystal and (b) higher carrier density crystal.
Fig. 1. Schematics of optical setup for THz wave generation from GaSe crystals.
and defects that scattered the THz waves. Therefore, a crystal with fewer defects (in this case, with the higher carrier density) yields a higher output power at every frequency over 1.3 THz, as shown in the experimental results (Fig. 4). Fig. 4 illustrates the frequency dependence of THz wave output power on the GaSe crystals. At frequencies below
ARTICLE IN PRESS A. Kenmochi et al. / Journal of Physics and Chemistry of Solids 69 (2008) 605–607
higher output power may be obtained from a crystal with a lower carrier density because low-frequency regions have little free carrier absorption. Fig. 5 illustrates the dependence of the phase-matching angle on the peak frequency position of THz wave output power. The angles were different at frequencies below 2 THz, probably because of a difference in the refractive index, which is related to free carrier density.
10 THz power (mW)
Ratio
(1010 cm-3) / (1014 cm-3)
3
2
1
0.1
0.01 1
2 3 4 Frequency (THz)
5
4. Conclusion
1
1
2
3
4
5
Frequency (THz) Fig. 4. Power ratio of THz waves in lower carrier density crystal to higher carrier density crystal. Inset: frequency dependence of THz wave output power from GaSe crystals.
50 External PM angle (deg)
607
θ
THz wave
c-axis
40
We investigated THz wave output power generated from different e-GaSe crystals. Results of X-ray diffraction indicate that the two crystals had an almost identical lattice strain, while the crystal with the lower carrier density had a greater dislocation. At frequencies over 1.3 THz, greater output power was produced by the GaSe crystal with fewer defects, because the defects scattered the THz waves. Because free carrier absorption is greater at low frequencies (below 2 THz), output power was enhanced at a lower carrier density. Our results suggest that defects in the free carriers and the lattice of GaSe crystals should be minimized to achieve higher THz wave output power. GaSe crystal growth and heat treatment under stoichiometry control with Se vapor should be effective.
OPO+YAG GaSe crystal
30
References
20 1014 cm-3 10
1010 cm-3 1
2
3 4 Frequency (THz)
5
6
Fig. 5. Dependence of the phase-matching angle on the peak frequency position of THz wave output power.
1.3 THz, the crystal with the lower carrier density (1010 cm3) yielded higher THz wave output power than that of the higher carrier density (1014 cm3). THz wave absorption by the crystal was mainly attributable to a free carrier. In particular, free carrier absorption was proportional to lp, where l is THz wavelength and p is constant from 1.5 to 3.5 [15]. It is possible that free carrier absorption was dominant in low-frequency regions. In frequencies below 1 THz, the absorption coefficient increases steeply [11], due to free carrier absorption. Thus, a
[1] J. Nishizawa, T. Sasaki, K. Suto, T. Tanabe, K. Saito, T. Yamada, T. Kimura, Opt. Commun. 246 (2005) 229. [2] J. Nishizawa, T. Sasaki, K. Suto, T. Yamada, T. Tanabe, T. Tanno, T. Sawai, Yasuhiro Miura, Opt. Commun. 244 (2005) 469. [3] J. Nishizawa, Denshi Kagaku 14 (1963) 17–30 (in Japanese). [4] J. Nishizawa, Denshi Gijutu 7 (1965) 101 (in Japanese). [5] J. Nishizawa, K. Suto, J. Appl. Phys. 51 (1980) 2429. [6] K. Suto, J. Nishizawa, IEEE J. Quantum Electron. 19 (1983) 1251. [7] J. Nishizawa, J. Acoust. Soc. Jpn. 57 (2001) 163 (in Japanese). [8] T. Tanabe, K. Suto, J. Nishizawa, T. Kimura, K. Saito, J. Appl. Phys. 93 (2003) 4610. [9] T. Tanabe, K. Suto, J. Nishizawa, K. Saito, T. Kimura, J. Phys. D: Appl. Phys. 36 (2003) 953. [10] J. Nishizawa, K. Suto, T. Sasaki, T. Tanabe, T. Kimura, J. Phys. D: Appl. Phys. 36 (2003) 2958. [11] T. Tanabe, K. Suto, J. Nishizawa, T. Sasaki, J. Phys. D: Appl. Phys. 37 (2004) 155. [12] V.G. Dmitriev, G.G. Gurzadyan, D.N. Nikogosyan, Handbook of Nonlinear Optical Crystals, Springer, Berlin, 1997, 68p. [13] W. Shi, Y.J. Ding, N. Fernelius, K. Vodopyanov, Opt. Lett. 27 (2002) 1454. [14] T. Tanabe, K. Suto, J. Nishizawa, T. Sasaki, H. Yasuda, Y. Oyama, in: Proceeding of International Symposium on Compound Semiconductors (ISCS2004). [15] A. Jacques, I. Pankove, Optical Processes in Semiconductors, Dover Publications, New York, 1975 (Translated by J. Nishizawa to Japanese).
Journal of Physics and Chemistry of Solids 69 (2008) 605–607 www.elsevier.com/locate/jpcs
Terahertz wave generation from GaSe crystals and effects of crystallinity Atsushi Kenmochia,, Tadao Tanabea, Yutaka Oyamaa, Ken Sutob, Jun-ichi Nishizawab a
Department of Materials Science and Engineering, Graduate School of Engineering, Tohoku University, Aoba-yama 6-6-21-1021, Sendai 980-8579, Japan b Semiconductor Research Institute, Aramaki Aza Aoba 519-1176, Sendai 980-0845, Japan
Abstract Terahertz (THz)-wave output power generated by GaSe crystals was investigated with respect to crystallinity. GaSe crystals with an epsilon crystal structure have carrier densities of 1010 or 1014 cm3. According to X-ray diffraction measurements, the two crystals have an almost identical lattice strain, while the lower carrier density crystal has a greater dislocation density than that of the crystal with higher carrier density. At frequencies of less than 1.3 THz, the THz wave output power of the GaSe crystal with the lower carrier density was higher than that of the crystal with the higher density. At frequencies greater than 1.3 THz, the crystal with the lower dislocation had a higher output than the other crystal. Therefore, a GaSe crystal with a low carrier density and few lattice defects should generate more powerful THz waves. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Semiconductor; A. Optical materials; C. Infrared spectroscopy; D. Optical properties
1. Introduction The terahertz (THz) region is located between the light and millimeter waves, and is relatively underutilized, compared to other regions, because no effective source of light or detector has been reported in the region. THz waves have straightly propagating light-wave properties, and a diffracted electric wave. In addition, this region contains many vibration modes in macromolecules, such as polymers and biomolecules. Therefore, THz spectroscopy [1] and THz imaging technology [2] could potentially be useful in the fields of medicine and pharmacy. In 1963, Nishizawa proposed that THz waves could be generated by exciting phonons in compounds such as semiconductors and dielectric substances [3,4]. Based on this proposal, Nishizawa and Suto [5] in 1979, succeeded in producing oscillation of the Raman laser with a GaP semiconductor in 1983, they used this GaP Raman oscillator, containing a GaAs mixing crystal, to generate Corresponding author. Tel.: +81 22 795 7330; fax: +81 22 795 7329.
E-mail addresses: [email protected] (A. Kenmochi), [email protected] (T. Tanabe). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.07.047
a THz wave (12 THz) with a peak power as high as 3 W [6]. In 2000, Nishizawa [7] suggested that THz waves could be applied to the diagnosis and treatment of cancer. In 2002, Tanabe, Suto, and Nishizawa generated THz waves by exiting a transverse optical (TO) phonon in a GaP crystal [8,9]. A THz spectral measurement system has been constructed with an automatic control (GaP Raman THz: GRT spectrometer) [10]. In a previous study, we measured the absorption spectra of biomolecules at frequencies of 0.8–5.8 THz [1]. We also generated THz waves based on difference frequency generation (DFG) in GaSe [11], ZnGeP2, and GaP. GaSe has a high second-order nonlinear optical (NLO) coefficient (d22 ¼ 54 pm V1) [12], and therefore this crystal is the most efficient semiconductor for THz wave generation. Furthermore, GaSe yields a simpler collinear phase-matched THz wave generation system than the non-collinear phase-matched DFG from a GaP crystal. Generated THz wave output may be greatly dependent on crystallinity [13,14]. The GaSe crystal has a layered structure, and is thus prone to defects and dislocations. However, researchers have not investigated how crystallinity affects THz wave output power.
ARTICLE IN PRESS 606
A. Kenmochi et al. / Journal of Physics and Chemistry of Solids 69 (2008) 605–607
In this study, we examined the crystallinity of GaSe crystals using X-ray diffraction, and investigated how it affected THz wave output. Intensity (a. u.)
We used z-cut undoped GaSe crystals with a length of 2 mm along the c-axis, and carrier densities of either 1010 or 1014 cm3. These crystals were grown using the Bridgman method. Lattice parameters were measured using (00018) X-ray symmetric diffraction. Lattice strain and dislocation were measured using the (00018) reciprocal space map. The pump source of the DFG in the GaSe crystals was a b-BaB2O4 (BBO)-based OPO (l ¼ 1042–1064 nm) and the signal source was a Nd:YAG laser (l ¼ 1064 nm) (Fig. 1). The OPO was pumped with 355 nm line from the third harmonic generation of the Nd:YAG laser. The Nd:YAG and OPO had pulse widths and line widths of 11 ns and 0.003 cm1 and 6 ns and 0.2 cm1, respectively. The repetition rate was 10 Hz. Pulse energies of the YAG and OPO were attenuated to 3 mJ (3 mm beam diameter). Two beams were mixed using a cube beam splitter. The pump and signal beams had vertical and horizontal polarization directions, respectively. An optical pass was adjusted by a delay line. In the case of GaSe with birefingent, the collinear phase-matching condition was satisfied by rotating the GaSe crystal. The generated output signals were detected using a liquid helium-cooled Si bolometer. A Ge filter was used to filter out near-infrared incidence beams.
100
10
120.72 120.76 2 theta - omega (deg)
120.80
Fig. 2. (00018) X-ray symmetric diffraction curves. Solid line: lower carrier density crystal; broken line: higher carrier density crystal.
0.5 0.4 0.3 0.2 0.1 -0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.10
3. Results and discussion
-0.05
0.00
0.05
0.10
0.05
0.10
Omega/2 Theta 0.5 0.4
Omega
Fig. 2 shows the (00018) X-ray symmetric diffraction rocking curves. The c-axes of GaSe crystals with carrier densities of 1010 and 1014 cm3 had lattice constants of 15.9633 and 15.9636 A˚, respectively. We confirmed that both GaSe crystals had epsilon structures; the full-width at half-maximum (FWHM) was almost equal, suggesting that the two crystals shared an almost identical lattice strain. Fig. 3 presents the (00018) reciprocal space maps. The lattice strain was almost identical in the two maps, but the crystal with the lower carrier density had a greater dislocation than the other crystal; it also contained twins,
1010 cm-3 1014 cm-3
Omega
2. Experiment
1000
0.3 0.2 0.1 -0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.10
-0.05
0.00
Omega/2 Theta
Fig. 3. (00018) reciprocal space maps of GaSe crystals for: (a) lower carrier density crystal and (b) higher carrier density crystal.
Fig. 1. Schematics of optical setup for THz wave generation from GaSe crystals.
and defects that scattered the THz waves. Therefore, a crystal with fewer defects (in this case, with the higher carrier density) yields a higher output power at every frequency over 1.3 THz, as shown in the experimental results (Fig. 4). Fig. 4 illustrates the frequency dependence of THz wave output power on the GaSe crystals. At frequencies below
ARTICLE IN PRESS A. Kenmochi et al. / Journal of Physics and Chemistry of Solids 69 (2008) 605–607
higher output power may be obtained from a crystal with a lower carrier density because low-frequency regions have little free carrier absorption. Fig. 5 illustrates the dependence of the phase-matching angle on the peak frequency position of THz wave output power. The angles were different at frequencies below 2 THz, probably because of a difference in the refractive index, which is related to free carrier density.
10 THz power (mW)
Ratio
(1010 cm-3) / (1014 cm-3)
3
2
1
0.1
0.01 1
2 3 4 Frequency (THz)
5
4. Conclusion
1
1
2
3
4
5
Frequency (THz) Fig. 4. Power ratio of THz waves in lower carrier density crystal to higher carrier density crystal. Inset: frequency dependence of THz wave output power from GaSe crystals.
50 External PM angle (deg)
607
θ
THz wave
c-axis
40
We investigated THz wave output power generated from different e-GaSe crystals. Results of X-ray diffraction indicate that the two crystals had an almost identical lattice strain, while the crystal with the lower carrier density had a greater dislocation. At frequencies over 1.3 THz, greater output power was produced by the GaSe crystal with fewer defects, because the defects scattered the THz waves. Because free carrier absorption is greater at low frequencies (below 2 THz), output power was enhanced at a lower carrier density. Our results suggest that defects in the free carriers and the lattice of GaSe crystals should be minimized to achieve higher THz wave output power. GaSe crystal growth and heat treatment under stoichiometry control with Se vapor should be effective.
OPO+YAG GaSe crystal
30
References
20 1014 cm-3 10
1010 cm-3 1
2
3 4 Frequency (THz)
5
6
Fig. 5. Dependence of the phase-matching angle on the peak frequency position of THz wave output power.
1.3 THz, the crystal with the lower carrier density (1010 cm3) yielded higher THz wave output power than that of the higher carrier density (1014 cm3). THz wave absorption by the crystal was mainly attributable to a free carrier. In particular, free carrier absorption was proportional to lp, where l is THz wavelength and p is constant from 1.5 to 3.5 [15]. It is possible that free carrier absorption was dominant in low-frequency regions. In frequencies below 1 THz, the absorption coefficient increases steeply [11], due to free carrier absorption. Thus, a
[1] J. Nishizawa, T. Sasaki, K. Suto, T. Tanabe, K. Saito, T. Yamada, T. Kimura, Opt. Commun. 246 (2005) 229. [2] J. Nishizawa, T. Sasaki, K. Suto, T. Yamada, T. Tanabe, T. Tanno, T. Sawai, Yasuhiro Miura, Opt. Commun. 244 (2005) 469. [3] J. Nishizawa, Denshi Kagaku 14 (1963) 17–30 (in Japanese). [4] J. Nishizawa, Denshi Gijutu 7 (1965) 101 (in Japanese). [5] J. Nishizawa, K. Suto, J. Appl. Phys. 51 (1980) 2429. [6] K. Suto, J. Nishizawa, IEEE J. Quantum Electron. 19 (1983) 1251. [7] J. Nishizawa, J. Acoust. Soc. Jpn. 57 (2001) 163 (in Japanese). [8] T. Tanabe, K. Suto, J. Nishizawa, T. Kimura, K. Saito, J. Appl. Phys. 93 (2003) 4610. [9] T. Tanabe, K. Suto, J. Nishizawa, K. Saito, T. Kimura, J. Phys. D: Appl. Phys. 36 (2003) 953. [10] J. Nishizawa, K. Suto, T. Sasaki, T. Tanabe, T. Kimura, J. Phys. D: Appl. Phys. 36 (2003) 2958. [11] T. Tanabe, K. Suto, J. Nishizawa, T. Sasaki, J. Phys. D: Appl. Phys. 37 (2004) 155. [12] V.G. Dmitriev, G.G. Gurzadyan, D.N. Nikogosyan, Handbook of Nonlinear Optical Crystals, Springer, Berlin, 1997, 68p. [13] W. Shi, Y.J. Ding, N. Fernelius, K. Vodopyanov, Opt. Lett. 27 (2002) 1454. [14] T. Tanabe, K. Suto, J. Nishizawa, T. Sasaki, H. Yasuda, Y. Oyama, in: Proceeding of International Symposium on Compound Semiconductors (ISCS2004). [15] A. Jacques, I. Pankove, Optical Processes in Semiconductors, Dover Publications, New York, 1975 (Translated by J. Nishizawa to Japanese).