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The excitonic molecule state in red-HgI2: Its direct observation by two-photon absorption
Solid State Communications, Printed in Great Britain.
THE EXCITONIC
MOLECULE
Vo1.44,No.l,
pp.33-35,
STATE IN RED-Hg12: T. Ishihara,
Department
of Physics,
(Received
oo38-1098/82/370033-03$03.oo/o Pergamon Press Ltd.
1982.
ITS DIRECT
OBSERVATION
BY TWO-PHOTON
ABSORPTION
T. Mita and N. Nagasawa
Faculty of Science, Tokyo 113, Japan
University
of Tokyo
18 June 1982 by Y. Toyozawa)
The excitonic molecule (EM) in red-Hg12 at 1.6K is identified by twophoton absorption (TPA) via the nearly longitudinal mixed mode exciton The energy of state by the use of two laser beams of different energy. the EM is determined to be 4.6650?0.0001 eV. Assuming that an EM consists of two triplet excitons (2.3347 eV), the binding energy of the EM is estimated to be 4.4kO.l meV.
this configuration only the mixed mode exciton band appears as a sharp spike at the energy of the longitudinal exciton RL. But in this case, the oscillator strength of the exciton is suppressed and the matrix element responsible for the TPA is consequently reduced. To enhance the two-photon transition probability, we used two laser lights of different energy and chose the energy in such a way that the nearly longitudinal mixed mode exciton was a resonant intermediate state of TPA. Then, we found the TPA band as follows. Figure 1 is the schematic illustration of the present experiment. A platelet of red-Hg12 single crystal grown from the vapor phase was directly immersed in liquid He(l.6K). Its thickness was about O.lmm and its c-axis was in its plane. Two tunable dye lasers(Coumarine 522 in ethanol was used as tire active medium) were pumped by a N2 laser(Molectron UV-14). Each dye laser beam had a spectral width of 0.04meV and a pulse width of 3nsec. The beams were focused on the sample along a direction almost normal to its surface and the polarization of each beam The beam was set almost parallel to the c-axis. spots on the sample were alligned so as to excite the same region and time coincidence of them was carefully checked. The intensity of the probe beam was one or two order of magnitude lower than that of the excitation beam which was estimated as lMw/cm2 on the sample surface. The energy of the probe light R2 was fixed in the vicinity of the longitudinal exciton's energy The intensity of the transmitted probe %' light was measured throughthe sample under the irradiation of the excitation beam. In order to reduce Rayleigh scattering light from the excitation beam, the transmitted probe light was filtered by a monochromator(Jovin Yvon THR1500) with a band pass of 0.08meV. The signal was monitored by a photomultiplier and was averaged by a Boxcar integrator(PAR-162 with an S-2 sampling head). Fixing the probe light's energy at 2.34014 eV, its transmitted intensity was measured as a function of the energy of the excitation light which irradiated the sample simultaneously, and a spectrum as shown in Fig.2 was obtained. A
Red-Hg12 is a layer-type semiconductor, in which exciton structures are well known experiSelection rules show that the Amental1y.l) exciton state is active only for light whose electric field E is polarized perpendicular to In fact, according to Akopyan the c-axis(Elc). et a1.2), the absorption coefficient of the Aexciton is so large for the configuration Elc that only the absorption edge can be measured for the single crystals of O.lmm thickness usually obtained, while for an Ellc configuration, such a crystal is almost transparent except for a narrow spike observed at the ener y of the longitudinal exciton, R =2.3402 eV *k transmission spectrum.2) This spikeyi~na~~~ibed to the mixed mode exciton3) whose existence reflects the crystal's anisotropy. The presence of the excitonic molecule(EM) in this material has been asserted by some authors4)5) from the analysis of the so-called Mband, which results from the radiative annihilation of an EM leaving an exciton behind. On the other hand, it is well known that the EM state can be excited directly through the twophoton absorption(TPA) process6) and that the energy of the EM state can be determined more precisely from the energy position of the TPA band, as is the case of cucl.7)8) In red-HgI2, Kurtze and Klingshirn9) have tried to find the TPA band in the excitation spectrum of the Mband, but could not obtain clear evidence suggesting its presence. The purpose of the present experiment is to observe directly the TPA band due to the EM’S in red-Hg12, and to establish the existence of the EM in this material experimentally. For the configuration Elc, it seems to be difficult to detect the TPA band, because the strong one-photon absorption due to the transverse excitons will mask it. For the Ellc configuration the TPA band is expected to be observed separated in energy from the one-photon absorption band, because in *) In references 2,4,5, and 9, the refraction index of air (n=1.000278) is not taken into consideration in wavelength-energy conversion. We used the correctly converted energies in the text. 33
34
THE EXCITONIC
MOLECULE
STATE IN RED-HgI,
Vol. 44, No'.
i?
sample
:
2.324-
s s L a
2.322
Cl)
I
I
c-axis
Ii
1.0
9
F Fig.1
Experimental arrangement ments of TPA
b)
0.5
for the measure-
I4 B
_--_____-___~~_~~--~i~~~~ excitation
0.0,
photon
twophoton
W2
absorption
(1.6 K) scatter of theexcitation /
I
Fig.3
beam
Fig.2
I
2.340
I
I
I
2.342
energy II2 (eV) d
of TPA as a function
of R2.
nT hh
I
I
2.34 -
\
AL
dip a) Spectral position of a dip, Ql , appeared in the transmission spectrum as a function of the probe light energy (Q,). b) Intensity
J-k
I
2.338
$error -=-o
I
2.33
excitation light, and
2.32
light and the other from the probe that the constant is the energy of the EM's level, fin. Assuming that an EM consists of two triplet excitons of 2.3347eV*)2), we obtained the binding energy of an EM:
excitation enegy fl, (eV)
Transmission of the probe light (R2'2.34014eV'U?L) under irradiation the excitation light.
of
remarkable dip is found at 2.32484eV. The dashed horizontal line shows the transmission intensity without the irradiation of the excitation beam. Fixing R2 at various energies in the vicinity of QL, we obtained a similar spectrum-for each R2. The energy position of the dip, fidlp varied depending on R2. It was found that hhe' sum of nd'p and R2 remains constant as shown in Fig.3 (a+; ndip + R = 4.6650 + 0.0001 2 1
Eb = 4.4 + 0.1 meV. m This value agrees approximately with the one deduced from the M-band analysis of Nakaoka et 3.8f0.2meV*), and the theoretically exal!) pected one5), 4.6meV. But it is inconsistent with the value of 9.6meV*) obtained from a hump which appeared in the M-band excitation spectrum of Kurtze and Klingshirng). This hump seems to be of different origin. Figure 3(b) shows the intensity of the TPA, Here n is defined as n as a function of n2.
eV.
We consider that the dip which appears in each spectrum originates from the TPA due to the EM's by the combination of two photons, one from the
where I2 is the transmitted intensity of the probe beam and Rt is tentatively taken to be
Voi. 44, No.
.
I
THE EXCITONIC MOLECULE STATE IN RED-Hg12
. npp
- 4 meV in ehich case the contribution of TPA to the I2 can be neglected. As can be seen in the figure, n is enhanced remarkably near the resonance at R2=RL, but is suppressed at perfect resonance. The dip could not be observed in the ranges of R2
35
effect there is too small to detect any TPA. The measurement of the two-photon resonant Raman scattering (TRRS) was also carried out by a one-beam method to confirm the energy of the EM estimated above. The Elc configuration was adopted to improve the Raman efficiency. TRRS was enhanced when the energy of the incident light was 2.3328+0.0002eV, which corresponds to one half of the energy of the EM, 4.665OCO.0001 eV, mentioned above. Details on TRRS will be published elsewhere.
References
1) K. Kanzaki and I. Imai, .I. Phys. Sot. Jpn. & 1003 (1972).
6) E. Hanamura, (1973).
2) I. Akopyan, B. Novikov, S. Permogorov, A. Selkin and V. Travnikov, Phys. Status Solidi 70, 353 (1975)
7) N. Nagasawa, T. Mita and M. Ueta, .I. Phys. Sot. Jpn. 4l, 929 (1976).
3) J.J. Hopfield Solids 2,276
and D.G. Thomas, (1960).
J. Phys. Chem.
4) I.Kh. Akopyan, B.V. Novikov, M.M. Pimonenko and B.S. Razbirin, JETP Lett. 17, 299 (1973). 5) K. Nakaoka, Cimento @,
T. Goto and Y. Nishina, 588 (1977).
I1 Nuovo
Solid State Commun.
12,
951
8) V.D. Phach, A. Bivas, B. H'dnerlage and J.B. Grun, Phys. Status Solidi 84, 731 (1977). 9) G. Kurtze and C. Klingshirn, Solid State Commun. 28, 337 (1978). They reported 6 + 1meV for Ek in their paper. But if one takes 2.3347 eVas the triplet exciton energy and makes correction for the refractive index of air, 6meVshould be read as 9.6meV.
THE EXCITONIC
MOLECULE
Vo1.44,No.l,
pp.33-35,
STATE IN RED-Hg12: T. Ishihara,
Department
of Physics,
(Received
oo38-1098/82/370033-03$03.oo/o Pergamon Press Ltd.
1982.
ITS DIRECT
OBSERVATION
BY TWO-PHOTON
ABSORPTION
T. Mita and N. Nagasawa
Faculty of Science, Tokyo 113, Japan
University
of Tokyo
18 June 1982 by Y. Toyozawa)
The excitonic molecule (EM) in red-Hg12 at 1.6K is identified by twophoton absorption (TPA) via the nearly longitudinal mixed mode exciton The energy of state by the use of two laser beams of different energy. the EM is determined to be 4.6650?0.0001 eV. Assuming that an EM consists of two triplet excitons (2.3347 eV), the binding energy of the EM is estimated to be 4.4kO.l meV.
this configuration only the mixed mode exciton band appears as a sharp spike at the energy of the longitudinal exciton RL. But in this case, the oscillator strength of the exciton is suppressed and the matrix element responsible for the TPA is consequently reduced. To enhance the two-photon transition probability, we used two laser lights of different energy and chose the energy in such a way that the nearly longitudinal mixed mode exciton was a resonant intermediate state of TPA. Then, we found the TPA band as follows. Figure 1 is the schematic illustration of the present experiment. A platelet of red-Hg12 single crystal grown from the vapor phase was directly immersed in liquid He(l.6K). Its thickness was about O.lmm and its c-axis was in its plane. Two tunable dye lasers(Coumarine 522 in ethanol was used as tire active medium) were pumped by a N2 laser(Molectron UV-14). Each dye laser beam had a spectral width of 0.04meV and a pulse width of 3nsec. The beams were focused on the sample along a direction almost normal to its surface and the polarization of each beam The beam was set almost parallel to the c-axis. spots on the sample were alligned so as to excite the same region and time coincidence of them was carefully checked. The intensity of the probe beam was one or two order of magnitude lower than that of the excitation beam which was estimated as lMw/cm2 on the sample surface. The energy of the probe light R2 was fixed in the vicinity of the longitudinal exciton's energy The intensity of the transmitted probe %' light was measured throughthe sample under the irradiation of the excitation beam. In order to reduce Rayleigh scattering light from the excitation beam, the transmitted probe light was filtered by a monochromator(Jovin Yvon THR1500) with a band pass of 0.08meV. The signal was monitored by a photomultiplier and was averaged by a Boxcar integrator(PAR-162 with an S-2 sampling head). Fixing the probe light's energy at 2.34014 eV, its transmitted intensity was measured as a function of the energy of the excitation light which irradiated the sample simultaneously, and a spectrum as shown in Fig.2 was obtained. A
Red-Hg12 is a layer-type semiconductor, in which exciton structures are well known experiSelection rules show that the Amental1y.l) exciton state is active only for light whose electric field E is polarized perpendicular to In fact, according to Akopyan the c-axis(Elc). et a1.2), the absorption coefficient of the Aexciton is so large for the configuration Elc that only the absorption edge can be measured for the single crystals of O.lmm thickness usually obtained, while for an Ellc configuration, such a crystal is almost transparent except for a narrow spike observed at the ener y of the longitudinal exciton, R =2.3402 eV *k transmission spectrum.2) This spikeyi~na~~~ibed to the mixed mode exciton3) whose existence reflects the crystal's anisotropy. The presence of the excitonic molecule(EM) in this material has been asserted by some authors4)5) from the analysis of the so-called Mband, which results from the radiative annihilation of an EM leaving an exciton behind. On the other hand, it is well known that the EM state can be excited directly through the twophoton absorption(TPA) process6) and that the energy of the EM state can be determined more precisely from the energy position of the TPA band, as is the case of cucl.7)8) In red-HgI2, Kurtze and Klingshirn9) have tried to find the TPA band in the excitation spectrum of the Mband, but could not obtain clear evidence suggesting its presence. The purpose of the present experiment is to observe directly the TPA band due to the EM’S in red-Hg12, and to establish the existence of the EM in this material experimentally. For the configuration Elc, it seems to be difficult to detect the TPA band, because the strong one-photon absorption due to the transverse excitons will mask it. For the Ellc configuration the TPA band is expected to be observed separated in energy from the one-photon absorption band, because in *) In references 2,4,5, and 9, the refraction index of air (n=1.000278) is not taken into consideration in wavelength-energy conversion. We used the correctly converted energies in the text. 33
34
THE EXCITONIC
MOLECULE
STATE IN RED-HgI,
Vol. 44, No'.
i?
sample
:
2.324-
s s L a
2.322
Cl)
I
I
c-axis
Ii
1.0
9
F Fig.1
Experimental arrangement ments of TPA
b)
0.5
for the measure-
I4 B
_--_____-___~~_~~--~i~~~~ excitation
0.0,
photon
twophoton
W2
absorption
(1.6 K) scatter of theexcitation /
I
Fig.3
beam
Fig.2
I
2.340
I
I
I
2.342
energy II2 (eV) d
of TPA as a function
of R2.
nT hh
I
I
2.34 -
\
AL
dip a) Spectral position of a dip, Ql , appeared in the transmission spectrum as a function of the probe light energy (Q,). b) Intensity
J-k
I
2.338
$error -=-o
I
2.33
excitation light, and
2.32
light and the other from the probe that the constant is the energy of the EM's level, fin. Assuming that an EM consists of two triplet excitons of 2.3347eV*)2), we obtained the binding energy of an EM:
excitation enegy fl, (eV)
Transmission of the probe light (R2'2.34014eV'U?L) under irradiation the excitation light.
of
remarkable dip is found at 2.32484eV. The dashed horizontal line shows the transmission intensity without the irradiation of the excitation beam. Fixing R2 at various energies in the vicinity of QL, we obtained a similar spectrum-for each R2. The energy position of the dip, fidlp varied depending on R2. It was found that hhe' sum of nd'p and R2 remains constant as shown in Fig.3 (a+; ndip + R = 4.6650 + 0.0001 2 1
Eb = 4.4 + 0.1 meV. m This value agrees approximately with the one deduced from the M-band analysis of Nakaoka et 3.8f0.2meV*), and the theoretically exal!) pected one5), 4.6meV. But it is inconsistent with the value of 9.6meV*) obtained from a hump which appeared in the M-band excitation spectrum of Kurtze and Klingshirng). This hump seems to be of different origin. Figure 3(b) shows the intensity of the TPA, Here n is defined as n as a function of n2.
eV.
We consider that the dip which appears in each spectrum originates from the TPA due to the EM's by the combination of two photons, one from the
where I2 is the transmitted intensity of the probe beam and Rt is tentatively taken to be
Voi. 44, No.
.
I
THE EXCITONIC MOLECULE STATE IN RED-Hg12
. npp
- 4 meV in ehich case the contribution of TPA to the I2 can be neglected. As can be seen in the figure, n is enhanced remarkably near the resonance at R2=RL, but is suppressed at perfect resonance. The dip could not be observed in the ranges of R2
35
effect there is too small to detect any TPA. The measurement of the two-photon resonant Raman scattering (TRRS) was also carried out by a one-beam method to confirm the energy of the EM estimated above. The Elc configuration was adopted to improve the Raman efficiency. TRRS was enhanced when the energy of the incident light was 2.3328+0.0002eV, which corresponds to one half of the energy of the EM, 4.665OCO.0001 eV, mentioned above. Details on TRRS will be published elsewhere.
References
1) K. Kanzaki and I. Imai, .I. Phys. Sot. Jpn. & 1003 (1972).
6) E. Hanamura, (1973).
2) I. Akopyan, B. Novikov, S. Permogorov, A. Selkin and V. Travnikov, Phys. Status Solidi 70, 353 (1975)
7) N. Nagasawa, T. Mita and M. Ueta, .I. Phys. Sot. Jpn. 4l, 929 (1976).
3) J.J. Hopfield Solids 2,276
and D.G. Thomas, (1960).
J. Phys. Chem.
4) I.Kh. Akopyan, B.V. Novikov, M.M. Pimonenko and B.S. Razbirin, JETP Lett. 17, 299 (1973). 5) K. Nakaoka, Cimento @,
T. Goto and Y. Nishina, 588 (1977).
I1 Nuovo
Solid State Commun.
12,
951
8) V.D. Phach, A. Bivas, B. H'dnerlage and J.B. Grun, Phys. Status Solidi 84, 731 (1977). 9) G. Kurtze and C. Klingshirn, Solid State Commun. 28, 337 (1978). They reported 6 + 1meV for Ek in their paper. But if one takes 2.3347 eVas the triplet exciton energy and makes correction for the refractive index of air, 6meVshould be read as 9.6meV.