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Luminescence of mercury iodide studied by holographic spectroscopy
Journal of Luminescence 18/19 (1979) 713—718 © North-Holland Publishing Company
LUMINESCENCE OF MERCURY IODIDE STUDIED BY HOLOGRAPHIC SPECTROSCOPY M. GROSMANN, V. GUBANOV and J. FONTAINE Laboratoire d’Optique et Spectroscopie des Solides (associé ULP-CNRS), 3, rue de I’Université, 67000 Strasbourg, France
The principles of holography and holographic spectroscopy are briefly stated. According to these principles a set up has been constructed for holographic spectroscopy of luminescent crystals in cryostat. This set up has been used to obtain luminescence spectra of mercury iodide crystals at low temperature. The obtained spectra have been compared to spectra obtained with conventional spectrograph.
1. Introduction The words hologram and holography have been created to underline the possibilities of 3 dimensional imaging offered by the techniques of phase recording and restitution in optics. The great achievements obtained in optical imaging by these techniques seem to justify asking whether they could not be also useful in spectroscopy. Proposals have been made recently by Caulfield [1] and Vienot [2] in the case of atomic emission spectroscopy. We have considered the case of emission spectroscopy of crystals and present some results obtained in mercury iodide.
2. Phenomenology of holography The simplest way of taking a hologram and restituting the recorded wave front is described on fig. I: a) recording the phase modulation of wave front 02 coming from object 0 is changed in amplitude modulation by interference with reference wave front R. It can therefore be recorded as an interferogram on photosensitive medium H. For a good recording, sufficient stability must be achieved during exposure and the photosensitive medium should have, if possible, linear sensitivity. But the photosensitivity can be either a change of the real or of the imaginary part of the dielectric b) restitution: the hologram having been exposed and (if necessary) developed, and being in exactly the same position as during recording is 713
M. Grosmann et aI./ Luminescence of mercury iodide
714
~
~
objec~ beam 2
recordrng
/
~/
iA..obJec~
a
0
7/
b)
/
/
/
~
/
beam 2
recons~rucrod
d~voIoped
~ho~ogrom
~ beam 3 -
Fig. I. Simplest way of taking a hologram and restituting the recorded wave front.
illuminated by reference wave front R. Through the hologram the object wave front 0 is then reconstructed by diffraction of R on the recorded interference fringes.
3. Mathematical description Let 0~and 02 be two waves incident on the holographic plate, both keeping temporal and spatial coherence during exposure time. They can be considered as elements written 0 E() exp i (K r Ut). The intensity recorded on the plate or (luminous flux) can be written =
H H
=(Ol+02)(0l+07)*, I(0l)+I(02)+0l0~+0~02.
=
If the recording is correctly performed, with exposure time and development time well chosen, the transmission of the resulting hologram can be T x H i.e. T=cH, orT=H. If we send 0~in this hologram we will get after it OtT
=
=
0/I(0~)+ 1(02) a01 + 1302 +
+
0~0~0~ + 010?02
that shows that we obtain (with intensities a, /3, y) the three wave 0~,0~and 02 by shining 0~on the hologram. Wave 02 is therefore reconstructed when we shine 0~ on the hologram.
M. Grosmann et al./Luminescence of mercury iodide
715
4. Fourier transform description If distances are great a hologram is an interferogram which records the Fourier Transform of the object. At great distance of the hologram the light diffracted by the hologram has a distribution of phases and amplitudes which is the Fourier transform of the one that exists on the hologram. Therefore at great distance of the hologram the diffracted light has a structure which is Fourier Transform of the light on the object, that is to say the diffracted light has the structure or “is” the light from the object. The use of the words “Fourier Transform” and “interferogram” make it clear that we are dealing which something closely connected with spectroscopy.
5. Holographic spectroscopy Fig. 2 shows enlarged views of three holograms. The first one is obtained from two plane waves, the second one is obtained from one plane and one non-plane wave, the third one is obtained from two non-plane waves. This is similar to what we obtain with two wave interferometers. Instead of using two different monochromatic waves one can try to use two dephased polychromatic waves. If the monochromatic components do not have similar intensities it will not be possible to fulfill simultaneously, for all of them, the simplest conditions and the equations for the resulting transparency will be somewhat complex. Fig. 3 shows the schematics of a setup for recording the spectral hologram of a source. In practice a Michelson or Mach—Zenhder type of interferometer is the best suited for studying sources of low temporal coherence. It is clear how the two plane waves interact on the plate. If a is the half angle of the two waves, x the distance from the middle of the plate along the
:j
~ 4
‘.
~‘~: 4~P~
~. .0
-
tI~
~
.
~
,,..rI
~~~rt*
V ~,‘uj’. Fig. 2. Enlarged views of three holograms.
ft.’!. Grosmann et al./ Luminescence of mercury iodide
716
LST S
N
7N~
Fig. 3. Setup for recording the spectral hologram of a source.
perpendicular to the central fringe and f the focal length of the lens we have 6
=
~,
T(x)
=
{H(x)}~2,
with y being the contrast constant of the plate H(x,
ii) =
J f
~(x){l
+
cos[2~vx sin(6
+
a)]} dx dt’.
If ‘P = 0 outside the limits the integrals can easily be extended. The variable part of the transmittance recorded with y = 2 will then be the Fourier Transform of the luminance of the source. If the hologram is positioned against the lens and illuminated with a monochromatic parallel beam at frequency v, in the focal plane of the lens we will obtain the Fourier Transform of T as is shown on fig. 4. Evidently we cannot chose the best suited exposure times for each line of the spectrum. This limitation is similar to that which exists in conventional spectrophotography. But with a cylindrical lens it is easy to illuminate the plate with intensities decreasing along the fringes’ axis. Therefore each will be recorded on a certain line normal to the fringes’ axis. In the more exposed plate regions weak lines will be recorded over a continuum of strong lines which will help to bring the weak lines in the linear part of the emulsion’s sensitivity. In the less exposed plate regions only strong lines spectra will be recorded. lens
pa~IleI
beam
~:~
spectral hologram Pourrer ~ran~Farm plane
Fig. 4. Fourier Transform of T.
M. Grosmann et al/Luminescence of mercury iodide Recon,rucrion Laser (He-Ne)
,.~
fl
~
~
717
Excirarron laser (argon) Cryosro~
//
/
M ~ ~ ~
~
;
Hologram ~-
-
~FSrde~5pec~ro C
5L ens
Zero ~rder
Fig. 5. Setup for recording luminescence spectra at low temperatures.
6. Experimental set up We have designed an experimental setup which enables us to record luminescence spectra at low temperatures (fig. 5). The lateral source dimension is defined by ,2 < hA/2, where h is the source-recording plate distance. Practically for focused—laser—excited luminescence we do not have problems. For wider sources it is clear that another setup (non parallel beams with equal paths) should be used. The luminescence of the crystal in cryostat is excited by an argon or krypton laser. Retrodiffused or transmitted luminescence is studied. The diffused light is focused at infinity by a lens of 35 cm of focal length with aperture f/3. A semi transparent mirror S divides the Beam in two parts which are recombined (under an angle of 12°)by mirrors M, M, and M2 to form the interferogram. The hologram can be recorded either on photographic plates, which produce an amplitude hologram with easy saturation or on a thermo-plastic film which, produces a phase hologram. The advantage of this last method is that the development process is instantaneous and that there is not much saturation. Thus during recording the evolution of the spectrum can be observed and optimal recording can be achieved. The interferogram being satisfactorily recorded, the spectrum can be at leisure studied, either by recording of a plate in the Fourier plane and subsequent densitometry or by an optical Multichannel Analyser in the Fourier plane.
7. Results obtained with the luminescence spectrum of Hg!2 The spectrum obtained by this technique is fairly different from the spectra obtained by conventional methods. The amplitude of the recorded lines covers
718
M. Grosmann et al/Luminescence of mercury iodide
Fig. 6. Spectra obtained by two methods.
only a very limited range. Fig. 6 shows the spectra obtained by the two methods. The detailed analysis shows that phonon replicas seem to occur for the line A~1 and that some lines have a fine structure which had not been observed previously. These structures could be due to phonons other than T—O [3—5]. To check the various possible hypothesis we plan to study the spectra of perturbed crystals by conventional and holographic spectroscopy. Further infrared and Raman studies should also enable us to check which phonons can play a role in the luminescence.
References [1] Hi. Caulfield, Applications of holography (Wiley—Interscience, New York, 1970). [2} J.Ch. Vienot and G. Perrin, C.R. Acad. Sd. 267 B (l%8) I 137. [31 BV Novikov and M.M. Pimonenko, Soy. Phys. Semicond. 4 (1971) p. 2077. [4] I. Akopyan et al., Phys. Stat. Sol. (b) 70 (1975) 353. [51J. Bielmann and B. Prevot, J. de Phys. (1978) to be published.
LUMINESCENCE OF MERCURY IODIDE STUDIED BY HOLOGRAPHIC SPECTROSCOPY M. GROSMANN, V. GUBANOV and J. FONTAINE Laboratoire d’Optique et Spectroscopie des Solides (associé ULP-CNRS), 3, rue de I’Université, 67000 Strasbourg, France
The principles of holography and holographic spectroscopy are briefly stated. According to these principles a set up has been constructed for holographic spectroscopy of luminescent crystals in cryostat. This set up has been used to obtain luminescence spectra of mercury iodide crystals at low temperature. The obtained spectra have been compared to spectra obtained with conventional spectrograph.
1. Introduction The words hologram and holography have been created to underline the possibilities of 3 dimensional imaging offered by the techniques of phase recording and restitution in optics. The great achievements obtained in optical imaging by these techniques seem to justify asking whether they could not be also useful in spectroscopy. Proposals have been made recently by Caulfield [1] and Vienot [2] in the case of atomic emission spectroscopy. We have considered the case of emission spectroscopy of crystals and present some results obtained in mercury iodide.
2. Phenomenology of holography The simplest way of taking a hologram and restituting the recorded wave front is described on fig. I: a) recording the phase modulation of wave front 02 coming from object 0 is changed in amplitude modulation by interference with reference wave front R. It can therefore be recorded as an interferogram on photosensitive medium H. For a good recording, sufficient stability must be achieved during exposure and the photosensitive medium should have, if possible, linear sensitivity. But the photosensitivity can be either a change of the real or of the imaginary part of the dielectric b) restitution: the hologram having been exposed and (if necessary) developed, and being in exactly the same position as during recording is 713
M. Grosmann et aI./ Luminescence of mercury iodide
714
~
~
objec~ beam 2
recordrng
/
~/
iA..obJec~
a
0
7/
b)
/
/
/
~
/
beam 2
recons~rucrod
d~voIoped
~ho~ogrom
~ beam 3 -
Fig. I. Simplest way of taking a hologram and restituting the recorded wave front.
illuminated by reference wave front R. Through the hologram the object wave front 0 is then reconstructed by diffraction of R on the recorded interference fringes.
3. Mathematical description Let 0~and 02 be two waves incident on the holographic plate, both keeping temporal and spatial coherence during exposure time. They can be considered as elements written 0 E() exp i (K r Ut). The intensity recorded on the plate or (luminous flux) can be written =
H H
=(Ol+02)(0l+07)*, I(0l)+I(02)+0l0~+0~02.
=
If the recording is correctly performed, with exposure time and development time well chosen, the transmission of the resulting hologram can be T x H i.e. T=cH, orT=H. If we send 0~in this hologram we will get after it OtT
=
=
0/I(0~)+ 1(02) a01 + 1302 +
+
0~0~0~ + 010?02
that shows that we obtain (with intensities a, /3, y) the three wave 0~,0~and 02 by shining 0~on the hologram. Wave 02 is therefore reconstructed when we shine 0~ on the hologram.
M. Grosmann et al./Luminescence of mercury iodide
715
4. Fourier transform description If distances are great a hologram is an interferogram which records the Fourier Transform of the object. At great distance of the hologram the light diffracted by the hologram has a distribution of phases and amplitudes which is the Fourier transform of the one that exists on the hologram. Therefore at great distance of the hologram the diffracted light has a structure which is Fourier Transform of the light on the object, that is to say the diffracted light has the structure or “is” the light from the object. The use of the words “Fourier Transform” and “interferogram” make it clear that we are dealing which something closely connected with spectroscopy.
5. Holographic spectroscopy Fig. 2 shows enlarged views of three holograms. The first one is obtained from two plane waves, the second one is obtained from one plane and one non-plane wave, the third one is obtained from two non-plane waves. This is similar to what we obtain with two wave interferometers. Instead of using two different monochromatic waves one can try to use two dephased polychromatic waves. If the monochromatic components do not have similar intensities it will not be possible to fulfill simultaneously, for all of them, the simplest conditions and the equations for the resulting transparency will be somewhat complex. Fig. 3 shows the schematics of a setup for recording the spectral hologram of a source. In practice a Michelson or Mach—Zenhder type of interferometer is the best suited for studying sources of low temporal coherence. It is clear how the two plane waves interact on the plate. If a is the half angle of the two waves, x the distance from the middle of the plate along the
:j
~ 4
‘.
~‘~: 4~P~
~. .0
-
tI~
~
.
~
,,..rI
~~~rt*
V ~,‘uj’. Fig. 2. Enlarged views of three holograms.
ft.’!. Grosmann et al./ Luminescence of mercury iodide
716
LST S
N
7N~
Fig. 3. Setup for recording the spectral hologram of a source.
perpendicular to the central fringe and f the focal length of the lens we have 6
=
~,
T(x)
=
{H(x)}~2,
with y being the contrast constant of the plate H(x,
ii) =
J f
~(x){l
+
cos[2~vx sin(6
+
a)]} dx dt’.
If ‘P = 0 outside the limits the integrals can easily be extended. The variable part of the transmittance recorded with y = 2 will then be the Fourier Transform of the luminance of the source. If the hologram is positioned against the lens and illuminated with a monochromatic parallel beam at frequency v, in the focal plane of the lens we will obtain the Fourier Transform of T as is shown on fig. 4. Evidently we cannot chose the best suited exposure times for each line of the spectrum. This limitation is similar to that which exists in conventional spectrophotography. But with a cylindrical lens it is easy to illuminate the plate with intensities decreasing along the fringes’ axis. Therefore each will be recorded on a certain line normal to the fringes’ axis. In the more exposed plate regions weak lines will be recorded over a continuum of strong lines which will help to bring the weak lines in the linear part of the emulsion’s sensitivity. In the less exposed plate regions only strong lines spectra will be recorded. lens
pa~IleI
beam
~:~
spectral hologram Pourrer ~ran~Farm plane
Fig. 4. Fourier Transform of T.
M. Grosmann et al/Luminescence of mercury iodide Recon,rucrion Laser (He-Ne)
,.~
fl
~
~
717
Excirarron laser (argon) Cryosro~
//
/
M ~ ~ ~
~
;
Hologram ~-
-
~FSrde~5pec~ro C
5L ens
Zero ~rder
Fig. 5. Setup for recording luminescence spectra at low temperatures.
6. Experimental set up We have designed an experimental setup which enables us to record luminescence spectra at low temperatures (fig. 5). The lateral source dimension is defined by ,2 < hA/2, where h is the source-recording plate distance. Practically for focused—laser—excited luminescence we do not have problems. For wider sources it is clear that another setup (non parallel beams with equal paths) should be used. The luminescence of the crystal in cryostat is excited by an argon or krypton laser. Retrodiffused or transmitted luminescence is studied. The diffused light is focused at infinity by a lens of 35 cm of focal length with aperture f/3. A semi transparent mirror S divides the Beam in two parts which are recombined (under an angle of 12°)by mirrors M, M, and M2 to form the interferogram. The hologram can be recorded either on photographic plates, which produce an amplitude hologram with easy saturation or on a thermo-plastic film which, produces a phase hologram. The advantage of this last method is that the development process is instantaneous and that there is not much saturation. Thus during recording the evolution of the spectrum can be observed and optimal recording can be achieved. The interferogram being satisfactorily recorded, the spectrum can be at leisure studied, either by recording of a plate in the Fourier plane and subsequent densitometry or by an optical Multichannel Analyser in the Fourier plane.
7. Results obtained with the luminescence spectrum of Hg!2 The spectrum obtained by this technique is fairly different from the spectra obtained by conventional methods. The amplitude of the recorded lines covers
718
M. Grosmann et al/Luminescence of mercury iodide
Fig. 6. Spectra obtained by two methods.
only a very limited range. Fig. 6 shows the spectra obtained by the two methods. The detailed analysis shows that phonon replicas seem to occur for the line A~1 and that some lines have a fine structure which had not been observed previously. These structures could be due to phonons other than T—O [3—5]. To check the various possible hypothesis we plan to study the spectra of perturbed crystals by conventional and holographic spectroscopy. Further infrared and Raman studies should also enable us to check which phonons can play a role in the luminescence.
References [1] Hi. Caulfield, Applications of holography (Wiley—Interscience, New York, 1970). [2} J.Ch. Vienot and G. Perrin, C.R. Acad. Sd. 267 B (l%8) I 137. [31 BV Novikov and M.M. Pimonenko, Soy. Phys. Semicond. 4 (1971) p. 2077. [4] I. Akopyan et al., Phys. Stat. Sol. (b) 70 (1975) 353. [51J. Bielmann and B. Prevot, J. de Phys. (1978) to be published.