1. Introduction

Photo-thermo-refractive (PTR) glass is a sodium-zinc-aluminum silicate glass doped with several cations (Ce, Ag, Sn, and Sb) and anions (F and Br) [1]. This glass is extensively used for holographic recording of different types of volume Bragg gratings (VBGs) [2–6] and phase masks [7] that have wide applications in lasers and spectroscopy. The hologram recording process consists of the two main stages, exposure to UV radiation and thermal development. Photo-structural transformations of PTR glass [1] are triggered by excitation of trivalent cerium ions (Ce³⁺), having an absorption band in near UV region between 280 nm and 350 nm that corresponds to transition from the ground state to the 5d¹ band. In PTR glass, this band is placed above the mobility threshold for electrons in the glass matrix. As a result of such an excitation, Ce³⁺ ion converts to Ce⁴⁺ ion, releasing an electron. The electron is then captured by the silver ion Ag⁺, converting it to a neutral Ag⁰ atom. Spatial distribution of silver atoms does not produce noticeable refractive index change and presents a latent image of the excitation pattern. The thermal treatment comprises two steps. At the first step glass is heated to the temperatures above 480°C, causing silver atoms to diffuse, creating silver containing clusters. Those clusters serve as nucleation centers for consequent growth of sodium fluoride nanocrystals that takes place at the second step, at the temperatures above 500°C. Upon that, refractive index decreases in the exposed areas of the glass sample. As a result, spatial modulation of refractive index in accordance with the latent image can reach 10⁻³ (1000 ppm) or more. Spatial frequency of recorded VBGs can be higher than 9000 /mm.

Refractive index change in PTR glass as a result of thermal treatment gives the advantage of the hologram stability [2]. Thus such a hologram cannot be destroyed by exposure to optical radiation of any wavelength in such manner as it happens with materials similar to Fe-doped LiNBO₃. Moreover, VBGs recorded in PTR glass can withstand temperatures up to 350°C. Thermal stability, low absorption in near IR spectral region (below 10⁻⁴ cm⁻¹), and high laser damage threshold make PTR VBGs a remarkable solution for high power laser applications. Stable behavior of VBGs in multi-kilowatt beams provides an opportunity for such applications as spectral stabilization of high-power LD bars and stacks, spectral beam combining, transverse and longitudinal mode selection in laser cavities, stretching/compressing of femtosecond pulses, filtering of spontaneous and Raman shifted emissions etc [5].

As it was mentioned above, photosensitivity range of PTR glass is limited by the UV absorption band of Ce³⁺, located in the 280-350 nm spectral region. As a result, only trivial planar holographic elements, such as volume Bragg gratings, can be recorded in PTR glass for applications in visible and IR spectral regions while complex holographic elements, e.g. curved Bragg mirrors, could be used only within the photosensitivity range in near UV. It would be very beneficial to find a way for recording complex holograms for visible and near IR regions in material similar to PTR glass, which can provide imaging combined with fine filtering and be stable and reliable for high-power laser applications. Because a complex hologram could be completely restored only at the wavelength of recording, a new material similar to PTR glass has to have photosensitivity in visible and near IR regions while being able to withstand exposure to such radiation. The first step in this direction has been demonstrated in the present paper.

The long history of dielectric photosensitive glasses development has shown that Ce³⁺ used as a photosensitizer in PTR glasses provides the longest wavelengths of photoionization and no ways for further shifting of photosensitivity were described. This is why a PTR-like glass was studied in this work where a photosensitizer has a more complex structure of upper electron states that could be used for some types of nonlinear excitation with longer wavelengths. An excited state absorption (ESA) mechanism could be used to populate an electron level, which is situated above the mobility threshold of electrons in a glass matrix, by concurrent exposure to two optical beams with longer wavelengths that correspond to excitation from a ground state to one of the upper levels with a reasonably long lifetime and then excitation from this level to the desirable one. Such an excitation should provide generation of free electrons that are necessary for triggering the chain of photo-structural transformations.

A Tb³⁺ ion was chosen for the experiments. An energy diagram of Tb³⁺ is shown in Fig. 1. The broad group of levels that produces wide 5d¹4f⁷ band is placed only 5 eV above the ground level ⁷F₆ while it is beyond electron mobility threshold of a number of wide bandgap inorganic compounds [8, 9]. Trivalent terbium has two metastable levels: ⁵D₃ which is placed 3.3 eV above the ground state, and ⁵D₄ - 2.6 eV above the ground state. Both levels reportedly have long millisecond lifetimes in different crystals or glass matrices [10]. Long lifetime of the excited state is one of the key parameters to influence the efficiency of the ESA process. ESA is an upconversion process, which allows for excitation of higher energy level via intermediate level by consecutive absorption of two photons. It should be noted that no absorption increase in the range of 400-1500 nm with an exception of the small area around 480 nm was reported in Tb doped glasses after exposure to UV radiation [11]. This feature could be useful to avoid induced losses in holograms recorded for longer wavelengths.

 

Fig. 1 Energy diagram for Tb3 + ions in silicate glass and excited state absorption scheme for different doping concentrations: 0.08 at% Tb^{3 +} (A) and 0.7 at.% Tb^{3 +} (B).

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2. Experimental

A PTR glass (Tb:PTRG) similar to that described in [12] but containing Tb₂O₃ instead of CeO₂ was fabricated. Three types of glass were prepared for this work. Two PTR glasses where cerium was replaced with different terbium concentration (0.08 and 0.7 at.%Tb³⁺ Tb0.08 and Tb0.7) were made in order to characterize the concentration effect on photosensitivity. The third glass was a PTR glass matrix (no Ce, Ag, Sn, and Sb) doped with 0.27 at.%Tb³⁺ to characterize a UV absorption in Tb-doped glass. The glasses were melted in an electrical furnace in a platinum crucible at the temperature of 1460 °C for 5 hours. A platinum stirrer was used to provide high optical homogeneity. The glass boule was annealed at the temperature of 460 °C for 6 hours and then was slowly cooled down to room temperature for 24 hours [12]. Polished wafers of 25 × 8 × 2 mm³ were fabricated. Surface flatness was better than λ/4 at 633 nm and refractive index uniformity across the aperture was of 30 ppm or better. Surface flatness and optical homogeneity were measured with a Fizeau interferometer (Zygo GPI). Thermal development was produced in two steps aging the samples sequentially at temperatures of 480°C and 515°C for periods from a few minutes to several hours.

Absorption spectra in UV-vis-IR regions were measured with a Perkin Elmer Lambda 950 spectrophotometer.

3. Results and discussion

Near UV and visible region absorption spectrum of Tb0.7 glass is presented in Fig. 2A. The spectrum consists of a band with maximum at 486 nm (2.55 eV), a group of overlapped bands from 310 to 390 nm (4.00-3.18 eV) with the most intense line at 379 nm (3.27 eV) and a sharp absorption edge at 290 nm (4.28 eV). The longest wavelength band at 486 nm could be ascribed to a transition from the ground ⁷F₆ state to a first metastable level ⁵D₄ (Fig. 1). The group of lines at shorter wavelengths corresponds to the transitions from the ground ⁷F₆ state to ⁵G₀-⁵G₆ and ⁵D₃ levels (Fig. 1). According to references [11, 13], the upper metastable ⁵D₃ state cannot be clearly separated from the lowest in the G group ⁵G₆ level in silicate glass due to inhomogeneous line broadening in glass and small ~26 meV energy gap between the levels. Therefore one can suppose that they both contribute to the absorption band with maximum at 379 nm.

 

Fig. 2 UV (A) and IR (B) absorption spectra of PTR glass doped with 0.7 at.%Tb³⁺.

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Near IR absorption spectrum of Tb:PTRG is given in the Fig. 2B and contains long wavelength absorption edge at 2700 nm, which is determined by contamination of hydroxyl groups resulted from interaction of glass melt with atmosphere. In near IR highest energy transitions are related to a complex absorption band with maximum at 1903 nm (0.65 eV) and spectral width of 248 nm, which is a superposition of three inhomogeneous lines corresponding to the transitions within the ground state multiplet from ⁷F₆ to ⁷F₀, ⁷F₁, and ⁷F₂. Absorption bands corresponding to the transitions from the ground state to lower levels are masked by hydroxyl absorption and could not be identified. Thus, PTR-glass doped with terbium does not show any detectable absorption in 550-1500 nm region.

An absorption band corresponding to the transition from the ground ⁷F₆ state to 5d¹4f⁷ state of Tb³⁺ could not be defined directly in the absorption spectrum of Tb³⁺-doped PTR glass. According to studies in different glasses and crystals [14, 15] absorption band corresponding to ⁷F₆-5d¹4f⁷ transition should be placed in the range of 220-260 nm (4.77-5.16eV). However, that band in PTR glass should overlap with absorption bands of Sb⁵⁺ and Ag⁺ dopants and could not be distinguished within sharp absorption edge near 290 nm (Fig. 2A). To clarify absorption of Tb³⁺ in the short wavelength region, a PTR glass matrix without conventional dopants (Ce, Ag, Sb) but with 0.27 at.%Tb³⁺ was melted and a number of samples with different thickness were prepared to enable measurements in vicinity of PTR glass matrix intrinsic absorption edge which is placed near 210 nm [16]. Absorption spectrum of this glass in logarithmic scale is shown in Fig. 3. One can see small absorption bands of Tb³⁺ described earlier and placed between 290 and 550 nm. The sharp absorption edge in vicinity of 210 nm could be mainly ascribed to intrinsic absorption of PTR glass matrix. A high intensity absorption band with maximum at 230 nm and a shoulder near 260 nm could be ascribed to transitions from the ground state of Tb³⁺ to different components of 5d¹4f⁷ band or higher excited states.

 

Fig. 3 Absorption spectrum (logarithmic scale) of PTR glass matrix doped with 0.27 at.%Tb3 + . Arrows correspond to positions in the absorption spectrum for excitation from 5D4 level with wavelengths of 522 nm (1), 449 nm (2) and 375 nm (3).

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Luminescence properties of glasses of both types were measured using an Ocean Optics S2000 spectrometer in order to study the kinetic characteristics of the ⁵D₃ and ⁵D₄ metastable levels. A beam from a 375 nm LED was focused into a 2 mm square spot inside the glass sample, and the consequent fluorescence was collected by a fiber coupled to the spectrometer. It was found that luminescence spectra of PTR-glass and PTR-glass matrix doped with Tb³⁺ look similar to those in different silicate glasses [11, 13]. The spectra consist of two separated groups of lines corresponding to blue and green luminescence bands. Blue luminescence lines ranged from 380 nm to 440 nm with the most intense line at 449 nm are related to ⁵D₃-⁷F_j transitions, and the green luminescence lines are located in the spectral region from 485 nm to 620 nm (most intense line at 543 nm) and correspond to ⁵D₄-⁷F_j transitions. Relative intensities of blue and green luminesce were found to differ significantly for two types of glass. While the ratio of the green and the blue peak intensities was measured to be 1.6 for low concentration Tb0.08 glass, in Tb0.7 glass the strongest green line had intensity by more than ten times higher than the blue one. The difference is attributed to the terbium concentration effect on the luminescence properties [13, 17]. Intensity of blue and green luminescence in Tb³⁺ doped aluminosilicate glasses is shown as a function of terbium concentration in Fig. 4. It can be seen that while blue luminescence prevails at low concentrations it is eventually quenched, and the green luminesce intensity is increased at the expense of the blue one. Blue luminescence quenching can be accounted for the cross relaxation process taking place between two neighboring terbium ions. The cross-relaxation process is concentration dependent and comes into effect once terbium concentration exceeds 0.07 at.%Tb³⁺. As a result the lifetime of the upper metastable ⁵D₃ level is decreased causing quenching of luminescence from the level. Hence while in Tb0.08 glass both ⁵D₃ and ⁵D₄ levels can be considered metastable, only ⁵D₄ level is metastable and can be used for upconversion in Tb0.7 glass.

 

Fig. 4 Green and blue luminescence intensity dependence on Tb3 + concentration [13].

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Luminescence excitation spectra were measured over the range from 200 to 340 nm using a Perkin Elmer LS45 Fluorimeter in reflection geometry. It was found that short wavelength excitation spectra were identical for both types of luminescence. One can see (Fig. 5) a small maximum at 330 nm, a main maximum at 253 nm (4.90 eV) with spectral width of ~35 nm (0.66 eV) and a tail of a short wavelength band. The band with maximum at 253 nm in the excitation spectrum corresponds to the shoulder in vicinity of 260 nm in the absorption spectrum of Tb³⁺ (Fig. 3). This band could be associated with transition from the ground ⁷F₆ state to 5d¹4f⁷ band. This assignment is in close agreement with the literature data for such transition in different type of glasses [15]. An inconsistency has to be noted, as location of the 5d band is shifted towards longer wavelengths in the luminescence excitation spectrum compared to the absorption spectrum. An inconsistency will be addressed, and explanation will be provided below. Thus the study of absorption and luminescence in Tb3 + doped PTR glass has shown that similarly to other silicate glasses it has a weak absorption band at 486 nm and a group of overlapped weak bands between 310 and 390 nm that correspond to excitation from the ground state to 5D4 and 5D3 levels correspondingly. Relaxation from those levels to the ground state results in green and blue luminescence. Strong absorption at wavelengths shorter than 300 nm includes a band with maximum at 230 nm and a shoulder at 260 nm that could be assigned to transitions from the ground state to 5d14f7 band and higher levels of Tb3 + and glass matrix. Excitation to those UV bands results in further relaxation to 5D3 and 5D4 levels with consequent blue and green luminescence.

 

Fig. 5 Spectrum of luminescence excitation in PTR glass matrix doped with 0.27 at.%Tb3 + . Arrows correspond to positions in the luminescence excitation spectrum for excitation from 5D4 level with wavelengths of 522 nm (1), 449 nm (2) and 375 nm (3).

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It was found in Ref [18–20]. that excitation of Tb-doped calcium aluminate and borosilicate glasses in short wavelength absorption bands (λ<300 nm) of Tb³⁺ caused not only luminescence but induced absorption band with maximum in vicinity of 350 nm. Comparison of this band with absorption spectra of Tb doped glasses melted in strong oxidizing conditions has proved that this is absorption of Tb⁴⁺. Therefore excitation to the short wavelength absorption bands of Tb³⁺ results in its photoionization and conversion from Tb³⁺ to Tb⁴⁺. This means that for those glasses the 5d¹4f⁷ band associated with absorption at short wavelengths (λ<300 nm) is placed above the mobility threshold of electrons in glass matrix.

To study long wavelength excitation and photoionization in Tb:PTRG, we produced irradiation of the samples with an LED emitting at 375 nm (3.30 eV). The LED provided up to 1 W/cm² power density in a spot of 7.5 × 7.5 mm. According to the energy diagram of Tb³⁺ (Fig. 1), absorption of a 3.30 eV photon should result in excitation from the ground level to a ⁵D₃ level of Tb³⁺. Subsequently a certain population undergoes a nonradiative downward transition to the lower metastable ⁵D₄ level placed at 2.55 eV above the ground state. Absorption of the second 3.30 eV photon should result in excitation from the metastable levels to upper states of 5d¹4f⁷ band and in photoionization of Tb³⁺. It was found that while Tb:PTRG is not sensitive to low power irradiation at 375 nm, power density in the range of 1 W/cm² is enough to produce additional absorption. Figure 6 shows an additional absorption spectrum of the sample, which was irradiated by LED emission at 375 nm and 0.84 W/cm² power density for 5 hours. The wide absorption band with a maximum at 340-350 nm corresponds to the absorption band of Tb⁴⁺ and confirms the supposition that Tb:PTRG can be ionized by two-step excited state absorption (ESA) at 375 nm. This phenomenon can trigger a chain of structural transformations in PTR glass and cause refractive index change.

 

Fig. 6 Spectrum of additional absorption in Tb-doped PTR glass after exposure to 375 nm radiation at power density of 0.84 W/cm2 for 15 kJ/cm2 exposing dosage.

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As a first step to observe difference in refractive index between irradiated and non-irradiated areas in Tb:PTRG, the samples were irradiated by different dosages at 375 nm using the same LED source. The exposed samples were treated using the two-step thermal development procedure, similar to that used for conventional PTR glass. Refractive index difference between an exposed stripe of about 1 mm thickness and an unexposed area at the sample (Δn) was measured with a Shearing interferometer which provided precision of better than 5 ppm [21]. After treatment, difference in refractive index up to 220 ppm between two areas was observed in Tb0.7 glass. . The obtained result verifies that the upconversion processes resulted in photoionization of Tb3 + provides refractive index change (Δn) in Tb-doped PTR-glass. However, an effect of refractive index change by a single source irradiation is unwanted from the point of view of hologram recording by exposure to long wavelength sources with the use of ESA. It is clear that ESA by near UV radiation would cause some undesired background refractive index change, and therefore, decrease the dynamic range for two-beam hologram recording. To mitigate this effect, one can decrease power density and dosage of UV irradiation. For our two-beam experiments in Tb0.7 glass exposure to 375 nm UV radiation with the power density of 0.84 W/cm2 was performed for 120 min, which corresponded to absorbed dosage of 139 J/cm2 for a 2 mm thick sample. Such irradiation by a single 375 nm source resulted in refractive index change of 50-60 ppm that provided enough dynamic range for two-wavelength experiments. Due to absorption in Tb0.08 glass which was by ten times lower compared to high concentration Tb0.7 glass, the dosage was adjusted for experiments with Tb0.08 glass so that the same refractive index modulation is achieved.

Multimode broad area blue and green laser diodes were used for two-wavelength experiments for the study of the ESA photo-induced process in Tb0.7 glass. A blue LD emitted 1.5 W CW power at 449 nm (2.76 eV) and a green LD had 1 W CW power at 522 nm (2.37 eV). The beams from LDs were focused onto a sample in a 5 mm long stripe with 0.9 mm close-to-Gaussian lateral profile of intensity. Maximum power density was 59 W/cm² at 450 nm and 39 W/cm² for 520 nm. 2 mm thick PTR-glass samples were placed at the intersection of UV and visible beam paths. A beam alignment was performed so that a stripe-shaped beam from LD was imposed upon the center of a uniform 7.5 × 7.5 mm² square-shaped LED beam at 375 nm.

To prove that there is no nonlinear photoionization of Tb³⁺ by blue or green light, the samples were exposed to visible radiation only with the LDs operating at maximum power for several hours. No refractive index change was observed after such illumination followed by thermal development.

Thereafter the samples were concurrently irradiated by the UV 375 nm LED beam and a 522 nm green beam for 120 min. Exposure was followed by thermal development resulting in refractive index change of 223 ppm. Concurrent irradiation by UV and a 449 nm blue beams for 240 min followed by thermal development resulted in refractive index change of 190 ppm. Thus, spatial modulation of refractive index about 200 ppm allows recording of any type of VBGs with high diffraction efficiency for use in the blue/green spectral region in a 2 mm thick blank.

Unlike Tb0.7 glass the low concentration Tb0.08 glass allows for the upper ⁵D₃ metastable state to be employed for upconversion photoexcitation. Hence it was possible to use low energy IR signal sources in Tb0.08 glass experiments. ESA was performed by two different LD sources operating at wavelengths of 808 and 975 nm. Due to lower absorption in the glass as well as limited source power the beam size had to be decreased in order to provide similar amount of absorbed radiation. For that reason, the IR beam was focused into a circular spot as opposed to the Gaussian stripe pattern used for Tb0.7 glass experiments. After 10 h concurrent exposure to the 375 nm UV LED and the 808 nm LD followed by thermal treatment, a refractive index modulation of approximately 300 ppm was estimated. Similar exposure with the use of the 975 nm LD and subsequent thermal development resulted in refractive index change on the order of 150 ppm. Thus the experiments showed significantly lower photosensitivity in Tb0.08 glass compared to Tb0.7 glass, however photosensitivity can be shifted further into the IR region if Tb0.07 glass is used.

The surprising result is that photosensitivity for longer wavelength of 522 nm is ~3.5 times higher than that for 449 nm while the simplest supposition should be that higher excitation should provide more efficient ionization. This surprising result could be explained by a complex structure of high electronic states of Tb³⁺ and their interaction with electronic states of glass matrix. Here, concurrent exposure to the 375 nm UV and a 522 nm visible radiation delivers an electron to a position at 4.93 eV. If blue light at 449 nm is used instead of green as a signal, the electron is elevated to 5.31 eV. One can see in Fig. 3 and Fig. 6 that energy of 4.93 eV corresponds to a shoulder of the peak at 260 nm in the absorption spectrum and to the maximum in the luminescence excitation spectrum. One can suppose that excitation to those states near 4.9 eV provides both relaxation to lower levels with consequent luminescence as well as photoionization. It looks like the short wavelength absorption band at 230 nm (5.5 eV) corresponds to some localized states that prevents both relaxation to lower levels of Tb³⁺ (it corresponds to the minimum in the luminescence excitation spectrum) and electron release to glass matrix. An interesting correlation is observed: the ratio of intensities of luminescence for 4.9 and 5.3 eV is 3.8 that is close to ratio of values of photosensitivity for 522 and 449 nm. Similar explanation could be provided for the difference in IR photosensitivity of Tb0.08 glass. Thus, these results support the supposition that the band at 260 nm in absorption and luminescence excitation spectra could correspond to 5d¹4f⁷ band of Tb³⁺.

It is clear that the used visible LDs were not suitable for complex hologram recording in conventional two-beam geometry due to their short coherence length resulted from poor spectral and spatial characteristics. To demonstrate a visible complex hologram in Tb:PTRG, we decided to record a reference-free hologram where interference is produced by diffracted and transmitted fractions of the same beam [22]. The similar hologram was demonstrated in the first publication on hologram recording in PTR glass [23]. Such holograms do not require long coherence length and high beam quality of the laser. A thin metal grid with 100 µm cells was chosen as an object. A diffraction pattern at the metal grid produced by radiation of an LD emitting at 522 nm is shown in Fig. 7A. Then the grid was set against a Tb:PTRG blank and illuminated by the same ~1 mm beam at 522 nm. At the same time, the glass blank was illuminated from the backside by the 375 nm LD. Parameters and time of exposure were kept the same as the previous experiments. After thermal development, the sample was returned to the same place and illuminated by the same beam. Figure 7B shows a pattern of complex green hologram reconstruction. Resemblance of the two images can be clearly seen. A similar hologram was created using the blue LD. We can state that the first complex visible holograms are recorded in Tb doped PTR-glass.

 

Fig. 7 Diffraction on a metal grid (A) and reference-free complex hologram of “difraction on a metal grid” (B).

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4. Conclusion

A new phase holographic material with sensitivity to visible and near IR radiation is demonstrated. It is photo-thermo-refractive glass doped with Tb³⁺. Volume holograms recorded in this material are not sensitive to visible radiation and cannot be bleached. The mechanism of photosensitivity is photoionization of Tb³⁺ ions by means of excited state absorption resulted from concurrent exposure to near UV and visible (or near IR) radiation. As the first step, near UV radiation with power density below 1 W/cm² excites electron to a ⁵D₃ level. For glasses with low concentration of Tb, where this level has long lifetime, the second step is excitation to 5d¹4f⁷ band by near IR irradiation that causes photoionization and triggers further photo-structural transformations. For glasses with high concentration of Tb, where ⁵D₃ level has short lifetime, fast relaxation to ⁵D₄ level occurs. As the second step, visible radiation excites electron to 5d¹4f⁷ band placed above the electron mobility threshold of PTR glass. The released electron triggers a chain of structural transformation in glass resulted in permanent refractive index change. Complex reference-free holograms in blue and green regions are demonstrated.