Open Access
Add to BibSonomyAbstract
Picosecond blue-light induced infrared absorption (BLIIRA) was investigated in periodically poled KTiOPO4, Rb:KTiOPO4, RbTiOPO4, KTiOAsO4 and RbTiOAsO4. The dependence of BLIIRA on blue light average power and intensity as well as on crystal temperature was studied. The results show the presence of at least two different types of color centers. A higher level of remnant absorption was observed in the phosphates compared to the arsenates. We attribute the largest portion of the induced absorption to photo-generated electrons and holes self-trapped in the proximity to Ti4+ and O2- ions, respectively, forming polaron color centers. Stabilization of these centers is aided by the presence of mobile alkali metal vacancies in the crystal. The lower level of remnant absorption in arsenates is related to higher population of thermal phonons related to TiO6 group vibrations in arsenates.
© 2015 Optical Society of America
1. Introduction
Since the first high-quality crystal growth of KTiOPO4 (KTP) by the hydrothermal-growth method [1] and especially after the introduction of the commercially more viable flux-growth [2], this nonlinear crystal was increasingly used as an optical frequency converter to the green spectral range, owing to its beneficial phase matching properties and apparent absence of photorefractive damage. Periodic structuring of the spontaneous polarization by electric field poling of KTP [3,4 ] and its isomorphs RbTiOPO4 (RTP) [5,6 ], RbTiOAsO4 (RTA) [7], KTiOAsO4 (KTA) [8,9 ], substantially widened the application range of this crystal family. Arsenate members of the isomorph family, owing to the lower phonon energies, are characterized by an increased transmission range in the mid-infrared and are actively used in few-cycle mid-infrared chirped optical parametric amplifiers [10,11 ]. The orthorhombic crystal structure of these crystals allows fabrication of high-fidelity quasi-phase-matched (QPM) structures in crystals with large optical apertures for high-energy optical parametric sources, as was demonstrated with periodically poled (PP)-KTP, PP-RTA [12], and PP Rb-doped KTP (Rb:KTP) [13], which appear as a viable alternative to the recently introduced large-aperture periodically poled samples of Mg-doped LiNbO3 [14] and MgO-doped LiTaO3 [15].
Early on it was observed that injection of free charge carriers in KTP, either by direct optical absorption [16–24 ], by passing through electric current [25], or by thermal reduction in H2 atmosphere [25,26 ] leads to formation of color centers, the so called grey tracks, in the bulk of the crystal with characteristic broad absorption bands in the visible range with absorption tails extending to the near-infrared. Electron-paramagnetic-resonance (EPR) studies indicated that the presence of color centers is associated with the emergence of Ti4+/Ti3+ electron traps and O2-/O- hole traps on the bridging oxygen adjacent to Ti – ion [21,25,27,28 ]. These electron and hole traps are inherently unstable and require charge compensation mechanism to stabilize them. In KTP, the stabilization of the centers can be provided by native stoichiometry defects: potassium vacancies (VK)- and oxygen vacancies (VO)2+. These defects are naturally present in flux-grown KTP, while the concentration of the defects depends on the crystal growth temperature [29]. Importantly, the VK are readily mobile vacancy species even at room temperature and are mainly responsible for the quasi-one-dimensional ionic conductivity in KTP [30]. Attempts to reduce the ionic conductivity by doping KTP with Ga, Ce and other elements have been explored [29,31 ], but such doping renders the material unsuitable for periodic poling and, therefore, of little interest for many applications. Impurities, primarily Fe3+ which can act as hole traps have also been identified in KTP by EPR and spark mass spectrometry measurements [21,26,29 ]. Although Fe3+ centers cannot be totally ruled out as contributing to the induced absorption, the concentration of these impurities is quite low (of the order of 1016 cm−3) [29] and it was shown [32] the their most pronounced effect in KTP is to slightly shift the fundamental bandgap to lower energies.
QPM devices pumped in the near infrared typically generate some visible radiation due to parasitic frequency conversion processes [33]. Even non-phase matched parasitic second harmonic and sum-frequency mixing output can reach substantial intensities in parametric devices pumped with picosecond-pulses. Visible radiation, either through two-photon absorption, or, if it is in the blue-UV range, by single-photon absorption would produce free carriers and will contribute to the creation of color centers absorbing in the near infrared. Such induced absorption can be detrimental for the device operation especially if the relaxation rate of the induced color centers is slow and there is pulse-to-pulse accumulation of the absorbing centers. Previously, we investigated green-light induced infrared absorption (GRIIRA) excited by 5 ns pulses at 532 nm [34] and blue-light induced infrared absorption (BLIIRA) excited by 1 ps at 410 nm [35] in single-domain KTP and PP-KTP as well as other periodically poled oxide ferroelectrics. PP-KTP showed significant susceptibility to grey tracking with slow absorption relaxation rates at room temperature.
Recent work on second harmonic generation in QPM structures fabricated in Rb:KTP [36] and KTA [37] has indicated that those crystals should be substantially less susceptible to grey tracking. In this work we report on the results of investigation of BLIIRA induced by picosecond pulses in periodically poled KTP isomorphs, namely, PP-KTP, PP-Rb:KTP, PP-KTA, PP-RTP, and PP-RTA.
The results show that in all isomorphs the induced infrared absorption during the presence of the blue beam depends essentially on the generated free-carrier concentration and that there is some correlation with the ionic conductivity for a given isomorph. In phosphates (KTP, Rb:KTP, RTP) the relaxation dynamics of the induced absorption is strongly dependent on the ionic conductivity. Here, the remnant absorption after the blue beam is switched off is much higher in crystals with high ionic conductivity, indicating that the vacancies VK indeed play an important role in stabilizing the induced color centers. In arsenates, the relaxation of the induced infrared absorption is drastically faster than in phosphates even at room temperature. The results show that there is hardly any long-term accumulation of stable absorbing centers in arsenates in strong contrast to phosphate isomorphs. We attribute the largest amount of the induced absorption as originating from photo-generated electrons and holes self-trapped at the titanyl bonds forming polaron-like excitations [38–41 ].
2. Experiment
The crystals included in this study were: two KTP, two RTP, four Rb:KTP, three KTA and one RTA, where the Rb doping concentration in the Rb:KTP crystals was approximately 0.3 at.%. All the samples were periodically poled and are listed in Table 1 , which shows ionic conductivity, poling period, small-signal absorption at 398 nm, intrinsic absorption at 1.04 µm and vendor.
Table 1. Measured sample parameters: ionic conductivity (σ) along the c-axis, poling period, absorption coefficient at the excitation wavelength (398 nm) and the absorption coefficient at pump wavelength (1.04 µm). These absorption coefficients apply to linearly polarized light propagating along the a-axis with the electric field parallel to the c-axis.
All the samples were flux grown, however we have limited knowledge of the actual growth conditions and the post growth processing. The crystals were cut parallel to their crystallographic axes to dimensions of approximately 10 mm × 5 mm × 1 mm (a,b,c), and the end faces in the bc-plane were polished to optical quality for beam propagation along the a-axis. We measured BLIIRA using thermal lens spectroscopy by employing a common-path interferometer using a setup similar to the one used in [35].
The setup is depicted in Fig. 1 . Here, three laser beams were combined by dichroic mirrors DM1 and DM2 for collinear propagation through the sample: a probe beam at 632.8 nm for measuring the thermal lens, a near-infrared (NIR) pump beam at 1.04 μm whose absorption was measured, and a blue beam at 398 nm for inducing infrared-absorbing color centers. All the beams were in the fundamental TEM00 mode. The probe beam was provided by a cw HeNe laser with 3.7 mW of output power, focused to a beam waist radius of 150 μm in the sample. The NIR pump beam was generated by a diode-pumped CW Yb:KYW laser, with up to 1.8 W of output power. The beam was focused to a waist radius of 50 μm in the sample and coinciding with the waist of the probe beam. Finally, the blue beam was acquired by frequency doubling the 796 nm pulses of a Ti:Sapphire regenerative amplifier (RA) in abeta-barium borate (BBO) crystal. This RA configuration with tunable pulse length enabled blue pulses with durations ranging between 2 ps and 40 ps to be employed at a repetition frequency of 1kHz. The blue beam was collimated over the samples with a radius of 350 µm. All of the beams were polarized parallel to the crystallographic c-axis of the samples as this is the most common way for QPM generation of visible radiation in KTP and its isomorphs, and it allows utilization of the largest nonlinear coefficient.
Fig. 1 Schematic of the common-path interferometer employed to measure the thermal lens induced by BLIIRA.
Absorption of the NIR pump beam creates a thermal lens in the samples, which distorts the phase front of the probe beam. This phase front distortion modulates the amplitude distribution of the spatial Fourier transform performed by a lens (f = 250 mm) mounted in a focus-to-focus configuration. In order to measure the intensity modulation, an aperture with a 200 µm diameter was placed approximately 2 mm off-axis in the Fourier plane, allowing only higher spatial frequencies to reach the photodetector. The signal from the photodetector was measured using a lock-in amplifier (Stanford Research Systems, SR830 DSP) whose output was buffered in an oscilloscope memory and continuously logged in a computer. The pump beam was modulated by a chopper at ~20 Hz. The probe beam was separated from the other beams using both a dichroic mirror (DM3) and a diffraction grating. In addition, Schott filters (OG550 and BG19) were used to further prevent radiation from the pump beam and the blue beam from reaching the photodetector (Si p-i-n, Thorlabs DET 36A). The crystal was housed in a copper holder whose temperature was regulated by a temperature controller (TEC2000) with the use of a Peltier element and a thermistor (AD590).
In the experiments we varied the average power of the 398 nm beam between 1 and 9 mW. The maximum carrier concentration generated using a 9 µJ blue light pulse was about 1.8 × 1015 cm−3. It should be noted that this concentration is substantially lower than the expected VK vacancy concentration, which varies between 4.5 × 1018 cm−3 and 7.2 × 1018 cm−3 in KTP flux-grown at temperatures between 920 °C and 970 °C, respectively [29]. This low free carrier concentration ensures that the above-mentioned induced color center stabilization mechanism by vacancies is not limited by the vacancy concentration.
The absorption coefficients, α, of the different samples were extracted from the recorded lock-in signals by comparison to a well-defined reference sample (Schott glass N-BK7) using the following equation [34,35 ]:
where the index refers to the material, S is the amplitude of the lock-in signal, L is the length, κ is the thermal conductivity and dn/dT refers to the thermo-optic coefficient. The calculations were performed using values of the material parameters acquired from [42–44 ]. However, the thermal conductivity of RTP is unknown to us. Thus, it was estimated by assuming that replacing K with Rb would change the thermal conductivity of phosphates in a similar manner as for arsenates, i.e., κRTP = κKTP × κRTA / κKTA. Furthermore, we estimate the lowest detectable absorption to ~1 × 10−5 cm−1. The relaxation of the induced absorption after the blue-light was cut off, could be fit by a double exponential decay function:where τslow/fast are the slow and fast decay constants, Aslow/fast are the amplitudes of the slow and fast decaying absorption component and αend is the remaining absorption. This αend represents color-centers with long life-times, whose decay constants are too long to be resolved within the timeframe of these measurements. Also, t0 is the point in time where the blue beam is switched off, and the absorption at t0 is denoted αmax, i.e., α(t0) = αend + Aslow + Afast = αmax.3. Experimental results
3.1 Time dependence
We studied the time dependent BLIIRA by illuminating the samples with the absorption-inducing blue beam for 50 minutes, after which the beam was blocked and the relaxation of BLIIRA was observed for another 50 minutes. A blue beam with an average power of 3 mW and pulse duration of 30 ps was used, resulting in a peak intensity of approximately 50 MW/cm2. For the absorption coefficient of 0.2 cm−1 at 398 nm this would correspond to the generated free carrier concentration of about 6 × 1014 cm−3. The temperature of the crystal holder was kept at 25 °C.
Representative BLIIRA traces measured under above-mentioned conditions are shown in Fig. 2 . In all isomorphs, the absorption at 1.04 µm increased by more than one order of magnitude and reached rather similar values when the blue light was illuminating the samples over the first 50 min. In isomorphs with higher ionic conductivity, KTP, Rb:KTP, KTA, the maximum value of BLIIRA, αmax, and the remnant absorption after 50 min of relaxation, αend, are correlated with the value of ionic conductivity. From Figs. 2(a) and 2(c), it is evident that KTP2 with higher conductivity than KTP1 showed higher values of αmax and αend, as is the case with Rb:KTP4 compared to Rb:KTP1, or similarly in the case of KTA3 as compared to the KTA1 and KTA2 samples. Although this “intra-isomorph” correlation of the level of BLIIRA with ionic conductivity does exist, it is a rather small effect. A much stronger effect of ionic conductivity is revealed if we consider the values of αend and the BLIIRA relaxation speed in different isomorphs. In Fig. 3 , αend normalized to αmax is shown for all samples against their ionic conductivity.
Fig. 2 Absorption at 1.04 µm measured as the 398 nm excitation beam was switched on at t = 0 min and switched of at t = 50 min. T = 25 °C. (a) PP-KTP and PP-Rb:KTP, (b) PP-RTP, (c) PP-KTA, (d) PP-RTA.
Fig. 3 Remnant absorption normalized to the maximum induced absorption. The samples were exposed to the blue light with an average power of 3 mW and pulse duration of 30 ps for ~50 minutes followed by a relaxation time of ~50 minutes.
It is instructive to compare KTP and Rb:KTP isomorphs in this respect. Those two isomorphs come from the same supplier with resembling growth technique and flux compositions. In Rb:KTP, there is only a small admixture of Rb in the flux (1.4 mol%) which renders Rb:KTP with much lower ionic conductivity than KTP. This is due to the fact that the ionic radius of Rb+ is larger than that of K+ which substantially impedes the vacancy hopping motion in the Rb:KTP. In KTP the accumulation of slow-relaxing induced absorption centers increases much more prominently during the exposure as compared to Rb:KTP. Moreover, at the end of the 50 min relaxation period, 90% of the induced absorption remains in KTP in long-lived color centers. For comparison, the remnant absorption in Rb:KTP is 60% and only about 30% in RTP, which has even lower ionic conductivity than Rb:KTP. Thus, the ionic conductivity appears to be crucial for stabilizing induced color centers. On the other hand, in arsenate isomorphs, the prevailing induced absorption is due to fast-relaxing color centers and there seems to be little accumulation of remnant absorption in KTA or RTA as can be seen from virtually constant induced absorption during the exposure in KTA. In RTP and too a much lower degree in RTA there are other effects apart from induced absorption which affect the measured signal dynamics. We attribute them to the buildup of an internal electric field possibly owing to the photo-galvanic effect and associated electro-optic modulation of the probe beam. Both these materials have very low ionic conductivity and therefore the field built-up by separated charge carriers cannot be readily screened at room temperature. In RTA the color centers relax much faster than in RTP therefore the electric-field related effects are much weaker. These effects disappear in RTP totally at temperatures of 80 °C and higher.
In this context it is important to compare remnant absorption accumulation in different isomorphs during repeated exposure to the blue light. For that purpose, we measured the BLIIRA dynamics by repeating five consecutive times a cycle of ~10 min exposure followed by a ~20 min relaxation time. The measured remnant absorption, αend after each relaxation period is shown in Fig. 4(a) against the accumulated exposure time at each point. In KTP and Rb:KTP there is clear accumulation with repeated exposure of the long-lived stable color centers. Due to the built-in electric field effects in RTP the absorption accumulation data is not reliable and therefore was not analyzed. On the other hand, arsenates do not show clear accumulation behavior indicating that the vast majority of color centers which were induced by the blue light have fast relaxation. The remaining absorption in arsenates was approximately constant and is probably related to the absorption by impurity related traps, (e.g. Fe4+/Fe3+).
Fig. 4 (a). Remnant induced infrared absorption as a function of total exposure time to the blue light. (b). Remnant induced infrared absorption normalized to the maximum absorption as a function of crystal temperature. The samples were exposed to the blue light with an average power of 3 mW and pulse duration of 30 ps for ~10 minutes followed by a relaxation time of ~20 minutes.
3.2 Temperature dependence
It is well known that the induced absorption centers in KTP can be removed at elevated temperatures. Measurements of the BLIIRA dynamics show that the induced absorption level during exposure with the blue light, αmax, does not change appreciably as the temperature of the crystals was varied between 25 °C and 80 °C. On the other hand, the induced absorption relaxation dynamics becomes substantially faster as the temperature is increased. As shown in Fig. 4(b) the remnant absorption in phosphate isomorphs drops drastically at 80 °C temperature as compared to 25 °C. In arsenates the remnant absorption does not change as much as in phosphates but the relaxation of absorption after the blue light is switched off becomes increasingly rapid at elevated temperatures. Illustration of the BLIIRA dynamics at different temperatures in KTP and KTA are shown in Fig. 5 . The temperature dependencies indicate that the free carrier trapping is largely unaffected by the temperature increase, thestability of the centers is strongly reduced and that there is an increasingly efficient mechanism leading to delocalization of the charge carriers and their recombination.
Fig. 5 BLIIRA traces measured at different crystal temperatures. KTP (a), KTA (b). The samples were exposed to the blue light with an average power of 3 mW and a pulse duration of 30 ps for ~10 minutes followed by a relaxation time of ~20 minutes.
3.3 Intensity dependence
In order to elicit a possible dependence of the BLIIRA on the peak intensity of the blue light we measure traces in all isomorphs by using blue excitation pulses ranging from 40 ps to 2 ps, while maintaining the average power at 3 mW. This varied the peak intensity between 37 and 733 MW/cm2. The crystal holder temperature was kept at 25 °C in all cases. The measurements reveal that the maximum absorption during the exposure and the remnant absorption after 50 min relaxation remained the same within the accuracy of the measurement, regardless of the variation of peak intensity. This indicates, first, that it is a linear absorption at 398 nm which is mostly responsible for creation of free carriers; second, that the carrier recombination time is longer than 40 ps. This makes the BLIIRA amplitude, and therefore the concentration of the induced color centers primarily dependent only on the pulse-integrated generated carrier concentration, i.e., the pulse fluence.
3.4 Power dependence
The dependences of the αmax and αend on the average power of 398 nm at a constant pulse duration of 30 ps, i.e. on the generated carrier concentration at 25 °C are shown in Fig. 6(a) . The blue power range between 1 mW and 9 mW corresponds to the intensity ranging from 16 MW/cm2 to 147 MW/cm2. High ionic conductivity samples KTP2, Rb:KTP3, KTA3 (see Table 1) were used here to investigate the “worst case” scenario. The value of αmax reflects contribution from both the unstable (short-lived) and stabilized (slowly relaxing) induced color centers, while that of αend stems mostly from long lived color centers and stationary impurity traps. After each measurement at a certain power level the samples were annealed at 250 °C in air for 15h, which reset the IR absorption to the same level as prior to the exposure to the blue light, thus we assume that all color centers were removed. The αmax clearly shows sublinear dependence with clear saturation in Rb:KTP and KTA. Within the power range of the blue light αmax increases in KTP by 2.4 times, while in Rb:KTP 1.4 times and in KTA only 1.3 times. The αend in KTA does not change with the blue power, while in Rb:KTP it proportionally follows the increase in αmax. In KTP, on the other hand, αend rises 4.5 times as an increasing proportion of the generated color centers are stabilized. Such stabilization doesnot appear to happen in KTA, although ionic conductivity in this sample was even higher than in KTP. In the next section we explain this by different rates of delocalization of the self-trapped charge carriers in arsenates and phosphates. In Rb:KTP, as discussed previously the stabilization of the color centers is much less efficient compared to KTP due to the reduced ionic conductivity.
Fig. 6 (a) BLIIRA after 50 min of exposure to 30 ps long pulses of the blue light, αmax (solid symbols), and at the end of 50 min of relaxation at 25 °C, αend (open symbols), as a function of average power of the blue light at 398 nm. Absorption coefficients normalized to the αmax at 1 mW of the blue power for each isomorph. (b) Room temperature fluorescence spectrum measured in RTA.
3.5 Color center fluorescence
Finally, we attempted to verify the existence of Ti3+ centers in the KTP isomorphs by measuring a spectrum of the fluorescence generated during color center creation. Samples were excited by 350 mW, 150 fs pulses at 400 nm and 80 MHz repetition rate. The beam was tightly focused inside the crystal using a microscope objective. The fluorescence was collected in the direction (along crystal c-axis) perpendicular to the excitation beam propagation (crystal a-axis) by simply placing a cleaved tip of a SMF28 fiber end in proximity to the focus. Due to the low collection efficiency, we employed a spectrometer with a photon counting detection system. The measured fluorescence spectrum at room temperature in RTA is shown in Fig. 6(b). The spectral position and broad spectral width correspond to those expected from Ti3+ centers in KTP [45]. Similar spectra were measured in KTP and KTA.
4. Discussion
Electrons and holes injected, respectively, into the conduction and valence bands of polar dielectrics, like KTP isomorphs, would induce bound electron polarization, primarily from the electronic orbitals contributing to the top of the valence and bottom of the conduction bands. X-ray photoemission spectroscopy and theoretical analysis show that in KTP isomorphs it is mostly hybridized Ti 3d and O 2p states in Ti-O bonds of the TiO6 octahedra which contribute to the density of states at the valence and conduction band extrema, forming a direct bandgap at the Γ point of the Brillouin zone [46,47 ]. The induced electronic polarization elicits local lattice deformation around the charge carrier. This should happen on the characteristic time scale of lattice vibrations. This creates a potential well and will result in self-trapping of the charge carrier, forming a polaron [38,48 ]. Electronic band structure calculations show that the conduction and valence bands in KTP are quite shallow and flat [46]. Therefore, free carriers would have large effective masses which would promote self-trapping and polaron formation. Due to the absence of reliable data on the dispersion of electronic bands in KTP we can only approximately estimate the polaron binding energy. Assuming long-range electron-lattice interaction via Fröhlich coupling the binding potential is given by [49] (in SI units):
where e is the electron charge, ε(∞) = 3.35 and ε(0) = 10 are the high-frequency and low-frequency dielectric constants, apol is the polaron radius:Here, mc is the effective conduction band mass, which we estimated as 0.72 m0 (m0 is the free electron mass) using a quasi-free electron approximation with a bandgap energy Eg = 3.5 eV and a unit lattice constant along the KTP crystallographic z-direction c = 10.616 Å by utilizing the relation [50]:It is quite likely that the effective mass is underestimated. Nevertheless, with the above parameters, the estimated polaron binding energy would be 0.387 eV and the polaron radius would be 3.6 Å. The polaron would then be contained within the extension of O-Ti-O bonds in KTP isomorphs [51]. Carrier self-trapping and polaron formation have been investigated before in wide-bandgap dielectrics containing similar TiO6 structural groups [39–41 ]. For instance, detailed quantum chemistry calculations [40] in rutile-TiO2 which contains the same Ti4+ spatial coordination as KTP isomorphs, give an electron polaron radius of 4-5 Å and a binding energy of 0.38 eV. Calculations of the electron and hole trapping in anatase-TiO2 [41] give single electron and single hole binding energies of 0.23 eV and 0.74 eV, respectively. In TiO2 an electron is trapped at the Ti4+ site, thus forming a Ti3+ center, while a hole is trapped at the adjacent O2- site forming an O- center. In [41] it was conjectured that close co-localization of the electrons and holes would make them susceptible to recombination, especially if appropriate thermal TiO6 group phonons are present. The peak of the optical absorption related to the ionization of these, so called, large polarons occurs at photon energies equal to approximately 3-times the binding polaron potential [49]. In any case, with the above binding energy estimates one should expect near infrared absorption at 1 µm originating from these self-trapped charge carriers.In KTP isomorphs alkali metal vacancies (VK) and associated oxygen vacancies can stabilize single-hole and single-electron traps, respectively. On the other hand the relaxation dynamics of the induced absorption in polarons via delocalization and recombination would be strongly assisted by thermal population of polar phonons associated with the TiO6 group. The strongest A1 symmetry polar phonons associated with TiO6 group vibrations have somewhat different energies in phosphate and arsenate isomorphs, 270 cm−1 and 230 cm−1, respectively [52,53 ]. At 25 °C the average thermal occupation numbers for this phonon mode 〈nLO〉 = [exp(ℏωLO / kBT) - 1]−1 are 0.35 and 0.5 in phosphates and arsenates, respectively. The higher probability of thermal phonon generation in arsenates would ensure faster relaxation rate of the induced absorbing centers in arsenates at room temperature. Although in KTA, the ionic conductivity is similar to that in KTP, most of the self-trapped carriers there have recombined before the color centers could be stabilized by mobile VK vacancies. As the temperature is increased to 80 °C the average thermal phonon occupation number of 0.5 will be reached in phosphates with a concomitant speed-up in the induced center relaxation and a reduction of the concentration of long-lived color centers. Stabilization by vacancies seems to be an essential prerequisite for formation of long-lived color centers. This stabilization mechanism competes with the relaxation channel mediated by the thermal phonons. As can be observed, in RTP and Rb:KTP, where the mobility of the vacancies is substantially hindered due to a larger ionic radius of Rb+, the accumulation of long-lived induced absorption centers is substantially lower than in KTP, although the phonon-mediated relaxation rate should be similar.
5. Conclusions
In summary, we investigated blue light induced infrared absorption dynamics in periodically poled KTP isomorphs.
The largest portion of the induced absorption is attributed to photo-generated electrons and holes self-trapped in the proximity to Ti4+ and O2- ions, respectively, forming polaron excitation. Stabilization of these centers is aided by the presence of mobile alkali metal vacancies in the crystal, making flux-grown KTP highly susceptible to formation of slowly-relaxing absorption centers and accumulation of BLIIRA with repeated exposures to blue light. In arsenate isomorphs the delocalization and/or recombination of self-trapped charge carriers is much faster than in phosphates, in fact, fast enough to largely prevent formation and accumulation of the long-lived color centers at room temperature. This fast relaxation of the induced absorption is related to higher population of thermal phonons related to TiO6 group vibrations in arsenates.
Finally, room-temperature fluorescence attributed to Ti3+ centers has been measured in KTP isomorphs. Although the fluorescence signal is rather weak, it has to be taken into account in nonlinear devices used in quantum optics experiments, which might employ blue-pumped degenerate downconversion schemes.
Acknowledgments
This work was supported by the Swedish Research Council (VR) through both its Linnæus Center of Excellence ADOPT and VR grant 621-2012-2430 as well as the Swedish Foundation for Strategic Research.
References and links
1. F. C. Zumsteg, J. D. Bierlein, and T. E. Gier, “KxRb1−xTiOPO4: A new nonlinear optical material,” J. Appl. Phys. 47(11), 4980–4985 (1976). [CrossRef]
2. P. F. Bordui, J. C. Jacco, G. M. Loiacono, R. A. Stolzenberger, and J. J. Zola, “Growth of large single crystals of KTiOPO4 (KTP) from high-temperature solution using heat pipe based furnace system,” J. Cryst. Growth 84(3), 403–408 (1987). [CrossRef]
3. Q. Chen and W. P. Risk, “Periodic poling of KTiOPO4 using an applied electric field,” Electron. Lett. 30(18), 1516–1517 (1994). [CrossRef]
4. H. Karlsson and F. Laurell, “Electric field poling of flux grown KTiOPO4,” Appl. Phys. Lett. 71(24), 3474–3476 (1997). [CrossRef]
5. Q. Jiang, A. Lovejoy, P. A. Thomas, K. B. Hutton, and R. C. C. Ward, “Ferroelectricity, conductivity, domain structure and poling conditions of rubidium titanyl phosphate,” J. Phys. D Appl. Phys. 33(21), 2831–2836 (2000). [CrossRef]
6. A. Fragemann, V. Pasiskevicius, J. Nordborg, J. Hellström, H. Karlsson, and F. Laurell, “Frequency converters from visible to mid-infrared with periodically poled RbTiOPO4,” Appl. Phys. Lett. 83(15), 3090–3092 (2003). [CrossRef]
7. H. Karlsson, F. Laurell, P. Henriksson, and G. Arvidsson, “Frequency doubling in periodically poled RbTiOAsO4,” Electron. Lett. 32(6), 556–557 (1996). [CrossRef]
8. G. Rosenman, A. Skliar, Y. Findling, P. Urenski, A. Englander, P. A. Thomas, and Z. W. Hu, “Periodically poled KTiOAsO4 crystals for optical parametric oscillation,” J. Phys. D Appl. Phys. 32(14), L49–L52 (1999). [CrossRef]
9. A. Zukauskas, N. Thilmann, V. Pasiskevicius, F. Laurell, and C. Canalias, “Periodically poled KTiOAsO4 for highly efficient midinfrared optical parametric devices,” Appl. Phys. Lett. 95(19), 191103 (2009). [CrossRef]
10. G. Andriukaitis, T. Balčiūnas, S. Ališauskas, A. Pugžlys, A. Baltuška, T. Popmintchev, M.-C. Chen, M. M. Murnane, and H. C. Kapteyn, “90 GW peak power few-cycle mid-infrared pulses from an optical parametric amplifier,” Opt. Lett. 36(15), 2755–2757 (2011). [CrossRef] [PubMed]
11. M. Baudisch, M. Hemmer, H. Pires, and J. Biegert, “Performance of MgO:PPLN, KTA, and KNbO₃ for mid-wave infrared broadband parametric amplification at high average power,” Opt. Lett. 39(20), 5802–5805 (2014). [CrossRef] [PubMed]
12. M. Peltz, U. Bäder, A. Borsutzky, R. Wallenstein, J. Hellström, H. Karlsson, V. Pasiskevicius, and F. Laurell, “Optical parametric oscillators for high pulse energy and high average power operation based on large aperture periodically poled KTP and RTA,” Appl. Phys. B 73(7), 663–670 (2001). [CrossRef]
13. A. Zukauskas, N. Thilmann, V. Pasiskevicius, F. Laurell, and C. Canalias, “5 mm thick periodically poled Rb-doped KTP for high energy optical parametric frequency conversion,” Opt. Mater. Express 1(2), 201 (2011). [CrossRef]
14. H. Ishizuki and T. Taira, “Half-joule output optical-parametric oscillation by using 10-mm-thick periodically poled Mg-doped congruent LiNbO3.,” Opt. Express 20(18), 20002–20010 (2012). [CrossRef] [PubMed]
15. H. Ishizuki and T. Taira, “High energy quasi-phase matched optical parametric oscillation using Mg-doped congruent LiTaO(3) crystal,” Opt. Express 18(1), 253–258 (2010). [CrossRef] [PubMed]
16. R. Blachman, P. F. Bordui, and M. M. Fejer, “Laser-induced photochromic damage in potassium titanyl phosphate,” Appl. Phys. Lett. 64(11), 1318–1320 (1994). [CrossRef]
17. B. Boulanger, I. Rousseau, J. P. Fève, M. Maglione, B. Ménaert, and G. Marnier, “Optical Studies of Laser-Induced Gray-Tracking in KTP,” IEEE J. Quantum Electron. 35(3), 281–286 (1999). [CrossRef]
18. J. P. Fève, B. Boulanger, G. Marnier, and H. Albrecht, “Repetition rate dependence of gray-tracking in KTiOPO4 during second-harmonic generation at 532 nm,” Appl. Phys. Lett. 70(3), 277–279 (1997). [CrossRef]
19. L. Carrion and J.-P. Girardeau-Montaut, “Gray-track damage in potassium titanyl phosphate under a picosecond regime at 532 nm,” Appl. Phys. Lett. 77(8), 1074–1076 (2000). [CrossRef]
20. V. Mürk, V. Denks, A. Dudelzak, P.-P. Proulx, and V. Vassiltsenko, “Gray tracks in KTiOPO4: Mechanism of creation and bleaching,” Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms 141, 472–476 (1998).
21. G. J. Edwards, M. P. Scripsick, L. E. Halliburton, and R. F. Belt, “Identification of a radiation-induced hole center in KTiOPO4,” Phys. Rev. B 48(10), 6884–6891 (1993). [CrossRef]
22. G. M. Loiacono, D. N. Loiacono, T. McGee, and M. Babb, “Laser damage formation in KTiOPO4 and KTiOAsO4 crystals: Grey tracks,” J. Appl. Phys. 72(7), 2705–2712 (1992). [CrossRef]
23. B. Boulanger, M. M. Fejer, R. Blachman, and P. F. Bordui, “Study of KTiOPO4 gray-tracking at 1064, 532, and 355 nm,” Appl. Phys. Lett. 65(19), 2401–2403 (1994). [CrossRef]
24. J. C. Jacco, D. R. Rockafellow, and E. A. Teppo, “Bulk-darkening threshold of flux-grown KTiOPO4.,” Opt. Lett. 16(17), 1307–1309 (1991). [CrossRef] [PubMed]
25. M. G. Roelofs, “Identification of Ti3+ in potassium titanyl phosphate and its possible role in laser damage,” J. Appl. Phys. 65(12), 4976–4982 (1989). [CrossRef]
26. M. J. Martín, D. Bravo, R. Solé, F. Díaz, F. J. López, and C. Zaldo, “Thermal reduction of KTiOPO4 single crystals,” J. Appl. Phys. 76(11), 7510–7518 (1994). [CrossRef]
27. M. P. Scripsick, D. N. LoIacono, J. Rottenberg, S. H. Goellner, L. E. Halliburton, and F. K. Hopkins, “Defects responsible for gray tracks in flux-grown KTiOPO4,” Appl. Phys. Lett. 66(25), 3428 (1995). [CrossRef]
28. S. D. Setzler, K. T. Stevens, N. C. Fernelius, M. P. Scripsick, G. J. Edwards, and L. E. Halliburton, “Electron Paramagnetic Resonance and Electron-nuclear Double-resonance Study of Ti3+ Centres in KTiOPO4,” J. Phys. Condens. Matter 15(23), 3969–3984 (2003). [CrossRef]
29. P. A. Morris, A. Ferretti, J. D. Bierlein, and G. M. Loiacono, “Reduction of the ionic conductivity of flux grown KTiOPO4 crystals,” J. Cryst. Growth 109(1-4), 361–366 (1991). [CrossRef]
30. S. Furusawa, H. Hayasi, Y. Ishibashi, A. Miyamoto, and T. Sasaki, “Ionic conductivity of quai-one-dimensional superionic conductor KTiOPO4 (KTP) single crystal,” J. Phys. Soc. Jpn. 62(1), 183–195 (1993). [CrossRef]
31. J. X. Zhang, J. Y. Wang, X. B. Hu, H. J. Zang, X. X. Wang, Y. G. Liu, and M. H. Jiang, “Growth, defects, conductivity and other properties of and crystals,” J. Cryst. Growth 275(1-2), e2113–e2116 (2005). [CrossRef]
32. C. V. Kannan, H. Kimura, A. Miyazaki, and P. Ramasamy, “Optical and electrical studies on trivalent-ion (Cr, Fe)-doped potassium titanyl phosphate single crystals,” Jpn. J. Appl. Phys. 43(9B), 6667–6671 (2004). [CrossRef]
33. H. H. Lim, S. Kurimura, and N. E. Yu, “Parasitic-light-suppressed quasi-phase-matched optical parametric oscillation device,” Opt. Express 22(5), 5209–5215 (2014). [CrossRef] [PubMed]
34. S. Wang, V. Pasiskevicius, and F. Laurell, “Dynamics of green light-induced infrared absorption in KTiOPO4 and periodically poled KTiOPO4,” J. Appl. Phys. 96(4), 2023 (2004). [CrossRef]
35. J. Hirohashi, V. Pasiskevicius, S. Wang, and F. Laurell, “Picosecond blue-light-induced infrared absorption in single-domain and periodically poled ferroelectrics,” J. Appl. Phys. 101(3), 033105 (2007). [CrossRef]
36. A. Zukauskas, V. Pasiskevicius, and C. Canalias, “Second-harmonic generation in periodically poled bulk Rb-doped KTiOPO₄ below 400 nm at high peak-intensities,” Opt. Express 21(2), 1395–1403 (2013). [CrossRef] [PubMed]
37. P. Zeil, A. Zukauskas, S. Tjörnhammar, C. Canalias, V. Pasiskevicius, and F. Laurell, “High-power continuous-wave frequency-doubling in KTiOAsO4.,” Opt. Express 21(25), 30453–30459 (2013). [CrossRef] [PubMed]
38. R. P. Feynman, “Slow electrons in a polar crystal,” Phys. Rev. 97(3), 660–665 (1955). [CrossRef]
39. W. S. Baer, “Free-carrier absorption in reduced SrTiO3,” Phys. Rev. 144(2), 734–738 (1966). [CrossRef]
40. H. Sezen, M. Buchholz, A. Nefedov, C. Natzeck, S. Heissler, C. Di Valentin, and C. Wöll, “Probing electrons in TiO2 polaronic trap states by IR-absorption: evidence for the existence of hydrogenic states,” Sci. Rep. 4, 3808 (2014). [CrossRef] [PubMed]
41. C. Di Valentin and A. Selloni, “Bulk and Surface Polarons in Photoexcited Anatase TiO2,” J. Phys. Chem. Lett. 2(17), 2223–2228 (2011). [CrossRef]
42. C. A. Ebbers and S. P. Velsko, “Optical and thermo-optical characterization of KTP and its isomorphs for 1.06 im pumped OPO’s,” in Photonics West ’96, M. C. Gupta, W. J. Kozlovsky, and D. C. MacPherson, eds. (International Society for Optics and Photonics, 1996), pp. 227–239.
43. S. Emanueli and A. Arie, “Temperature-Dependent Dispersion Equations for KTiOPO4 and KTiOAsO4.,” Appl. Opt. 42(33), 6661–6665 (2003). [CrossRef] [PubMed]
44. I. Yutsis, B. Kirshner, and A. Arie, “Temperature-dependent dispersion relations for RbTiOPO4 and RbTiOAsO4,” Appl. Phys. B Lasers Opt. 79(1), 77–81 (2004). [CrossRef]
45. A. Deepthy, M. N. Satyanarayan, K. S. R. K. Rao, and H. L. Bhat, “Photoluminescence studies on gray tracked KTiOPO4 single crystals,” J. Appl. Phys. 85(12), 8332–8336 (1999). [CrossRef]
46. W. Y. Ching and Y.-N. Xu, “Band structure and linear optical properties of KTiOPO4.,” Phys. Rev. B Condens. Matter 44(10), 5332–5335 (1991). [CrossRef] [PubMed]
47. V. V. Atuchin, V. G. Kesler, G. Meng, and Z. S. Lin, “The electronic structure of RbTiOPO4 and the effects of the A-site cation substitution in KTiOPO4-family crystals,” J. Phys. Condens. Matter 24(40), 405503 (2012). [CrossRef] [PubMed]
48. L. D. Landau, “Über die Bewegung der Elektronen in Kristallgitter,” Phys. Z. Sowjetunion 3, 644–645 (1933).
49. D. Emin, “Optical properties of large and small polarons and bipolarons,” Phys. Rev. B Condens. Matter 48(18), 13691–13702 (1993). [CrossRef] [PubMed]
50. K. Seeger, Semicondictor Physics, 2nd ed. (Springer-Verlag, 1982).
51. R. J. Bolt and P. Bennema, “Potassium tutanyl phosphate KTiOPO4 (KTP): Relation between crystal structure and morphology,” J. Cryst. Growth 102(1-2), 329–340 (1990). [CrossRef]
52. C.-S. Tu, R. Guo, R. Tao, R. S. Katiyar, R. Guo, and S. Bhalla, “Temperature dependent Raman scattering in KTiOPO4 and KTiOAsO4 single crystals,” J. Appl. Phys. 79(6), 3235–3240 (1996). [CrossRef]
53. G. E. Kugel, F. Bréhat, B. Wyncke, M. D. Fontana, G. Marnier, C. Carabatos-Nedlec, and J. Mangin, “The vibrational spectrum of a KTiOPO4, single crystal studied by Raman and infrared reflectivity spectroscopy,” J. Phys. C Solid State Phys. 21(32), 5565–5583 (1988). [CrossRef]
