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Sci Rep. 2017; 7: 45650.
Published online 2017 April 3. doi:  10.1038/srep45650
PMCID: PMC5377312

Controllable optical modulation of blue/green up-conversion fluorescence from Tm3+ (Er3+) single-doped glass ceramics upon two-step excitation of two-wavelengths

Abstract

Optical modulation is a crucial operation in photonics for network data processing with the aim to overcome information bottleneck in terms of speed, energy consumption, dispersion and cross-talking from conventional electronic interconnection approach. However, due to the weak interactions between photons, a facile physical approach is required to efficiently manipulate photon-photon interactions. Herein, we demonstrate that transparent glass ceramics containing LaF3: Tm3+ (Er3+) nanocrystals can enable fast-slow optical modulation of blue/green up-conversion fluorescence upon two-step excitation of two-wavelengths at telecom windows (0.8–1.8 μm). We show an optical modulation of more than 1500% (800%) of the green (blue) up-conversion fluorescence intensity, and fast response of 280 μs (367 μs) as well as slow response of 5.82 ms (618 μs) in the green (blue) up-conversion fluorescence signal, respectively. The success of manipulating laser at telecom windows for fast-slow optical modulation from rear-earth single-doped glass ceramics may find application in all-optical fiber telecommunication areas.

As the internet applications continue to develop at an extremely fast pace, the dominant electronic interconnection approach suffers from issues of bandwidth and loss due to its performance restrictions in terms of speed, energy consumption, dispersion and cross-talking. Photonic technologies are central to our information-based society. Among photonic technologies, optical modulation is one of the most essential operations, which offers intrinsic advantages of higher bandwidth and lower loss1. Therefore, substantial research efforts are being directed towards optical modulation to exploit compact, cost-effective, efficient, fast and broadband light modulators for high-performance optical interconnects2.

In recent years, intense research efforts on optical modulation have concentrated on finding the fast and highly nonlinear media, including graphene and other two-dimensional layered materials3,4,5,6,7,8,9,10, gain nonlinear active media11, carrier-induced nonlinear semiconductor photonic crystal cavities12, nonlinear metals, semiconductors and low-dimensional carbon13, and optomechanical and phase-change metamaterials14,15. However, such materials suffer from defects of difficult fabrication, low production, high expense, low chemical durability, and are detrimental to environment1,16. On the contrary, rare-earth (RE) ions doped glass ceramics (GCs) can overcome these drawbacks. Besides, the well-engineered RE-doped GCs, combining the merits of glass (low expense, easy fabrication, good homogeneity and optical transparency) and crystals (high chemical durability and mechanical strength, intense crystal field effect)17,18,19,20, can be effortless for fiber drawing and will greatly impact the application in future all-optical fiber telecommunication2,21,22. As is known to all, photons interact weakly with each other, which requires the mediation of a physical system to produce efficient photon-photon interactions16,23,24,25. RE3+ ions possess vast amounts of energy levels, which provide convenience for us to realize optical modulation by adopting facile physical approaches26,27,28,29,30,31,32. Moreover, tunable excitation and emission wavelengths from visible to near-infrared (NIR) of RE3+-doped GCs guarantee optical modulation operating at telecom windows or at the visible range for emerging Li-Fi technology, showing notable advantages for optical data transmission systems1,33. Nevertheless, for a long time, fast-slow optical modulation of up-conversion (UC) fluorescence from RE3+ ion single-doped GCs by utilizing a strategy named “two-step excitation of two-wavelengths” has been overlooked34,35. The inclusion of electronic structure of the single-doping RE3+ ion has been argued to be particularly advantageous to manipulate the speed of electrons populated fully in the excited state29,36, by controlling one ground state absorption (GSA) or excited state absorption (ESA) wavelength laser as a gating beam combined simultaneously with another continuous-wave (C.W.) ESA or GSA wavelength laser beam, which in turn affect the optical switching “on-off” response. Hence, it gives an efficacious physical method for the realization of fast-slow optical modulation of UC fluorescence.

In this work, we introduce an approach for future all-optical information processing using two-step excitation of two-wavelengths at telecom windows from germanate oxyfluoride GCs containing LaF3: Tm3+ (Er3+) nanocrystals, which enables optical modulation of blue/green UC fluorescence with fast-slow response. The optical switching “on-off” response is relatively fast by manipulating the ESA wavelength laser as gating beam coupled simultaneously with a C. W. laser beam of GSA wavelength for both Tm3+ and Er3+ single-doped GCs. Conversely, the “on-off” response becomes much slower through modulating the GSA wavelength laser as gating beam combined simultaneously with a C. W. laser beam of ESA wavelength. Furthermore, we put insight into the mechanism responsible for this fast-slow optical modulation, which reveals the presence of differentiation of the speed of electrons populated fully in the excited state manipulated by various pumping tactic29,36. Importantly, the success of manipulating light at telecom windows for fast-slow optical modulation of blue/green UC fluorescence from LaF3:Tm3+ (Er3+) nanocrystals embedded germanate oxyfluoride GCs has been empowered by imaginative designs, which may provide powerful opportunities for novel all-optical fiber data processing in future optical telecommunication fields37,38.

Results

Theory, design, and concept for fast-slow UC fluorescence modulation

As sketched in Fig. 1(a–c), the concept of fast-slow optical modulation via two-step excitation of two-wavelengths is proposed. To validate this judicious design, the well-engineered Tm3+ (Er3+) single-doped GCs with lower optical losses (12.87 dB/cm at 800 nm and 3.64 dB/cm at 1064 nm for Tm3+ doped GCs, and 4.65 dB/cm at 850 nm and 8.22 dB/cm at 1530 nm for Tm3+ doped GCs. See Supplementary Material, Figs S1–S5, Table S1) were selected to study the fast-slow optical modulation properties. The room-temperature absorption and emission spectra of the Tm3+ (Er3+) single-doped GCs are shown in Fig. 1(d–e). The first GSA occurs at a wavelength of 800 nm for Tm3+ and 1530 nm for Er3+, which is a hallmark of the 3H6  3H4 and 4I15/2  4I13/2 transition of the Tm3+ and Er3+ single dopants, respectively29,31,36. The UC fluorescence emission can be triggered and expedited while selectively pumping with another non-resonant NIR laser through an efficient ESA step (Here, there’s no other ESA processes involved in this pumping strategy showing in Fig. S6). For the realization of this fast-slow optical modulation, we demonstrated a facile approach to controlling the speed of electrons populated fully in the excited state through selectively adjusting the NIR laser of GSA wavelength as C. W. or gating beam.

Figure 1
Concept of fast-slow optical modulation of blue/green UC fluorescence from Tm3+ (Er3+) single-doped GCs upon two-step excitation of two-wavelengths.

Fast-slow optical modulation features for the blue/green UC fluorescence

Figure 2 demonstrates fatigue-free switching of the blue/green UC fluorescence from Tm3+ (Er3+) single-doped GCs by two-step excitation of two-wavelengths. Using a home-built coaxial optical setup, bright UC fluorescence signal is detectable only when irradiating with both of two wavelengths laser, while single-wavelength laser irradiation alone cannot elicits a strong signal (Fig. S7). For the fast-slow optical modulation of blue fluorescence, we continuously excited Tm3+ single-doped GCs with 33.37 KW/cm2 of 800 nm laser, and periodically added 3.65 MW/cm2 of 1064 nm laser (Fig. 2(a)). The “on-off” cycling is reproducible and follows the modulation of 1064 nm laser, showing a faster response of 367 μs (Fig. 2(c)). Instead, when continuously excited Tm3+ single-doped GCs with 3.65 MW/cm2 of 1064 nm laser and periodically added 33.37 KW/cm2 of 800 nm laser (Fig. 2(b)), the “on-off” cycling is reproducible and follows the modulation of the 800 nm laser, showing a slower response of 618 μs (Fig. 2(c)). Vice versa for the fast-slow optical modulation of green fluorescence, we continuously excited Er3+ single-doped GCs with 22.88 KW/cm2 of 1530 nm laser, and periodically added 3.57 MW/cm2 of 850 nm laser (Fig. 2(d)), the “on-off” cycling is reproducible and follows the modulation of 850 nm laser, showing a faster response of 280 μs (Fig. 2(f)). Conversely, when continuously excited Er3+ single-doped GCs with 3.57 MW/cm2 of 850 nm laser and periodically added 22.88 KW/cm2 of 1530 nm laser (Fig. 2(e)), the “on-off” cycling is reproducible and follows the modulation of 1530 nm laser, showing an extremely slower response of 5.82 ms (Fig. 2(f)). Intriguingly, both of the fast-slow optical modulation can be performed, and the differentiation for the fast-slow response of green fluorescence modulating is as high as an order of magnitude.

Figure 2
Fast-slow “on-off” optical modulation of blue/green UC fluorescence from Tm3+ (Er3+) single-doped GCs upon two-step excitation of two-wavelengths.

Blue/green UC fluorescence and efficiency manipulation

To validate the feasibility of the fast-slow optical modulation of blue/green UC fluorescence, we put insights into the performances of the UC fluorescence produced by two-step excitation of two-wavelengths. The co-irradiation by simultaneous two-wavelengths laser generates a notably enhanced UC emission spectrum from Tm3+ (Er3+) single-doped GCs (Fig. 3(a–c)), which is effortlessly distinguishable from that generated by single-wavelength excitation. The UC fluorescence emission intensity shows flexible modulating region. What’s more, the optimize UC fluorescence efficiency can be tuned over 800% for Tm3+ single-doped GCs and by up to 1500% for Er3+ single-doped GCs (Fig. 4(a–d)). In addition, the power of the output UC fluorescence can be tuned by changing the optical power of the input laser combined with and without another laser power fixed (Fig. 3(b–d)), where bright UC fluorescence can be obtained only by two-step excitation of two-wavelengths. The microscopic mechanisms of enhanced UC fluorescence tuned by two-step excitation of two-wavelengths has been shed light on the investigation for NIR laser power dependence of the fluorescence counts. As plotted in Fig. S8, using least-squares fitting41,42, only one photon is required for the blue/green UC fluorescence upon two-step excitation of two-wavelengths, which results in high efficient UC luminescence owing to the existing of an effective ESA process. Therefore, using different pumping methods through one NIR laser controlling another NIR laser, the blue/green UC fluorescence can be tailored for the exploitation of the “on-off” optical switching with fast-slow response.

Figure 3
UC fluorescence spectra of Tm3+ (a) and Er3+ (c) single-doped GCs at room temperature following single-wavelength excitation, and two-step excitation of two-wavelengths. Fluorescence images of Tm3+ (b) and Er3+ (d) single-doped GCs with and without one ...
Figure 4
Possibility of modulating the blue/green UC fluorescence emission from the Tm3+ (a,b) and Er3+ (c,d) single-doped GCs. Setting the excitation power of one NIR laser to a fixed value while increasing the power of another NIR laser, a ITSTW/(ISW1+ISW2) ...

Dynamic evolution for the fast-slow optical modulation of blue/green UC fluorescence

For a closer insight into the dynamic evolution processes of this fast-slow optical modulation, the kinetics of the UC fluorescence was thoroughly confirmed by time-resolved photoluminescence studies in Fig. 5. Under two-step excitation of 80 MHz 800 nm (as GSA wavelength) fs laser (that can be roughly recognized as a C. W. laser beam) combined simultaneously with 1064 nm (as ESA wavelength) gating laser with 150 Hz repetition frequency, the rise time is 2.45 ms to approach the steady-state for the blue UC fluorescence from the Tm3+ single-doped GCs (Fig. 5(a)). Instead, a little longer rise time of 2.82 ms is required to reach the steady-state for the blue UC fluorescence when tuning the GSA wavelength of 800 nm as gating laser beam at the same time changing the ESA wavelength of 1064 nm as C. W. laser beam (Fig. 5(b)). Quite surprisingly, in the case of optical modulation of green UC fluorescence from the Er3+ single-doped GCs, the rise time is just 1.46 ms to come to the steady-state upon two-step excitation of 1530 nm (as GSA wavelength) C. W. laser coupled simultaneously with 850 nm (as ESA wavelength) gating laser with 100 Hz repetition frequency (Fig. 5(c)). On the contrary, more than one order of magnitude rise time of 25.05 ms is needed for the steady-state under two-step excitation of 1530 nm (as GSA wavelength) gating laser combined simultaneously with 850 nm (as ESA wavelength) C. W. laser (Fig. 5(d)).

Figure 5
Time-dependent blue/green UC fluorescence intensity profiles of Tm3+ (a,b) and Er3+ (c,d) single-doped GCs under two-step excitation of one gating laser combined simultaneously with another C. W. laser with a fixed power. The rise time is defined as the ...

Discussion

The full dependence of the fluorescence rate (F) for the fast-slow optical modulation upon two-step excitation of two-wavelengths is predicted by the rate-equation model (see Supplementary Material for details):

An external file that holds a picture, illustration, etc.
Object name is srep45650-m1.jpg

For the fast optical modulation,

An external file that holds a picture, illustration, etc.
Object name is srep45650-m2.jpg

For the slow optical modulation,

An external file that holds a picture, illustration, etc.
Object name is srep45650-m3.jpg

Where η is the collection efficiency of the detector, ω31 is intrinsic decay rates, N is the total numbers of ions, IG is the GSA wavelength laser intensity, IE is ESA wavelength laser intensity, α and β are the proportionality constants. The main assumption of the model is the excitation rates proportional to the power of laser, which is plausibly mediated by the speed of electrons populated fully in the first excited state rooting from the effect of GSA wavelength laser. For the fast optical modulation, the GSA wavelength laser is modulated as C. W. beam combined simultaneously with a gating laser of ESA wavelength. IG is larger, IE is lower and γ is lower in comparison with that for the slow optical modulation, resulting in the fast fluorescence rate of the “on-off” switching. Conversely, when the GSA wavelength laser is manipulated as gating beam coupled simultaneously with a C. W. laser of ESA wavelength, the IG is lower, IE is larger and γ is larger comparing with that for the fast optical modulation, leading to the appearance of slow fluorescence rate of the “on-off” switching. From the experimental observations we find excellent agreement with our analysis from the analytical solution of the model based on rate equation involving three energy levels, strongly verifying the feasibility of this fast-slow optical modulation. In addition, the investigation for fluorescence lifetime (Fig. S9), and time response of the switch affected by laser pulse duration (Fig. S10), can also provide proofs from dynamic evolution aspect for the fast-slow optical modulation of blue/green UC fluorescence.

In summary, we introduced an approach to future all-optical information processing using two-step excitation of two-wavelengths operating at telecom windows that enables fast-slow optical modulation of blue/green UC fluorescence from Tm3+ (Er3+) single-doped transparent GCs. We showed an optical modulation of more than 1500% (800%) of the green (blue) UC fluorescence intensity and a fast response of 280 μs (367 μs) as well as a slow response of 5.82 ms (618 μs) in the green (blue) UC fluorescence signal of LaF3:Tm3+ (Er3+) nanocrystals embedded germanate oxyfluoride GCs through two-step excitation of two-wavelengths. The study on dynamic evolution mechanism was indicated that the differentiation of the speed of electrons populated fully in the excited state manipulated by various pumping strategy was responsible for this fast-slow optical modulation. This fast-slow optical modulation of blue/green UC fluorescence from Tm3+ (Er3+) single-doped GCs was successfully manipulated by two-step excitation of two-wavelengths at telecom windows, which may provide a strategy for constructing all-optical fiber data processing in future optical telecommunication realms.

Methods

Fabrication of Samples

The preparations of germanate oxyfluoride GCs precipitating LaF3:Tm3+ (Er3+) nanocrystals are analogous to our previous works31. The precursor glasses, with a composition of 50GeO2-22Al2O3-13LaF3-15LiF-1XF3 (X = Tm or Er), were prepared at 1450 °C for 1 h by melt quenching technique. The precursor glasses were cut into blocks and heat-treated at 680 °C for 4 h to achieve GCs through crystallization. The samples were optically polished for further measurements of optical performances.

Measurements and Characterization

X-ray diffraction (XRD) pattern of the samples were obtained on an X’Pert PRO X-ray diffractometer (PANalytical, Netherland) using Cu Kα (λ = 1.5418 Å) radiation, as shown in Fig. S1. A Lambda 900 spectrophotometer (PerkinElmer, USA) was employed to record the absorption spectra of the samples depicted in Fig. 2(d,e) and Fig. S2. The microstructures of the samples were analyzed by utilizing a high-resolution transmission electron microscope (HRTEM) 2100 F (JEOL, Japan), as illustrated in Figs S3 and S4. The optical loss of the GCs were measured by home-built optical setup with optical power meter PM320E (THORLABS, USA), as sketched in Fig. S5 and Table S1. The UC fluorescence spectra were recorded by a spectrometer HR4000 (Ocean Optics, USA).

Optical Setup

To investigate the fast-slow optical modulation of blue/green UC fluorescence, we used a coaxial optical path coupling two laser beams with dichroic mirror DMLP950 (THORLABS, USA) to illuminate samples at the confocal point, as sketched in Fig. 6. The size of the laser focal spot radius is determined by: ref. 34For Gauss laser beam:

Figure 6
Optical setup.
An external file that holds a picture, illustration, etc.
Object name is srep45650-m4.jpg

For monochromatic parallel laser beam:

An external file that holds a picture, illustration, etc.
Object name is srep45650-m5.jpg

Where ω is laser focal spot radius, λ is the wavelength of the laser, f is the effective focal length of the lens, and ω0 is the entrance beam radius. The fluorescence signal is collected through vertical direction of the sample and sent to the photomultiplier tube (PMT) with high voltage of −500 V. The laser signal is detected by a Si detector (SD) or an avalanche photodiode (APD). For Tm3+, 1,064 nm laser (LEO Photoelectric, China) beam is superimposed with 80 MHz 800 nm femtosecond laser (COHERENT, USA) beam, after one of which passing through an optical chopper (THORLABS, USA) that allows us to temporally modulate its frequency and pulse width. For Er3+, 1530 nm laser beam (LEO Photoelectric, China) is superimposed with 850 nm laser (LEO Photoelectric, China) beam, after one of which using a signal-generation (Tektronix, USA) that allows us to temporally modulate its frequency and pulse width. The optical modulation signal and optical switching “on-off” response was collected with a TDS 3012B digital oscilloscope (Tektronix, USA). All the measurements were performed at room temperature.

Additional Information

How to cite this article: Chen, Z. et al. Controllable optical modulation of blue/green up-conversion fluorescence from Tm3+ (Er3+) single-doped glass ceramics upon two-step excitation of two-wavelengths. Sci. Rep. 7, 45650; doi: 10.1038/srep45650 (2017).

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary Material

Supplementary Information:

Acknowledgments

This work is financially supported by the National Natural Science Foundation of China (Grants 51472091, 61475047, 11404114) and the Guangdong Natural Science Foundation (Grants S2011030001349, 2014A030306045, 1045106410104887). The authors acknowledge the open fund from the State Key Laboratory of High Field Laser Physics of the Shanghai Institute of Optics and Fine Mechanics of Chinese Academy of Science, State Key Laboratory of Precision Spectroscopy of East China Normal University, and Science and Technology Department of Zhejiang Province, China.

Footnotes

The authors declare no competing financial interests.

Author Contributions J.R.Q. and G.P.D. proposed and guided the overall project. Z.C., S.L.K., H.Z., T.W., S.C.L. and Q.Q.C. performed all the experiments and analyzed the results. All the authors discussed the results. Z.C. wrote the manuscript, with discussion from D.P.D. and J.R.Q.

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