All-optically controllable nanoparticle random laser in a well-aligned laser-dye-doped liquid crystal

This study reports for the first time an all-optically controllable nanoparticle random laser (NPRL) in a well-aligned laser-dye-doped liquid crystal (LDDLC) cell added with NPs and azo-dyes. Experimental results display that the NPRL can be obtained when the pumped energy exceeds the energy threshold (~3.5 μJ/pulse). The occurrence of the NPRL is attributable to the enhancement of the fluorescence by the multi-scattering events of the fluorescence photons from the randomly distributed NPs in the diffusion rout of the wellaligned LDDLC cell. In addition, the lasing intensity of the NPRL can decrease with increasing irradiation time of one UV beam. Continuing irradiation of one green beam following the UV illumination can increasingly recover the lasing intensity of the NPRL. The all-optically reversible controllability of the NPRL is basically attributed to the successive UV-beam-induced increase and green-beam-induced decrease in the randomness of the LDDLC via their interactions with the curved cis and rod-like trans isomers after the accumulation of the trans→cis and cis→trans back isomerizations of the azo-dyes, respectively. The former and latter mechanisms can decrease and increase the laser-dye’s absorption and thus the induced spontaneous emission, respectively. These consequences can decrease and increase the lasing intensity, or equivalently, increase and decrease the energy threshold for the occurrence of the NPRL, respectively. © 2016 Optical Society of America OCIS codes: (230.3720) Liquid-crystal devices; (290.4210) Multiple scattering; (140.3600) Lasers, tunable. References and links 1. N. M. Lawandy, R. M. Balachandran, A. S. L. Gomes, and E. Sauvain, “Laser action in strongly scattering media,” Nature 368(6470), 436–438 (1994). 2. H. 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Introduction
Random lasers have received much attention in recent years, not only because of their unique mechanisms, but also their useful application in photonics and bio-medicine [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15].Many randomly dispersive media, such as TiO 2 and ZnO powders [2,16], human tissues [5], polymers [6], dye-doped liquid crystals (DDLCs) [8][9][10]15,17], and dye-doped polymerdispersed LCs [11,12,18], can be used to generate random lasing emission.Different from the traditional lasers which need a cavity with two high-reflective mirrors to resonate, random lasers are cavity-free and can be obtained with either extended or localized modes by a multiscattering mechanism in a disorder active medium.A sufficiently long dwelling time for the fluorescence photons in the multi-scattering process in the active medium can induce a gain sufficiently high to overcome the optical loss, leading to the generation of the random lasing [1,13].In the materials mentioned above, only those LC-based random lasers possesses controllability in their lasing features such as lasing intensity (or energy threshold) and polarization by electrically, thermally, or optically change the orientation or structure of the LCs [7][8][9][10][11][12]14,15,17,18].Among these controlling ways, the optical control has some advantages over the other ways such as remote, precise and reversible controllability in a noncontact way, and thus is especially suitable for use in photonic applications [15,18,19], including controllable random lasers [15,18].
This study reports for the first time an all-optically controllable nanoparticle random laser (NPRL) in a well-aligned laser-dye-doped LC (LDDLC) cell added with azo-dyes.Experimental results indicate that the lasing intensity of the NPRL, and thus, the associated energy threshold, can be all-optically controlled by successive exposures of the UV and green beams.The lasing intensity of the NPRL can be reduced with increasing irradiation time of one UV beam.Continuing irradiation of one green beam following the UV illumination can increasingly recover the lasing intensity of the NPRL.This all-optically reversible controllability of the NPRL is ascribed to the successive UV-irradiation-induced decrease and green-beam-irradiation-induced increase in the order of the LDDLC via their interactions with the bent and rod-like azo-dyes after their accumulative effects of trans→cis and cis→trans back isomerizations, respectively.The former and latter mechanisms can decrease and increase the laser-dye's absorption and thus the induced spontaneous emission, respectively.These outcomes can decrease and increase the lasing intensity, or equivalently, increase and decrease the lasing threshold for the occurrence of the NPRL.

Sample preparation and experimental setup
Fig. 1.Top view of the experimental setup for examining the optically controllable NPRL based on an azo-dye-added LDDLC cell doping with NPs.One pumped pulse beam is propagated along the + x direction and then focused by a cylindrical lens on a striped region near one of the edges of the cell.One lateral random lasing emission can be stimulated along the + z direction normally out of that edge of the cell.The polarization of the incident pulses is pre-set parallel to the LC-aligned y-direction of the LDDLC cell such that the absorption of the laser-dye is maximum.One non-polarized UV beam and one cw circularly-polarized green beam are installed to pre-irradiate the pumped stripe region of the cell for investigating the alloptical controllability of the NPRL.
This work develops one homogeneously-aligned LDDLC cell added with NPs and azo-dyes.The materials used in the above-mentioned cell include 87.1-wt% nematics E7 (birefringence, n o = 1.5216 and n e = 1.7462 at 20 °C and 589 nm; dielectric constants ε // = 19.0 and ε ⊥ = 5.2 at 20 °C and 1 kHz), 0.6-wt% laser-dye DCM (4-dicyanmethylene-2-methyl-6-(pdimethylaminostyryl)-4-H-pyran, from Exciton), 12-wt% azo-dye 4MAB (4-Methoxyazobenzene, from Fluka), and 0.3-wt% BaT i O 3 NPs (barium titanate nanoparticles, particle size ≅ 100 nm, refractive index ≅ 2.2, from Inframat Advanced Materials).BaTiO 3 NP is not ferroelectric because its structure is cubic.Two cleaning glass slides with precoated PVA film are pre-rubbed in antiparallel directions and then combined with each other to form an empty cell in which two 23-μm-thick plastic spacers are placed in the cell.The cell is injected with the above-mentioned mixture via the capillary effect to form a homogeneously-aligned azo-dye-added LDDLC cell doping with NPs.Given the guest-host effect, the laser-dye and the azo-dye can be confirmed to be roughly orient themselves along with the long axes of the LCs by the polarizing optical microscope.
Figure 1 shows the experimental setup for examining the all-optically controllable NPRL based on the homogeneously-aligned azo-dye-added LDDLC cell doping with NPs (represented simply by "LDDLC cell" in the following content).One pumped laser beam, derived from a Q-switched Nd:YAG second harmonic generation pulse laser (λ = 532 nm) with a pulse duration of 8 ns, repetition rate of 10 Hz, and pumped energy E p , propagates along the + x direction and is focused by a cylindrical lens (focal length ~15 cm) on a striped region near the edge of the cell.This pumped stripe is roughly 3 mm long and 0.3 mm wide.A fiber-optic probe of a fiber-based spectrometer (Jaz-Combo-2, Ocean Optics, optical resolution: ~0.9 nm) is placed such that it faces the edge of the cell to record the lateral fluorescence emission output emitted by the NPRL along the + z direction.A half-wave plate (for 532 nm), a polarizer, and a non-polarizing beam splitter (NBS) are placed in order in front of the cylindrical lens to vary the incident pulse energy.The NBS can split the incident pulse beam with half the energy into the detector of the energy meter for measuring the energy of the incident pulses.The polarization of the incident pulses is set parallel to the LCaligned direction of the cell (in the y direction) such that the laser-dye in the cell has maximum absorption of the incident pulses.One non-polarized UV beam (λ < 400 nm) with a fixed intensity of I UV = 300 mW/cm 2 and a variable exposure time t UV and one circularlypolarized green beam [from a continuous-wave (cw) diode-pumped solid-state (DPSS) laser, λ = 532 nm] with a fixed intensity of I G = 8.28 mW/cm 2 and a variable exposure time t G are set up on the xz plane to pre-illuminate the pumped stripe of the cell in the examining experiment of the all-optically controllable NPRL.Apparently, the peak intensity of the emission output nonlinearly increases with increasing E p , and an energy threshold of 3.5 μJ/pulse can be obtained in Fig. 2(b).At low pumped energy, the emission spectra are weak and broad, as shown at bottom right of Fig. 2(a).When the pumped energy is higher than the threshold, the fluorescence intensity significantly increases and FWHM becomes narrower.At this moment, a non-resonant random laser is generated [7,16].As the pumped energy increases further, multiple narrow spikes appear simultaneously and randomly at the emission spectrum curves, as shown in Fig. 2(a).Multiple spikes represent the generation of resonant random laser.These spikes shown at E p = 12 μJ/pulse can be as narrow as ~1 nm.All these features indicate the occurrence of an NPRL in the well-aligned LDDLC.In last year, Lee et al. has proved that no random lasing signal can be obtained if no NPs are added in the LDDLC cell (with no azo-dye).Furthermore, the formation of the abovementioned NPRL was resulted from the diffusion route of the fluorescence photons via the multi-scattering from the NPs added in the LDDLC cell.This result was obtained by measuring the transport mean free path of the fluorescence photons in the multi-scattering process from the NPs based on a coherent backscattering experiment, where the cell is probed by a weak 633 nm He-Ne laser beam [17].Because the LDDLC cell employed in the present work is similar to that used by Lee et al. (the only difference between the two cells is whether or not the azo-dyes are added), it is no doubt that the random lasing in the present work is attributable to the same mechanism mentioned above.

Emission features of the NPRL in the LDDLC cell
Figures 3(a) and 3(b) show the experimental results associated with the all-optical reversible controllability of the NPRL based on the LDDLC cell under successive illuminations of the UV beam with a fixed irradiation intensity of I UV = 300 mW/cm 2 and green beam with a fixed intensity of I G = 8.28 mW/cm 2 , respectively.The NPRL appears initially [black curve in Fig. 3(a)] when the cell is excited by the pumped pulses (E p = 12 μJ/pulse).When the irradiation time of UV light increases to t UV = 5, 10, and 15 mins at I UV = 300 mW/cm 2 (I G = 0), the lasing emission output gradually decreases to almost vanish (red, orange, and blue curves shown in Fig. 3(a), respectively).Afterwards, the UV beam is turned off and the green beam is turned on to irradiate the cell at t G = 0, following the UV irradiation for 15 mins.When the irradiation time of green beam increases to t G = 5, 10, 15, 20, and 25 s at I G = 8.28 mW/cm 2 (I UV = 0), the lasing emission output gradually increases to almost the original value [presented by the blue, green, orange, red, black, and pink curves in order in Fig. 3(b)].This finding indicates that the nanoparticle random laser possesses an all-optically reversible controllability, i.e., the random lasing can gradually decay and rise back all-optically.Equivalently, the energy threshold of the NPRL can be controlled reversely in an all-optical way under the successive irradiations of the UV and green beams.Because no similar phenomenon of all-optically controllable random lasing described above can be observed based on a 4MAB-free LDDLC cell doping with the NPs (associated results are not presented herein), the azo-dye must play a key role in the all-optical controllability of the NPRL shown in Fig. 3.The azo-dyes are well known in their alloptically controllable molecular conformation; that is, their structure can be exchanged between the rod-like trans state and curved cis state if illuminated with a short-and a longwavelength lights (e.g., UV and green lights) [19,20].The absorption spectra of the 4MAB at the two states are quite different.Figure 4 shows the variation of the measured absorption spectrum of 12 wt% 4MAB dissolved in E7 (homogeneous alignment) under successive irradiations of UV and green beams.The blue, red, and black (dotted) curves represents the absorption spectra measured before and after the UV irradiation of 300 mW/cm 2 for 15 mins and the green-beam-irradiation of 8.28 mW/cm 2 for 25 s following the UV irradiation, respectively.The blue curve shows that the 4MAB in E7 originally has two absorption peaks at UV (around 380 nm) and visible (around 448 nm) regions before UV irradiation.The two peaks are associated with π-π* and n-π* transitions of the 4MAB molecule, respectively.By the UV irradiation, the isomeric concentration ratio of trans/cis decreases through the accumulation of the UV-irradiation-induced trans→cis isomerization of 4MAB, which leads to the effective drop and rise of the two absorption peaks at 380 and 448 nm, respectively, as displayed by the red curve in Fig. 4.After turning off the UV irradiation, the absorption spectrum apparently recovers back to the original state in 25 s under the green-beam irradiation (as presented by the dotted-black curve in Fig. 4).This is because of the increase of the concentration ratio of trans/cis through the accumulation of the green-beam-irradiationinduced cis→trans back isomerization of 4MAB.The above-mentioned all-optical controlling phenomenon in the NPRL is just matched with the all-optical controllability for the molecular configuration of the azo-dyes.The following, with the models shown in Fig. 5, describes the detailed process for the all-optical controllability of the NPRL signal based on the azo-dye-added LDDLC cell doping with NPs.As displayed in Fig. 5(a), the 4MAB dyes are generally in a stable trans state in darkness.The rod-like trans-4MAB dyes are well-aligned with LC molecules in y-direction via the guesthost effect in the LDDLC cell before light illumination.The trans dyes may absorb the nonpolarized UV light and convert to curved cis state via trans-cis isomerization.The curved cis 4MAB dyes may then interact with and disturb both the LC host and laser-dyes.When the t UV increases, the concentration of the cis 4MAB dyes increases through the accumulation of trans-cis isomerization such that the randomness of the LDDLC increases isothermally.Once the UV light is turned off and then the green beam is turned on, the concentration of the trans dyes increases back via the accumulative events of the cis-trans back isomerization such that the order of the LDDLC recovers isothermally.As shown in the model in Fig. 5(b), a strong absorption of laser-dye in the well-aligned LDDLC cell can be obtained under the excitation of y-polarized pumped pulses before UV irradiation.This case can induce a strong NPRL through the multi-scattering events of a strong spontaneously-emitted fluorescence emission from the randomly distributed NPs in the diffusion rout of the well-aligned LDDLC cell [corresponding to the black curve at t UV = 0 in Fig. 3(a)].The randomness of the LDDLC increases with increasing t UV , resulting in the isothermal decrease of the laser-dye's absorption and thus of associated spontaneous emission.This result of the decay for the spontaneous emission of the LDDLC cell after the UV-irradiation can be confirmed by examining the variations of the spontaneous emission spectra before and after the UVirradiation for 300 mW/cm 2 for 15 mins measured under the excitation of the cw DPSS laserbeam (532 nm) with 4.96 mW/cm 2 for 1 s.As shown in Fig. 6, the spontaneously-emitted intensity of the cell after the UV-irradiation is significantly weaker than that before the UVirradiation.Certainly, the result decreases the lasing intensity, or equivalently, increases the energy threshold isothermally for the occurrence of the NPRL after the UV-irradiation [corresponding to the red, orange, and blue curves in order at t UV = 5, 10, and 15 min in Fig. 3(a)].When t UV is long enough (say, 15 mins), the random lasing is very weak or even vanish even if the weak spontaneously-emitted fluorescence emission can undergo multiple scattering from the randomly distributed NPs in the diffusion rout of the poorly-ordered LDDLC cell [corresponding to the blue curve at t UV = 15 min in Fig. 3(a)].After the UV beam is turned off and the cell is irradiated instead by the green beam, the isothermal recovery in the order of the LDDLC can lead to the increase of the laser-dye's absorption and thus of the corresponding spontaneous emission, resulting in the isothermal recovery of the lasing intensity and thus of the energy threshold for the NPRL [corresponding to the blue, green, orange, red, black, and pink curves in order at t G = 0, 5, 10, 15, 20, and 25 min in Fig. 3(b)].

Exclusion from the thermal effect-induced controllability of the NPRL of the LDDLC cell
A separate experiment can be quickly examined to demonstrate that the controllability of the above-mentioned NPRL shown in Fig. 3 is not attributed to the thermal effect of the dye's absorption of UV light that can induce the LCs to fluctuate.At first, a strong random lasing signal is stimulated (as displayed by the black curve of Fig. 7) at E p = 12 μJ/pulse with no pre-illumination of the UV and green beams.This random lasing signal becomes very weak [as displayed by the red curve in Fig. 7 or the blue curve in Fig. 3(b)] after the UV beam (300 mW/cm 2 ) irradiates the cell for 15 mins and is then turned off.Thereafter, the random lasing signal pretty slowly increases from a very low to a very high strength approaching the original state (as displayed by the red to the gray curve in order in Fig. 7) with increasing time for the cell in dark state at t dark = 0 min to 80 min.The following explains the experimental results shown in Fig. 5. Massive 4MAB dyes can convert to the cis state in the cell after the UV illumination for 15 mins.These cis-4MAB isomers may slowly change to trans form via the thermal cis→trans back isomerization if with no post-illumination of green beam.The LDDLCs can slowly change from a fluctuating to an ordered state.This phenomenon can induce the slow increases of the laser-dye's absorption and associated spontaneous emission, resulting in the slow rise of the lasing output of the NPRL.The time needed for the lasing output of the NPRL to recover completely though the thermal cis→trans back isomerization effect exceeding 80 mins is much larger than that required by the relaxation of the photoinduced thermal effect (~a few minutes).Therefore, the all-optical controllability of the NPRL presented in Fig. 3 is not due to the thermal effect but the photoisomerization effect.

Conclusion
An all-optically controllable random laser based on a well-aligned azo-dye-added LDDLC cell doping with NPs is reported for the first time.The outcome in the experiment exhibits that the lasing intensity of the NPRL and thus the associated energy threshold can be alloptically controlled by successive exposures of the UV and green beams.The all-optically reversible controllability of the NPRL is ascribed to the increase and decrease of the isothermally fluctuating effect of the LDDLC via the accumulation of the trans→cis and cis→trans back isomerizations of the azo-dye with the UV and green-beam irradiations, respectively.The former and latter effects may decrease and increase the laser-dye's absorption and thus associated spontaneous emission, resulting in the decay and rise of the NPRL output originated from the fluorescence amplification via the multi-scattering of the NPs in the LDDLC cell.This facile and inexpensive random laser gives flexibility in the use of tunable laser sources and has a high potential in the application of integrated photonics.

Figure 2 (
Figure2(a) presents the variation of the measured intensity spectra of the lateral fluorescence emission output from the NPRL of the LDDLC cell with pumped energy E p = 0.5-12 μJ/pulse.A summary of the experimental result in Fig.2(a) is displayed in Fig.2(b), where the variations of the peak intensity of the fluorescence emission output and the corresponding full widths at half-maximum (FWHM) with incident pumped energy are presented.Apparently, the peak intensity of the emission output nonlinearly increases with increasing E p , and an energy threshold of 3.5 μJ/pulse can be obtained in Fig.2(b).At low pumped energy, the emission spectra are weak and broad, as shown at bottom right of Fig.2(a).When the pumped energy is higher than the threshold, the fluorescence intensity significantly increases and FWHM becomes narrower.At this moment, a non-resonant random laser is generated[7,16].As the pumped energy increases further, multiple narrow spikes appear simultaneously and randomly at the emission spectrum curves, as shown in Fig.2(a).Multiple spikes represent the generation of resonant random laser.These spikes shown at E p = 12 μJ/pulse can be as narrow as ~1 nm.All these features indicate the occurrence of an NPRL in the well-aligned LDDLC.

Fig. 2 .
Fig. 2. Variations of (a) the measured intensity spectra and (b) the peak intensity of the lateral fluorescence emission output and corresponding FWHM with incident pumped energy for the NPRL of the well-aligned azo-dye-added LDDLC cell doping with NPs.The inset in (a) shows the obtained lateral random lasing pattern on the screen at E p = 12 μJ/pulse.The energy threshold is around 3.5 μJ/pulse.

Fig. 3 .
Fig. 3. All-optically-reversible controllability of the NPRL based on the LDDLC cell under the successive irradiations of UV and green beams.Variations of the measured intensity spectra of fluorescence emission output (a) with increasing t UV from 0 min to 15 min (I UV = 300 mW/cm 2 , I G = 0) and then (b) with increasing t G from 0 s to 25 s (I G = 8.28 mW/cm 2 , I UV = 0) at E p = 12 μJ/pulse for the NPRL in the LDDLC cell.

Fig. 4 .
Fig. 4. Evolutions of measured absorption spectrum of 12 wt% 4MAB dissolved in E7 (a) before and after the UV irradiation with 300 mW/cm 2 for 15 mins and (b) after the greenbeam-irradiation with 8.28 mW/cm 2 for 25 s following the UV irradiation.

Fig. 5 .
Fig. 5. (a) Schema for the all-optically isothermal change in the order of the LC orientation.The order of the LC orientation decreases and increases when the added azo-dyes are excited successively by UV and green beams, respectively, via accumulative effects of trans-cis and cis-trans back isomerizations, respectively.(b) A model describing the all-optical controllability of the NPRL based on the azo-dye-added LDDLC doping with NPs.Under the UV-beam (green-beam) irradiation, the increase of the randomness (order) of the LDDLC decreases (increases) the laser-dye's absorption and thus the associated spontaneous emission under the excitation of y-polarized pumped pulses.The consequence induces the decrease (increase) of the lasing intensity of the NPRL which is generated by the multiple scattering events of spontaneously-emitted fluorescence photons from the randomly-distributed NPs in the diffusion routes of the LDDLC cell.

Fig. 6 .
Fig.6.Spontaneous emission spectra for azo-dye-added LDDLC cell doping with NPs before and after the UV-irradiation with 300 mW/cm 2 for 15 mins (presented by black and green curves, respectively) measured under the excitation of the cw DPSS laser-beam (532 nm) with 4.96 mW/cm 2 for 1 s.

Fig. 7 .
Fig. 7. Recovery of the NPRL signal of the LDDLC cell in dark after the UV pre-irradiation.Variations of the measured intensity spectra of the fluorescence emission output of the LDDLC cell at E p = 12 μJ/pulse with increasing the time as the cell is in dark from t dark = 0 min to 15 min after the cell is pre-illuminated by the UV light with 300mW/cm 2 for 15 mins.