This investigation elucidates for the first time electrically controllable random lasers below the threshold voltage in dye-doped liquid crystal (DDLC) cells with and without adding an azo-dye. Experimental results show that the lasing intensities and the energy thresholds of the random lasers can be decreased and increased, respectively, by increasing the applied voltage below the Fréedericksz transition threshold. The below-threshold-electric-controllability of the random lasers is attributable to the effective decrease of the spatial fluctuation of the orientational order and thus of the dielectric tensor of LCs by increasing the electric-field-aligned order of LCs below the threshold, thereby increasing the diffusion constant and decreasing the scattering strength of the fluorescence photons in their recurrent multiple scattering. This can result in the decrease in the lasing intensity of the random lasers and the increase in their energy thresholds. Furthermore, the addition of an azo-dye in DDLC cell can induce the range of the working voltage below the threshold for the control of the random laser to reduce.
© 2011 OSA
Random lasers have become considerably attractive in recent years, not only because of their unusual lasing mechanisms and properties but also because of their potential applications in micro-size optical sources and bio-medicine [1–20]. Compared with conventional lasers, the resonant cavity in random lasers is built on recurrent multiple scattering instead of two mirrors with high reflectivity. The fluorescence photons can be repeatedly multi-scattered in random directions as they propagate in an active medium where the scattering particles or domains are distributed disorderly. The recurrent multiple scattering may prolong the duration of the photons remaining in the active disordered medium. The duration can be sufficiently long such that the gain of the fluorescence via the enhancements of the rates of spontaneous and stimulated emissions may exceed the optical losses and random lasing may occur subsequently [4,5,20].
In recent years, random lasers have been developed using liquid crystal (LC)-associated disordered materials [5,10–19,21,22]. Such random lasers possess peculiar merits of flexible controllability or tunability in their lasing characteristics (e.g., energy threshold or lasing wavelength) by thermal [5,10,13,14], electric [11,16,17], or optic [21,22] means because the orientation of LCs with large anisotropies and its macroscopic physical properties, e.g., the refractive index and dielectric tensor, can be easily modified externally. The electric method is the simplest and most possible way to realize the application of controllable or tunable LC-based photonic devices, including random lasers. Among the electrically controllable LC-based random lasers developed [11,16,17], the applied voltages needed to control the lasing features should be very high to lower the potential significantly in real application. Therefore, this paper develops and investigates, for the first time, electrically controllable dye-doped LC (DDLC) random lasers operated at a very low voltage range below the Fréedericksz transition threshold. Experimental results demonstrate that the lasing intensities of the random lasers can decrease, and their energy thresholds can increase by increasing the applied voltage below the threshold. The mechanism for the below-threshold-electric controllability of the random laser is attributed to the effective decrease of the spatial fluctuation of the orientational order and thus of the dielectric property of LCs, with the increase in the electric-field-aligned LC order below the threshold. Accordingly, the diffusion constant is increased, and the scattering strength of the fluorescence photons in their recurrent multi-scattering is reduced, leading to the decrease in the lasing intensity of the random lasers and the increase in their energy thresholds. Moreover, the addition of an azo-dye in the DDLC cell can induce the drop in the range of the working voltage below the threshold for the control of the random laser.
2. Sample preparation and experimental setup
The materials used in this work are NLC E7 (ne = 1.7462 and no = 1.5216 at 20 °C for λ = 589 nm, from Merck), laser-dye P650 (1, 2, 3, 5, 6, 7-hexamethyl-8-cyanopyrromethene-difluoroborate, from Exciton), and azo-dye 4MAB (4-Methoxyazobenzene, from Fluka). Each empty cell is fabricated with two PVA-coated ITO glass slides separated by two 188 μm thick plastic spacers, with one of the two glass slides pre-rubbed but the other not. Two different uniform DDLC mixtures with different prescriptions of E7 (91.7wt%)–P650 (0.3wt%)–4MAB (8.0wt%) and E7 (99.7wt%)–P650 (0.3wt%) are both injected into the empty cells. The cells are then filled up the mixtures through the capillary effect to form homogeneously aligned DDLC cells with and without an azo-dye.
Figure 1 presents the experimental setup for examining the electrically controllable random lasing based on the DDLC cells. One pumped laser beam, derived from a Q-switched Nd:YAG second harmonic generation pulse laser (wavelength, 532 nm) with a pulse duration of 8 ns, repetition rate of 10 Hz, and pumped energy, E, is focused by a lens (focal length: 20 cm) on the cell with an incident angle of 20° relative to the cell normal (N). A fiber-optic probe of a fiber-based spectrometer (Jaz-Combo-2, Ocean Optics, resolution: ~0.9 nm) is placed to face N with a 6 cm distance from the cell to measure the random lasing signal. The combination with a half-wave plate (λ/2 WP, for 532 nm), a polarizer (P) and a nonpolarizing beam splitter (NBS) is placed in front of the lens for varying the incident pulse energy. The NBS is used to split the incident beam with half the energy into the detector of the energy meter for measuring the energy of the incident pulses. The transmission axis of the polarizer is
set parallel to R. An external AC voltage (Va, 1 kHz), as displayed in the inset of Fig. 1, is applied on the DDLC cells to examine their electric controllabilities for the obtained random lasing emissions. Notably, each random lasing signal is measured in 1 min of pumped time to prevent the generation of significant accumulative pumped-pulses-induced thermal and photoisomerization effects of the azo dye in the DDLC cell. The two effects probably significantly influence the LC structure, which in turn may affect the performance of the obtained random laser [13,22]. Further discussions of these issues are presented in Section 3.
3. Results and discussion
Before examining the experiment of the electric controllability of the obtained random lasing emission, the energy threshold for the production of the random laser needs to be identified first. Figure 2(a) shows the variation in the measured fluorescence spectra with a pumped energy E = 13.5–16 μJ/pulse in a DDLC with the addition of an azo-dye. Figure 2(b) summarizes the experimental data in Fig. 2(a), where the variations in peak intensity of the fluorescence output and the corresponding full widths at half-maximum (FWHM) with the pumped energy are given. Apparently, the peak intensity of the fluorescence output nonlinearly increases with increasing the pumped energy, and an energy threshold (Eth) ~14.8 μJ/pulse can be obtained, which is a symptom for the occurrence of a random lasing emission. The inset in Fig. 2(b) indicates the emission pattern of the random lasing at E = 16 μJ/pulse. In Fig. 2(a), the narrowest random lasing spike can be as narrow as Δλ ≤ 1 nm at E = 16 μJ/pulse, which exceeds Eth. The black dotted curve, shown in Fig. 2(a), represents the fluorescence emission curve for the DDLC cell between 600 and 660 nm. The discrete spikes of the random lasing are distributed around 640 nm, which is near the wavelength (~638 nm) for the maxima of the fluorescence emission. To identify the essential mechanism for the occurrence of the random lasing presented in Fig. 2, we also perform a coherent backscattering experiment by probing the DDLC cell by exploiting a weak 633 nm laser beam to determine the mean free path of the fluorescence photons in the scattering process in the LC bulk [23–26]. The measured coherent cone width of the backscattering light is ~7 mrad. According to Refs. 23–26, the coherent cone φ is dependent on the transport mean free path ℓ* (defined as the average distance a photon travels before its propagation direction is completely randomized) by the following approximate relation:
Substituting λ = 633 nm and φ = 7 mrad into Eq. (1), the transport mean free path is calculated as roughly ℓ*≅14.4 μm. By satisfying the condition kℓ* = 2πℓ*/λ ≅140 > 1, the random lasing observed in the present investigation results from the weak localization of the fluorescence photons through the recurrent multi-scattering from the spatial fluctuation of the orientational order and thus of the dielectric property of LCs in our DDLC films [1,4,5,12,13,20].
The following experimental data present the investigation of the electric controllability for the occurred random lasing in the presence of an external AC voltage applied to the DDLC cell with the addition of an azo-dye. Figures 3(a) and 3(b) show the variations in the fluorescence emission spectra at E = 16 μJ/pulse and the peak intensity of the fluorescence output versus incident pumped energy with the increase in the applied voltage, respectively. Clearly, the intensity of the random lasing signal can be controlled to decrease from a high peak value [~55260 a. u. in Fig. 3(a)] to zero, and the energy threshold can also be controlled to increase from 14.8 to ∞ μJ/pulse by increasing the applied voltage from 0 to 1.2 V. The range of the operating voltage capable of entirely controlling the energy threshold of the LC random laser is very low. To determine the reason for the low-voltage-controllability of the random laser, this paper also uses a simple setup [Fig. 4(a) ] to measure the variation in the cell transmission with the applied voltage. The cell is normally probed by a very weak collimated He-Ne laser beam under a configuration with a crossed polarizer and an analyzer placed in front of and behind the cell, where the included angle of R relative to the transmission axis of the polarizer is 45°. A photo-detector is placed behind the analyzer to record the transmission of the probe beam, and one AC voltage (Va, 1 kHz) with variable magnitudes is applied to the cell during this measurement. Experimental results presented in Fig. 4(b) indicate that one regular oscillation with an increasing period in the cell transmission generates at Va ≥ 2.1V, which is indicative of a threshold voltage (Vth) of a Fréedericksz transition for a typical homogeneously-aligned LC cell . The small fluctuation in the cell transmission at Va < 1.2V (notably, Eth→∞ for the random laser at Va≥1.2V) results from the multiple scattering effect of the LCs, which dominates the generation of the random lasing shown in Figs. 2 and 3. This multiple scattering effect will be further discussed in next paragraph. Note that the threshold voltage (2.1V) exceeds the voltage range (0–1.2 V) for the controllability of this random laser. This result is different from that of most cases of traditional Fréedericksz LC devices, in that the voltage range of controllability must exceed the corresponding threshold voltage, where a collective and consistent reorientation of LCs must occur . Due to this speciality of the below-threshold controllability, the present random laser can be controlled at a very low voltage range (0–1.2 V).
Based on the experimental results in Figs. 3(b) and 4(b), we can identify the existence of two regimes: the multi-scattering regime at Va < 1.2V and the no multi-scattering regime at Va ≥ 1.2V. The physical model sketched in Fig. 5(a) further explains the mechanisms of the multi-scattering of LCs for inducing the random lasing and of its electric controllability below the Fréedericksz threshold. Due to the weak anchoring force from the inner surfaces of the substrates of the thick cell, the LCs in the cell bulk fluctuate in position to form many LC micro-domains with different orientations relative to R at Va = 0 V, resulting in a spatial non- uniformity of the orientational order [δS = S(r + δr)–S(r)≠0] and thus of the dielectric tensor [δε = ε(r + δr)–ε(r)≠0] for LCs, where δr, δS, and δε represent the differential displacement, differential orientational order, and differential dielectric tensor of LCs between adjacent micro-domains of LCs in the cell bulk, respectively. Figure 5(b) presents a transmission image of the DDLC recorded under a polarizing optical microscope with crossed polarisers (P⊥A) at Va = 0 V. The rubbing direction R is set parallel to the transmission axis of the polariser. The length of the whit bar in the image is 50 μm. The contrast of the obtained image has been appropriately adjusted using a computer. In this image, the presence of numerous micro-sized non-uniform regions with different brightness levels confirms the existence of the LC micro-domains with different orientations. The diffusion constant (D) for photons in a random medium can be expressed by the following relation :
where ℓ* denotes the transport mean free path, and υ is the average velocity of photons transporting in a disordered LC medium. While the applied voltage increases, all LC micro- domains with different orientations gradually reorient roughly to the direction of R by increasing the aligned order of LCs by the applied electric field (E a) below the Fréedericksz threshold, such that the δS and thus δε for LCs decrease, causing the increase in the diffusion constant (or ℓ*). This can induce the decay of the scattering strength and the decrease in the random lasing intensity and the increase in the energy threshold. Since the spatial fluctuation of the orientation for the local LC micro-domain has already been cancelled at Va ≥ 1.2 V, the random lasing cannot be generated; thus, Eth→∞. Notably, while the applied voltage decreases from 1.2 to 0 V, the electric-field-aligned order of LCs can decrease, such that the LC micro-domains gradually fluctuate and the multiple scattering gradually increases. Thus, the random lasing can rise, and the energy threshold can decrease.
This study also performs additional experiments using another DDLC cell without adding an azo-dye to obtain similar results as those shown in Figs. 3(a), 3(b), and 4(b) based on the DDLC cell with an azo-dye. Experimental results are presented in Figs. 6(a) , 6(b), and 7 . Figure 6(b) shows that the energy threshold increases from 13 to ∞ μJ/pulse by increasing the applied voltage from 0 to 2.0 V, where 2.0 V is also below Vth = 2.1 V of the azo-dye-free DDLC cell (as exhibited in Fig. 7). Simply, the DDLC random laser with or without the azo-dye presents a similar electric-controllable feature of random lasing below the same threshold voltage (2.1V). Tables 1(a) and 1(b) present the comparative results in terms of measured transport mean free path (ℓ*), energy threshold (Eth), and peak wavelength (λpeak) and FWHM of E = 16 μJ/pulse for the cases of DDLC cell, with and without the addition of the azo dye, at different applied voltages (Va). As described in the model [Fig. 5(a)], both the measured transport mean free paths increase; thus, the measured energy thresholds and FWHMs increase accordingly with increasing applied voltage in the two cells. The peak wavelength of the measured fluorescence output for the two cases at different voltages is sustained at roughly 640 nm. Specifically, the operating voltage range for the entire control of the random laser drops from 0–2.0 to 0–1.2 V when the azo dye is added to the DDLC random laser. This is because of the decrease of the fluorescence emission intensity of the excited laser dye due to the competitive absorption of the azo dye, which results in the rise of overall Eth at different voltages, as well as the drop of the applied voltage for obtaining an infinite Eth. Moreover, both the pumped-pulses-induced thermal and photoisomerization effects of the azo dye have negligible influence on the LC structure and on those parameters mentioned above. Further evidence of these negligible phenomena is presented later in the document. The threshold voltage for a homogeneously-aligned pure LC cell can be expressed as the following formula:Eq. (3), we obtain the calculated Vth to be roughly 0.943V. The value is considerably smaller than the experimental one (2.1V) based on our DDLC cells. This discrepancy in the Vth is attributed to the spatial fluctuation of the LC orientation in our DDLC cells, resulting in the reduction of the effective Δε for the LCs and thus the increase in the threshold voltage.
This work also measures the temperature at the pumped spots of the DDLC cells, with and without the addition of azo dye, using a thermal imager (Fluke, Ti10) after the cell is excited by the pumped pulses with 16 μJ/pulse for over 5 min. Experimental results show that the measured temperature at the pumped spots is sustained at approximately room temperature (25 ± 0.5 °C). Moreover, to determine whether a significant photoisomerization effect of the azo dye can be induced by the pumped pulses , this work also measures the absorption spectra in the region of 400–700 nm for the 8 wt% 4MAB-added E7 cell before and after the cell is excited by the pumped pulses with 16 μJ/pulse for 1 min (presented as red and black curves, respectively, in Fig. 8 ). The absorption spectrum of the azo dye does not show any obvious change after the excitation of the pumped pulses. The same result can be obtained if the above step is performed repeatedly. The above results indicate that no significant pumped-pulses-induced thermal and trans–cis isomerization effects can be induced to disturb the LCs, and, thus, to influence the performance of the random laser. Such a result is reasonable; according to the measured absorption spectra for the 8 wt% 4MAB-added E7 cell (red or black curve) and the 0.3 wt% P650-doped E7 cell (blue curve) (Fig. 8), the absorbance of the azo dye at the pumped wavelength (532 nm) is lower than that of the laser dye. Roughly 70% of the photons of the incident-pumped-pulses were absorbed by the laser dyes, thereby strongly inducing fluorescence emission (especially in the orange-to-red region). Thus, the pumped-pulses-induced thermal and photoisomerization effects of the azo dye can be ignored in the present work of electrically controlled random laser. Good stability and repeatability of the random laser can be obtained if the DDLC cells are pumped continuously by 600 pulses (16 μJ/pulse) for 1 min.
In summary, the electrically controllable random lasers below Fréedericksz threshold based on thick DDLC cells with and without adding an azo-dye are investigated and reported for the first time. Experimental results show that the externally applied voltage on the cells can control the lasing intensities of the generated random lasers and their energy thresholds below the threshold voltage. The reason for the below-threshold electric controllability of the random lasers is attributable to the effective decrease of the spatial fluctuation of the orientation of the LC micro-domains and of the dielectric property of the LCs with the increase in the electric-field-aligned order of LCs below the threshold, leading to the decay of the scattering strength, the decrease in the lasing intensity of the random lasers, and the increase in their energy thresholds. Moreover, such easily fabricated and low-cost random lasers can potentially be applied to integrated photonics (e.g., controllable micro-laser sources) without the need for polarizing films and with a low working voltage range below the threshold.
The authors would like to acknowledge the financial support provided by the National Science Council of Taiwan (contract NSC 97-2112-M-006-013-MY3) and the Advanced Optoelectronic Technology Center in National Cheng Kung University under the projects of the Ministry of Education. The authors are also grateful to KGSupport for their editorial assistance.
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