1. Introduction

Photonic crystals (PCs) have been of interest to scientists because their potential optical properties offer ways of controlling light [1–4] by disallowing the propagation of photons of certain energies. Using theoretical predictions as a basis, promising applications of PCs have been proposed in optical devices that use 3-D [5–7], 2-D [8, 9], and 1-D [10–13] photonic bandgap (PBG) materials. 1-D PC materials have many applications. They are easy to make, and hence are potentially inexpensive. Among them, cholesteric liquid crystals (CLCs) are one of the most interesting 1-D PCs and are characterized by their spontaneous self-assembly into periodic helical structures. Thus, CLCs show the unusual optical property of selectively reflecting circularly polarized (CP) light; only light that has a handedness that is different from that of the CLC helix can propagate in the PBG frequency range [11–13]. These optical characteristics make the CLCs attractive for many applications, such as in mirrorless lasing, reflective color displays, circular polarizers, and color filters [11–16]. Mirrorless lasing is of particular interest because the group velocity of the photon approaches zero at the edges of the PBG. Thus, a low-threshold laser may be possible. A number of investigations into lasing using CLCs have been reported [11–13, 17–22]. Recently, multilayered structures consisting of CLCs were constructed for the efficient generation of laser light [23, 24]. 1-D hybrid PCs composed of a CLC defect layer between the two 1-D PCs (“1-D PC/CLC defect layer/1-D PC”) have also been proposed in order to obtain low threshold single mode lasing [17, 18]. In this structure, forward and backward bidirectional lasing was achieved with linear polarization at a particular defect mode. The electrically controllable omnidirectional propagation of laser emission from a photonic composite film containing helical-polymer networks was also recently reported [25].

In the study reported herein, we have proposed a new structure of placing a simple hybrid photonic helical CLC layer on a 1-D PC layer (“CLC/1-D PC”). Using this structure, we have demonstrated highly circularly polarized unidirectional laser emission from a device with a “CLC/1-D PC” structure. It may be noted that the control of the polarization of the emitting light could be of use in a number of optical applications, such as optical data storage, optical communication, and stereoscopic 3-D imaging systems [26, 27]. Thus, the fabrication of a simple polarized unidirectional lasing device using a reliable CLC system is attractive.

The device configurations for lasing from hybrid photonic helical CLC on 1-D PC devices are depicted in Fig. 1. In Fig. 1(a), a right-handed CLC (R-CLC) layer was set apart from a 1-D PC reflecting mirror film with a spacer (thickness d) between the CLC layer and the 1-D PC film. Here the thickness d of the spacer is defined to be larger than the coherent length. This device configuration is quite similar to the configuration of the CLC laser with a mirror reported in Ref. [19]. In this structure (“CLC/spacer/1-D PC”) shown in Fig. 1(a), bidirectional R-CP lasing emission (forward and backward) can be generated from the R-CLC layer by external optical pumping. The generated backward R-CP lasing light is reflected from the 1-D PC and the reflected light changes its polarization to L-CP light due to the π phase change upon reflection, so that it is then transmitted through the R-CLC layer. Thus in the forward propagation sense, laser light is characterized by both kinds of rotation (R-CP and L-CP) at the same time, i.e. by the generation of unpolarized lasing light based on incoherent superposition [19]. When the CLC reflector is used instead of the 1-D PC [19, 20], one may obtain the single directional (either forward or backward) propagation of a circularly polarized laser light by recycling the laser emission, but the CLC reflector cannot control the characteristic lasing mode in any way and the handedness of helical axis and reflection band of the CLC reflector used must be carefully selected. On the other hand, when the spacing d between the 1-D PC and the CLC layer is reduced to zero, i.e. when the CLC layer was in direct contact with the 1-D PC, the combined layers of the 1-D PC and the CLC layer can generate a new “lasing mode” and produce coherent polarization interference effects. Thus, by appropriate adjustment of the refractive indices and thicknesses of the hybrid structure of CLC/1-D PC, one may construct a circularly polarized unidirectional laser device.

 

Fig. 1. Schematic illustrations of hybrid photonic helical devices with spacing d (“CLC/spacer/1-D PC”) (a) and d=0 (“CLC/1-D PC”) (b). The blue and red arrows correspond to forward- and backward-propagating directions, respectively.

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2. Experimental methods

For fabrication of the CLC layer, commercially available CLC materials were used. The optical pitch of the used CLC at room temperature is about 660 nm. The pitch was controlled by adjusting the concentration ratio of chiral compound to a host LC material. The CLC mixture was introduced into an empty test cell by capillary action. The empty test cell was fabricated by using a pair of indium-tin-oxide (ITO) coated substrates, which were subjected to an antiparallel rubbing treatment. A vacant cell was sealed with 9 µm thick spacers. After the CLC mixture was introduced, the sample was sheared slightly in order to achieve a homogeneous planar structure.

For the active medium layer for lasing, we prepared a host CLC that was doped with guest fluorescent dye. The dye used was highly fluorescent 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7- tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB, a commercial fluorescent dye). The fluorescence emission peak was about 635 nm. The full width at half maximum (FWHM) of the emission was about 80 nm. In order to mix the fluorescent dye and CLC homogeneously, the materials were dissolved in chloroform by stirring for one hour. After the solvent had evaporated completely, the sample laser cell was fabricated by sandwiching the homogeneous mixture layer of dye-containing CLC (2 wt % of the dye) as an active medium layer between a transparent glass substrate and a 1-D PC reflector film, which consisted of five multilayers of TiO₂ (62 nm) and SiO₂ (84.5 nm) bilayers coated on a glass substrate. To form a uniform helical CLC structure with the helical axis along the substrate normal, the used substrates were also rubbed unidirectionally and stacked with 9 µm thick spacers.

Once the lasing sample devices were complete, the optical reflectance spectra were measured using a Cary 1E (Varian) UV-vis spectrometer and a multichannel spectrometer (HR 4000CG-UV-NIR, Ocean Optics Inc., 0.25 nm resolution). As an optical pumping source for the lasing experiments, a 532 nm pulsed laser beam of second-harmonic light from a Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (Surelite III; Continuum) was used. The laser beam was loosely focused on the sample cell surface at oblique incidence (about 30°) and the emission from the sample cell was collected by a lens along the normal to the cell surface, focused on a fiber bundle to send the light signal to the multichannel spectrometer. A combination of a quarter-wave plate and an analyzer were placed at the front and/or back of the sample cell to investigate the polarization characteristics of the light emitted from the sample.

 

Fig. 2. (a) Reflection spectra of the used CLC layer and 1-D PC reflector film. (b) Measured reflection spectra of the combined CLC/1-D PC device for the incident light polarized linearly along the directions parallel (blue curve) and perpendicular (red curve) to the rubbing direction. (c) Simulated reflection spectra of the combined CLC/1-D PC device for the incident light polarized linearly along directions parallel (blue curve) and perpendicular (red curve) to the rubbing direction.

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3. Results and discussion

First, we observed the reflection spectra of the prepared 1-D PC reflector film and the used CLC layer, as shown in Fig. 2(a). It is clear from the figure that the 1-D PC film made from five multilayers of TiO₂ (62 nm) and SiO₂ (84.5 nm) bilayers on a glass substrate shows a wide optical reflection band in the 440 to 660 nm wavelength region with a FWHM of 220 nm. This wide selective reflection band is due to the large difference between the refractive indices of the used TiO₂ and SiO₂. In the inset of Fig. 2(a), a scanning electron microscopy (SEM) image of the cross-sectional structure of the 1-D PC film may be seen. The SEM image shows clearly that uniform layers of two alternating layered elements of [TiO₂/SiO₂] are well-formed in five multiple stacks with different refractive indices. For the CLC layer, the selective reflection band was observed from 620 to 700 nm with a FWHM of 80 nm only for right circularly polarized (R-CP) light, and thus the maximum reflectance reached near 0.5. After the two optical components had been combined, i.e., hybrid CLC/1-D PC devices, polarized reflectance spectra from the hybrid device were then observed for the two forward-propagating incident light polarized linearly along the directions parallel (x-direction) and perpendicular (y-direction) to the rubbing direction, as shown in Fig. 2(b). Note that the x-direction is consistent with the rubbing direction for the alignment of CLC molecules. In the used measurement geometry, linearly polarized light was incident on the CLC layer of the hybrid CLC/1-D PC device. From Fig. 2(b), it is clear that the reflection bands depend strongly on the polarization of the incident light and thus the polarized reflectance spectra are quite different from each other, especially for the PBG band wavelength region of the CLC layer; polarized reflectance along the x-direction shows a strong and increased reflection, while polarized reflectance along the y-direction is reduced in the wavelength range (620~700 nm). This large difference indicates clearly that in the hybrid device, the reflectance spectra of the combined device cannot be explained by simply summing the reflectance spectra of the CLC layer and the 1-D PC film. Thus, it is evident that the phase delay due to the different optical thicknesses of polarized light may cause the differences in the reflectance spectra. To make the optical characteristics of the hybrid device structure clear, polarized reflection spectra were simulated by using the Berreman 4×4 matrix [28]. In the simulation, the used parameters were as follows: n_TiO2=2.349 with thickness d_TiO2 of 61.9 nm, n_SiO2=1.462 with thickness d_SiO2 of 84.5 nm for the 1-D PC layer, and n_e=1.65, n_o=1.55 with an optical pitch of 670 nm for the CLC layer (10 µm). The simulation results are shown in Fig. 2(c). The figure reveals clear characteristics: for the PBG band wavelength region of the CLC layer, the simulated polarized reflectance spectra are also quite different from each other; the reflectance maxima for polarization along the x-direction correspond to the reflectance minima for polarization along the y-direction. Comparison of the simulated results with the experimental results in Fig. 2(b) shows that the measured polarized reflectance was reasonably reproduced theoretically, although the fine structures deviated slightly from each other due to the uncertainty in the values for the used parameters in the simulation.

On the basis of the above information, we prepared hybrid CLC/1-D PC lasing devices, by doping the CLC host layer with DCJTB fluorescent dye. The dye-doped active-medium CLC layer was sandwiched between a glass substrate and a 1-D PC films. Figure 3(a) shows the measured fluorescence emission spectra from the device at the normal incidence. The emission spectra shown here were taken through L and R-circular polarizers. As shown in Fig. 3(a), upon optical pumping at 532 nm, a strong sharp light emission occurred at ~630 nm for the R-CP light (blue curve), whose handedness corresponds to the handedness of the helix in the used CLC layer. The observed FWHM of the sharp emission peak was about ~10 nm. The opposite situation was observed for the L-CP emission light; a relatively weak emission of L-CP light was observed (red curve). The strong circularly polarized light emission attests to the well-aligned planar structure of the helical CLC layer on the 1-D PCs. For evaluating the degree of circular polarization at a certain wavelength, the g factor was defined as g=2(I_R-I_L)/(I_R+I_L) with I _L/R as the intensity of left/right-handed CP light. It is evident that g is zero for unpolarized light and is 2 for pure, single-handed circularly polarized light. The g factor of the studied hybrid devices for circularly polarized light emission at the peak wavelength of 630 nm was approximately 1.14. This value is significantly higher than that of unpolarized light emission, which showed a g of 0. This result indicates that the R-CPL component dominates, which accounts for more than ~80% of the agreement with the handedness of the CLC helical structure used. The L-CPL component represents only ~20% of the light, which is believed to result from imperfections in the CLC planar structure. In the vicinities of the spacer balls and the high viscosity of the CLC, the CLC alignment has been disturbed. Thus, one may obtain a higher g value for highly circular polarized emission when a well-aligned planar CLC layer is formed on the 1-D PC. In contrast, the g value falls abruptly to zero for wavelength far from that of the stop band of the CLC layer. In order to understand the light emission behavior of the hybrid device, we also observed the dependence of the emitting light intensity on the pumping power for the fabricated device. Figure 3(b) shows the peak intensity of the emitted light (630 nm) at various pump energies. The intensity of light emitted from the CLC/1-D PC device shows the threshold behavior and the threshold pumping intensity was about 2.5 mJ/pulse. This combination of strongly circularly polarized emission, narrow FWHM, and clear threshold behavior implies that the sharp peak of the polarized light emission may be ascribed to lasing emission [29]. As can be seen, the observed lasing threshold is fairly low and comparable to those of previous devices reported elsewhere. We also took a photograph of the operating polarized CLC/1-D PC laser sample (see the inset of Fig. 3(b)). The photograph shows clearly the lasing emission (red light) from the laser sample that is produced by optical pumping (green light). It may be seen from the figure that lasing from the fabricated sample is fairly luminous and is directed along the normal to the CLC layer. Note that the strong lasing light were propagated through the CLC layer side, but that no lasing light was observed through the 1-D PC layer side at all, which indicates unidirectional lasing. Note also that for the R-CP of emitting light, the sharp emission occurs near the wavelength of the high-energy band edge (at 630 nm) of the photonic gap of the CLC layer, as indicated by the reflection spectra. At this wavelength, the high reflectance of the 1-D PC reflector layer may result in unpolarized laser emission, as shown in Fig. 1(a). However, in this case, the hybrid laser device shows the highly circular polarized lasing operation in R-CP mode.

 

Fig. 3. (a) R- and L- CP light emission spectra of the combined CLC/1-D PC devices in the forward direction. Blue and red curves represent R- and L-handed CP emission, respectively. (b) Threshold power behavior for R-CP light emission. The dependence of the emission-peak intensity of the sample cells on the pump power; clear threshold behavior is exhibited. The inset shows a photograph of light emission (left red light) from a 1-D PC/CLC sample and reflected optical pumping light (right green light).

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The high value of g and lasing at the wavelength of the high energy band edge of the CLC layer may be caused by the increased density of mode (DOM) of R-CP light for the sample structure, because the lasing emission rate is proportional to the DOM [30]. From the reflection spectra shown in Fig. 2(c), we could deduce DOM spectra by using theoretical descriptions [30]. The obtained DOM spectra are plotted in Fig. 4(a) for R- and L-CP light. It is clear from the figure that for R-CP light, the high DOM was deduced at the high-energy band edge (640 nm), which is nearly the same as the peak wavelengths of lasing emission (630 nm). Note that the small difference in the wavelengths may be due to the slight mismatches of the values for the optical parameters that were used in the simulation. By contrast, for L-CP light, the DOM at the high-energy band edges is not high enough. This indicates that the lasing with R-CP polarization is much more efficient than that with L-CP polarization, in that the increased photon dwell time at the band edge allows the R-CP light to be amplified by stimulated emission of radiation. When the wavelength is far from the peak DOM wavelength, the DOMs for both polarizations decrease and converge to a common value, with small differences. Note that in the longer wavelength region outside the PBG of the 1-D PC film (λ=690 nm), another peak in DOM can be found. This mode may be attributed to the conventional low-energy band edge mode of the CLC layer. (In this study, we are interested in the unidirectional hybrid mode and thus we omit details of the conventional bidirectional propagating low-energy band edge mode.) Next, we studied the internal field distribution in the hybrid device at the peak DOM wavelength of 640 nm [30]. The estimated field strengths are shown in Fig. 4(b). As shown in the figure, the field strength for the R-CP light is much stronger than that for the L-CP light, not only in the CLC layer, but also in the 1-D PC layer. The strong oscillating electric field through the device indicates that the combined hybrid structure is quite efficient for producing circularly polarized unidirectional lasing.

 

Fig. 4. (a) The calculated DOM from the simulated reflection spectra for the R- and L- CP light. Blue and red curves represent R- and L-handed CP light, respectively. (b) Square modulus of the field distribution corresponding to the wavelength that identifies the high energy band edge of the CLC layer.

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Therefore, from the above results, it may be concluded that a unidirectional lasing device with a high circular polarization state was fabricated successfully by using the 1-D PC/CLC lasing device structure.

4. Conclusions

In summary, efficient laser emission from a hybrid CLC/1-D PC laser device that consisted of a helical CLC layer doped with a fluorescent dye on a 1-D reflector film was investigated. It was demonstrated that highly circularly polarized laser emission were produced from the hybrid device by optical pumping with a relatively low lasing threshold. The wavelength of the emitted laser light corresponded to the high-energy band edge of the CLC layer. Moreover, the direction of propagation of the laser emission could be set to be unidirectional. With its advantages of easy fabrication, a combination of the device reported here with optical devices reported elsewhere will lead to highly efficient polarized organic lasers that could have a wide range of optical applications. Furthermore, the device structure used in this study can be applied to the design of special light-emitting devices, such as organic surface-emitting lasers, 3-D displays, polarized light sources of optical waveguide devices, and/or light sources that have a directional viewing angle.