**3.1 Electrically switchable liquid crystal-based photonic crystals for a white light laser**

Laser source is the most unique light source with many special optical properties such as coherence and collimation. The laser emission needs both the elements: stimulating source and the gain media. Today, various solid and gas materials have been employed as gain media for lasing. However, white light lasers that span the visible spectrum (red, green, and blue colors) are important for lighting, imaging, and communication applications. Recently, the organic white light laser source was successfully demonstrated [32]. Recently, an inorganic semiconductor laser source has also been proposed with a monolithic multi-segment semiconductor nanostructure [33]. Huang et al. also shows that PC/CLC hybrid structure (**Figure 11a**) is a new way to achieve white light laser [34]. In addition, the complex stacking PC/CLC structure is designed (see **Figure 11a**) and can be simply coded as [GI(HL)4HH(LH)3]−P(D)P−[(HL)3HH(LH)4IG], where D means the dye-doped CLC (DDCLC); P is the polyimide alignment layer; H and L are the high and low refractive indices of dielectrics; G represents the glass substrate; and I is the ITO. In addition, the high and low refractive indices of dielectric materials are Ta2O5 (nH = 2.18) and SiO2 (nL = 1.47). The configurations of the CLCs in three states (P, FC, and H states) are shown in **Figure 11b**. Note that the voltage V1 leads to the FC state exhibiting an optical scattering property, and a larger voltage V2 induces the H state. The transmission spectra of CLC and a PC substrate are also displayed in **Figure 12a**. In addition, the PBG is divided by a defect mode peak at the 640 nm of PBG. The Bragg reflection of DDCLC is located at right half of PBG in hybrid PC cell. The dye composition (C540A, PM580, and LD688) in the PC/DDCLC device was adjusted to fluoresce in three wavelengths (red, green, and blue lasing emissions). However, the artificial defect mode peak in the PC is at 446nm, which allowed the pumping light to penetrate the PC cell. An organo-inorganic white light laser from PC/DDCLC composed of three colors red, green, and blue lasing emissions is therefore achieved, as displayed in **Figure 12b**. A genuine photo of the PC/ DDCLC laser is shown in **Figure 12c**, which is accompanied by the CIE1931 chromaticity diagram. In addition, the color of red, green, and blue are mixed as the discrete

#### **Figure 11.**

*Schematics of (a) the hybrid phonic structure and (b) the configurations of the three CLC states in the multilayers device (adapted from [32]).*

*Photonic Crystals - A Glimpse of the Current Research Trends*

*various voltage at a fixed frequency of 1 kHz (adapted from [22–24]).*

intense defect modes in the empty PC cell because of the transparent air defect. With a PSCT defect layer embedded in the PC device, the PC/PSCT device initially in the H state and the more spectral defect windows in the PBG due to the higher (ordinary) refractive index no in the LC layer. And the FC state is demonstrated when a 30-kHz voltage pulse is applied. The lower transmission of the defect modes in the PBG is also shown in **Figure 10a**. We can employ the defect modes of FC to switch off the PC device by the light scattering property. When the frequency still increases to 100 kHz, the PC/PSCT will be in the P state. The redshifted defect modes is shown and the increasing defect mode number is exhibited (**Figure 10a**). Moreover, **Figure 10b** illustrates the spectra of the PC/PSCT device in the H and FC states induced by various voltage amplitudes at a fixed high frequency of 100 kHz. We can observe that the H state of the cell is the initial state. The intensity strength of the defect modes can be tuned by increasing the voltage. **Figure 10c** demonstrates the transmission spectra of the defect modes by applying various voltage of 0, 10, 25, 40 Vrms at a low frequency of 1 kHz. We can easily modulate the strength of defect modes between FC and P states. This powerful photonic device has the potential to expand optics applications, making it use as an electrically tunable

*(a) Transmission spectra of the empty PC cell and the PC/PSCT structure in three different states P, FC, and H states. In addition, (b) transmittance of the defect modes from H to FC in the PBG induced by a 100-kHz various voltage amplitudes. (c) Transmittance of the defect modes from P to FC in PC/PSCT induced by* 

device and optically tristable filter based on these special properties.

which let the PC/PSCT device more potential for applications.

To conclude, the electrically tunable PC/PSCT devices have been investigated. In addition, the tunability is caused by the incorporation of a PSCT material as a new defect layer in PC structure. This hybrid PC/PSCT owns three stable P, FC, and H states. The electrically tunable PC device has been investigated, and it can be directly switched from one to another stable state by just applying a voltage pulse. Due to the tristability, the optical defect modes of PC/PSCT remain at zero voltages. This PC/PSCT composite device exhibits many different defect mode transmission spectra when we switch among P, FC, and H states. In addition, the intensity of the defect modes can be tuned by the amplitude of voltage as well as the wavelengths can be switched by the frequency in the H and P states. Based on the properties of tristable switching, wavelength controlling, and intensity tunability in the defect modes, the novel PC/PSCT device can be used as a low-power consumption optical filter, light shutter or an electrically intensity modulator without any polarizers,

**60**

**Figure 10.**

**Figure 12.**

*Spectra of PC/DDCLC. (b) The white-light lasing spectrum and the spectra of PC and the CLC in the planar state. (c) Photograph of a tricolor laser device and the color space coordinates of the PC laser on the CIE 1931 chromaticity diagram. (d) the pumping energy-dependent the lasing emitted from PC/DDCLC device.*

white light laser and depicted in **Figure 12c**. One can tell that the novel PC/DDCLC structure can be really lasing in white light. **Figure 12d** shows the relation between lasing intensity of PC device with the pumping energy. The threshold is about 7.4 μJ/ pulse in this PC lasing device.

This is the first demonstration of a discrete white light source (three-colors: red, green, and blue) lasing. The organo-inorganic PC/DDCLC cannot only generate three colors in lasers with a single pump, but also be electrically switched among the three modes lasing. With such properties, lasing wavelength can be altered back and forth in a wavelength range and in a very short response time. In addition, PC/DDCLC lasing device is also cost effective, color tunable, and can be fabricated easily. Moreover, it has been shown that the PC device can be pumped using a simple CW laser. The ability to generate a single-color, two-color, three-color or white-light laser makes a new way to full color display, lighting, and other optics applications. By employing PC/DDCLC lasing device, a small size laser system can be achieved to make the proposed PC/DDCLC applications more feasible and potential.
