Speaker:Guo-Shuen Wen Advicer:Ja-Hon Lin

1. Mirrorless lasing from nematic liquid crystals in the plane waveguide geometry without refractive index or gain modulation Speaker:Guo-Shuen Wen Advicer:Ja-Hon Lin

2. Outline Introduction Methodology Results and discussion Conclusions

3. Introduction • The growing interest in lasing effect in dye doped liquid crystals is stimulated by the prospects for • building compact • all-organic mirrorless lasers with low threshold, • Tunability • High sensitivity to various external factors • light , pressure, electric and magnetic field, • A periodic modulation of refractive index provides a distributed feedback necessary for the generation • Cholesteric or smetic C* • A periodic modulation of light amplification • Nematic liquid crystal and isotropic LC

4. Introduction • NLC is confined between two glasses forming a waveguide and illuminated with the interference (holographic) pattern produced by two powerful beams • We present the results on the observation of lasing in the same in-plane waveguide geometry • Without special modulation of the gain or refraction index of NLC by a holographic or other technique • The light amplification occurs along the length of a narrow stripe produced by the pump beam • The feedback sufficient for generation is provided by the walls of the same waveguide

5. Methodology • Typical liquid crystal cells consisting of • a pair of glasses • indium tin dioxide (ITO) • The glasses were separated by Teflon stripes of different thicknesses • NLC material was introduced into the flat capillary between the glasses • NLCs were used, both based on the same standard mixture • E7 (BDH), doped with 0.5%of either • Chromene (3-diethylamino-7-imino-7H-chromene[3’,2’-3,4]Pyrido[1,2a]-(Et2-benzimidazole-6-carbonitryl) • oxazine-700(2,3,5,6-tetrahydro-1H,4Hquinolizino[9,9a,1-bc]Benzo[i]-phenoxazinone-13) • The dyes reduced the clearing point57 °C of pure E7 mixture insignificantly by 1°–2°

6. Waveguide NLC cell • Light source: second harmonic of a Q-switched Nd:YAG • The light beam was incident normally to the cell from the z direction • Wavelength l0: 532 nm • Pulse width :5 ns • Repetition rate : 5 Hz • Focal lens: a cylindrical lens of 200 mm focal length • Focal area: a rectangular narrow stripe of dimensions x*y=7*0.8 mm Pump intensity threshold :0.1–0.5 mJ/cm2

7. Luminescence and lasing spectrum Black(1,2,3):chromene Blue(1`,2`): oxazine-700 • In both mixtures • the lasing is observed at the long-wavelength slope of the luminescent curves • The fine multimode structure 2` • The lines within the group exchange intensity from one pump pulse to another • In the nematic phase, emission intensity within the same spectral location of the fluctuating modes can be controlled by a voltage applied to the ITO electrodes • Inset of Figure • Curves 1 and 1’:at low pump intensity (luminescence) • Curves 2: P=0.3 mJ/cm2 pulse • Curves 2’:P=1.2 mJ/cm2 pulse • Curve 3: P=0.3 mJ/cm2 pulse (Isotropic phase)

8. Dependence of emission intensity on the pump beam pulses • 10 mm cell filled with the E7-chromene mixture • voltage U=0, • Threshold=0.36 mJ/cm2 pulse • Inset: (x, z) plane angle for the same cell with • P=0.3 mJ/cm2 pulse • voltage Urms=7 V • Pthreshold=0.15 mJ/cm2 pulse • Thevoltage markedly reduce the threshold • About 0.6% of pump energy is emitted in both direction (x and -x)

9. Results and discussion • For the distributed feedback, the experiment was carried out with special modulation of the • refractive index • amplification coefficient • The NLC layer may play the role of low quality mirrors • The light exist the cell is guided by two glasses • The glasses play a very important role: • provide a selection of the proper lasing modes • The exit of the generated light from the cell • The edges of glasses cannot play the role of mirrors • because they are deliberately misaligned with respect to the lasing direction.

10. Lasing mechansim in “cold” waveguide • Due to the self-consistency condition for the guided eigenmodes, cos qm =ml/2d, such a waveguide can support the infinite number • The lasing occurs at l0=nl*=625nm • For the isotropic phase E7 (d=6.4 m, niso=1.59) • cold waveguide supports 32 discrete modes(cos qm =ml/2d<1, qm =10.6°) • From the Snell law, we find that • the first 10 modes remain in the Waveguide (qc = Sin-1 (n1/n2), n1(glass)=1.5, n2(E7)= 1.59, qc =70.6°)) • 15 remain in the glasses (n=1.5) (n1(air)=1, n2(glass)= 1.5, qc =38.97°) • 7 leave for air.

11. Results and discussion • Among all the cold modes, only one has a minimum threshold for lasing(leaky mode:first not totally reflected mode )(m*=11) • in waveguide qm=11 =70.26° (cos qm =ml/2d) • total reflection qw-g=70.6° (qc = Sin-1 (n1/n2)) • qglass=86° (n1 sinq1=n2 sinq2) • At a very grazing angle of 4° to the glass surface, • and leaves the glass at an angle 6° • Among all other modes in the waveguide, this leaky mode has the minimum group velocity

12. Conclusions • The mechanism for lasing on leaky modes suggested here is principally different and is basically valid only for the thin-film structures with lateral dimensions essentially exceeding the waveguide thickness • At this wavelength lG the optimum conditions for lasing • easily be satisfied by a proper choice of the eigenmode angle in the waveguide • The fine structure of lasing bands , it is sufficient to assume some inhomogeneity in the waveguide thickness. • Fine structure could be either a time dependence of the amplification factor • The nonstationary nature of the pulse excitation • A fluctuating amplitude of the pump pulses themselves.