A low threshold and high efficiency laser based on dye-doped cholesteric liquid crystals (CLCs) is demonstrated using an input excitation with the same handedness of circular polarization as the helical structure of the sample at the shorter wavelength band edge of the reflection band. The responsible mechanism originates from the dramatic increase of the optical density of state (DOS) at the band edges. The calculated DOS of the CLC system confirms the authors\u27 experimental results

Fujii, Akihiko

Huang, Yuhua

Matsuhisa, Yuko

Ozaki, Masanori

Ozaki, Ryotaro

Takao, Yuuki

Wu, Shin-Tson

Zhou, Ying

English

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University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2007 Low-threshold and high efficiency lasing upon band-edge excitation in a cholesteric liquid crystal Yuko Matsuhisa University of Central Florida Yuhua Huang University of Central Florida Ying Zhou University of Central Florida Shin-Tson Wu University of Central Florida Ryotaro Ozaki See next page for additional authors Find similar works at: https://stars.library.ucf.edu/facultybib2000 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Matsuhisa, Yuko; Huang, Yuhua; Zhou, Ying; Wu, Shin-Tson; Ozaki, Ryotaro; Takao, Yuuki; Fujii, Akihiko; and Ozaki, Masanori, "Low-threshold and high efficiency lasing upon band-edge excitation in a cholesteric liquid crystal" (2007). Faculty Bibliography 2000s. 7412. https://stars.library.ucf.edu/facultybib2000/7412 Authors Yuko Matsuhisa, Yuhua Huang, Ying Zhou, Shin-Tson Wu, Ryotaro Ozaki, Yuuki Takao, Akihiko Fujii, and Masanori Ozaki This article is available at STARS: https://stars.library.ucf.edu/facultybib2000/7412 Appl. Phys. Lett. 90, 091114 (2007); https://doi.org/10.1063/1.2710777 90, 091114© 2007 American Institute of Physics.Low-threshold and high efficiency lasingupon band-edge excitation in a cholestericliquid crystalCite as: Appl. Phys. Lett. 90, 091114 (2007); https://doi.org/10.1063/1.2710777Submitted: 09 October 2006 . Accepted: 28 January 2007 . Published Online: 01 March 2007Yuko Matsuhisa, Yuhua Huang, Ying Zhou, Shin-Tson Wu, Ryotaro Ozaki, Yuuki Takao, Akihiko Fujii, andMasanori OzakiARTICLES YOU MAY BE INTERESTED INThe photonic band edge laser: A new approach to gain enhancementJournal of Applied Physics 75, 1896 (1994); https://doi.org/10.1063/1.356336How doping a cholesteric liquid crystal with polymeric dye improves an order parameter andmakes possible low threshold lasingJournal of Applied Physics 94, 279 (2003); https://doi.org/10.1063/1.1578534Cholesteric liquid crystal laser with wide tuning capabilityApplied Physics Letters 86, 161120 (2005); https://doi.org/10.1063/1.1897439Low-threshold and high efficiency lasing upon band-edge excitationin a cholesteric liquid crystalYuko Matsuhisa, Yuhua Huang, Ying Zhou, and Shin-Tson WuaCollege of Optics and Photonics, University of Central Florida, Orlando, Florida 32816Ryotaro OzakiElectrical and Electronic Engineering, National Defense Academy, 1-10-20 Hashirimizu,Yokosuka, 239-8686 JapanYuuki Takao, Akihiko Fujii, and Masanori OzakiDepartment of Electrical, Electronic and Information Engineering, Graduate School of Engineering,Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, JapanReceived 9 October 2006; accepted 28 January 2007; published online 1 March 2007A low threshold and high efficiency laser based on dye-doped cholesteric liquid crystals CLCs isdemonstrated using an input excitation with the same handedness of circular polarization as thehelical structure of the sample at the shorter wavelength band edge of the reflection band. Theresponsible mechanism originates from the dramatic increase of the optical density of state DOSat the band edges. The calculated DOS of the CLC system confirms the authors’ experimentalresults. © 2007 American Institute of Physics. DOI: 10.1063/1.2710777Liquid crystals containing chiral molecules such as cho-lesteric liquid crystals CLCs, ferroelectric liquid crystals,cholesteric blue phases, and cholesteric glasses have recentlyattracted great attention as self-organized photonic crystals.Different from other photonic crystals, the photonic band gapof the CLC is polarization dependent. A circularly polarizedlight in the opposite handedness to the liquid crystal helixtransmits through the CLC, while for the one in the samehandedness a photon density of state DOS vanishes in thephotonic band gap.1,2 Since the photon group velocity ap-proaches zero and DOS is enhanced at the edge of the pho-tonic band gap, low-threshold lasers have been theoreticallypredicted3 and experimentally demonstrated.4–16 Moreover,an anomalous strong absorption occurs at the band edge ofan absorbing CLC. A low-threshold CLC laser upon suchphenomena has been theoretically predicted.17 So far, how-ever, no experimental demonstration has been carried out.In this letter, we experimentally demonstrate the low-threshold laser action upon the band-edge effect due to theanomalous absorption in a dye-doped CLC. To enhance theband-edge effect, we match the pump laser wavelength to thehigher energy band edge of the photonic band gap of thedye-doped CLC by controlling the operating temperature.We also perform theoretical calculation about the DOS,which shows good agreement with the experimental result.The CLC host was prepared by mixing 72.5% nematicliquid crystal BL006 with 27.5% of a right-handed chiraldopant MLC6248 both are from Merck. Afterwards, a0.1 wt % laser dye PM597 exciton was doped into the mix-ture. The CLC was filled into a 12-m-thick LC cell bycapillary action. The inner surfaces of the glass substrateswere coated with a thin polyimide alignment layer andrubbed in antiparallel directions. The pretilt angle is 3°.The helical axis of the CLC was perpendicular to the glasssubstrates. The temperature of the sample was controlled bya temperature controller.To investigate the laser emission properties, a secondharmonic laser from a Q-switched neodymium-doped yt-trium aluminum garnet pulsed laser MiniliteII, Continuumwith vertical linear polarization was used for the excitation.The wavelength, pulse width, and pulse repetition frequencyare =532 nm, 4 ns, and 1 Hz, respectively. The linear po-larized light beam was converted into right-handed circularlypolarized RCP or left-handed circularly polarized LCPlight by a quarter-wave plate and was focused by a lens ontothe sample. The spot diameter on the sample was measuredto be about 160 m. The emitted laser light from the samplewas collected into a fiber-based spectrometer Ocean Optics,HR4000.The dashed lines in Fig. 1a show the transmissionaElectronic mail: swu@mail.ucf.eduFIG. 1. a Transmission spectra of the CLC system for the RCP and LCPincident lights and the emission spectrum at 23.0 °C. b and c Trans-mission spectra for the RCP incident light and emission spectra at 36.0 and42.0 °C.APPLIED PHYSICS LETTERS 90, 091114 20070003-6951/2007/909/091114/3/$23.00 © 2007 American Institute of Physics90, 091114-1spectra of the dye-doped CLC for the LCP and RCP incidentlasers at 23.0 °C. For the LCP incident light, the absorptionof the laser dye was observed at 530 nm. On the otherhand, for the RCP incident light, the suppressed transmit-tance in the spectral range from 519 to 590 nm is due to thestop band of the CLC. The stop band covers the wavelengthof the excitation beam =532 nm at room temperature.The dashed lines in Figs. 1b and 1c show the transmissionspectra for the RCP incident light at 36.0 and 42.0 °C, re-spectively. The stop band shifts toward the longer wave-length side as the temperature increases, which is attributedto the temperature dependence of the helical pitch length ofthe CLC. At 36.0 °C, the high-energy band edge is located atthe wavelength of the excitation beam, as Fig. 1b shows.When the temperature is increased to 42.0 °C, the stop bandis shifted to the 540–607 nm spectral range. In this case, thepumping laser wavelength is outside the stop band, as shownin Fig. 1c.The solid lines in Fig. 1 show the emission spectra as afunction of temperature when the pump energy is22 J /pulse. Above a threshold, the laser action was ob-served at the low-energy band edge. Continue to increasingthe temperature, the lasing wavelength shifts to a longerwavelength, which is in accordance with the redshift of theband edge. Figure 2 shows the emission intensities as a func-tion of pumping energy. The lasing efficiency strongly de-pends on the temperature. As shown in the inset of Fig. 2, thelasing efficiency at 23.0 °C is approximately 4.4 times lessthan that at 42.0 °C. From Fig. 1, the stop band overlaps theexcitation wavelength and a portion of the pump energy isreflected at 23.0 °C. While at 42.0 °C, since the excitationwavelength is outside the reflection band, all the excitationenergy is able to pump the dye molecules. As a result, higheroptical gain is achieved in the sample at 42.0 °C than that at23.0 °C. Consequently, higher laser efficiency is obtainedwhen the sample temperature is controlled at 42.0 °C.The most interesting discovery is that the lasing effi-ciency at 36.0 °C is further enhanced by 17 times as com-pared to that at 42.0 °C. This is contributed by the band-edgeeffect. At the edges of the photonic band gap, the DOS andphoton dwell time are significantly enhanced because of themultiple internal reflections of the CLC. Therefore, when theband edge is within the absorption spectrum of the laser dye,the excitation of the dye molecules is dramatically enhanced,which means much more dye molecules are elevated to theexcited state by anomalous absorption of the pumping pho-tons. As a result, the laser efficiency is dramatically en-hanced.We also investigated the lasing threshold of the dye-doped CLC sample for the RCP and LCP pumping beams asa function of the wavelength of the high-energy band edgeby changing the temperature. The results are shown in Fig. 3.For the LCP excitation beam, the lasing threshold graduallyincreases with the redshift of the bandedge. This stems fromthe spontaneous emission spectrum of the doped dye, whichdetermines the optical gain. The dye used in this experimentexhibits its maximum spontaneous emission at =580 nm.Therefore, the lasing threshold reaches the lowest when thelasing wavelength occurs at 580 nm and then graduallyincreases when the lasing wavelength departs from=580 nm.On the other hand, for the RCP excitation beam the las-ing threshold is high when the shorter band edge of the bandgap is below the pumping wavelength =532 nm. This phe-nomenon can be explained as follows. At a short wavelength,the stop band covers the wavelength of the excitation beam,as Fig. 1a shows. As a result, the incident beam is reflectedby the stop band, which causes the lasing threshold toincrease.18 Note that the lasing threshold is significantly de-creased from 21 to 1.9 J/pulse when the shorter band edgeis located at the pumping wavelength of 532 nm. Moreover,the lasing threshold for the RCP excitation beam is 2.3times smaller than that for the LCP one. This is contributedby the above-mentioned band-edge effects. When the band-edge wavelength gets longer than 532 nm, the lasing thresh-old increases again and becomes higher than the thresholdfor the LCP excitation beam, which is attributed to the re-flection induced by interference. From the result, the band-edge excitation is remarkably effective to increase the lasingefficiency and lower the lasing threshold.To confirm the experimental results, we calculated theDOS of the CLC system19,20 using the 44 matrixmethod.21 In our calculations, the thickness and the extraor-dinary and ordinary refractive indices of the CLC are12.0 m and 1.700 and 1.506, respectively, according to ourexperimental conditions. The absorption of the CLC mediumis neglected. We investigated the DOS as a function of wave-length of the high-energy band edge for LCP or RCP incidentlight by changing the helical pitch. As plotted in Fig. 4, theDOS for the LCP incident light is insensitive to the wave-length of the band edge. This is because a circularly polar-ized light in the opposite handedness to the CLC helix couldnot interact with the CLC medium. On the other hand, for theRCP incident light, the DOS at a shorter wavelength issmaller than that of the LCP light due to the overlapping withthe stop band. The low DOS reduces the optical gain. How-ever, in the vicinities of =532 nm the DOS is drasticallyFIG. 2. Emission intensity as a function of pumping energy at 23.0, 36.0,and 42.0 °C.FIG. 3. Lasing threshold as a function of wavelength of high-energy bandedge for the LCP and RCP excitation beams.091114-2 Matsuhisa et al. Appl. Phys. Lett. 90, 091114 2007increased to 20 because the band edge coincides with thepumping wavelength. This high DOS greatly enhances thelasing efficiency and decreases the lasing threshold at532 nm, as shown in Fig. 3. The calculated results agree wellwith our experimental observations.In conclusion, we have demonstrated a low-threshold la-ser action in dye-doped CLC based on band-edge excitation.The lasing efficiency is dramatically enhanced and the lasingthreshold significantly decreased by the band-edge excita-tion. Our theoretical calculations on the DOS in the CLCconfirm our experimental results.This work is partially supported by Grant-in-Aids forScientific Research from the Ministry of Education, Culture,Sports, Science and Technology of Japan and JSPS. TheUCF part is supported by AFOSR.1E. Yablonovitch, Phys. Rev. Lett. 58, 2059 1987.2S. John, Phys. Rev. Lett. 58, 2486 1987.3J. P. Dowling, M. Scalora, M. J. Bloemer, and C. M. Bowden, J. Appl.Phys. 75, 1896 1994.4V. I. Kopp, Z. Zhang, and A. Z. Genack, Prog. Quantum Electron. 27, 3692003.5Y. Huang, Y. Zhou, and S. T. Wu, Appl. Phys. Lett. 88, 011107 2006.6V. I. Kopp, B. Fan, H. K. M. Vithana, and A. Z. Genack, Opt. Lett. 23,1707 1998.7M. Ozaki, M. Kasano, D. Ganzke, W. Haase, and K. Yoshino, Adv. Mater.Weinheim, Ger. 14, 306 2002.8M. Kasano, M. Ozaki, K. Yoshino, D. Ganzke, and W. Haase, Appl. Phys.Lett. 82, 4026 2003.9M. Ozaki, M. Kasano, T. Kitasho, D. Ganzke, W. Haase, and K. Yoshino,Adv. Mater. Weinheim, Ger. 15, 974 2003.10W. Cao, A. Munoz, P. Palffy-Muhoray, and B. Taheri, Nat. Mater. 1, 1112002.11P. V. Shibaev, V. I. Kopp, A. Genack, and E. Hanelt, Liq. Cryst. 30, 13912003.12A. F. Munoz, P. Palffy-Muhoray, and B. Taheri, Opt. Lett. 26, 804 2001.13Y. Huang, Y. Zhou, and S. T. Wu, Opt. Express 14, 1236 2006.14S. Furumi, S. Yokoyama, A. Otomo, and S. Mashiko, Appl. Phys. Lett.82, 16 2003.15K. Funamoto, M. Ozaki, and K. Yoshino, Jpn. J. Appl. Phys., Part 1 42,1523 2003.16J. Schmidtke, S. Kniesel, and H. Finkelmann, Macromolecules 38, 13572005.17V. A. Belyakov, Mol. Cryst. Liq. Cryst. 453, 43 2006.18S. Furumi and Y. Sakka, Adv. Mater. Weinheim, Ger. 18, 775 2006.19J. Schmidtke and W. Still, Eur. Phys. J. B 31, 179 2003.20M. D. Tocci, M. Scalora, M. J. Bloemer, J. P. Dowling, and C. M.Bowden, Phys. Rev. A 53, 2799 1996.21D. W. Berreman, J. Opt. Soc. Am. 63, 1374 1973.FIG. 4. DOS as a function of wavelength of high-energy band edge for theLCP or RCP incident light.091114-3 Matsuhisa et al. Appl. Phys. Lett. 90, 091114 2007

Low-threshold and high efficiency lasing upon band-edge excitation in a cholesteric liquid crystal

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Low-threshold and high efficiency lasing upon band-edge excitation in a cholesteric liquid crystal

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