Microlasers and Microamplifiers on Liquid Crystals. L.M. Blinov, Phys. Dept., Calabria University

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1 OLC-7, Puebla, Mexico Microlasers and Microamplifiers on Liquid id Crystals L.M. Blinov, Phys. Dept., Calabria University Collaborators (with great acknowledgement!) Calabria University: G.Cipparrone, P.Pagliusi, A. Mazzulla, T.Rugiero Inst. of Crystallography (Moscow): V.Lazarev, B.Umansky, S.Palto, N.Shtykov 1. Motivation. Lasers based on leaky modes in NLC and CLC 3. Simple electric field tunable NLC laser 4. ASE gain measurements in NLC 5. Planar light amplifiers 1

2 1. Motivation: why NLC and why to amplify? DFB-CLC dye laser Pump Advantages: CLC as lasing medium: L.Goldberg, J. Schnur, US Patent,3,771,65,, (Nov, 1973) First observation of lasing: I.Il chishin et al JETP Letters 3, 7 (198) Lasing simple, planar, mirrorless; any wavelength in near UV- Vis-near IR are possible; sensitive to external agents Hence, lasers & amplifiers on NLC! Disadvantages: Low thermal stability (low pump energy density) Leaky modes do not allow large diameters of pump beam: Low output energy (1 nj/p) No possibility for tuning by the electric field E. Tikhonov, M. Bertolotti, F. Scudieri, Appl. Phys. 11, 357 (1976).

3 . Observation of lasing on leaky modes (top view) a) nematics 53nm, 5ns Cylindr. lens Laser YAG Uω Filters Glan mm PC Spectrometer ϕ 5mm Fiber 3

4 Em mission Inten nsity I (arb. u.) 3 1 Nematic materials: E7 (BDH)+.5% Chromene (NIOPIK) Oxazine 7 (NIOPIK) DCM (Exciton) Chromen Oxazine λ (nm) Luminescence (1,3) and lasing (,4); nematic, d=6.4±.5 μm I las (arb. un nits) Pump: Chromene P=.3 mj/p.cm, Oxazine-7 P=1. mj/p.cm ) 1 5 Nematic E7+Chromen.5% d=6.3μm, α=7deg Iso Wavelength (nm) Lasing in E7 + Chromene Nematic and diso-phase P=.3 mj/cm pulse Emission polarization: close to TM (p), voltage dependence 4

5 Waveguide cell and Leaky Modes NLC α Glass+ITO pump z y x ϑ m lasing lasing Pump beam propagates along the z-direction and forms a 7 x.8 mm stripe in the xy-plane x,y-plane of the cell. The light amplification appears in the LC layer in the x- direction where waves are partially reflected by ITO layers. Amplified modes at an angle close to the total reflection angle leak into glasses, propagate within them and exit from their edges. Which mode has the maximum gain? 5

6 Answer: Our modeling shows that it is the first non-waveguided mode, which escaped into the glass (the most gliding mode within the glass) This is an additional mechanism of lasing mode selection (the first is narrowing of ASE spectrum, as in traveling wave lasers). L.M. Blinov et al, Appl. Phys. Lett, 89, (6) S.P. Palto, JETP 13, 47 (6) We believe that we observe a new regime of lasing in NLC which is valid also for the isotropic and any other phases of liquid crystals. Let us see what happens in a cholesteric LC 6

7 b) Cholesterics liquid crystals: competition between Bragg and Quasi- In-Plane Leaky (QIPL) modes Geometry (helix axis along z) Absorbance spectrum MLC6815/ZLI %DCM d=47 μm Bragg band Absorbance Bragg QIPL Wavelength (nm) 7

8 CLC: wide elliptic pump p spot Luminescence: angular dependence Emiss sion (counts) 6 Typical spectra 4 Pump 96 o, R=17 3 o, R=1 φ= o, R= λ (nm) φ is angle of observation, counted from the cell normal R is polarization ratio (counts) Emission F1,F3, F3 Sph.Lens1-dist195, Pump Inc.angle b45deg, fb d6 (48μJ/p), OS1 15 Cell plane λ m (nm) Cell normal Stop-band shift Angle φ (deg) For organic solid films luminescence see: Penzhofer et al Opt. Comms. 9, 79 (4) 1 8

9 CLC: Cylindrical Lens. Focused beam (7x.5mm ) Spectra and pump dependence of emission intensity in Bragg and QIPL modes QIPL φ=87 o 6 4 Bragg 1 Pump: 8 mj/cm pulse QIPL Scale x1 Emission Int. (counts) λ (nm) Bragg, φ= Pump energy (mj/cm pulse) 9

10 So we can conclude that In CLC, for an expanded pump beam area, the Quasi-In-plane Leaky modes dominate over the Bragg mode (orders of magnitude!) Leakage of energy must be reduced for DBF LC structures (for example by using multi-channel pump, a kind of light sieve or to use amplifiers L.M. Blinov et al J. Appl. Phys. 11, (7) L.M. Blinov et al MCLC 465, 37-5 (7) But what about tuning laser bands by electric field? In this case CLC is not proper p material (CLC helix is not smoothly unwound due to principal limitations, as discussed in my tutorial lecture). We decided to follow the work of T. Matsui, M. Osaki et al, Electro-tunable laser in dye-doped NLC waveguide under holographic excitation, APL 83, 4 (3), but to do without holography! 1

11 3. Simple tunable DFB laser (b) T Cell size (1 x5 mm) Pump n y R Period is 15 μm (Large!!) Odd (shadow, E): 5 μm Even (illum, E=): 1 μm What does electrode mask make? - modulates gain even without field - modulates refraction I=1 U y index by field n z Λ odd even x z 11

12 Laser generation in the isotropic phase of E7/dye (.5%) (a) 3 3' 1' ' Experiment 1 n=1.569, Λ=15μm EI (arb. u.).5.1. (b) * R (%) λ (nm) Experimental & calculated lasing spectra Accuracy is determined by a) spectrometer resolution (.6 nm) b) dependence of n on the type of dye 69 λ m = Model (no fitting parameters!) n Λ sin iso m ϕ 1

13 Evolution of lasing spectra with applied voltage (E7/DCM, nematic, d=1.6 μm, pump1.8 mj/cm ) V 6 (nm) 1.4 V 61 EI (counts) λmax 5. V U (V).5 V λ (nm) How to understand d this trend ( ) rms with increasing voltage? 13

14 Calculated voltage dependence of spectral positions of the z- polarized Bragg modes of different order m E7/DCM Calculation m= Experiment λmax (nm) U (V) Exper U rms (V) av Λ n, z ( U) y, z y m ( U ) λ = m 14 λm (nm)

15 Suggested: simple DFB microlasers with spectral positions of the emission band controlled by an electric field (shift 5 nm) Understood: the role of the high order resonator modes and the basic mechanism (gain modulation and additional influence of refraction index modulation) L.M. Blinov et al, Appl. Phys. Lett. 9, (7) L.M. Blinov et al, J. Nonlinear Opt. Phys. & Mat. 16, 75-9 (7) 15

16 4. ASE gain measurements of dye doped NLC in polarized light with an electric field applied I λ g l ( ) ( λ) = ln 1 l Il ( ) + / λ s λ= 53 nm; τ = 5 ns Mirror Laser YAG Optical filters (f 1 ) Polarizing prism Cylindrical lens { Shutter Cell with NLC position l position ll Rotating stage Technique: C.V. Shank et al, Appl. Phys. Lett. 1, 37 (197) Rotating λ/4-plate L =.7 cm CCD-spectrometer 1 cm Polarizer fiber Optical filters (f ) 16

17 e y A Polarization spectra of gain at variable pump energy per pulse e z IS p 5 z y 1 (a) x e y y L x L x,p y Material: E7 + DCM (.5%) Cell thickness 7 μm (a) e z e y L y,p y I (1 3 counts) I (1 3 counts) 6 4 e z (b) 5.9 mj/p (b) 5.9 mj/p.68 Gai Gai in (cm -1 ) n (cm -1 ) λ (nm) λ (nm) 17

18 Field-controlled gain Material: E7 + DCM (.16%), cell thickness 4 μm (a) L y -> L z thickness 4 μm (b) U= 7 V 7 V The field reorients the director from planar (y) to homeotropic (z) position Here we see a possibility to U= switch the gain from negative to positive, ON and OFF λ (nm) 18 Gain (cm -1 ) I (1 3 counts)

19 5. Planar micro-amplifier for CLC laser Oscillator: MLC-6815 (Merck) + chiral ZLI-811 (Merck) +DCM.5% Amplifier: Rh64 c=1.5.1^- M/L λ= 53 nm; τ = 5ns Mirror Laser YAG Optical filters (f 1 ) Lens Polarizing prism Mirror CCD-spectrometer Optical filters (f 3 ) CLC cell Optical filters (f ) (5 mkm) Rotating stage Amplifying cell ( mkm) fiber 19

20 B1. Absorbance spectra Red and blue: 5 μm ChLC-DCM cells Black: μm cell Rh64 dense solution in glycerine (1 - M/L) 5.5. Rh64 band DCM 1.5 band λ pump 1. 1 A C B λmax(dcm)=467 nm. Lasing at λ 61 and 614 nm Absorbance 5.5. Bragg band λ (nm) λmax (Rh64)=573 nm Dmax=6.4 Rh64 ASE at λ nm, depends on pump

21 B. Laser emission energy and vs pump energy Black: from CLC61 and CLC614 without amplifier Blue: for same cells with the amplifying i Rh Layer B3. Inset: Gain coefficient at λ=61(1) and 614 () nm ' 6 times amplification with locked λ= constant Emission Energy (μj) Gain (cm -1 ) Pump energy (μj) Pump energy (μj) ' 1 Pump energy scale W=1 μj corresponds to W p = 3 mj/cm and P p =6 MW/cm. 1

22 5b. CLC emission amplification by a nematic cell Spectra of Ox17 absorption (1) and luminescence at pump 7.4 () and 11 (3) mj/pulse У CLC oscillator: LJK+Leitsine+ Oxazine 17 (.3%) Cell thickness d=3 μm NLC amplifier: LJK+Chromene (.5%) Cell thickness d=5 μm I (arb.u.).5. 3 Г 5.5 D λ (nm)

23 W (nj/pulse e) Light amplification spectra Input signal 1. nj, utput 9 nj (W) 1 (W ) (μm) -1 γ, Gain coefficient anisotropy γ γ I =69 mj/cm p W =1.9nJ d=5μm W (nj/pulse) λ (nm) Max amplification 15 Gain: up to 5 cm -1 Planar amplifier on Rhodamine 645: L.M.Blinov et al, Appl. Phys. Lett. 91, 1611 (7) Amplifier on NLC: N.M. Shtykov et al, Pis'ma Zh. Eksp. Teor. Fiz. 85, (7) 3

24 Conclusion Lasers: A new field controllable regime of lasing based on QIPL modes is found in NLC (valid for isotropic and any other LC phases) At certain geometries of CLC lasers, QIPL modes may have consumed almost all energy of excitation Simple voltage tunable DFB laser is suggested with interdigitated electrode system, which operates on high order Bragg reflection (m>7) Amplifiers: Shown a possibility to control the ASE gain magnitude by the electric field using NLCs. Weak emission of CLC is amplified 6 times by Rhodamine64-glycerin solution in the planar (chip) configuration Weak emission of CLC is 15 times amplifies by a 5 μm nematic cell Acknowledgements. Mr. A. Pane (LiCryL-CNR) and S. Yakovlev (IKRAN) for the help in experiment. Support from CNR-INFM and CEMIF.CAL funds (Italy), OFN RAN program (Russia) 4