Tunable Laser Emission Research Articles

Single-mode tunable laser emission in the single-exciton regime from colloidal nanocrystals

Whispering-gallery-mode resonators have been extensively used in conjunction with different materials for the development of a variety of photonic devices. Among the latter, hybrid structures, consisting of dielectric microspheres and colloidal core/shell semiconductor nanocrystals as gain media, have attracted interest for the development of microlasers and studies of cavity quantum electrodynamic effects. Here we demonstrate single-exciton, single-mode, spectrally tuned lasing from ensembles of optical antenna-designed, colloidal core/shell CdSe/CdS quantum rods deposited on silica microspheres. We obtain single-exciton emission by capitalizing on the band structure of the specific core/shell architecture that strongly localizes holes in the core, and the two-dimensional quantum confinement of electrons across the elongated shell. This creates a type-II conduction band alignment driven by coulombic repulsion that eliminates non-radiative multi-exciton Auger recombination processes, thereby inducing a large exciton–bi-exciton energy shift. Their ultra-low thresholds and single-mode, single-exciton emission make these hybrid lasers appealing for various applications, including quantum information processing.

Electronically tunable aperiodic distributed feedback terahertz lasers

Focussed ion beam milling can be used to introduce aperiodic distributed feedback (ADFB) gratings into fully packaged, operational terahertz (THZ) quantum cascade lasers to achieve electronically controlled, discretely tunable laser emission. These aperiodic gratings—designed using computer-generated hologram techniques—consist of multiple slits in the surface plasmon waveguide, distributed along the length of the laser cavity. Tuning behaviour and output power in ADFB lasers operating around 2.9 THz are investigated with a variety of slit dimensions and grating scales. Mode selectivity and grating losses are found to be strongly dependent on milling depth into the upper waveguide layers, dramatically increasing as the metallic layers are penetrated, then rising more slowly with deeper milling into the laser active region. Grating scale and placement along the laser cavity length are also shown to influence mode selection.

Transmission of tunable diode laser emission, through rubidium vapor, as a function of laser intensity

We study laser intensity absorption, at the wavelength of 780 nm, in neutral rubidium vapor using a tunable external cavity diode laser close to the Rb85D2 line. The dependence of laser transmission, as a function of laser intensity, was calculated using the optical Bloch equations and compared with the experiment. The calculations were extended to the saturated absorption case and compared to saturated absorption spectra. Excellent agreement between theory and experiment is observed.

Continuously tunable solution-processed organic semiconductor DFB lasers pumped by laser diode

The fabrication and characterization of continuously tunable, solution-processed distributed feedback (DFB) lasers in the visible regime is reported. Continuous thin film thickness gradients were achieved by means of horizontal dipping of several conjugated polymer and blended small molecule solutions on cm-scale surface gratings of different periods. We report optically pumped continuously tunable laser emission of 13 nm in the blue, 16 nm in the green and 19 nm in the red spectral region on a single chip respectively. Tuning behavior can be described with the Bragg-equation and the measured thickness profile. The laser threshold is low enough that inexpensive laser diodes can be used as pump sources.

Wavelength tunable ultraviolet laser emission via intra-cavity frequency doubling of an AlGaInP vertical external-cavity surface-emitting laser down to 328 nm

We demonstrate an optically pumped vertical external-cavity surface-emitting laser in a compact v-shaped cavity configuration for frequency doubling to the ultraviolet (UV) spectral range at ∼330 nm. The fundamental red laser emission is realized with a metal-organic vapor-phase epitaxy grown (GaxIn1−x)0.5P0.5/[(AlxGa1−x)yIn1−y]0.5P0.5 multi-quantum-well structure. Second harmonic generation is accomplished by using a beta barium borate non-linear crystal to generate maximum UV output powers exceeding 100 mW. By using a birefringent filter, we are able to tune the fundamental laser resonance to realize a maximum tuning range of 7.5 nm of the second harmonic.

Infrared absorption of methanol clusters (CH3OH)n with n = 2−6 recorded with a time-of-flight mass spectrometer using infrared depletion and vacuum-ultraviolet ionization

We investigated IR spectra in the CH- and OH-stretching regions of size-selected methanol clusters, (CH(3)OH)(n) with n = 2-6, in a pulsed supersonic jet by using the IR-VUV (vacuum-ultraviolet) ionization technique. VUV emission at 118 nm served as the source of ionization in a time-of-flight mass spectrometer. The tunable IR laser emission served as a source of predissociation or excitation before ionization. The variations of intensity of protonated methanol cluster ions (CH(3)OH)(n)H(+) and CH(3)OH(+) and (CH(3)OH)(2)(+) were monitored as the IR laser light was tuned across the range 2650-3750 cm(-1). Careful processing of these action spectra based on photoionization efficiencies and the production and loss of each cluster due to photodissociation yielded IR spectra of the size-selected clusters. Spectra of methanol clusters in the OH region have been extensively investigated; our results are consistent with previous reports, except that the band near 3675 cm(-1) is identified as being associated with the proton acceptor of (CH(3)OH)(2). Spectra in the CH region are new. In the region 2800-3050 cm(-1), bands near 2845, 2956, and 3007 cm(-1) for CH(3)OH split into 2823, 2849, 2934, 2955, 2984, and 3006 cm(-1) for (CH(3)OH)(2) that correspond to proton donor and proton acceptor, indicating that the methanol dimer has a preferred open-chain structure. In contrast, for (CH(3)OH)(3), the splitting diminishes and the bands near 2837, 2954, and 2987 cm(-1) become narrower, indicating a preferred cyclic structure. Anharmonic vibrational wavenumbers predicted for the methanol open-chain dimer and the cyclic trimer with the B3LYP∕VPT2∕ANO1 level of theory are consistent with experimental results. For the tetramer and pentamer, the spectral pattern similar to that of the trimer but with greater widths was observed, indicating that the most stable structures are also cyclic.

Continuous Spatial Tuning of Laser Emissions in a Full Visible Spectral Range

In order to achieve a continuous tuning of laser emission, the authors designed and fabricated three types of cholesteric liquid crystal cells with pitch gradient, a wedge cell with positive slope, a wedge cell with negative slope, and a parallel cell. The length of the cholesteric liquid crystal pitch could be elongated up to 10 nm, allowing the lasing behavior of continuous or discontinuous spatial tuning determined by the boundary conditions of the cholesteric liquid crystal cell. In the wedge cell with positive slope, the authors demonstrated a continuous spatial laser tuning in the near full visible spectral range, with a tuning resolution less than 1 nm by pumping with only a single 355 nm laser beam. This continuous tuning behavior is due to the fact that the concentration of pitch gradient matches the fixed helical pitch determined by the cell thickness. This characteristic continuous spatial laser tuning could be confirmed again by pumping with a 532 nm laser beam, over 90 nm in the visible spectral range. The scheme of the spatial laser tuning in the wedge cell bearing a pitch gradient enabled a route to designing small-sized optical devices that allow for a wide tunability of single-mode laser emissions.

Tunable two-photon pumped lasing from alloyed semiconductor nanoribbons

Single-crystal ZnS1−xSexnanoribbons with various compositions were synthesized through an Au-catalyzed vapor transport method. Under two-photon pumping with an 800 nm femtosecond pulsed laser, the spontaneous emission peak of the ternary nanoribbons undergoes a gradual red-shift from 422 to 464 nm with the increase of Se content. When the pump energy densities are increased above the thresholds, the nanoribbons exhibit a tunable upconverted laser emission accompanied by a dramatic decrease in spectral linewidth as a set of sharp peaks on the corresponding wavelengths of each sample, featuring the occurrence of stimulated emission. Two components appear in the fluorescence lifetimes, which indicate that the ZnS1−xSexnanoribbons have two different recombination centers. Both lifetime components decrease with the increase of pump energy, further confirming the occurrence of stimulated emission. The two-photon pumped lasing behavior was also observed from single nanoribbons, in which the photoluminescence (PL) spectra are composed of multimode of lasing. The distinct modes show gain competition and pronounced shifts as a function of excitation density.

Red-edge-wavelength finely-tunable laser action from new BODIPY dyes

New BODIPY dyes with two 4-formylphenyl, 4-(2,2-dimethoxycarbonylvinyl)phenyl and 4-(2,2-dicyanovinyl)phenyl groups at the 3- and 5-positions have been successfully designed and synthesized via palladium-catalyzed coupling reaction or Knoevenagel-type condensations. Structural modification of the BODIPY core via conjugation-extending residues significantly affects the spectroscopy and photophysical properties of the BODIPY fluorophore. These substituents cause the largest bathochromic shift in both absorption and emission spectra, which are shifted toward the red compared to its 4-phenylsubstituted analogue. Additionally, the fluorescence quantum yields and the Stokes shifts are also significantly higher than the corresponding phenyl-substituted dye. New BODIPY dyes have a high laser photostability, superior to that of commercial dyes with laser emission in the same spectral region, such as Perylene Red and Rhodamine 640. The substitution introduced in these derivatives allows to obtain tunable laser emission with a bandwidth of 0.15 cm(-1) and a tuning range of up to 50 nm. So with these three dyes it is possible to cover the spectral range 590-680 nm in a continuous way and with stable laser emission and small linewidth.

Electrical tunable random laser emission from a liquid-crystal infiltrated disordered planar microcavity

Here we report electric-field-induced, wavelength-tunable random lasing action in a liquid-crystal infiltrated disordered planar microcavity. With the increasing of external voltage from 0 V to 30 V, the emission wavelength of the random laser was continuously tuned from 648 nm to 597 nm. It was also tuned back to 648 nm again by reducing the voltage back to 0 V.

Continuously tunable laser emission from a wedge-shaped organic microcavity

We present an organic microcavity laser with wide tunability in the range of 595–650nm, having a threshold as low as 2nJ/pulse. The active medium consists of the organic composite tris(8-hydroxy quinoline) aluminum (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) embedded in between two distributed Bragg reflectors. The inhomogeneously broadened emission from DCM is exploited for lasing by means of a tunable Fabry–Perot-type interferometer. Continuous tuning is achieved by varying the thickness of the active layer. The device shows a high photostability under direct excitation in the absorption band of the guest molecule, thus, allowing to monitor the optical gain spectrum of the active medium.

Multimode laser emission from dye doped polymer optical fiber

Multimode laser emission is observed in a polymer optical fiber doped with a mixture of Rhodamine 6G (Rh 6G) and Rhodamine B (Rh B) dyes. Tuning of laser emission is achieved by using the mixture of dyes due to the energy transfer occurring from donor molecule (Rh 6G) to acceptor molecule (Rh B). The dye doped poly(methyl methacrylate)-based polymer optical fiber is pumped axially at one end of the fiber using a 532 nm pulsed laser beam from a Nd:YAG laser and the fluorescence emission is collected from the other end. At low pump energy levels, fluorescence emission is observed. When the energy is increased beyond a threshold value, laser emission occurs with a multimode structure. The optical feedback for the gain medium is provided by the cylindrical surface of the optical fiber, which acts as a cavity. This fact is confirmed by the mode spacing dependence on the diameter of the fiber.

Tuning in to dye laser origins

Brewer, Mooradian, and Stoicheff reply: We attributed the first demonstration of laser emission in organic dyes to the two independent work groups led by Peter Sorokin and Fritz Schäfer (reference 7 in our article).We also erroneously attributed tunability to them. Although Schäfer and coauthors noted that the laser wavelength of each dye was tunable over a great part of its fluorescence band with changing dye concentration, that was not the narrowband tunability suitable for sweeping over an atomic or molecular spectrum. Indeed, Bernard Soffer and B. B. McFarland first showed that pulsed dye laser radiation could be tuned with a reflective diffraction grating at one end of the laser cavity; that generally more useful method provided tuning over 400 Å in rhodamine 6G with the requisite narrow output of 0.6 Å (50 GHz).Later, others replaced the grating with an echelon or Lyot filter to generate much narrower output. However, the technique used most frequently for precision spectroscopy was the extremely monochromatic (MHz) and tunable dye laser emission produced by using the grating at grazing incidence, a method still in use. Also, continuous-wave and broadly tunable dye lasers with linewidths as low as 50 kHz have been observed by frequency-offset locking the dye frequency to a stable reference.Currently, broad tunability in the near-IR and visible regions with narrow linewidths between 50 and 300 kHz is obtained with a continuous-wave diode laser—a semiconductor chip—using an external cavity that consists of a diffraction grating and a retroreflector. Resolution of this magnitude has permitted precision measurements in fundamental investigations.© 2007 American Institute of Physics.

Single glycerol/water microdroplets standing on a superhydrophobic surface: Optical microcavities promising original applications

Glycerol/water microdroplets take almost spherical shapes when standing on a superhydrophobic surface. Hence they are suitable to function as optical microcavities. Using Rhodamine B doped water microdroplets, large spectral tunability of the whispering gallery modes (WGMs) (>5 nm) was observed. Tunability was achieved by evaporation/condensation in a current controlled mini humidity chamber. Experiments revealed a mechanism stabilizing the volume of these microdroplets with femtoliter resolution. The mechanism relied on the interplay between the condensation rate that was kept constant and the size dependent laser induced heating. The radii of individual water microdroplets (>5 µm) stayed within a few nanometers during long time periods (several minutes). By blocking the laser excitation for 500 msec, the stable volume of individual microdroplets were changed stepwise. Laser emission was also observed from Rhodamine B doped glycerol/water microdroplets using a pulsed, frequency-doubled Nd:YAG laser (λ=532 nm) as the excitation source. The observed largely tunable WGMs and laser emission can pave way for novel applications in optical communication systems. Besides due to the sensitivity of the WGMs to the size and shape of the microdroplets, the results can find applications in characterizing superhydrophobic surfaces and investigating liquid-solid surfaces.

Spatially tunable laser emission in dye-doped cholesteric polymer films

A spatially tunable laser emission of the dye-doped cholesteric polymer film is demonstrated by generating a one-dimensional gradient photonic band gap which is cured by an ultraviolet light. A frequency-doubled pulsed Nd:YAG laser is used to pump the film. The lasing wavelength is tunable from 595to643nm by shifting the position of the film with respect to the pumping beam. Since the spatial distribution of the reflection band is fixed in the cholesteric polymer film, the tuning of the laser wavelength does not require any external field. Therefore, the film is portable and has many applications.

Widely tunable green laser emission using the coumarin 545 tetramethyl dye as the gain medium

We report on widely tunable laser emission obtained using a solution of coumarin 545 tetramethyl dye in ethanol. Interest in this dye derives from its use as an efficient emitter in electrically excited high-brightness organic light emitting diodes. The laser tuning range, at a concentration of 3 mM, was found by measurement to be in the 501–574 nm range, while exhibiting an intensity dynamic range of over four orders of magnitude.

Spatially tunable laser emission in dye-doped photonic liquid crystals

A spatially tunable laser emission of the dye-doped cholesteric liquid crystal (CLC) cell using a one-dimensional temperature gradient is demonstrated. The photoexcitation of dye-doped CLC device using a frequency-doubled pulsed Nd: yttrium–aluminium–garnet laser gives rise to laser emission in the yellow-red spectral range. The lasing wavelength is widely tunable from 577 to 670 nm by shifting the position of the dye-doped CLC cell with respect to the pumping beam. The lowest excitation energy and maximum lasing efficiency occur at λ∼605nm which corresponds to the peak fluorescence emission of the dye.

Wavelength tunable two-photon-pumped distributed feedback zirconia waveguide lasers

Two-photon-pumped distributed feedback laser action of narrow linewidth output was demonstrated in zirconia waveguides doped with trans-4-[p-(N-hydroxyethyl-N-methylamino)styryl]-N-methylpyridinium p-toluene sulfonate, a compound showing strong two-photon upconverted emission around 620 nm. Transient gratings were generated in the waveguides by crossing two nanosecond 1.06 µm beams. Operated at the third-order Bragg resonance condition, tunable narrow linewidth laser emission was observed from 618 to 640 nm by varying the intersection angle of the crossing beams around 61°. Tunable lasing at the second-order Bragg condition was also obtained when the zirconia waveguides were pumped at 532 nm.

Measurement of the refractive index dispersion around an absorbing line

The “anomalous” dispersion of the refractive index has been determined around the 10P18 CO2 absorbing line by recording the spectrum of a mode tunable CO2 laser emission transmitted through a high quality Fabry–Perot resonator. The weak phase shift observed agrees well with the usual quantum mechanic treatment of a two level transition.

1A08 強誘電性液晶の選択反射ストップバンドの電界応答特性とレーザー発振波長制御(物理・物性)

1A08 強誘電性液晶の選択反射ストップバンドの電界応答特性とレーザー発振波長制御(物理・物性)