We report a detailed physical characterization of a novel array of organic distributed feedback microcavity lasers possessing a high ratio between the quality factor Q of the resonant cavity and its volume V. The optical microcavity was obtained by confining self-organized mesophases doped with fluorescent guest molecules into holographically patterned polymeric microchannels. The liquid crystal microchannels act as mirror-less cavity lasers, where the emitted laser light propagates along the liquid crystal helical axis behaving as Bragg resonator. This miniaturization process allows us to obtain a micro-laser array possessing an ultralow lasing threshold (25nJ/pulse) while having directional control on the lasing emission, a fine wavelength tunability and the control over the emission intensity.
©2006 Optical Society of America
The concept of wave transport in complex media, in particular the propagation of electromagnetic waves in dielectric periodic structures represents a topic of great interest in various fields of scientific research. Periodic structured materials, namely “photonic crystals” that strongly localize light, are finding applications in many technological areas, providing an intriguing stage for controlling radiation fields [1–3]. Photonic crystals were introduced by Yablonovitch  and John  and have attracted a huge interest because of their remarkable potential confirmed in fundamental physics and emerging applications.
The optical properties of photonic materials are governed by a refractive index spatial modulation, on length scales comparable to wavelengths of light, similar to the periodic potential for an electron in a crystal. This variation of the dielectric constant gives rise to Bragg selective reflection effect, irrespective of the direction of light propagation, within a certain frequency band, allowing the confinement, manipulation and guiding of light photons in such ordered structures. Analogous to the energy gap in semiconductors, periodic photonic crystals exhibit stop bands, representing forbidden windows for electromagnetic wave propagation within a given frequency range . This phenomenon is mainly due to coherent Bragg scattering of the coupled waves inside the photonic bandgap (PBG) crystal. In PBG microstructures light can be confined and manipulated by engineering quasi-ideal distributed feedback (DFB) micro-resonators [6,7]. Helixed liquid crystals materials are self-organized mesophases which possess all the peculiar optical properties for achieving organic mirror-less optical resonant cavities. The birefringence and natural ability to form periodic structures make chiral liquid crystals interesting one-dimensional photonic bandgap materials. In fact, chiral liquid crystals (CLC) possess a helical superstructure which provides a 1D spatial modulation of the refractive index giving rise to Bragg selective reflection for circularly polarized light having the same handedness as the LC structure. Therefore a photonic pseudo-bandgap structure is present, being centred at λc= pnavg and having the bandwidth ∆λ = (ne-no) p, where no and ne are the ordinary and extraordinary refractive indices of the locally uniaxial structure, navg is the average refractive index of the cholesteric planes which have a birefringence of ∆n = (ne-no) and p is the pitch of the helix.
In a CLC doped with fluorescent guest molecules, the spontaneous emission is suppressed for the reflection stop band and it is enhanced at the band edges where a series of narrow long-lived transmission modes are expected [8,9]. Within the bandgap, the wave is evanescent and decays exponentially, so that the density of states (DOS) within the gap vanishes in large structures. At the stop band edges the density of states is expected to diverge. The group velocity vg is real, and at the photonic band edge tends to zero, while the photon dwell time is greatly increased, so that the gain is significantly enhanced at these spectral positions . This gain enhancement together with distributed feedback mechanism can give rise to low threshold mirror-less lasing at the band edge in a variety of liquid crystal materials. In chiral liquid crystals the index modulation is low so that sufficient feedback can be achieved by increasing the length of the resonant cavity. Kogelnik and Shank  were the first to report laser action in mirrorless periodic Bragg DFB structures, while laser action in chiral liquid crystals was predicted by Goldberg and Schnur in 1973 .
The inclination toward miniaturization in the emerging generation of nanoscale optoelectronic devices leads to a growing interest in morphologically periodic photonic hetero-structures like photonic bandgap materials and special microcavity geometries [13–15]. A relevant example is represented by the distributed feedback microstructures which play a fundamental role in confining and manipulating light for obtaining lasing in media with gain.
Spontaneous emission is not an immutable property of the emitter, but it relies on the density of modes of the light field at the location of the emitter. It was predicted by Purcell  that an atom in a wavelength-size cavity can radiate much faster than in the free space. Thus, the rate at which a dipole spontaneously emits radiation can be enhanced by placing the emitter inside an opportune optical cavity. This phenomenon, known as the Purcell effect, has been verified by numerous experiments that involve atoms passing through cavities. Optical microcavities confine light spatially and temporally ; a microcavity’s great potential is not only determined by the increased photon dwell time (or the equivalent quality-factor, Q), but also by the simultaneous achievement of a small modal volume (V). The Purcell factor describes the amount by which the spontaneous emission rate is enhanced for an emitter on resonance with a cavity mode:
This factor is proportional to the quality factor of the mode and is inversely proportional to the mode volume. The quality factor of a mirrorless resonant cavity Q is defined as:
where n is the medium index of refraction; l is the length of the resonant cavity; c is the vacuum speed of light; k is the extinction coefficient; λ is the radiation wavelength; R is the cavity reflectance and ω is the resonant frequency. So, the larger the quality factor is, the longer the radiation remains trapped inside the cavity resulting in a lower lasing threshold.
Here, we report a detailed physical characterization of the novel array of organic DFB microcavity laser possessing a high ratio between the quality factor Q of the resonant cavity and its volume V. The optical microcavity was obtained by embedding helixed liquid crystals doped with fluorescent guest molecules into holographically patterned polymeric microchannels. The employed single-step process in creating the microstructures imposed the orientation of the liquid crystal helical axis along the polymeric walls. A series of significant physical advantages yield from adopting this type of configuration: increase of the cavity length, very small modal volume per each resonant microcavity (providing also minimization of the optical losses), directional control of the lasing emission, wavelength tunability and control over the emission intensity. This miniaturization process allows to obtain a micro-laser array where the single channel might be selectively tuned or switched off.
3. Experiment and results
To accomplish the objective various suitable combinations of liquid crystal host, fluorescent guest molecules , alignment technique, and pumping conditions were considered. An initial important matter was to find a helixed material which presents a good spectral overlapping of its photonic stop band edge and the high efficiency region of the dye fluorescence spectrum. At the stop band edge the photonic density of states is expected to diverge, the group velocity of the wave-packets approaches zero giving rise to a pseudo-standing wave within the periodic medium, and the resulting long dwell time of the emitted photons strongly supports stimulated emission. This peculiar attribute was met by the BL088 liquid crystal (Merck) in the cholesteric phase and the Pyrromethene 597 dye (Exciton). The exact superposition was achieved by adding a small amount of BL001 (Merck) nematic liquid crystal. However, this procedure slightly decreases the orientational order parameter of the system producing a reduced feedback mechanism and for this reason this procedure was ruled out.
The experimental setup used for optically pumping and characterizing the systems is depicted in Fig. 1. The pump is represented by a frequency-doubled (532 nm) Nd:YAG (Neodimium doped : yttrium aluminium garnet) laser (New Wave, Tempest 20). The pump beam was focused onto the sample by means of a cylindrical lens (f = 100 mm). The emission spectra were aquired with a spectral resolution of about 3 nanometers using a multichannel charge-coupled device (Jobin-Yvon CP200).
The most interesting results were obtained by using a mixture containing 99.6 wt % of BL0088 liquid crystal and 0.4 wt % of Pyrromethene dye. The mixture was then confined between two indium tin oxide (ITO) parallel glass plates separated by 25 μm thick Mylar spacers. The inner polyimide coated sides of the ITO glass give a planar alignment to the liquid crystal molecules inducing an orientation of the liquid crystal helical axis normal to the glass plates. The sample was optically pumped with 3 ns duration pulses, at a frequency of 10-20 Hz, produced by the Nd:YAG laser (Fig. 2). As expected, the photonic bandgap structure of the self organized chiral liquid crystal clearly modifies the fluorescence spectrum of the dye molecules: at low pump power a dip appears in the emission at the position of the stop band (Fig. 2(a)). By increasing the pump power above a certain limit, very intense stimulated emission radiation was obtained in a direction perpendicular to the ITO glass plates (Fig. 2(b)). The lasing wavelength is 580 nm and the reported FWHM is about 2 nm, limited by the resolution of the CCD spectrometer. Further investigations reveal a very stable system which presents an average lifetime for the gain medium of more than 6 months under normal pumping conditions.
The obtained results indicate the above mixture as the appropriate candidate for the principal stage of this study: to exploit the capabilities as optical microcavity of the novel polymeric configuration. The engineered novel structure is known as PoLiCryPS (the abbreviation for Polymer Liquid CRYstal Polymeric Slides), the main difference with respect to the standard PoLiCryPS is the introduction of helixed liquid crystal to create the PBG structure. Complete details regarding the fabrication process are present in several previous works [13,18].
A thorough structural analysis of the system was performed by using the optical (Fig. 3(a)) and scanning electron microscopy (Fig. 3(b)) techniques. The study reveals a very well-defined morphology consisting in a string of side-by-side polymeric microchannels which act as optical microcavities by confining the active matrix of chiral liquid crystals doped with fluorescent guest molecules. Although the employed curing setup allows to obtain a spatial periodicity in the range 200 nm up to a few micrometers, the presented experimental results refer to a measured periodicity of the structure of about 5 μm with the microcavity width of 1.5 μm. A series of optical investigations (tilting compensator and photopolarimetry) confirmed the fact that the LC’s helix orientation is along the polymeric channels.
This complex system was optically pumped with the second harmonic (532nm) of a Nd:YAG laser. The laser beam was focused onto the sample by means of a cylindrical lens (f =100mm) and linearly polarized perpendicularly with respect to the microchannels. The long axis of the section was oriented perpendicularly to the orientation of the polymeric walls, therefore, the obtained profile (long axis of approx. 5 mm) assured the simultaneous excitation of multiple microchannels. Above a certain pump power, stimulated emission was achieved emerging from the microcavities in a direction parallel to the glass plates and along the microchannel structure. At this position highly sensitive spatial distribution emission measurements were performed in a restricted cone angle of about 0.1 rad. The sketch in Fig. 4 shows this lasing scenario of the micro-laser array. The stimulated emission emerging from the microchannels was circularly polarized demonstrating that the distributed feedback mechanism is behind the observed phenomenon.
The spectral position of the lasing line (580 nm) was not importantly modified with respect to the conventional cell (Fig. 5(a)), indicating that the mixture, in particular the helix of the liquid crystal was slightly affected by the confinement process. Lasing was obtained at red-edge of the photonic bandgap, where the photonic density of states is expected to diverge, the group velocity approaches zero and the resulting long dwell time of the emitted photons highly supports stimulated emission.
The lowest obtained FWHM of the lasing spectra was at the limit of our multi-channel charged coupled device (3 nm). One can also notice the remarkable spectral overlapping of the high efficiency of the dye fluorescence spectrum and the low energy edge of the stop band.
Figure 5(b) shows the graphic for the calculated density of states (DOS) function of our right handed chiral liquid crystals in the case of long microcavity (LMC) confinement, where one can ignore the multiple reflections at the boundary interfaces [8,13]. This calculus is in perfect agreement with the results: the experimentally measured band edges positions of the chiral structure match the spectral regions where the calculated DOS function diverges and lasing is obtained.
The dependence of the emission intensity and spectral linewidth (FWHM) on the input pump energy are reported in Fig. 6(a). At low excitation energies, both the emission intensity and the linewidth show a quasi linear dependence as function of the pump energy. Above a characteristic threshold, the pump energy per excited sample area was about 5 mJ/cm2 which corresponds to about 25 nJ/pulse, the emission intensity suddenly starts to increase nonlinearly. Also, above this energetic pump limit, the emission linewidth break off from the previous tendency and begins to significantly decrease. The observed pump energy value at which the explosion and narrowing effects occur (25 nJ/pulse) is an order of magnitude lower than in the case of other conventional dye doped systems in a similar environment and under the same pumping conditions.
Polarized fluorescence measurements put into evidence a strong dependence of the emission intensity (spontaneous emission) on the pump polarization, as it shown in the polar plot of Fig. 6(b). By maintaining the pump energy fixed, a four fold increase in the spontaneous emission intensity was recorded by rotating a linear state of polarization from parallel to the microchannels (0 deg) up to a perpendicular one (90 deg) and by analyzing the emitted light along the direction of the excitation polarization. This suggests that the PM 597 dye molecules possess an anisotropic orientational distribution of the transition dipole moments because of the strongly anisotropic environment in which has been dissolved, with a dye order parameter SD = 0.23 calculated as follows :
where F∥ and F⊥ are fluorescence intensities polarized parallel and perpendicular with respect to the microchannel walls.
Experimental studies revealed a couple of striking features for the presented system: the possibility to tune the lasing wavelength by varying the temperature and to control the lasing intensity by applying an electric field perpendicular to the helical axis orientation. These properties were found to be fully reversible.
By changing the temperature value from 25 °C up to 80 °C a fine-tuning of the laser emission wavelength was obtained (Fig. 7(a)). An average red-shift of 0.2 nm/°C was recorded in the spectral range 580-590 nm. It is demonstrated that the period of the chiral liquid crystal, the pitch p, changes with temperature variations , in this case, producing a low energy shift of the stop band edge where the lasing is expected. The experimental evidence definitely proves that the distributed feedback mechanism, provided by the dielectric tensor modulation of the helixed liquid crystal, is behind the observed effect. The inset of Fig. 7(a) shows the quasi linear dependence of the lasing wavelength on temperature. The peculiar behavior around 60 °C is attributed to a sudden unwind of the helical superstructure, a discharge of the surplus elastic energy accumulated upon increasing the temperature.
A very interesting feature of the presented system is the possibility to control the stimulated emission intensity by applying an electric field (Fig. 7(b)). A sinusoidal voltage (1 kHz) was applied to the sample between the two glass-ITO plates. By maintaining fixed the pump energy and by varying the voltage from 0 up to 50VRMS a significant decrease of the lasing intensity was observed. In addition, above a given field threshold the micro-laser switches off. Taking into account that the BL088 chiral liquid crystal possesses a positive dielectric anisotropy, the applied electric field perpendicular to the helical axis produces a local distortion of the periodic structure. Liquid crystal molecules tend to align along the electric field producing modifications in the sinusoidal modulation of the refractive index which assumes a low efficiency rectangular profile [20–21]. The overall result is a diminishing of the DFB mechanism, therefore a decrease in the lasing intensity is observed.
By further increasing the field, the regions where the molecules are energetically favorably oriented (locally nematic) keep expanding until a certain threshold value is reached and the chiral helix completely unwinds and the structure becomes nematic. The critical electric field Ec for this transition is:
being K2 is the Frank’s twist elastic modulus, ea is the dielectric anisotropy and P0 is the unperturbed chiral pitch. The estimated critical field results several timed larger than the electric field applied at the presented system. Therefore, by working in a low field regime the slight helical distortion is sufficient to greatly reduce the optical feedback process.
A striking scenario is presented in Fig. 8 showing the spatial distribution of the laser emission emerging from the microcavity laser array. A high sensitivity and resolution (1390 × 1024 12bit PixelFlyQe by PCO) imaging CCD camera was employed in order to check the near-field modal profile for the stimulated emission. The images were acquired by scanning in the proximity of the output edge of our sample cell, in a direction perpendicular to the micro channels. The mapped intensities profile hereby obtained indicates that the maxima lasing intensities have a spatial recurrence. The periodicity of the organic micro-laser array is found to be about 5 μm. This value is in perfect correlation with the initial tailoring configuration (i.e. the distance between the polymeric walls). The LC microchannels act as miniaturized mirror-less cavity lasers, where the emitted laser light propagates along the liquid crystal helical axis behaving as Bragg resonator.
In addition, by considering a single channel emission intensity profile chart (Fig. 9) and transposing it into the two main directions, transverse (X) and longitudinal (Y), we obtained that each single resonant mode have an elliptic Gaussian beam profile. The calculated ellipticity is about 7.
A color tunable DFB microcavity laser was created by embedding dye doped helixed liquid crystals in holographically sculptured polymeric microchannels. The unique optical and electrical properties of chiral liquid crystals play a determining role in the mirror-less cavity limited by polymeric walls. The laser action was observed at the low energy edge of the bandgap where a sharp increase of the photonic density of states (DOS) is expected. In fact, the calculated DOS was found to diverge at the spectral position where lasing was observed and the emitted light was circularly polarized corroborating the idea that distributed feedback is behind the presented lasing effect. The investigation of the spatial distribution of the emitted light shows a laser array where each microchannel acts as an astigmatic resonant cavity producing an elliptic Gaussian beam. This level of integration might lead to new photonic chip architectures and devices, such as zero-threshold micro-lasers, phased array, discrete cavity solitons, filters and routers.
The authors are grateful to Prof. L. M. Blinov for the fruitful discussions. This work was partly supported by CEMIF.CAL Center of Excellence of Functional Nanostructured Materials of the Italian Ministry of University and Research (MIUR).
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