Patent Application: US-201113021221-A

Abstract:
a method of producing a lasing microsource of colloidal nanocrystals . the method includes the steps of preparing a nanocrystal solution in a solvent ; depositing at least a drop of the nanocrystals solution with a drop volume below 1 nl on a flat substrate ; and evaporating the solvent to dryness thereby to obtain at the edge of the evaporated drop a single annular stripe including a domain wherein the nanocrystals are arranged in an ordered array , wherein the ordered nanocrystals in the domain constitute an active region capable of lasing and the radially inner and outer edges of the stripe define a resonant cavity in which the active region is inserted .

Description:
colloidal core - shell cdse / cds nanorods were obtained by the seeded growth method , published by l . carbone et al . in nano letters , 2007 , 7 , 2942 - 2950 . the obtained nanorods , which are soluble in toluene in a concentration of 1 - 3 exhibit both high fluorescent quantum yields ( up to 70 %) and linear polarized emission . the emission wavelength of the nanorods is in the range of 600 - 630 nm , but can in principle be controlled over a spectral window from 570 nm to 650 nm . the nanorods were dissolved in toluene at a concentration of 10 − 6 m . the obtained solution was jet - deposited by means of a capillary with 20 μm diameter on a previously cleaned glass slide ( cleaning by 20 min ultrasonic in acetone and 20 min ultrasonic in propanol , and low dry by nitrogen ). the jet - deposition was performed using a femtojet ( eppendorf ) system combined with an inverted optical microscope . the capillary - glass slide distance was adjusted to some hundreds of microns , which corresponds to the drop diameter which is formed at the apex of the capillary . preferably , the drop should touch the surface when it forms at the capillary tip . since the volume of a picoliter drop is 4 / 3 pi r 3 = le - 12 , le - 4 is a preferred sample capillary distance , in agreement with what we observed in the experiment . typical jet deposition parameters were injection pressure at 1000 hpa for 1 second , with droplet volume in the picoliter range . injection pressure and time had to be adjusted for each deposition series , as they depended very critically on the glass - sample distance . the drops can also be deposited on the substrate by means of an ink jet printer . in the experiments , three samples of nanorods were used with a length , respectively , of 17 , and 50 nm and a diameter of 3 - 5 nm . each nanorod sample was nearly monodisperse in terms of distribution of rod length and diameter ( size distribution n the nanocrystal samples was smaller than 10 %). once deposited on the glassy substrate and upon evaporation of the solvent ( room temperature ), it was observed that the nanorods start to self - organize into large - scale ordered superstructures that are reminiscent of nematic / smectic liquid crystal phases . due to the peculiar solvent evaporation in fluid convection dynamics usually referred to as the “ coffee stain ” effect , the whole process did not lead to uniform thickness of the nanoparticle film , but to a well - defined stripe ( a single stripe ) at the drop edge ; this is due to the pinning of the contact line during solvent evaporation ( cf . fig2 ( a - b )). the film thickness at the border of the dried droplet , i . e . in the outermost coffee stain ring , was between 30 nm and 90 nm , while the thickness immediately inside this ring was considerably smaller (˜ 10 nm ). the optical microscope images of the film ( cf . e . g . fig2 ( c - f )) show stripes at the border of the film that were darker than the inner region , which is due to higher concentration of the nanorods and higher film thickness in those stripes . in addition , the stripes were confined by two relatively sharp and dark lines , one at the inner edge and one at the outer edge of the border region . according to the invention , the radial size of the stripes ( i . e . the distance between the two dark lines ) may range from 0 . 5 μm to 50 μm and is preferably comprised between 8 and 12 μm . the stripes appeared birefringent when imaged by polarized optical microscopy . the anisotropy in the refractive index in the stripes can be explained by ordered arrays of laterally aligned close - packed nanorods ; best lasing properties were found , when in said ordered array the main longitudinal axis of the nanorods are tangential to the circular annular stripe . according to the invention it has been found that the dense self - assembled coffee stain ring , consisting of ordered nanorods , can form fabry - perot microresonators , in which sections of the annular stripe with two parallel planes partially reflecting the light give rise to interference . the confocal image ( fig3 ( a )) provides deeper insight into the structure at the edges of the stripe ; two dark edges limiting the stripes are clearly visible in the reflectivity image of the stripe region . the corresponding signal profile plotted in fig3 ( b ) indicates a strong and abrupt change of the reflectivity at the edges of the stripe , which occurred on a length scale of less than 1 μm . this rapid change in reflectivity , and thus also in the refractive index , is most likely responsible for the confinement of light inside the stripes and consequently for the fabry - perot effect , that was observed also in the ase spectra . the fabry - perot effect was observed more frequently on stripes prepared from nanorods 17 nm and 25 nm long and only occasionally on coffee stain fringes formed from nanorods with a 50 nm length , whereby nanorods with a length of from 10 nm to 30 nm are preferred in the method of the invention . the shape of the self assembled laser cavity is determined by the shape of contact line that is formed when the nanocrystal solution is deposited on the substrate surface . in the case of no lateral alterations of the substrate this shape is circular and the result of the coffee stain evaporation is an annular ring . the shape of the coffee stain rings can be manipulated to a certain extent via modifications of the substrate surface or shape . it is important for the lasing functionality that the modifications do not alter significantly the microfluidic dynamics in the coffee stain evaporation process . since the contact line pinning is enhanced at the edges of a substrate , the shape of the coffee stain can be manipulated via the borders of the substrate , or via artificial edges ( holes in the substrate , large vertical offsets ). for example , linear sections of the coffee stain edge can be obtained in a simple way by depositing the nanocrystal solution near the linear edge of a , preferably glassy , substrate , as illustrated in fig6 . this could be the external edge of the substrate , or edges that are produced for example by rectangular holes . instead of holes , also high step - like features produced for example by reactive ion etching could be used . another possibility is the manipulation of the contact line via different surface hydrophobic / hydrophilic functionalization , for example via adequate molecules . in the experiments , the threshold for lasing with respect to the pump power was of the order of few hundreds μj / cm 2 ( best value 200 μj / cm 2 ). the optical experiments probing the lasing properties were performed using a ti : sapphire chirped pulse amplified source that delivers pulses at a central wavelength of 800 nm with a 1 khz repetition rate . excitation pulses at 400 nm were obtained by doubling the fundamental wavelength in a β - barium borate ( bbo ) crystal . spectrograph acton , grating 300 g / mm , centered at 610 nm , slit , 26 μm , resolution 1 . 1 nm , acquisition time 500 ms . with the core - shell nanorods used in the experiments , the spectral range of the laser emission can be tuned in the visible range by the nanorod core size and rod - shape in the range from 550 nm to 650 nm ; additional experiments showed also lasing from the shell states of the core - shell nanorods which extends the spectral range for the laser device of the invention into the blue to 490 nm . also included within the scope of the invention is a lasing microdevice obtainable by the hereinbefore described method . in the microdevice of the invention , the active region , which can emit photons at optical frequencies , is an ordered array of nanocrystal and particularly an ordered array of laterally aligned close - packed nanorods and the semireflective means defining the resonant optical structure in which the said active region is inserted comprise the radially inner and outer edges of an annular , circular , stripe formed by said nanocrystals and obtained by evaporating to dryness a circular drop of a nanocrystal solution . in the microdevice of the invention , the excitation system can comprise a pumping laser focusing on the sector of the annular stripe formed by said ordered array of nanocrystals or , alternatively , a pumping laser illuminating the entire area of said annular stripes . also included within the scope of the invention is a light emitting microdevice or lasing microsource comprising a plurality of said annular stripes on a flat substrate , wherein the said annular stripes are obtained by the method hereinbefore described . 1 . s . coe , w . k . woo , m . bawendi , v . bulovic , nature 2002 , 420 . 800 - 803 . 2 . m . kazes , d . y . lewis , y . ebenstein , t . mokari , u . banin , adv . mater . 2002 , 14 . 317 -+. 3 . h . j . eisler , v . c . sundar , m . g . bawendi , m . walsh , h . i . smith , v . klimov , appl . phys . lett . 2002 , 80 . 4614 - 4616 . 4 . v . i . klimov , a . a . mikhailovsky , d . w . mcbranch , c . a . leatherdale , m . g . bawendi , science 2000 , 287 . 1011 - 1013 . 5 . a . a . mikhailovsky , a . v . malko , j . a . hollingsworth , m . g . bawendi , v . i . klimov , appl . phys . lett . 2002 , 80 . 2380 - 2382 ; 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