Abstract:
A process for fabricating a device capable of random lasing comprising a substrate and a rare earth-doped glass fabricated on the substrate in the form of a waveguide, wherein the glass comprises a germanium glass, a titanium glass or a chalcogenide glass, where the process comprises ablating a target glass with incident radiation from an ultrafast laser in the presence of the substrate to deposit a quantity of the target glass on the substrate and applying rastering to ablate the target glass uniformly. The ultrafast laser emits pulses of 15 ps or less and the relative position of the laser spot on the target glass with respect to the substrate is constant during the ablation and wherein the Gaussian intensity profile of the laser beam has a spot area less than 3000 μm 2 .

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 13/395,310 filed Mar. 9, 2012, which is the National Stage of International Patent Application No. PCT/GB2010/0051514, filed on Sep. 10, 2010, which claims priority to and all the advantages of Great Britain Patent Application No. GB0915944.3, filed on Sep. 10, 2009 and Great Britain Patent Application No. GB0919472.1, filed on Nov. 6, 2009, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a device capable of random lasing which comprises a rare-earth-doped glass waveguide, to a process for fabricating the device and to a laser comprising the device 
       BACKGROUND 
       [0003]    Lasers that rely on multiple scattering processes in a random scattering medium for optical feedback are known as random lasers. They differ from a conventional laser which relies on a resonant optical feedback by reflectors or mirrors. 
         [0004]    Random lasing has been the subject of enormous interest to researchers in the area of solid state physics and laser physics. Initial experiments were designed to prove the principles of random lasing and not to identify an efficient laser source for applications. A disordered amplifying medium was created using a fine powder of neodymium doped crystal and random lasing was demonstrated at liquid nitrogen temperature (Markushev, V. M., Zolin, V. F. &amp; Briskina, Ch. M. Powder laser. Zh. Prikl. Spektrosk. 45, 847-850 (1986)). Lasing was demonstrated in a liquid dye gain medium with dispersed scatterers (Martorell, J., Balachandran, R. M. &amp; Lawandy, N. M.  Radiative coupling between photonic paint layers . Opt. Lett. 21, 239-241 (1996)). In these experiments, a distinct narrowing of the emission spectrum was observed above a certain threshold which was attributed to stimulated emission in a random scattering medium leading to amplification that can be described using Letokhov&#39;s model (Letokhov, V. S.  Generation of light by a scattering medium with negative resonance absorption . Zh. Eksp. Teor. Fiz. 53, 1442-1447 (1967); Soy. Phys. JETP 26, 835-840 (1968)). The discrete modes of the laser that would have been present in a random scattering medium where light can be localised was first observed by Cao and co-workers (Cao, H. et al.  Random laser action in semiconductor powder . Phys. Rev. Lett. 82, 2278-2281 (1999)). The strong scattering and high gain was achieved in a ZnO nanopowder film of thickness in the range 6-10 μm and coherent lasing was observed with angular dependence when pumped using a mode-locked Nd:YAG laser. The lasing threshold was 763 kW/cm 2 . It has been proposed that etching glass or semiconductor crystal can produce very strong scattering media for random lasing (Schuurmans, F. J. P., Vanmaekelbergh, D., van de Lagemaat, J. &amp; Lagendijk, A.  Strongly photonic macroporous GaP networks . Science 284, 141-143 (1999) and D. S. Wiersma and S. Cavalieri, “ Light emission: A temperature - tunable random laser ” Nature 414(6865), 708-709 (2001)). 
         [0005]    The media which have been used to demonstrate random lasing have been generally 3-dimensional and require a very high pump threshold to initiate lasing. The media were mostly complex, non-portable, unstable and the distribution of scatterers was non-reproducible. Modes were difficult to preserve. 
         [0006]    There has been interest in theoretical studies of 2D structures for random lasers (Apalkov, V. M., Raikh, M. E. &amp; Shapiro, B.  Random resonators and prelocalized modes in disordered dielectric films . Phys. Rev. Lett. 89, 016802 (2002)). The use of a random laser deploying a Tm 3+  ion doped glass powder film claims to have achieved a threshold of several kilowatts per square centimetres (H. Fujiwara, and K. Sasaki, “ Observation of upconversion lasing within a thulium - ion - doped glass powder film containing titanium dioxide particles ” Jpn. J. Appl. Phys. 43(No. 10B), L1337-L1339 (2004)). The use of random lasers for practical applications as a coherent source has been limited because of their high threshold, low reliability and fabrication difficulties. 
         [0007]    U.S. Pat. No. 3,573,653, U.S. Pat. No. 3,579,142, U.S. Pat. No. 3,787,234, U.S. Pat. No. 6,574,249 and U.S. Pat. No. 3,747,021 disclose thin film lasers which are pumped electrically or optically. U.S. Pat. No. 5,306,385 discloses photoluminescent crystalline doped-CaF 2  thin films on a silicon substrate. U.S. Pat. No. 5,783,319 discloses a tunable dye laser comprising an organically doped-silica glass thin film on a silicon substrate. U.S. Pat. No. 6,656,588 discloses a process for preparing a doped nanocrystalline metal oxide powder in the form of a thin film which exhibits lasing. 
       SUMMARY 
       [0008]    The present invention is based on the recognition that by confining electromagnetic radiation in a non 3-dimensional doped-glass structure it is possible to achieve improvements in random lasing. In particular, the present invention relates to a device capable of random lasing which exploits a certain rare earth-doped glass waveguide. 
         [0009]    Thus viewed from one aspect the present invention provides a device capable of random lasing comprising:
       a substrate; and   a rare earth-doped glass fabricated on the substrate in the form of a waveguide, wherein the glass comprises a germanium glass, a titanium glass or a chalcogenide glass.       
 
         [0012]    The device of the present invention is robust and reproducible and advantageously does not require Bragg or conventional mirrors making it straightforwardly integratable with other light emitting devices based on semiconductors and polymers. It exhibits excellent mode stability and is tunable over a surprisingly broad wavelength range. 
         [0013]    The device may be optically pumpable. The device may be electrically pumpable. Typically, the device is optically pumped. 
         [0014]    When the device is optically pumped, the device may begin laser emission at a certain power output from the pump laser. The minimum power required from the pump laser which causes laser emission in the device may be known as the threshold pump power. This may also be known simply as the ‘threshold’. The threshold may also be expressed as a threshold pump power per unit of area of the incident area of the pump laser beam on the device (e.g. Watts per mm 2 ) This may also be known as a threshold power density. For example with a threshold pump laser power of 37 mW and a pump laser beam of incident area of approximately 1.45 mm 2 , the threshold pump power density is approximately 26 mW/mm 2 . 
         [0015]    The device may be capable of random lasing at a threshold of 1 mW or less to produce coherent output from an area less than 2 mm 2 . This is equivalent to a threshold power density of 0.5 mW/mm 2 . 
         [0016]    The threshold power density of the device may be less than 200 mW/mm 2 . Preferably the threshold power density of the device is less than 100 mW/mm 2 , particularly preferably less than 50 mW/mm 2 , more preferably less than 26 mW/mm 2 . 
         [0017]    The threshold of the device may be less than 1 mW. Preferably the threshold of the device is less than 500 μW, particularly preferably less than 300 μW, more preferably less than 200 μW, yet more preferably less than 100 μW, even more preferably less than 50 μW. 
         [0018]    The wavelength of the laser emission from the device may be dependent on the angle of incidence of the pump laser beam. The wavelength of the laser emission from the device may be tunable. The wavelength of the laser emission from the device may be tunable by varying the angle of incidence of the pump laser beam. 
         [0019]    The wavelength of the laser emission from the device may be tuneable in the range from 1510 nm to 1620 nm. 
         [0020]    The threshold pump power for the device may be dependent on the angle of incidence of the pump laser beam. The threshold pump power for the device may be tunable by varying the angle of incidence of the pump laser beam. 
         [0021]    The threshold of the device may vary in the range between 300 μW to 20 μW as the angle of incidence of the pump laser is varied between 39° and 51°. 
         [0022]    Reducing the threshold or threshold power density reduces the thermal load on the device. A reduced thermal load makes the device more practically applicable because the need for heat dissipation is reduced. 
         [0023]    Typically the glass is non-crystalline. Typically the glass is substantially without grain boundaries. 
         [0024]    Preferably the glass is capable of multiple scattering. The glass is typically dielectric. 
         [0025]    Preferably the glass is porous, particularly preferably nanoporous. A nanoporous glass promotes multiple scattering and facilitates the fabrication of the waveguide. 
         [0026]    Preferably the glass is a chalcogenide glass. Particularly preferably the chalcogenide glass comprises sulphur, selenium, tellurium or a mixture thereof. 
         [0027]    Preferably the glass is a germanium glass or a tellurium glass, particularly preferably a tellurium glass. 
         [0028]    The glass may comprise (or consist essentially of) germanium dioxide (GeO 2 ). 
         [0029]    GeO 2  may be present in the rare earth-doped glass in an amount over 30 mol %, preferably an amount in the range 60-90 mol %, particularly preferably 75-85 mol %. 
         [0030]    The glass may comprise (or consist essentially of) tellurium dioxide (TeO 2 ). TeO 2  is also known as tellurite. 
         [0031]    TeO 2  may be present in the rare earth-doped glass in an amount over 30 mol %, preferably an amount in the range 60-90 mol %, particularly preferably 75-85 mol %. 
         [0032]    The glass may further comprise one or more network modifiers. The glass may comprise four network modifiers, preferably three network modifiers, particularly preferably two network modifiers. The network modifier may open the network of the glass. 
         [0033]    The network modifier may be a network modifying ion. The network modifier may be a metal compound. The metal compound may provide a network modifying ion. The network modifier may be a metal fluoride or oxide. Each network modifier may be present in an amount of 30 mol % or less, preferably 20 mol % or less, particularly preferably 10 mol % or less. Each network modifier may be present in an amount of up to 30 mol %, preferably up to 20 mol %, particularly preferably up to 10 mol %. 
         [0034]    The network modifier may be an oxide of at least one of Ba, Bi, Pb, Zn, Al, Ga, La, Nb, Wo, Ta, Zr, Ti, V and mixtures thereof. 
         [0035]    Preferably the network modifier is selected from the group consisting of BaO, Bi 2 O 3 , PbO, ZnO, Ga 2 O 3 , Al 2 O 3 , La 2 O 3 , Nb 2 O 5 , WO 3 , Ta 2 O 5 , ZrO 2 , TiO 2 , V 2 O 5  and mixtures thereof. The network modifier may be present in an amount of 30 mol % or less, preferably 20 mol % or less, particularly preferably 10 mol % or less. 
         [0036]    Preferably the glass may further comprise one or more of MgO, CaO, SrO, BaO, ZnO, PbO and mixtures thereof. The one or more of MgO, CaO, SrO, BaO, ZnO, PbO and mixtures thereof may be present in an amount of 30 mol % or less, preferably 20 mol % or less, particularly preferably 10 mol % or less. These metal oxides may also be network modifiers. 
         [0037]    Preferably the glass further comprises one or more alkali metal oxides. Particularly preferably the glass further comprises one or more of Li 2 O, Na 2 O and K 2 O and mixtures thereof. The one or more alkali metal oxides may be present in an amount of 25 mol % or less, preferably 20 mol % or less, particularly preferably 10 mol % or less. For example, the alkali metal oxide may be present in an amount of approximately 9 mol %. The alkali metal oxide may be a network modifier. 
         [0038]    Preferably the glass further comprises one or more metal halides, particularly preferably an alkali metal halide. The metal halide may be a network modifier. 
         [0039]    The one or more metal halides may be selected from the group consisting of BaCl 2 , PbCl 2 , PbF 3 , LaF 3 , ZnF 2 , BaF 2 , NaCl, NaF, LiF and mixtures thereof. The one or more metal halides may be present in an amount of 20 mol % or less. 
         [0040]    The glass may further comprise an alkali metal or alkaline earth metal phosphate. 
         [0041]    The glass may further comprise an enhancing compound comprising phosphorous or boron. The enhancing compound may be an oxide of phosphorous or boron. Preferably the enhancing compound comprises P 2 O 5  or B 2 O 3  or a mixture thereof. The enhancing compound may enhance the refractive index of the glass. The enhancing compound may cause random light scattering. 
         [0042]    The glass may be doped with a lanthanide. The glass may be doped with a lanthanide oxide. 
         [0043]    The glass may be doped with at least one of erbium, ytterbium, neodymium, praseodymium, holmium, cerium, yttrium, samarium, europium, gadolinium, terbium, dysprosium or lutetium. 
         [0044]    The glass may be doped with one or more lanthanide ions. 
         [0045]    Preferably the glass is doped with one or more ions selected from the group consisting of Nd 3+ , Yb 3+ , Er 3+ , Tm 3+ , Pr 3+ , Ho 3+ , Sm 3+ , Eu 3+ , Tb 3+  and Ce 3+ . 
         [0046]    Preferably the glass is doped with Tm 3+  or Er 3+ , particularly preferably Er 3+ . 
         [0047]    The rare-earth dopant is typically present in an amount in the range 0.01 to 5 mol %, preferably 0.5 to 2 mol % (eg about 1 mol %). 
         [0048]    The wavelengths at which the device is capable of random lasing may be selectable. The wavelengths may be selectable by selecting a particular rare-earth dopant or mixture of rare-earth dopants. By selecting a particular rare-earth dopant or mixture of rare-earth dopants and using an appropriate excitation/pumping scheme, the wavelengths at which the device is capable of random lasing may be selected from a range of 600 nm to 5000 nm. 
         [0049]    The glass may have an emission peak in the range 1450 to 1650 nm (eg at about 1550 nm). 
         [0050]    The glass may have an emission cross-section greater than 1×10 −21  cm 2  at a wavelength of 1535 nm. 
         [0051]    The spectral full-width half maximum exhibited by the glass is typically 50 nm or more. 
         [0052]    The glass may be a high refractive index glass. The glass, when in the form of a film, may have an effective refractive index of 1.5 or more. The effective refractive index at a thickness of 770 nm and a wavelength of 633 nm is typically about 1.55. 
         [0053]    The waveguide may be a fiber, channel, planar or slab waveguide. Preferred is a planar waveguide. 
         [0054]    The waveguide is typically in the form of a thin film. The thin film may have a thickness less than 10,000 nm (10 μm), preferably less that 5,000 nm (5 μm), preferably less than 1000 nm, preferably less than 500 nm Preferably the thin film has a thickness in the range 115 to 777 nm. 
         [0055]    The substrate may be a silicon based substrate. The substrate may be a polymeric substrate. The substrate may comprise a polymer. The substrate may comprise an electroluminescing device. The substrate may comprise a photoluminescing device. 
         [0056]    Preferably the substrate is a silicon-based substrate. The silicon-based substrate may be or include silicon, silica glass, silicon oxide or silicon hydride. The silicon-based substrate may be a semiconductor. 
         [0057]    The effective refractive index of the silicon-based substrate may be about 1.455 at 633 nm. 
         [0058]    The device may be capable of random lasing by electrical or optical excitation. The device may be capable of random lasing by being optically excited by a 980 nm continuous wave laser diode at a threshold below 50 μW. 
         [0059]    The device may be capable of random lasing at a wavelength in the UV, visible, near-IR or mid-IR spectrum. 
         [0060]    The device may be obtainable by ablating a target glass with incident radiation from an ultrafast laser in the presence of a silicon-based substrate. 
         [0061]    Of independent patentable significance is the recognition that at certain excitation geometries, it is possible to achieve different lasing wavelengths from the device of the invention. 
         [0062]    Viewed from a further aspect the present invention provides a laser assembly comprising:
       a device as hereinbefore defined; and   an exciter downstream from the device and capable of exciting the device into a laser output.       
 
         [0065]    The exciter may be a focussed exciter. 
         [0066]    The assembly surprisingly achieves lasing at different wavelengths across the entire spontaneous emission line width (eg 1512.75 to 1612.15 nm for erbium) without mirrors simply by varying the relative disposition of the exciter and device. 
         [0067]    Preferably the laser assembly is a tunable laser assembly. The tunable laser assembly may be tunable over a range of up to 100 nm. 
         [0068]    Preferably the laser assembly further comprises a detector upstream from and capable of measuring the laser output from the device. 
         [0069]    Preferably the laser assembly further comprises a collector upstream from and capable of collecting the laser output from the device. 
         [0070]    Preferably the angular disposition of the exciter and device is adjustable. This permits the tunability of the laser assembly to be exploited. 
         [0071]    Preferably the exciter is a source of electromagnetic radiation. For example, the exciter may be a diode laser or light emitting diode (LED or SLED). The exciter may be a semiconductor laser. For example, the exciter may be a vertical cavity surface emitting laser (VCSEL). The exciter may be a continuous wave laser. 
         [0072]    The exciter may be a pump laser. The pump laser may have an output wavelength of 980 nm. 
         [0073]    Viewed from a yet further aspect the present invention provides a process for fabricating a device as hereinbefore defined comprising:
       ablating a target glass with incident radiation from an ultrafast laser in the presence of a substrate whereby to deposit a quantity of the target glass on the substrate.       
 
         [0075]    The substrate may be a silicon based substrate. The substrate may be a polymeric substrate. The substrate may comprise a polymer. The substrate may comprise an electroluminescing device. The substrate may comprise a photoluminescing device. 
         [0076]    Preferably the substrate is a silicon-based substrate. The silicon-based substrate may be or include silicon, silica glass, silicon oxide or silicon hydride. The silicon-based substrate may be a semiconductor. 
         [0077]    Preferably the target glass is mounted on a rotational platform. 
         [0078]    The substrate may be spaced apart from the target glass (eg by a distance of about 70 mm) The substrate may be heated (eg to a temperature of 600° C. or more). 
         [0079]    The incident radiation may be incident on the target glass at an angle in the range 40 to 80° (eg about 60°). 
         [0080]    The ultrafast laser may be a pulsed laser. Preferably the ultrafast laser is capable of emitting ultrashort pulses which are target glass ablative. The ultrafast laser may be a femtosecond or picosecond laser. Preferably the ultrafast laser is a femtosecond laser. In the process of the invention, the ultrafast laser may emit pulses of 15 ps or less (eg pulses in the range 5 fs to 15 ps). Preferably in the process of the invention the ultrafast laser emits pulses of 150 fs or less, preferably in the range 50 to 150 fs, particularly preferably about 100 fs. The pulses may be emitted with a repetition rate in the range 1 Hz to 100 MHz, preferably 1 kHz to 20 MHz, preferably 1 kHz to 1 MHz, preferably 1 kHz to 200 kHz. 
         [0081]    The ultrafast laser may be mode-locked. The average power of the ultrafast laser may be 80 W or less. The wavelength is typically about 800 nm. 
         [0082]    The ultrafast laser may be (for example) a Ti-sapphire laser, a diode pumped laser such as a Yb-doped or Cr-doped crystal laser or a fibre laser. 
         [0083]    The laser may be an excimer laser or an exciplex laser. 
         [0084]    The pulse energy is typically in the range 1 μJ to 100 mJ, preferably 10 μJ to 100 mJ (eg in the range 50 to 300 μJ). Pulse energy may be selectively adjusted using an attenuator. 
         [0085]    The process is typically carried out in a vacuum chamber. The process may be carried out at reduced pressure (eg in the presence of oxygen at a partial pressure of about 70 mTorr). The duration of the process may be 30 minutes or more. 
         [0086]    The device according to the invention may be used in communication, computer or display technology and in laser assemblies. The device according to the invention may be used in integrated optics (eg as a signal source), chemical sensing, environmental sensing, bio-sensing, micro-nano spectroscopy, optical communication, micro fluidic devices, opto-fluidic devices, terahertz amplifiers, lab-on-chip or optical tomography. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0087]    An embodiment of the invention will now be described in detail and by way of example only with reference to the accompanying drawings in which: 
           [0088]      FIG. 1  shows the optical transmittance spectrum (under normal incidence) recorded using UV-VIS-NIR wavelength region of a film in a device according to an embodiment of the present invention; 
           [0089]      FIG. 2   a  shows an optical micrograph of the film of  FIG. 1  at a magnification of 20×; 
           [0090]      FIG. 2   b  shows an optical micrograph of the film of  FIG. 1  at a magnification of 50×; 
           [0091]      FIG. 2   c  shows an optical micrograph of the inside of the film of  FIG. 1  at a magnification of 50×; 
           [0092]      FIG. 3  shows the fluorescence spectrum of the film in comparison to that of the glass target; 
           [0093]      FIG. 4  shows the absorption and emission of photons by the erbium ions which provide the erbium laser mechanism; 
           [0094]      FIG. 5   a  shows how the fluorescence of the film was monitored at different angles of collection θ by varying the angle of incidence of the pump laser by rotating the film clockwise; 
           [0095]      FIG. 5   b  shows the resulting output spectrum at the different angular positions defined in  FIG. 5   a;    
           [0096]      FIG. 6   a  shows the observed longest wavelength spectrum corresponding to an angle of 39° (error of)+/−0.5°); 
           [0097]      FIG. 6   b  shows lasing was also observed at wavelengths well below 1534 nm where the absorption cross-section is greater than the emission cross-section; 
           [0098]      FIG. 6   c  shows that lasing was observed at wavelengths as low as 1515 nm; 
           [0099]      FIG. 7  shows the spectrum of the laser at two different pump powers; 
           [0100]      FIG. 8  shows the integrated output intensity vs incident pump power; 
           [0101]      FIG. 9  shows an AFM image of a section of the surface of a sample of the glass film; 
           [0102]      FIG. 10  shows the laser emission output from a sample of the glass film; and 
           [0103]      FIG. 11  shows the threshold power varying with different angles of θ. 
       
    
    
     DETAILED DESCRIPTION 
     Example 
     Fabrication of a Nanoporous Tellurite Glass Film Doped with Erbium Ions on a Silica Glass Substrate by Pulsed Laser Deposition 
       [0104]    A target tellurite glass was prepared by batch melting and quenching TeO 2 , ZnO and Na 2 O at a relative molar composition of 80 mol % TeO 2  10 mol % ZnO and 10 mol % Na 2 O. The glass was doped with 1 mol % erbium oxide. The resulting target tellurite glass had the composition: 80 mol % TeO 2 , 10 mol % ZnO, 9 mol % Na 2 O and 1 mol % erbium oxide. The tellurite glass has a very high refractive index (&gt;2 at 633 nm) providing strong scattering. 
         [0105]    A silica glass substrate was placed at a distance of 70 mm above the target tellurite glass in a chamber. The chamber was pumped down to below 10 −6  Torr and oxygen atmosphere was maintained at a pressure of 70 mTorr. The substrate heater temperature was set at 700° C. and held at this temperature during deposition. 
         [0106]    Thin films of glass were prepared by an ablation process using a femtosecond laser. The laser with a pulse duration of 100 fs was focussed at an angle of incidence of 60° at the target tellurite glass. The energy of the laser with a repetition rate of 500 Hz was 52 μJ. 
         [0107]    Laser ablation was carried out for 3 hours. The target was rotated at a speed of 40 rpm and a proper rastering was applied to ablate the target uniformly while the substrate was at a speed of 20 rpm. The substrate was cooled down to room temperature rapidly after deposition. 
         [0108]    An empirical approach was deployed to optimize the process parameters to optimize the film quality. This was successful even though the relative position of the laser spot on the target with respect to the substrate was constant during the ablation. There was a thickness modulation that is attributable to the density profile of the laser plasma. 
       Properties 
       [0109]    The erbium-doped tellurite glass film fabricated on a silica glass substrate by the femtosecond laser ablation process described above was examined by different methods. 
         [0110]      FIG. 1  shows the optical transmittance spectrum (under normal incidence) of the film recorded using UV-VIS-NIR wavelength region. The film showed very high transmittance (&gt;98%) over the wavelength region and a distinct interference pattern indicative of good uniformity. 
         [0111]    Although the results of only a single film are presented below, lasing was observed in films having lower thicknesses and different porosities. The optical properties of the film were investigated using spectroscopic ellipsometry and showed some interesting characteristics. The thickness at the middle of a sample having dimensions 3×2 cm 2  obtained from the ellipsometry data was 777 nm and thickness variation was about 150 nm/cm across the film. The thickness variation is attributed to the fact that the relative position of the laser ablation spot and the substrate was constant during deposition and the laser beam has a Gaussian intensity profile with a spot area less than 3000 μm 2 . The surface roughness of the film was in the range 17 to 23 nm over the surface where the lasing experiments was performed. This roughness of the film resulted in a lower effective refractive index than that of the glass target used to prepare the film. At 633 nm the refractive index of the glass measured using spectroscopic ellipsometry was 1.935 whilst that of the film was 1.549 at a region where the surface roughness was 22.69 nm and 1.552 at a lower roughness of 17.16 nm. The results of the spectroscopic ellipsometry were a clear indication of a nanoporous film. 
         [0112]    The optical micrograph of the films shown in  FIG. 2  revealed no cracking or peeling. However the roughness of the surface is clearly evident. In order to further understand the film morphology, atomic force microscopy (AFM) was used.  FIG. 9  shows an AFM image of a section of the surface of a glass film. The surface roughness of the glass film in the nanoscale can be seen clearly from  FIG. 9 . The dimension of the glass strands formed on the substrate showed the random distribution. 
       Laser Performance 
       [0113]    The film was excited using a focussed beam of a fibre pigtailed laser diode with an output peaked at 980 nm and the fluorescence was collected transverse to the film and detected using a liquid nitrogen cooled photomultiplier tube attached to an Edinburgh instruments&#39; (UK) spectrofluorimeter. 
         [0114]      FIG. 3  shows the fluorescence spectrum of the film  10  in comparison to that of the bulk tellurite glass target  20 . 
         [0115]    The absorption and emission of photons by the erbium ions occurs by the process shown in  FIG. 4 . The pump laser at around 980 nm excites the ions to the  4 I 11/2  level and some ions relaxes non-radiatively to  4 I 13/2 . The erbium ions in this excited state decay radiatively to the ground state  4 I 15/2  by emitting photons in the wavelength band peaked around 1535 nm. This particular emission has been exploited successfully in optical fibre and waveguide amplifiers and is the core to the success of broadband fibre optic communication systems. 
         [0116]    The spectral width of the fluorescence signal for the film has been narrowed compared to that of the target tellurite glass. The FWHM for the glass is 72.6 nm while that of the film is only 31 nm. This narrowing of the spectrum is attributed to the amplification of spontaneous emission through scattering and stimulated emission in a random medium. 
         [0117]    The observed spectrum of the film is direct evidence of the model that predicts that the emission spectrum narrows down above threshold with a maximum intensity at the wavelength of maximum gain. This is manifested in the shifting of the peak of the spectrum towards longer wavelengths and also in the longer fluorescence decay lifetime of 11.9 ms observed for the film while the target was only 3.8 ms. 
         [0118]    Since the spectral narrowing and longer lifetime clearly pointed to random lasing with localisation, the angle of incidence (corresponding to θ) of the pump laser  30  was varied by rotating the film sample  40  clockwise and the fluorescence was monitored at different angles of collection θ by the detector  50  (see  FIG. 5   a ). 
         [0119]    Lasing was observed at an angle of collection around 39° at the longer wavelength tail of the erbium gain band. Discrete modes were present in the spectrum with the longest wavelength lasing mode at 1615.15 nm. The laser peaks were then systematically recorded by varying the angle at a constant incident pump power of 37.39 mW and the resulting output spectrum at different angles shown in  FIG. 5   b.    
         [0120]    The observed longest wavelength spectrum corresponding to an angle of 39° (error of +/−0.5° is shown enlarged in  FIG. 6   a . The laser emission band spanned the wavelength range of 1604.3 to 1615.15 nm. The resolution of the spectrometer was 0.3 and peaks with widths narrower than this were not resolved. The peak laser wavelength was shifted towards the lower wavelengths in the gain spectrum of the erbium ions when the angle of collection was increased. Lasing with narrow spikes in the spectrum predicted in random lasers were clearly observed in the entire gain spectrum of the erbium ions and lasing was also surprisingly observed at wavelengths well below 1534 nm where the absorption cross-section is greater than emission cross-section (see  FIG. 6   b ). Lasing was observed at wavelengths as low as 1515 nm (see  FIG. 6   c ). 
         [0121]      FIG. 10  shows the integrated laser intensity for the wavelength range 1610-1625 nm plotted against incident pump power. The threshold of the laser is approximately 290 μW but varies at different positions on the film and for different lasing wavelengths, nevertheless the threshold is in the sub-mW range, for wavelengths above 1520 nm, and is the lowest recorded for random lasers. 
         [0122]      FIG. 11  shows the threshold power varying with different angles of θ. The threshold power for different values of θ is tabulated in Table 1 below: 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 variation of threshold power with different angles of θ 
               
             
          
           
               
                   
                 θ° 
                 Threshold Power (μW) 
               
               
                   
                   
               
             
          
           
               
                   
                 39 
                 290 
               
               
                   
                 41 
                 65 
               
               
                   
                 43 
                 75 
               
               
                   
                 45 
                 25 
               
               
                   
                 47 
                 20 
               
               
                   
                 49 
                 50 
               
               
                   
                 51 
                 100 
               
               
                   
                   
               
             
          
         
       
     
         [0123]    This is a demonstration of amplification by localization ie a spontaneously emitted photon being amplified by the localization in a random media.  FIG. 6  also shows the broad amplified spontaneous emission spectrum. This is the lowest wavelength lasing ever recorded in an erbium doped system and definitive proof of light localization in the porous glass film. 
         [0124]    The spot size of the laser at the film when it was oriented at a right angle to the incident laser beam was 1.36 mm That means it is lasing from an area approximately 1.45 mm 2  or a volume of 1.13×10 −3  mm 3 . The shifting of the spectrum towards shorter wavelengths on decreasing the angle is attributed to the shortening of the photon mean free path of the photons that allow only shorter wavelengths to acquire the threshold to lase. 
       Dependence of the Laser Spectrum and Integrated Intensity on the Incident Laser Power 
       [0125]    The angle of collection was fixed at 48° and lasing was observed even below the threshold of the laser diode used in the experiment due to the spontaneous emission noise. The power was ˜15 μW. 
         [0126]      FIG. 7  shows the spectrum of the laser at two different pump powers. Excellent mode stability is evident from  FIG. 7  by comparing the peaks observed at two different powers well above the laser threshold. This is another characteristic of a random laser. 
         [0127]    The threshold power (&lt;26 mW/mm 2 ) is the lowest ever recorded for an erbium laser and for a random laser. The integrated output intensity vs incident pump power is shown in  FIG. 8 .