Femtosaniye Lazer İle İnce Metal Filmler Üzerinde Nano Boyutta Yapılar Oluşturma

Abstract

Lazerlerin teknolojik gelişmelerde önemli bir rol üstlendiği şu dönemlerde, femtosaniye (fs) darbeli lazerler (1 fs s) üstün özellikleri ile kendine ayrı bir yer edinmiştir. Fs lazerler, darbe sürelerinin son derece kısa olması nedeniyle çok hassas ve yüksek çözünürlükte işleme imkanı vermektedir. Bu lazerlerin malzeme işlemedeki en büyük avantajı termal difüzyon sürelerinden çok daha kısa süreye sahip darbe genişliklerinin olmasıdır. Bu özelliği sayesinde kusursuz malzeme işleme imkanı verir. Bu tip lazerler özellikle cam üzerinde kırılma indisi değiştirerek dalga kılavuzu oluşturma, saydam malzemeler üzerinde işleme, hassas dokularda (göz gibi) en az zararlı kesme gibi çok büyük hassasiyet gerektiren uygulama alanlarında aktif olarak kullanılmaktadır. Bu çalışmada, doğrusal olmayan optik yöntemlerden de yararlanıldı. İkinci harmonik üretimi ve frekans toplanması yöntemleri kullanılmıştır. Fs lazerin harmoniklerinin (lazer frekansının 2, 3 ve 4 katının) elde edilmesi ile daha kısa dalgaboyları, dolayısıyla daha yüksek işleme çözünürlüğü elde edilmiştir. Bu harmonikler temel lazer bileşeninin BBO (Beta Baryum Borat) kristali içerisinden geçirilmesiyle elde edildi. İkinci, üçüncü ve dördüncü harmoniklerin maksimum verimleri sırasıyla %25, %2 ve %0.7 dolaylarında elde edildi. Bu verimler düşük gibi görünmesine rağmen, bu harmoniklerden elde edilen enerji malzeme üzerinde mikro ve nano boyutta işleme yapmak için yeterli seviyededir. Bilindiği üzere lazerler genel olarak Gauss hüzme profilleri üretirler. Bu tez çalışmalarında malzeme işlemedeki önemli katkılarının olacağı öngörüldüğü için mevcut hüzme aksikon mercek yardımı ile şekillendirilerek Bessel hüzmesi elde edildi. Bessel hüzmelerinin kırınımsız ilerleme özelliği nedeniyle deneysel çalışmalardan büyük avantaj sağlamıştır. Bessel hüzmenin odaklanma mesafesinin Gauss hüzmesine göre çok daha uzun olması, lazerin ilerleme yönündeki hizalama problemlerinin malzeme işleme üzerindeki olumsuz etkisini ortadan kaldırır. Cam üzerine ince altın film kaplanarak elde edilen numuneler üzerinde nanometre hassasiyetinde yapılar elde etmek için lazerin elde edilen bütün harmonikleri Bessel hüzmelerine dönüştürüldü. Bu hüzmelerin hepsi malzeme işlemede kullanıldı. Lazerin dalgaboyundan daha küçük bir boyuta odaklanamaması nedeniyle, malzemenin aşındırma eşiğine yakın çalışılarak 200 nm’nin altında yapılar elde edildi. Bu işleme tekniği ile malzeme üzerinde periyodik yapılar oluşturarak plazmonik yapıların elde edilebileceği gösterildi. Böylelikle, plazmonik yapıların üretiminde, litografik yöntemlere alternatif olabilecek etkin bir yöntem geliştirilmiştir.

In this period when lasers play an important role in technological developments, femtosecond laser (1 fs ) has obtained a significant place with its superior qualities. Since its pulse duration is extremely short, femtosecond laser enables processing in much finer and high resolutions. The major advantage of this laser in processing materials is that it has much shorter pulse duration than thermal diffusion length. Owing to this feature, it enables excellent processing. It can be assumed that there is no heat affected zone on material when femtosecond laser is used for micro machining. This type of laser is used actively in application areas, which require much greater precision such as creation of waveguide by changing the refractive index on glass, process on transparent materials and the least harmful transection on sensitive tissues (e.g. eye tissues). Because of fs laser’s non-contact nature, allows the micromachining and surface patterning of materials with minimal mechanical and thermal deformation. In addition to its advantages, fs-laser nano-processing also has restrictions. First of all, since the fluence levels should be close to the threshold, the pulse to pulse fluctuations in optical energy becomes critical and they determine the ultimate resolution. Moreover, when laser beams are tightly focused, due to diffraction, their depth of focus becomes very short (proportional to the square of the focal spot size). As a result, very fine control of sample positioning is required. Angular misalignments of the sample and mechanical vibrations can become critical. In this thesis, gold thin film is used for demonstration the thermal evaporation method. First of all this thin film is fabricated on microscope glass substrates. Thickness of the thin gold film is around 25-30 µm. As known, the gold film has low adsorption ability to glass. For that reason while all studies are being done, the sample is damaged. To prevent this impact, thin Chrome layer with 5 µm thickness is placed between glass and gold film. Motorized three dimensional stages are used to move the sample which is placed in front of the laser. For precise machining on the sample, piezo motors are used for obtaining positioning resolution in the nanometer range. Piezo motors are programmed by Labview for sequential movements. By this means, no delay occurs between movements of motors so no additional energy accumulates on any point of sample. The laser pulse energy is controlled using a waveplate and polarizer. In this thesis, nonlinear optical methods are also made use of. Nonlinear optics is the working field that arises from result of the optical properties’ modification of a material system that is exposed to light. There is no light has enough intensity to modify the optical properties of a material system except the laser light. I can be accepted that discovery of second-harmonic generation is the beginning of the field of nonlinear. In the nonlinear optic the meaning of “nonlinear” comes from the response of a material system to an applied optical field depends in a nonlinear manner upon the strength of the optical field. As an example of second harmonic generation occurs as a result of the part of the atomic response that depends on the strength of the applied optical field quadratically. The intensity of the light generated at the second harmonic frequency tends to increase as the square of the intensity of the applied laser light. Second harmonic generation and also sum frequency methods are used. With the obtainment of femtosecond laser harmonics (2, 3 and 4 times of laser frequencies), shorter wavelengths and thus higher process resolution are obtained. All these harmonics are obtained by passing a frequency through a BBO (Beta Barium Borate) crystal. Efficiencies of all harmonics are demonstrated for each energy level of fundamental one. For second harmonic, maximum efficiency is around %25. The maximum efficiency decreases to %2 for third harmonic and highly decreases to %0.7 for fourth harmonic. Although the efficiencies of all harmonics don’t seem very high, the energies obtained from these harmonics are adequate to machine the material. As is known, lasers generally generate Gaussian beam profiles. Bessel beam is obtained by structuring the existing beam with the help of axicon lens since it is predicted that it would have significant contributions in the processing of materials. Due to non-diffracting propagation feature, Bessel beams had a substantial advantage in the experimental studies. The diffraction-free nature of Bessel beams means that their intensity stays high over much longer distances (compared to Gaussian beams of the same spot size), and also that the spot size, full-width half maximum (FWHM), does not change by propagation. Most applications of Bessel beams in the literature rely on the first property. Since the plasmonic structures typically require metal layer thicknesses less than 100 nm, long depth of focus of Bessel beams may not seem relevant. However, since their focal spot sizes do not change by propagation, Bessel beams practically remove the alignment constraint in the laser propagation direction. Moreover, provided that a transparent material is used, axicons work just as well for UV wavelengths. As opposed to Gaussian beams, tightly focused Bessel beams are less prone to aberrations. In order to get nanometer precise structures on the samples obtained by plating thin gold film on glass, entire harmonics of the laser are converted to Bessel beams. All these beams are used in material processing. Because of the reason that the laser cannot focus on a smaller dimension than the wavelength, structures under 200 nm are obtained by being studied close to the ablation threshold of the material. The sample under laser illumination and ablate 100 µm stripes of gold. The pulse energy between stripes is gradually decreased, and observed that their width gets smaller. At the fundamental wavelength (1030 nm) even though nanometer resolution is easily got, most of the stripes had discontinuities. The minimum width is 580 nm. At the second harmonic wavelength of 515 nm the ablation quality is much better and almost all stripes are continuous. As the pulse energy is decreased, the minimum width of 193.2 nm is obtained. Material processing with femtosecond laser pulses fabricates nano and micro sized structures with extraordinary precision. Result of this feature thermal and shock effects are minimized. In this pulse duration regime, the ablation threshold is well determined and due to the involvement of nonlinear optical effects, structures even smaller than the wavelength of the laser are generated. Fabrication of nano sized periodic structures on thin metal film generates surface plasmons. Surface plasmons are guided waves at the interface between a metal and a dielectric layers. Free electrons of a metal’s conduction band can respond collectively to an electromagnetic disturbance, whether induced by incident light or by incident fast electrons. Result of this trapped waves between metal and dielectric layers are generated. Surface plasmons can take various forms, ranging from freely propagating electron density waves along metal surfaces to localized electron oscillations on metal nanoparticles. The localized electron osilation on metal nanoparticles are called “localized surface plasmons”. Plasmons’ proper features enable a wide range of practical applications, including light guiding and manipulation at the nano scale, biodetection at the single molecule level, enhanced optical transmission through sub-wavelength apertures, and high resolution optical imaging below the diffraction limit. One of the most commonly exploited effects of surface plasmons is the spectral shift in their plasmonic resonances. This effect is foundation of most widely used sensor in nano plasmonic. Surface plasmons are used for detecting some molecules or proteins etc. in the biomedical applications. With this process technique, it is demonstrated that plasmonic structures can be obtained by generating periodic structures on the material. Therefore, in the production of plasmonic structures, an efficient method, which can be an alternative to lithography, is developed. The ability of generating plasmonic structures directly through laser ablation would be advantageous especially for rapid fabrication, since this method does not require lithographical masks and chemical processing. Results of this study can be improved by using laser’s third and fourth harmonics effectively. For this purpose another material can be found instead of microscope glass. Because of that when studying with laser’s third and fourth harmonics, most of laser’s energy is absorbed by this substrate and thin gold film burns and damages. Another thing is that more uniform thin gold film can be fabricated for machining on. Using the uniform samples can increase the quality of machining. In conclusion, it is shown that high resolution fs laser material processing combined with the advantages of Bessel beams opens the way to enhanced nano processing of metal thin films. These results show that Bessel beams can effectively be used for rapid fabrication of plasmonic structures.