Construction And Application Of Nonlinear Optical Absorption Device In Femtosecond Time Domain

Optical absorption is one of the fundamental processes in light-matter interaction.Usually,the linear optical absorption takes place in natural situation,where the intensity of acting light is weak and the extinction-coefficient is a constant.However,when the action light such as laser is strong enough,the absorption coefficient will change with the light intensity,and the so-called nonlinear optical absorption occurs in this situation.The measurements of nonlinear optical absorption are of significance for characterizing optical materials and devices.It has been demonstated that the femtosecond Z-scan and transient absorption spectroscopy are effective methods in the study of nonlinear optical absorption properties of materials,for instance,to obtain the nonlinear optical absorption coefficient,nonlinear refractive index,the contribution from excited state absorption or ground state bleaching,etc.Considering the high cost of commercial ultrafast spectrometer and software,we designed and constructed the femtosecond Z-scan and transient absorption systems in this work,in which a tunable femtosecond optical parametric amplifier and TOPAS were used as the femtosecond laser sources.This thesis consists of the fundamental theories of nonlinear optical absorption,the mechanical,optical and electrical elements,the setup schemes of Z-scan and transient absorption systems,the programming of control software,the optimization of all involved parameters and the exampling the application of these two systems.1,Firstly,we introduced the Z-scan technology.Then built and debugged emtosecond Z-scan device combined with scientific research requirements.Optical path design and control software design were the two main parts of the construction of the femtosecond Z-scan device.?1?Optical path design:The optical path of the femtosecond Z-scanning device was mainly composed of a 400 mm lens,an electronically controlled translation stage,an aperture and a photo-detector.The beam was focused on the sample by lens and the electronically controlled translation stage to control the movement of the sample near its focus.The nonlinear characteristic curve of the sample was obtained by reading the changing of the beam energy after passing the sample nearby the focus by the energy meter.?2?The core of control program was designed for collection and processing data.The realization and innovation of the control software for data acquisition and processing ware as follows:?1?we took the average value of the three pulses when the each step of the translation stage happened.This is a effective method to reduce the system noise and improve the signal to noise ratio.?2?Then the average for the multiple single-pass scans?+Z direction?was taken.Such as in the waveform graph showed the average energy value after multiple single-pass scans with the position of the sample in real time.Those results not only reduced the noisy of the system,but also made the relevant researchers to notice the average energy changes with sample position.?3?Under normal circumstances,the one-way Z-scan took about 4 minutes and the data was automatically saved after the measurement is completed.This greatly reduced the time of scientific research personnel and improved the related research efficiency.We also tested and verified the device by using the standard sample CS₂,which showed this system was simple in device and had higher automation,also revealed this system can be used to test the nonlinear light absorption and refraction of materials.2,Secondly,the ultrafast spectroscopy device was built and debugged combine with the principle of ultrafast spectroscopy and scientific research requirements,.The construction of the ultrafast spectroscopy device mainly included two parts:the optical path design and the control program design.?1?The optical path included pump light path and probe light path.?1?Pump light path:the monochromatic light emitted by the light source was focused on the sample after passing through the optical delay line to excite the sample.?2?Probe light path:fundamental light of 800 nm was focused on a sapphire sheet of 2 mm in thickness to produce a broad spectrum of super-continuous white light after it was attenuated to about 1?J,.The probe light was focused on the sample through the lens after passing through the sample,then optical fiber collected and introduced into spectrometer.?2?The core of control program was designed to collect and process data.The implementation and innovation of the control software for data acquisition and processing ware as follows:?1?We use a 1/2 frequency division chopper to synchronize the pumped optical path with the laser pulse and reduce its frequency from 1 KHz to 500 Hz.?2?The signal light is collected and entered into the spectrometer through optical fibers.And the external trigger port of the spectrometer was synchronized with the chopper's 500 Hz output signal.The exposure time of reading signal light by spectrometer is 1 ms.?3?In the process of ultra-fast spectral detection,every step forward have 1 second pause time.The intensity of detection light with or without pump light is read separately,and the sample is detected by averaging the reading of 100 consecutive absorbance differences.We tested and verified the function of the device,which show us that the device basically meet the pre-target of the design task,realized the expected basic functions,and could complete the ultra-fast dynamic process detection of some samples.In this paper,we built a femtosecond Z-scan device and a femtosecond ultrafast spectroscopy device,including the optical path design and control program design of the two devices.The construction of Z-scanning device and ultra-fast spectroscopy device provides a good experimental platform for studying nonlinear optical absorption properties of materials.We detect the change of the nonlinear absorption coefficient of the material through the Z-scan device,and further detect the photochemical and photo-physical mechanisms of the nonlinear absorption characteristics of the material through the ultrafast spectral detection device.