Ultrafast Optics

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

- How many millimetres light travels in vacuum in 1 ps.

- How the spectral bandwidth relates to the pulse duration and vice versa.

- How broad spectrum you need to generate a 100 fs pulse centered at 800 nm.

- Why it is easier to generate femtosecond laser pulses than picosecond ones.

- What is the order of magnitude of the temporal scale of molecular processes (e.g. vibrations period in simple diatomic molecules)?

- What is the peak intensity available with femtosecond pulses and how to estimate it from the average power, repetition rate and pulse duration.

- What is the fundamental concept behind nonlinear optical processes in transparent dielectric media.

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

- How a femtosecond pulse is described as the electric field amplitude vs. time.

- What is the concept of the pulse envelope, carrier frequency, instantaneous frequency and phase vs. time.

- What a Fourier transform limited pulse is.

- What a pulse chirp is and how it affect the pulse in time and frequency.

- The concept of the Time-Bandwidth product.

  Home assignment:

    1. The instantaneous frequency tells us how the frequency changes as we scan across the pulse in time.
        The group delay tells us how different frequencies arrive at different times.
        Using the steps similar to these that were used to derive the formula for the instantaneous frequency, derive the formula for the group delay.

    2. Draw the electric field amplitude for a Gaussian pulse with positive linear chirp.
        Draw the pulse intensity and phase vs. time.
        Draw the pulse intensity and phase vs. frequency.

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

- What a superposition of many monochromatic waves equally spaced in frequency, with their phases locked, is. What the relation between the number of these waves and their spacing and the resulting pulse width and spacing in time is.

- How a CW laser is built, what an amplification bandwidth is and what the cavity modes are and how their spacing relates to the cavity length.

- How to "turn" a CW laser into a pulsed (femtosecond) laser with passive mode-locking (nonlinear element + dispersion control).

  Home assignment:

    1. Find a commercial femtosecond laser (oscillator) specs and note the parameters: average power, pulse duration and repetition rate. From these three numbers, calculate:

    - the pulse energy,
    - the pulse instantaneous power,
    - the maximum possible pulse instantaneous intensity (assume reasonable focusing condition, with e.g. a single lens of moderate NA).

    2. Find at least three different approaches to passive mode-locking of fiber lasers (e.g. saturable absorbers deposited on fibre end/side, nonlinear polarization rotation, NOLM, NALM) . Make sure that you understand each technique at the basic (qualitative) level and that you can explain its fundamental concepts in a few sentences.

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

- The concepts of laser and parametric amplifiers and the differences between them.

- The concept of the multipass and the regenerative (regen) laser amplifier.

- The fundamental ideas behind Chirped Pulse Amplification (CPA).

- What limits the pulse stretching in time in a typical grating streatcher.

- Why many high-power amplifiers have two stages.

  Home assignment:

    1. Find a commercial femtosecond CPA amplifier (crystal of fiber) specs and note the parameters: average power, pulse duration and repetition rate. From these three numbers, calculate:

- the pulse energy,
- the pulse instantaneous power,
- the maximum possible pulse instantaneous intensity (assume reasonable focusing condition, with e.g. a single lens of moderate NA).

    Compare these numbers with the results for an ultrafast oscillator you got last week.

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

- How vibrational molecular wavepackets are generated in molecules and how we can study their dynamics with pump-probe techniques.

  Home assignment:

    1. Explain the fundamental concept of the pump-probe ultrafast spectroscopy. Draw the schematic of a basic experimental setup. If you want to scan the delays in this experiment up to 10 ps with 10 fs resolution, what are the parameters of the translation stage you need to use in the delay line? Find the appropriate stage in one of the suppliers on-line.

    2. What are the two very efficient processes in biology that involve light absorption and why are the initial stages of these processes ultrafast?

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

- Concepts of the phase velocity, the group velocity and the group velocity dispersion in the context of femtosecond light pulse propagation.

- How the dispersion curve looks like for a typical glass in the visible and IR region.

- How the dispersion curve looks like for a typical optical fiber and what determines its shape (apart from the refractive index/indices).

- How to interpret the units of GVD and GDD.

  Home assignment:

   1. Check what is the maximum negative GDD (in fs²) offered by broadband chirped mirrors at 800 nm available on the market.

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

1. How femtosecond pulses can be shaped.

2. How a typical 4f shaper is built.

3. What modulators can be used in 4f shapers (LC, AOM, deformable mirrors), how they work and what are their pros and cons.

4. What limits the shaper performance - theoretical and technical limits.

  Home assignment:

    1. Draw a sketch of the 4f zero-dispersion stretcher with diffraction gratings and lenses. Mark the important distances and position of the Fourier plane.

    2. Explain how the liquid crystal spatial light modulator changes the phase and amplitude of the transmitted light.

   Lecture video (YT link)

   Lecture notes (.pdf)

  After the lecture check if you know/understand: 

    1. How the anharmonic potential experienced by electrons leads to nonlinear polarization and thus to nonlinear optical response in transparent dielectrics (at a single atom level).

     2. What are the conditions required for a second-order nonlinear process (e.g. SHG) to occur in a medium.

     3. How third order (Kerr) nonlinearity manifests itself in space (Kerr lensing) and time (SPM, e.g. in fibers). 

Home assignment:

     1. Find out what are the maximum possible efficiencies available in the second harmonic generation with femtosecond laser pulses.

   Lecture video (YT link)

   Lecture notes (.pdf)

After the lecture check if you know/understand: 

     1. What the phase matching is and why it is important for macroscopic nonlinear response. How it can be understood as energy and momentum conservation in the "photon destruction/creation" picture.

      2. How phase matching is achieved in birefringent nonlinear crystals.

      3. What type I and type II phase matching in birefringent crystals are.

   Home assignment:

    1. Find at least five different crystals that are commonly used for SHG in the UV, VIS and NIR spectral range. Make sure you check their "commercial" names (such as "KDP") as well as their proper chemical names and the spectral tuning ranges.

    2. Find the characterists of a commercially available microstructured fiber for supercontinuum generation. What are their important parameters?

   Lecture video (YT link)

   Lecture notes (.pdf)

After the lecture check if you know/understand: 

    1. What the basic concept of coherent control is. How it is done and what it can achieve.

    2. Why genetic algorithms are used in coherent control experiments. How a simple genetic algorithm works.

   Lecture video (YT link)

   Lecture notes (.pdf)

After the lecture check if you know/understand: 

    1. How autocorrealtion is used to characterize ultrashorl laser pulses - you can draw a schematic of a simple scanning autocorrelator and explain how it works. What the difference is between intensity (collinear and non-collinear) and interferometric autocorrelations and how they are measured.

     2. How a single shot autocorrelator uses the spatial degree of freedom to measure the whole AC signal at once.

     3. How SHG FROG works - you can draw a schematic of a simple SHG FROG setup, explain how the measured signal is generated and explain (qualitatively) how this signal is processed to get the pulse electric field data.

Home assignment:

    1. Find a translation stage model that you would use to build a scanning interferometric autocorrelator for 100 fs pulses from a Ti:Sapphire oscillator. What are the most important parameters of the stage?

   Lecture video (YT link)

   Lecture notes (.pdf)

After the lecture check if you know/understand:    

    1. How it is possible that two pulses separated in time (they E-fields do not overlap) can interfere to produce fringes in the spectrum. Hint: look at how a short pulse looks like in time-space while it propagates in a prism compressor (Lecture 6). How this interference will look like if the pulse separation equals the inverse of the laser repetition rate.

     2. How SHG SPIDER works - you can draw a schematic of a simple SHG SPIDER setup, explain how the measured signal is generated and explain (qualitatively) how this signal is processed to get the pulse electric field data.

     3. How the two methods, FROG and SPIDER, compare. What their pros and cons are and what the limitations of these techniques are.

Home assignment:

    1. Find a commercially available SPIDER and check the parameters it offers. Think about what determines these parameters.

   Lecture video (YT link)

    Lecture notes (.pdf)

After the lecture check if you know/understand:    

    1.What the advantages of using short (femtosecond) laser pulses to machine various materials are, compared to using long (or CW) laser sources.

    2. How femtosecond pulses enable additive manufacturing in small scale (Direct Laser Writing).

    3. What the basic concept of generating attosecond pulse by high harmonic generation in gases is. You should be able to draw a schematic of a setup and explain the processes involved (Keldysh model of HHG).

   Home assignment:

    1. Find a few (2-3) companies offering laser micromachining (cutting) services with ultrashort pulses and check what they offer (materials, resolutions, quality, speed, etc.).

    2. Find a company that offers direct laser writing services and/or workstations and see what they offer (resolution, materials, speeds).