에너지환경화학
Table of Contents
Light, electron (microscope), electricity can be used to probe them
1. Spectroscopy
1.24 eV = 1000 nm = 10,000 cm-1 1 eV = 1240 nm = 8065 cm-1
- Non-destuctive
- High sensitivity (ppb level)
- Mapping Capable
The pratical methodology is given in spectroscopy (engineering).
1.1. Measurement
The shutter speed, delay is important to properly isolate the signal.
Frequency filter is also used to isolate desired signal.
2. Semiconductor
Wafer Spec
- Polished on one side or both side.
- Resistance of silicon
- type of doping
Photoluminescence is used to probe defects in crystal.
Classification
- Chemisty-based
- Elemental semiconductor (Si, Ge - interesting characteristic at IR)
- Compound semiconductor (GaAs, GaN)
- Physics
- Direct bandgap
- Indirect bandgap (silicon): not easily transition -> leads to photovoltaic phenomenon
2.1. LED
Every LED is based on GaN
Color of LED is relaated to temperature. Blue is hardly affected by thermal excitation, while red is heavily affected.
2.2. Applications
- Microelectronics
- Optoelectronics
- Power Electronics
- Sensors: MEMS
2.3. Characteristic
Optical
- Band gap
- Recombination dynamics
Chemjical/Electrical
- Composition and State
- Carrier transport
Structural
- Defects and Strain
- Crystallinity
- Morphology
2.4. Piezoelectricity
Piezostage: actuator, motor for precision equipment (sub nano)
- AFM, STM control
- Lithography control
3. Optics
Halas Nanophotonics group, Sherwingroup.itst.
3.1. Non-linear Crystals
BBO LiMBO3
Differential frequency generation (DFG)
3.2. 감광
Hamamatsu: photodiode, image sensors, PMT
PMT has been used for light detection for over 50 years.
Silicon detector works for 200 nm to 1000 nm, and slow varing signals. PMT can work for those edges cases.
When dealing with PMT, vary the voltage slowly, and not expose to intense light.
3.3. Laser
Coherent 미국 레이저 회사. 전세계 1, 2등을 다투는
Laser burns dust in air, producing popping sound.
\(10^14\, \rm J\) per pulse that lasts for nanoseconds of time.
광학 장비는 지문이 묻으면 탈 수 있는 경우가 있다.
CW laser generally has weak power, while pulse laser is strong.
CW laser to nanoseond laser can produe short wavelength laser, while picosecond and femtosecond laser cannot make precise wavelength (Heisenberg uncertainty)
Ideally, and with some post-processing in practice
- collimated
- monochromatic
- coherent
3.3.1. Mode locking laser
- Pulsed laser
A mode-locked laser is a pulsed laser that generates ultrashort pulses by locking the phases of many resonant modes together.
A mode-locked laser is a laser where many different longitudinal cavity modes are forced to keep a fixed phase relationship with each other.
What this does:
- In an ordinary laser, modes oscillate independently.
- In a mode-locked laser, the modes line up in phase.
- Their interference produces very short, intense pulses instead of continuous output.
So:
- Mode locking = phase-locking many cavity modes
- Result = a pulsed laser, often with extremely short pulses (picoseconds, femtoseconds, etc.)
Why it matters:
- Gives very high peak power
- Used in ultrafast spectroscopy, precision measurements, microscopy, and telecommunications
In one line:
3.3.2. Nd:YAG laser
Pulsed laser
Nanosecond pulse. Being superceded by femtosecond laser. Good for academic use, but not used for industrial use. American companies stopped producing this.
The power is supplyed through flash lamp on the side of Nd:YAG crystal.
3.3.3. Dye laser
Change the color of the indicent laser within specific range.
3.3.4. Pulse Width
Time duration and pulse width are related. 1 um is equivalent to 3 fs.
3.3.5. Application
Diode laser, nanowire laser.
3.3.6. Lasing material
- Amplification medium
Gas, liguid, solid-state, semiconductor.
3.3.7. Mechanism
10-7 at excited state, and 10-3 at metastable state
If there are multiple seeds, then the coherence is degraded.
Seed injection can be used in which another weak laser is used to seed the high-power laser.
Light in specific direction is selected by the resonance of cavity.
Two ways to extract the light out of the cavity:
- High reflection coefficient (up to 99.999%) and half reflecting mirror can be used on each side.
- Control the mirror's reflection coefficient using electrical signal (in pulse laser). Q-switching.
3.3.8. Laser Display
LG did produced laser display.
Technology to create display with lens to each pixel was also developed.
3.3.9. Cavity Type
Frequencies that match the Cavity (Resonance) Mode and Gain Bandwith gets amplified. Filtering is required for monochromatic light.
Plane-parallel Fabry-Perot
- Hard to align
- \(\infty-\infty\)
Conentric
- \(L/2-L/2\)
Confocal
- \(L-L\)
3.3.10. Modes
\[ \mathrm{TEM}_{nm} \] there exists \(n\) plane nodes horizontally, \(m\) plane nodes vertically.
3.3.11. Diffraction Angle
10-3 rad
3.4. Amplified Spontaneous Emission
- ASE
Spontaneous emission compete with stimulated emission in laser.
Linewidth of few to tens of nanometer, with less directionality. It can be more powerful than laser in some cases.
3.5. Lamp
Incoherent light source. LED used to be incoherent, but there are LED that emit coherent light. About tens of nanometers.
LED does not undergo stimulated emission and does not require cavity.
3.6. Coherence
Multimode HeNe has 10 m coherence length Single-mode HeNe has 100 m coherence length Single mode fiber laser has 100 km of coherence length.
A color filter increases spectral purity by passing a narrower range of wavelengths, which can increase the coherence time/length somewhat.
3.7. Consistent Source
Consistency is important for experiment \[ 1\, kW/m^2 = 1 Sun \]
AAA: power consistency, spatial consistency, spectral similarity to the solar radiation
3.8. Superresolution Microscope
STED
Resolve emission by single photon. Data processing for each photon.
Decision threshold for being a single photon is experimental.
It has been about 15 years since it started to be used in biology.
3.9. Optical Fiber
- Edge filter
- Long-pass filter
- Short-pass filter
- Band-pass filter
- Notch filter
3.10. Optical Density
Optical density of 7 means signal of decreased by the factor of 10 to the negative 7.
Optical density represent the absorbance. \[ \mathrm{OD} := - \log_{10}T = \log_{10} \frac{P_{\rm in}}{P_{\rm out}}. \]
For an optical fiber of length \(L\), \[ P(L) = P_0 e^{-\alpha L} \] and \[ \mathrm{OD} = \frac{\alpha L}{\ln 10} \] where \(\alpha\) is the power attenuation coefficient.
4. Luminescence
Photoluminescence is used in solid material, while fluerescence is used for moleules.
- Fluorescense 10-8 10-4 s
- Phosphorescence
Electron excited electrons can be both in singlet and triplet states.
Probe laser, pump laser.
4.1. Jablonski Diagram
Energy levels of electronic states and vibrational levels of a molecule are arranged vertically, grouped by spin-multiplicity horizontally. Ground vibrational states are indicated with thick lines.
Radiative transitions is drawn with straight arrow, and nonradiative transitions are drawn with squiggly arrows.
4.2. Grotrian Diagram
- Term Diagram
Show allowed electronic transitions between energy levels of atoms.
4.3. Franck-Condon Diagram
- Franck-Condon Energy Diagram
Potential energy diagram along a nuclear configuration.
4.4. Laser-Induced Fluorescense
Excitation LIF The wavelength of source laser sweeps certain range.
Emission LIF The source wavelength is fixed, and the emitted signal is analyzed with spectroscope (or monochromator).
4.5. PL Spectrum
Purity correlates to the intensity of signal. High density of nonradiative centers (point center, dislocation, surface center) is present if the intensity is low and broad
Thermal broadening occurs.
4.6. Micro Raman
4.7. Fluoresence Microscope
생물학에서:
- Bright-field microscope 명시야 현미경: 밝은 배경 위로 빛을 표본에 투과시켜 관찰. 염색 필요
- Dark-field microscope 암시야 현미경: 어두운 배경 속 빛을 비스듬히 빛을 산란시켜 관찰
- 형광 현미경
특정 부분을 염색할 수 있다. 레이저 (or lamp)로 형광을 발생시킨 후, 특정 파장만 필터링하여 관측
입사 파장과 형광 파장을 선택할 수 있도록 필터가 부품화되어 있다 (objective lense for turret).
다색사진은 색별로 각각 찍거나, 여러 카메라로 동시에 찍는다.
5. Others
Samsung is eletronics, and LG is chemistry.
물질의 퍼포먼스를 측정해야 하기 때문에 측정이 중요하다.
회사에서는 제품을 직접적으로 판매하는 쪽이 파워를 갖는다.
6. Textbooks
- An introduction to LASER and their application.