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Physics for Biologists (BAPHY102)

Course Information

• Professor: Dr. A. Joseph Nathanael, MSc, MPhil, PhD, PGDCA, MRSC.
• Institution: Vellore Institute of Technology (VIT), Vellore - 632 014.
• Semester: Winter Semester 2025-26

Module 4: Lasers

Detailed Syllabus in Module - 4

• Topics Covered:
  • Laser characteristics:
  • Spatial and Temporal Coherence
  • Absorption and Emission Process
  • Monochromaticity
  • Coherence
  • Directionality
  • Brightness
  • Short-time Duration
  • Einstein coefficients and their significance
  • Einstein Thermodynamic Treatment and correlation with Planck's Radiation Law
  • Population inversion - two, three, and four-level systems
  • Need for population inversion
  • Importance of stimulated emission
  • Types of lasers based on the number of levels and importance of metastable state
  • Pumping Schemes:
  • Threshold gain coefficient
  • Components of a laser
  • Different pumping schemes
  • Amplification of photons for sustained output
  • Basic components of lasers (He-Ne, CO2)
  • Types of Lasers:
  • Atomic and molecular lasers
  • Resonant energy transfer
  • Importance of Brewster's window
  • Lasers in Biology:
    • Principle, construction, and working of Nd:YAG laser
    • Applications in surgery, ophthalmology, and dentistry (qualitative)

Class Overview

Today's Class Topics

• Absorption and Emission Process:
• Monochromaticity
• Directionality
• Brightness
• Coherence (Temporal and Spatial Coherence)

Introduction to LASER

• LASER: Light Amplification by Stimulated Emission of Radiation
• History:
  • 1917: Einstein demonstrated stimulated emission
  • 1954: First MASER developed by Charles Townes
  • 1960: First LASER created by Theodore Maimen
• Analogy: Laser is analogous to an oscillator which consists of an amplifier and feedback.
  • An oscillator is a circuit that produces a continuous, repeated, alternating waveform without any input.

Applications of Lasers

1. Laser pointers
2. Printers
3. Barcode scanners
4. Optical tweezers
5. Laser cutting
6. Intruder detection
7. Tattoo removal
8. Eye surgery

Absorption and Emission Processes

Fundamental Concepts

• Stimulated Absorption
• Spontaneous Emission
• Stimulated Emission
• Average Lifetime: approx. $10^{-8} ext{ seconds}$ or $100 ext{ ns}$

Detailed Process Descriptions

• Spontaneous Absorption: Electron jumps from lower to higher energy level by absorbing a photon.
• Spontaneous Emission: Electron releases a photon while transitioning from higher to lower energy level.
• Stimulated Emission: A photon stimulates an electron to emit another photon, creating two identical photons that are always in phase with one another.

Characteristics of Lasers

Major Laser Characteristics

1. Monochromaticity:
   • Laser emits a single wavelength, in contrast to white light, which is composed of multiple wavelengths.
2. Coherence:
   • All photons share the same phase and polarization, resulting in high-intensity superposition.
3. Directionality:
   • Laser light is emitted in a single direction with minimal spreading compared to ordinary light.
4. Collimation:
   • Narrow, collimated rays result in powerful performance over long distances without significant divergence.
5. Polarization:
   • Laser light is polarized in a single plane.
6. High energy concentration:
   • Energy is concentrated in a small spatial region, defined as intensity—a measure of energy per unit time and area.
7. Bandwidth:
   • Defined as the frequency range of maximum intensity in the laser source.

Comparison: Laser Light vs. Thermal/Ordinary Light

Coherence

Definition

• Coherence: Ability of waves to interfere; relies on constant phase relationships between waves.

Types of Coherence

1. Temporal Coherence:
   • Measures the correlation between wave values at two different times, indicating how monochromatic the wave is.
   • Coherence Time ($au_c$): Time for which a wave maintains a phase relationship, defined as $au_c = \frac{1}{ riangle<br />\nu}$ where $riangle<br />\nu$ is the bandwidth.
   • Coherence Length ($L_c$): Distance a wave travels in time $au_c$, expressed as $L_c = c au_c$.
   • Example: For ordinary and laser lights, coherence times are approx. $10^{-10} ext{ s}$ and $10^{-3} ext{ s}$ respectively, leading to coherence lengths of $3 ext{ cm}$ and $300 ext{ km}$.
2. Spatial Coherence:
   • Refers to phase relationships between waves at different spatial points in the beam.
   • Maintains phase relationships independent of time, relevant to coherence area defined by limited spatial points.

Comparison of Temporal and Spatial Coherence

• Temporal Coherence
  • Focuses on phase correlation over time at a given point.
  • Measures with interferometers like Michelson.
• Spatial Coherence
  • Concerns phase correlation across different points at a specific time.
  • Measures with interferometers like Young's double-slit.

Example Calculation

• Problem Statement: Calculate coherence length and coherence time for light with wavelength $5890 ext{ Å}$, and wave trains of length $16 ext{ } ext{λ}$.