Closed-Cycle Cryostat for Optical Measurements
MCCHeC-4K
Closed-Cycle Cryostat for Optical Measurements MCCHeC-4K
 
This closed-cycle helium cryostat uses a Gifford-McMahon (GM) cryocooler. A compressor circulates helium gas in a closed loop, achieving temperatures of 4 K or below in approximately 2–3 hours without liquid cryogens such as liquid helium. A vacuum shroud with synthetic fused silica windows and a radiation shield support optical experiments such as transmission and photoluminescence (PL) measurements. The standard support stand includes sample angle adjustment and XYZ positioning mechanisms for flexible optical alignment. A temperature controller provides stable temperature control from 4 K to room temperature. The detachable tail section allows easy sample replacement by venting the sample vacuum space at room temperature. The air-cooled compressor operates on a 100 V AC power supply and requires no special installation work, making this cryostat a convenient platform for optical measurements.

■ Specifications
Minimum Temperature 4 K or below (at the heat exchanger)
Cooling Method Cryocooler (no liquid cryogens required)
Cooldown Time Approximately 150 minutes
Temperature Range 4–300 K (at the heat exchanger)
Temperature Stability ±0.1 K
Sample Environment Vacuum
Optical Windows Vacuum shroud with synthetic fused silica windows on four sides
Radiation Shield • From 270° to 90°: slit with a height of 10 mm and an angular width of 180°
• At 180°: 10 mm diameter aperture
Compressor Air-cooled, for indoor installation
Ambient Operating Temperature 4–38°C
Power Supply Single-phase 100 V AC (50/60 Hz)
Power Consumption Maximum 1.5 kW (during cooling)
Required Power Supply Capacity 2.5 kVA

*Specifications and appearance are subject to change without notice for product improvement.

■ Cryostat Overview and Applications in Optical Measurements
A cryostat is a system used to cool a sample to cryogenic temperatures and maintain it at those temperatures for optical measurements and material characterization. This product employs a closed-cycle system that requires no liquid cryogens. Helium gas is circulated through a cryocooler to achieve temperatures of 4 K or below in approximately 150 minutes. A vacuum chamber and radiation shield minimize heat input, while a temperature sensor and heater enable precise temperature control with a stability of ±0.1 K.

For optical measurements, synthetic fused silica windows on multiple sides accommodate a wide range of configurations, including laser excitation, emission observation, and transmission and reflection measurements. Representative applications are described below.
● Photoluminescence (PL) Measurements:
A low-temperature environment that suppresses thermal excitation is essential for precise evaluation of excitons, defect levels, and bandgap properties in materials. Measurements near 4 K enable high-resolution emission spectroscopy and the observation of fine spectral structures, making them useful for evaluating the quality of semiconductors, quantum dots, and two-dimensional materials.
● Electroluminescence (EL) Measurements:
When evaluating light-emitting devices such as LEDs and laser diodes under current injection, low-temperature measurements help clarify recombination processes and reveal the effects of suppressing nonradiative losses. Examining the temperature dependence of emission efficiency and spectral purity enables assessment of device performance limits.
● Photodetector Characterization:
Photodetectors such as photodiodes and infrared sensors exhibit significantly reduced dark current at low temperatures, allowing accurate evaluation of their intrinsic sensitivity and noise characteristics. Analysis of quantum efficiency, response speed, and temperature dependence supports the design of high-performance detectors and their use in advanced fields such as astronomical observation and quantum communications.
This product serves as a reliable cooling platform for stable, reproducible low-temperature optical measurements in a wide range of applications, including materials development, fundamental research, and product evaluation.

■ What Is Internal Quantum Efficiency (IQE)?

Internal quantum efficiency (IQE) is a measure of the fraction of carriers (electrons and holes) generated in a semiconductor that recombine radiatively, emitting light.

Definition:
IQE = Number of radiatively recombined carriers / Total number of recombined carriers

IQE is an important indicator for evaluating the effects of nonradiative recombination in materials and structures.

■ Evaluating Relative Internal Quantum Efficiency (IQE) Using Low-Temperature PL Measurements

In this method, the PL intensity at low temperature (typically around 10 K) is taken as the maximum emission due to radiative recombination. Relative internal quantum efficiency (IQE) is then calculated by comparing it with the PL intensity at room temperature (300 K).

Assumptions:
• Nonradiative recombination is suppressed at low temperatures, and radiative recombination dominates.
• The low-temperature PL intensity can therefore be regarded as the ideal, maximum emission intensity.
• At room temperature, increased nonradiative recombination reduces the PL intensity.

Approximate formula for relative IQE:
IQE300K = I300K / I10K

where:
I300K: Integrated PL intensity at room temperature
I10K: Integrated PL intensity at low temperature (maximum emission)

The IQE obtained using this method is a relative value rather than an absolute value, but it is highly useful for evaluating the temperature-dependent effects of nonradiative recombination.

■ Steps for Calculating Internal Quantum Efficiency (IQE) Using Low-Temperature PL Measurements
  1. Measure Temperature-Dependent PL Spectra
    Cool the sample using a cryostat and measure its photoluminescence (PL) spectra at multiple temperatures from 10 K (or below) to room temperature (300 K).
  2. Calculate the Integrated PL Intensity
    Integrate each PL spectrum over wavelength to obtain the total PL intensity (integrated intensity) at each temperature.
    I(T) = ∫ PL(λ, T) dλ
  3. Calculate Relative IQE
    Calculate relative IQE from the ratio of the integrated PL intensity at room temperature (300 K), I300K, to that at low temperature (10 K), I10K.

    Formula:
    IQE300K = I300K / I10K

    where:
    I300K: Integrated PL intensity at room temperature
    I10K: Integrated PL intensity at low temperature (maximum emission)

    This ratio serves as an indicator of the temperature-dependent effects of nonradiative recombination. The IQE obtained using this method provides a relative evaluation rather than an absolute value.
  4. Evaluate Nonradiative Recombination (Optional)
    Perform an Arrhenius analysis of the temperature dependence of PL intensity to extract the activation energy associated with nonradiative recombination.
    I(T) = I0 / (1 + C × exp(-Ea / kT))
    where:
    Ea: Activation energy of the nonradiative process
    k: Boltzmann constant

■ What Is External Quantum Efficiency (EQE)?

External quantum efficiency (EQE) is defined as the ratio of the number of photons emitted outside a semiconductor device (such as an LED or solar cell) to the number of injected or generated carriers (electrons).

Definition:
EQE = Number of externally emitted photons / Number of injected electrons

EQE represents the device's actual light-emission efficiency. Measurement methods include absolute PL measurements using an integrating sphere and measurements under electrical drive.

■ Measuring External Quantum Efficiency (EQE) Using an Integrating Sphere (2π Measurement)

External quantum efficiency (EQE) can be calculated by measuring the light emitted by a device (spectral radiant flux) with an integrating sphere and comparing the emitted photon rate with the injected electron rate. The following method uses radiant flux measured over the forward hemisphere (2π solid angle).

  1. Measure Spectral Radiant Flux Using an Integrating Sphere
    Use an integrating sphere to measure the spectral radiant flux emitted by the device, Φλ [W/nm]. Calculate the emitted photon rate from the measured wavelength-dependent radiant flux.

    Photon rate formula:
    Nphoton = ∫ (Φλ × λ) / (h × c) dλ
    where:
    λ: Wavelength [m]
    h: Planck constant
    c: Speed of light
  2. Correct for Emission in All Directions (4π Conversion)
    The integrating sphere measures only the light emitted into the forward hemisphere (2π solid angle). Assuming isotropic (Lambertian) emission, the total photon emission rate into all directions can be approximated as twice the measured value.

    Ntotal ≈ 2 × Nphoton
  3. Calculate the Injected Electron Rate
    Calculate the number of electrons injected per unit time from the current I [A] flowing through the device.

    Nelectron = I / q
    where q = 1.602 × 10-19 C (the magnitude of the electron charge)
  4. Calculate External Quantum Efficiency (EQE)
    Calculate EQE from the ratio of the total photon emission rate to the injected electron rate.

    EQE = (2 × Nphoton) / Nelectron
    or:
    EQE = (2 × Nphoton × q) / I

    This allows the device's actual external light-emission efficiency to be evaluated using integrating sphere measurements.

■ Relationship Between External Quantum Efficiency (EQE) and Internal Quantum Efficiency (IQE)

EQE and IQE are related by the following equation:

EQE = IQE × ηout

where:
ηout is the light extraction efficiency,
representing the fraction of photons generated in the emitting layer that escape from the device.

Light extraction efficiency is affected by total internal reflection caused by refractive index differences, device structures such as reflective layers and surface patterns, and optical absorption.

IQE describes the light-emission efficiency within the material, while EQE describes the efficiency of light actually escaping from the device. IQE can reach a maximum of 100%, whereas EQE is typically lower.