Inserting quantum well (QW) structures into a p-n junction endows the same QW diode with the capability to be used as a light emission device, a modulator, an energy harvester or a detection device. The same QW active region generates an emission-detection overlap phenomenon when the QW diode functions as a light-emitting diode and a photodiode. Therefore, the QW diode has the ability to sense the light emitted from another diode with an identical QW active region. A variety of promising QW-based optoelectronic systems, including monolithic GaN photonic circuits and simultaneous illumination-imaging systems, have been demonstrated. An irreversible process exists between light emission and detection of the QW diode because the device can only detect, modulate and harvest shorter-wavelength light than that emitted by itself. Consequently, the electroluminescence (EL) spectrum partially overlaps with the responsivity spectrum in reality. It is of great interest to investigate the underlying mechanism of the irreversible process. On the basis of the partial emission-detection overlap of the QW diode, we unite energy conservation, gravitational field and energy diagram theory to study the irreversibility.
Figure 1. Measured EL and responsivity spectra of the InGaN-based QW diode; (b) comparisons between PRBS signals transmitted and received.
Figure 1 merges the EL and responsivity spectra of the InGaN-based QW diode (CREE XRE-Q5). The device has a thin-film flip-chip geometry with a square size of 948 µm × 948 µm. The wavelength-dependent responsivity spectrum allows the device to function as a wavelength-selective photodetector . The responsivity spectrum is also voltage dependent, endowing the diode with the potential for light detection, modulation or harvesting. As a light-emitting device, the emission intensity depends on the injection current, which makes the device suitable as an optical transmitter for light communication. A partial overlap exists between the EL and responsivity spectra. Therefore, a proof-of-concept wireless light communication system is established using two identical QW diodes, wherein one is used as the transmitter and the other functions as the receiver. The transmitter pulses its light according to a pseudorandom binary sequence (PRBS) data stream, and the receiver senses the light to convert the encoded light into electrical signals. The decoded PRBS signals that are directly characterized without additional preshaping or circuit amplification agree well with the transmitted signals, as shown in Fig. 1(b).
Figure 2. Schematic illustration of different operation modes of the QW diode: On the left, the device emits light. On the right, the device absorbs light.
Figure 2 schematically illustrates the light-emission and light-detection operation modes of the QW diode. In principle, there is a discrete set of energy levels in the conduction band. The quantized states become nearly indistinguishable in the real light-emission process, leading to a broad and continuous electroluminescence spectrum. The transitions from different conduction bands to the valence band, for example, from energy state E1 to energy state E0, occur by emitting light. The frequency of the light is determined by the QW energy gap and given by
ωc-v =（E1-E0）/h (1)
h is the Planck constant, which is the proportionality constant relating the energy of a photon to its frequency. The Planck constant h can be accurately determined through experimental measurements. In fact, the individual constituents that make up our world strongly interact with the environment. According to the law of motion in a gravitational field, an object exists in the space-time fabric by occupying a position and taking up a certain amount of time. The position that the object takes depends on the gravitational field, which in turn depends on the position of the object. The situation can become quite complicated because the object always moves from one place to another. Therefore, photons at different positions have different quantized states in a gravitation field even if they have the same frequencies. Now, consider a transition from conduction band E1 to valence band E0 in a gravitational field. In falling distance H from energy state E1 to energy state E0, some work will be done to move the mass. The amount of work done is
g is the free fall acceleration in a gravitational field, and c is the velocity of light. Conversely, the transition from energy state E0 to energy state E1 occurs when the photon with frequency ωv-c is absorbed. During this return trip, the mass is , and thus, the work against the gravitational force is
Since the energy is conserved, the following must hold:
hωc-v+E1gH/c2 =hωv-c+E0gH/c2 (4)
Because the height H must be positive and energy state E1 is higher than energy state E0, we have
The required frequencies of photons that allow holes in the valence band to transition to different conduction bands cannot be less than the frequencies of photons in a gravitational field, leading to an irreversible process. The heights are normally very small, and consequently, the frequency differences between and are small. Irreversibility, which is the most natural characteristic of all phenomena, comes from the gravitational field. It is a natural law. It is the basic mechanism for the emission-detection overlap phenomenon, in which the QW diode can only detect and modulate higher-energy photons than those emitted by itself.
On the other hand, we use the mean molecular kinetic energy as the definition of the temperature T. The scale of temperature is arbitrarily defined, and thus, the mean energy is linearly proportional to the temperature. Because the kinetic energy must be positive, the efficiency of an engine is always less than unity. During an irreversible process, the total entropy of the system always increases, which is caused by the gravitational field. Furthermore, there is a fundamental interchangeability between the mass and energy of an object in the space-time fabric. According to the law of conservation of energy, a very large amount of energy will be liberated when we make the mass of the object disappear in the form of photon radiation. Conversely, if we have the ability to convert photons back into the object, then we can come up with an equation regarding the required energy E and velocity v:
Then, we can obtain
s∙v ≥h⁄m (8)
The symbols t and s are the time and space that the force acts through, respectively. Physically, we can conclude that the irreversible process is a differential manifestation of the Heisenberg uncertainty principle. An individual object cannot be completely isolated from the environment, and their interactions are complicated and manifest in various forms through energy, mass, space and time.
By utilizing their spectral emission-detection overlap, two identical QW diodes separately act as a transmitter and a receiver to establish a wireless light communication system. Our postulations regarding energy conservation, the gravitational field, quantum physics, the second law of thermodynamics and energy diagram theory all fit together and explain the experimental results well. The irreversible process is the underlying mechanism for the partial overlap between the EL and responsivity spectra of the InGaN-based QW diode. Irreversibility, the most natural characteristic of all phenomena, is caused by the gravitational field.